The preservation of antiquities

by Friedrich Rathgen

I. Preservation of Objects composed of Inorganic Substances.

(a) Limestone.

The method formerly employed for the preservation of decaying and crumbling limestones was that of simple impregnation, and this is still followed in some cases which will be subsequently described. But as the active agents of destruction are not removed by this method the result is not always satisfactory, and an attempt is now made where possible to remove those salts which are soluble in water, especially the sodium chloride, by the simple process of steeping in water.

If the presence of salt in a limestone is evidenced by a crumbling surface, or by the taste when touched with the tip of the tongue, the question will arise whether it will bear steeping, or whether the destruction is so far advanced that, on being placed in water, the limestone will fall to pieces.

If fractures or cracks can be actually seen in the stone, steeping is contra-indicated, but if the condition is less manifest, a preliminary test should be applied.

A large drop of water, e.g. about 25 cubic centimetre in volume, should be placed on the surface of the stone, and any changes which take place should be carefully noted. If the drop is not absorbed by the stone, it may be due to a layer of dust or to previous saturation with solutions of resin or varnish. Dust may be removed with a moderately hard brush or by rubbing with the finger, but if a limestone has been previously saturated with a varnish solution it will not absorb the water, and is therefore hardly suitable for this treatment. If the drop is absorbed, an iron or a steel point, such as the thick end of a medium-sized needle, should be used to ascertain whether the limestone at the moistened spot shows the same degree of hardness as elsewhere. If this is found to be the case, especially if the pieces are of a large size, the test should be repeated at other spots, including the back of the stone, for a hardened layer on the front aspect may be the result of former treatment. If the result of this examination is satisfactory and no soluble colouring is observed on the limestone, the process of steeping may be applied. If on the other hand the moistened area has become softer, or has become to any extent swollen, or if any colours which may be present show signs of disappearance or fading, treatment with water must be abandoned.

The difference of behaviour is easily explained, for limestones do not always consist of lime only, or, more correctly, carbonate of lime (CaCO₃), but often contain sand or clay, and the greater the amount of clay the more readily the stone softens or swells. Even when a limestone has borne this preliminary test satisfactorily it should be carefully watched for an hour or two after immersion and should be at once removed from the water should any further changes appear.

Steeping. The procedure to be observed is as follows. The rapidity with which the salts may be removed varies directly with the quantity of water used in steeping. The treatment of objects of small size presents no difficulties; any vessel of glass, porcelain, or earthenware will serve the purpose. Towards the end of the treatment distilled water should be used, or in default of this, clean rain water should be used in preference to that from a well. For larger objects (as, for example, the large limestone blocks of the Meten Chamber mentioned above, some of which were 1 metre in length and 1⁄2 metre or more in breadth and thickness), it is convenient to use wooden tubs fitted with a tap in front to draw off the water, and so tilted by means of stones placed underneath that the tap may be at the lowest point. The objects should not touch the bottom of the vessel. Smaller pieces may be suspended or may be made to rest on glass rings or supports of glass rods; while large objects should be laid on blocks of wood so placed as to allow the tub to be cleaned when necessary without removal of the blocks, the weight of which would otherwise entail much labour. The blocks should be as near to the surface of the water as possible, leaving a considerable depth of water beneath, for the heavier salt-laden water sinks to the bottom, thus bringing into contact with the limestone water with a smaller salt-content. The length of the steeping must depend upon the size and porosity of the limestone.

Under certain circumstances phenomena make their appearance which must not be neglected. Thus if the treatment extends over a considerable length of time, the wooden tubs should be provided with lids to prevent the access of light. This was found indispensable in the treatment of the blocks from the Meten Chamber when Berlin tap-water[83] was used, for when the tubs were open a large quantity of brown hydrated ferric oxide appeared on the limestone, the roughness of which rendered its removal an impossibility even with brushes. This oxide is produced by various forms of algae and bacteria which developed in such numbers that the sides of the tubs were frequently covered with a layer of slime, which under the microscope appeared as a confused web of transparent threads[84]. This was brushed off with soft brushes at least once a fortnight, for the slimy covering impeded the access of the water to the limestone. That in this case the ferric oxide was the result of the action of light was proved by the fact that only those blocks which were placed near the windows were discoloured, and that the discolouration was proportionate to the amount of light which fell upon them[85]. Again, after the treatment in the covered tubs some of the blocks became so black that they resembled blocks of coal rather than limestone. After exposure to light for a day or two, especially when the water had been drawn off, the discolouration disappeared without leaving any traces. The colour was doubtless due to a minute quantity of iron in the form of sulphide which, after oxidation in the air and light, became invisible upon the light yellow limestone. Under these circumstances the presence of sulphuretted hydrogen in the water, possibly produced by bacterial action upon the sulphates, was attested by the characteristic smell.

The enormous number of bacteria which develop in the water constitute a great hindrance to the process of steeping, and as to boil such a quantity of water as is required for these large objects is out of the question, frequent changes of water and frequent cleaning of the stone, wooden blocks, and tubs are the only remedies.

Examination of the Progress of the Steeping. The water should be changed at first daily, then by degrees every two, three, or four days, later on weekly only, until finally once a fortnight is sufficient. To ascertain the progress and completion of the elimination the quantity of chlorine in the wash-water may be determined by a simple method of titration[ [86].

The following short explanation may be of use, and the method is easily learnt. If a solution of silver nitrate is poured into a solution of common salt (sodium chloride), a white curdy precipitate is produced, a process which the following equation will explain:

NaCl + AgNO₃ = AgCl + NaNO₃.

The white precipitate is the silver chloride, whilst the sodium nitrate which is produced at the same time remains in solution and is therefore not visible. As always definite proportions of the two substances, silver nitrate and sodium chloride, react upon one another, by the use of a solution containing a known amount of silver nitrate we can determine the amount of salt, and hence of chlorine present. By cautiously inclining a burette (Fig. [13]) divided into tenths of cubic centimetres[87], the silver solution should be dropped into a beaker containing a definite volume of the solution to be examined for chlorine; the level of the silver solution in the burette should be read off before and after pouring out, and the number of cubic centimetres of the silver solution required to precipitate the chlorine will thus be known.

Fig. 13. Gay-Lussac’s Burette. 1⁄6 nat. size.

The process when carried out in this manner has one defect, for it is necessary to allow the precipitate to settle in order to see clearly whether an additional drop of the silver solution will produce further precipitation, or whether it will merely cloud the fluid; this defect can, however, be remedied by means of a so-called “indicator.” A few drops of a concentrated solution of neutral yellow potassium chromate should be added to the fluid to be examined, which is thereby coloured yellow; the solution is then shaken or stirred with a glass rod, while the silver nitrate is dropped in. Every drop of the silver solution will cause a red precipitate, the colour of which however disappears on stirring so long as there is any chlorine present; only when the silver solution has precipitated all the chlorine does the red colour become permanent, and thus the change of colour of the whole fluid from yellow to red shows with exactness the complete precipitation of the chlorine. For practical purposes all that is required is the so-called decinormal silver solution, and from the number of cubic centimetres of this solution which are required to precipitate all the chlorine the total amount of chlorine present can be readily calculated.

In steeping smaller objects before examination the whole of the water should be well stirred with a glass rod or poured two or three times from one vessel into another: 100, 50 or 25 cubic centimetres are then poured into a graduated glass or drawn up into a pipette. The water should be drawn up by suction slightly above the level of the mark upon the stem of the pipette, the upper end of which is immediately closed with the thumb. By slightly raising the thumb the water is allowed to run off until its upper surface is exactly level with the mark. The amount taken is then placed into a beaker (for 100 cubic centimetres a beaker of 400 c.c. capacity should be used), and, after the addition of a few drops of a solution of potassium chromate, is examined by titration. In the treatment of large objects, for which tubs are required, the necessary quantity of water may be drawn by means of a long pipette from the bottom of the tub, where the quantity of salt is always greatest, or through the tap at the bottom of the tub (as was done with the blocks from the Meten Chamber, in which case about 1 litre was drawn off into a glass out of which 100 c.c. were taken for titration). To obtain results which are comparable, care must be taken that the object is always as nearly as possible in the same quantity of water. After placing the larger blocks in the water, one examination should be made during the first few days, when the titration may require 20 c.c. or more of silver solution. There is no need to examine for chlorine while the water is being frequently changed: indeed, in order to economise the silver solution, this need not be begun until the second month, when the water is changed every fortnight.

As has been stated above, it is only necessary to read off the number of cubic centimetres of the solution used in the titration, for the decrease in these figures is a sufficient indication of the progress of the operation, while the diminution of the chlorine-content may be taken as an indication of the simultaneous removal of the sulphates[88]. In the treatment of small objects in distilled water, the process may be regarded as complete if the red colour is obtained on the addition of from one to two drops (i.e. about 1 ⁄10 to 1⁄ 5 cubic centimetre) of the silver solution. If, when tap-water is used, and is being changed at intervals of a fortnight or a month, the estimations give a constant result between 0·6 and 1·0, the treatment need not be carried further.

The accompanying table shows the figures obtained from three large blocks from the Meten Chamber. They represent the number of cubic centimetres of decinormal silver solution used for 100 c.c. of the water, which was changed every fortnight. The first column on the left shows the dates upon which the stones were placed in the tubs:

3 Feb. 1890	7 Apr.	4 May	2 June	25 Aug.	22 Sep.	4 Nov.	1 Dec.	12 Jan. 1891
	3·0	2·5	2·3	1·4	1·0	0·9	0·8	0·8
25 Apr. 1893	9 May	18 July	1 Aug.	25 Nov.	23 Dec.	16 Feb. 1894	29 Aug.	12 Sep.
	6·0	5·6	4·9	2·0	1·7	1·5	0·8	0·8
29 July 1893	28 Oct.	25 Nov.	23 Dec.	20 Jan. 1894	16 Feb.	1 Aug.	29 Aug.	12 Sep.
	3·0	2·0	1·5	1·3	1·1	0·7	0·8	0·7

When repeated examinations gave a fairly constant result of 0·7-0·8 cubic centimetre the process was regarded as complete, for Berlin tap-water itself contains small quantities of chlorine compounds, 100 c.c. requiring from 0·4 to 0·6 c.c. decinormal silver solution. Before using the water from a well or from waterworks, it should be examined to ascertain the number of cubic centimetres of silver solution required to produce the red colouration. As the amount of chlorine compounds in the water may vary it is advisable to repeat the examination[89].

The following table shows the rapidity with which salts can be completely extracted from small pieces of limestone. The limestones were placed in tap-water in three glass cylinders, each containing 2 litres; the amount of silver solution required for the water was 0·45 c.c. per 100 c.c.

	No. of days of soaking in water	1	1	1	2	2	4
1	Weight in grammes 107	3·4 c.c.	1·4	1·1	0·7	0·5	0·5	cubic centimetres of decinormal silver solution.
2	Weight in grammes 66	3·0 c.c.	1·2	0·7	0·5	—	0·5	cubic centimetres of decinormal silver solution.
3	Weight in grammes 47	3·6 c.c.	0·8	0·6	0·5	—	0·5	cubic centimetres of decinormal silver solution.

These figures show the numbers of the c.c. of silver solution used for every 100 c.c. of the water, which was changed after 1, 2, etc. days as shown above. After 9 to 11 days therefore the stones could be declared free from salt.

If the accuracy of the titration method be considered unnecessary, either on account of the small number or size of the objects to be treated, or for reasons of expense (the outlay required is however very small), a solution of unknown strength may be used. A comparison between the degree of turbidity produced on mixing the silver nitrate solution with the tap-water, and that produced with the wash-water, will enable the progress of the operation to be gauged.

Advantages and Disadvantages. Although steeping removes the cause of decay, i.e. the salts contained in the limestone, and although permanence may be considered as certain, there are certainly some disadvantages connected with the process, especially when the pieces, on account of their size, must remain in the water for some length of time. Some large and very thick blocks from the Meten Chamber required to be soaked for more than a year.

The small quantity of carbonic acid which is always found in water dissolves small quantities of calcium carbonate, thus the sharp contours of prominent parts may become somewhat rounded. Limestones which have developed fissures may, on immersion, lose small portions which might otherwise have remained attached, though probably for a while only. In such cases it must be carefully noted from which block, and from which part of it, the fragment has broken off, in order that it may be replaced[90].

Limestones which are much cracked, or which are likely to fall to pieces, should be wrapped round with gauze, or held together with twine, before they are put in the water.

In addition to the permanent preservation of the object some other smaller advantages of this method may be mentioned: for example, the layer of dust which is often present is removed and thus traces of colours may be brought out by the steeping which had been concealed by it. Thus certain remains of colour mentioned by Lepsius[91] as being still visible in his time upon some of the blocks from the Meten Chamber were no longer visible when we took them in hand. Moreover traces of green colouring which were visible after the treatment in the eyes of a few large figures in relief were probably evidence that colours had formerly been present.

Drying. When the steeping is finished the limestone is taken out to be dried. Small objects may be placed upon a glass ring, wooden tripod or some such appliance, which admits air on all sides, and may thus be dried by the air only. A piece of paper laid loosely over them will protect them from dust. In winter a hot stove, or similar source of heat, affords a satisfactory method of drying, but wet stones must not of course be placed directly upon the hot iron stove plate lest spots of rust should be produced upon the stone. Large blocks are preferably dried in drying chambers in which in summer time a strong draught is obtained by opening windows on opposite sides, and which in winter are strongly heated and opened every now and then for a short time. The limestones should be laid upon wooden blocks to allow air to pass beneath them, while they must be guarded from dust both above and at the sides with sheets of paper. Several months are often required to dry large blocks completely.

Impregnation. When limestones have been completely dried, especially if they are soft, it is often advisable to impregnate them with one or other of the impregnation agents. To economize material, large objects may be painted over once or twice with a solution of the material chosen, but smaller objects should be immersed in the solution until air-bubbles are no longer formed. If there is a supply of tap-water with sufficiently good pressure, rapid and complete penetration by the fluid can be ensured by placing the object in a vessel containing the necessary fluid under a bell glass, the air from which is then exhausted by a water air-pump[ [92]. Figure [14] illustrates the application of such an air-pump fixed to the water-tap by means of an india-rubber tube which is firmly bound with wire. An india-rubber stopper perforated to admit a glass tube is fixed in the top of the bell glass, while the smooth ground edge and the thick ground glass plate upon which it rests are smeared with grease or vaseline. The side tube of the air-pump is connected with the interior of the bell glass by an india-rubber tube which is sufficiently strong to resist the pressure of the outer air, and thus when the tap is opened the pressure of the flow of water carries with it the air from the bell glass with which the pump is connected. If the water-tap is suddenly turned off when the air is exhausted the pressure of the outer air will force the water into the bell and cause it to mix with the solution of resin or varnish. To prevent this, a stop-cock or valve should be inserted, or the water-tap should not be turned off until the stopper of the bell-glass has been cautiously raised. A second glass tube provided with a stop-cock may be passed through the india-rubber cork and connected with a manometer to measure the progressive action of the pump (Figure [15]). When air-bubbles cease to come from the object under treatment, the glass tap should be closed and the manometer removed, after which the glass tap should be again opened and the water-tap closed[93].

Fig. 14. Air-pump fixed to water-tap.

Fig. 15. Apparatus for impregnation by extraction of air fitted to manometer.

If the object is of some length but not too thick, the bell-glass may be fixed on a strong glass cylinder of a similar diameter having a ground edge (Fig. [15]), into which the object and the impregnating solution are then placed.

The following solutions, amongst others, may be recommended as suitable for impregnation:

(1) Shellac dissolved in alcohol.

(2) Solution of gum-dammar[94].

15 grammes of dammar are dissolved in 130 grammes of benzine, to which is added a solution of 20 grammes of clarified poppy seed oil in turpentine. If the solution becomes too thick it should be diluted with benzine and a small quantity of turpentine.

(3) Rice water or tapioca water[95].

(4) Dilute size.

(5) Waterglass solution.

(6) Linseed oil dissolved in benzine.

(7) Linseed varnish dissolved in 3 parts of benzine or petroleum ether.

(8) Solutions of stearine or paraffin wax in benzine.

(9) Collodion (free from acid). Zapon[96].

(10) Kessler’s fluate.

It may be added that, as a general rule, solutions for this purpose must be used as dilute as possible, for two immersions in a dilute solution are preferable to a single soaking in a concentrated one, which often scarcely penetrates into the pores.

As the preparation of solutions of shellac, gum-dammar, and of such substances as resin, stearine, and paraffin, necessitates heating, and as the solvents are very inflammable, it is advisable to make use of the solution of linseed varnish in benzine. This solution may be obtained at any time at any degree of concentration without the use of heat. Although it has the advantage that it hardens more rapidly than a simple solution of linseed oil it has also one disadvantage, for it gives a somewhat darker colour to light-coloured limestones. No more of the mixture of varnish and benzine should be prepared than is required for the impregnation, for this solution, on standing, throws down a gelatinous precipitate which is not re-dissolved even by heating. As this alteration is accelerated by the action of light, the mixture should always be kept in a dark place.

Collodion and zapon[96], on account of the expense, should only be used for small objects. After impregnation the objects should be covered with glass jars, cardboard boxes, etc., to prevent the precipitation of moisture upon them, as the result of the rapid evaporation of such volatile substances as benzine and ether upon exposure to the open air.

Rice water, tapioca water, or size (the latter of no greater strength than 2%) are only applicable to specimens which are kept in dry rooms, for in damp rooms they readily become sticky, and are liable to be attacked by moulds. Waterglass solution, probably because it is generally applied in too concentrated a form, instead of penetrating the object has a tendency to form a pellicle, which readily strips off. Even dilute solutions, however, are said to be unsuitable, from the liability to the efflorescence of alkali salts.

In the case of marble objects and antique statues of porous limestone, showing colours which are still bright on excavation, but which would soon fade, Rhousopulos[97] recommends impregnation with a very dilute solution (1 in 1000) of waterglass to preserve the colour. The solution should be as neutral as possible: in any case not alkaline. This is several times sprayed upon the object, which is allowed to completely dry between each spraying.

A material which is suitable for large objects to which the solution can only be applied upon the surface is Kessler’s fluate[98], which is soluble in water, and which hardens the limestone without completely closing the pores. It offers the additional advantage that it is applicable to thick limestone blocks, the dryness of which is not certain. The solutions numbered 1-9 must only be used when the limestone is dry throughout its mass. The fluate to be used in any particular instance must be decided from the nature of the case. Those most generally applicable are magnesium and zinc fluates and the so-called “double fluate.”

The stones from the Meten Chamber were hardened in the following manner: The limestone blocks were placed upright and the surface dusted by the air-current from a Dechend’s spray apparatus[99] which was then used to spray them repeatedly with a solution of “double fluate” of sp. gr. 1·16. Owing, however, to the injurious effect of the fine spray of the fluate upon the nose and lungs the stones were turned to a horizontal position, and a solution of fluate of sp. gr. 1·38 was applied by means of a large brush until the fluid was no longer absorbed. For the treatment of limestones on which there are remains of colours the use of a solution of shellac, gum-dammar, or collodion is recommended. Fluates should not be applied until their suitability for the particular purpose has been tested.

All specimens should be kept after impregnation in rooms which as far as possible are free from dust, for the dust which falls upon the surface will set in the varnish whilst it is hardening.

Impregnation without Previous Steeping. If a preliminary examination has shown that specimens of limestone will not bear steeping in water, recourse can be had to impregnation only. The treatment of such specimens must be thorough, for merely to paint the fluid upon the surface with a brush almost invariably proves a failure. Instead of penetrating the stone the impregnating medium forms a firm coating which is liable to be lifted, and in parts broken, by the crystallisation of salts, and thus allows the destructive processes to continue uninterrupted. Aqueous solutions, e.g. size, cannot of course be applied, and as it is necessary to make a preliminary trial of a fluate spray, it is generally found preferable to make use of the varnish-benzine mixture. In spite of this, salts may still make their appearance in the form of a crystalline powdery layer on the surface, which can be wiped off with a wet sponge; any moisture must however be removed with a soft dry linen cloth.

Removal of Incrustations and Dust. Incrustations of earth, lime, or gypsum should be washed off with water or removed by mechanical means, such as gentle rubbing with the finger. The solvent action of acids upon limestone precludes their use for this purpose. Any dust which adheres can be removed by rubbing with stale bread-crumb.

(b) Marble and Alabaster.

It is usually only necessary to clean marble with a soft brush and warm water, with the addition perhaps of some good neutral soap. In rare cases the presence of sulphates may perhaps cause some friability. The crystalline structure of marble renders steeping futile, and accordingly impregnation is resorted to. The use of Kessler’s fluates may be recommended. Adherent pitch or resin is best removed by a mixture of alcohol and ether. Alabaster seems to remain permanently sound and may be cleaned in the same way.

(c) Earthenware.

Steeping. The same line of treatment should be followed as in the case of limestones. A preliminary examination should always be made to test the power of resistance in water, which is always satisfactory if the clay has been sufficiently baked.

In the case of coloured terra-cotta care should be taken to ascertain whether the colours are likely to suffer during steeping. There is no danger of injury if the steeping is not too prolonged; in fact, the removal of the dust during the procedure often brings out the colours more clearly. If the Egyptian ostraca (clay fragments with black script) require to be washed they should be carefully watched in order to preserve the script, and therefore should be placed in the bath in such a way that the lettering is visible.

These fragments are usually curved and bear the script upon the convex side, care should therefore be taken that they are completely immersed, and that no large air-bubbles prevent the access of the water to any part of the under-surface. The writing is done with either lamp-black or more rarely some form of iron ink, and is retained mechanically by the porous character of the ostraca. In the latter case the characters may be enhanced by the application of a dilute solution of tannic acid, which sometimes proves useful also for limestone pieces.

If these fragments are sufficiently few in number to allow each to be put into a separate glass vessel, the washing out of the salts is completed so quickly that there need be little danger of obscuring the script. When large numbers were to be washed and when the script was already indistinct I have employed the following method: After examination as to their fitness for immersion the fragments are placed on a wooden grating in a tub, in which they remain for a couple of days, during which the water is renewed once. They are then taken out and allowed to dry. All those which still show the script distinctly are separated and their steeping is completed, but the remainder, having been completely dried, perhaps on the top of a warm stove, are brushed over once or twice with a dilute (1:6) mixture of varnish and benzine in such a way that the surface is only moistened, and when dry shows no gloss. The pieces thus superficially varnished are kept in a dry place for about two months, until the varnish is hardened; the process of washing out the salt is then begun again. The thin coat of varnish fixes the script without interfering with the steeping. The varnish solution must be dilute, for a thick coating will partially peel off from the object in the course of the steeping, or will remain in the pores in the form of opaque particles, and thus render the script illegible.

The same difficulty which arose in the treatment of the Meten limestones was frequently met with in the treatment of these ostraca. Those which were of a dark brown colour especially, and to a less degree also the red and the yellow, were covered with a slimy growth of algae. As the script is easily destroyed no attempt should be made to remove these algae from the side which bears the script even with the softest brush, although they should from time to time during steeping be brushed from the underside. The inconvenience caused by algae is, however, less marked in the treatment of earthenware, the light and porous character of which renders prolonged steeping needless, nor is there the same necessity to continue the steeping for the purpose of chlorine estimation. The following results were obtained in the treatment of 13 fragments, the average thickness of which was 1 cm. [3⁄8th inch], with an average superficial area of 1⁄10 sq. metre [4 inches]. The tub in which they were steeped contained 85 litres [181⁄2 gallons] of water.

100 cubic centimetres of the tap-water used were found to require 0·5 c.c. of the silver solution, and on each occasion this quantity of the water was tested.

Water changed after	1	1	1	1	2	2	2	4	5 days
100 c.c. of the water used in steeping required	3·0	1·3	1·0	0·8	0·9	0·7	0·7	0·6	0·6 c.c. of silver solution

The water was changed at first daily, then every two days, and so on: the steeping could therefore be regarded as complete at the end of a fortnight.

A small figure of earthenware, which weighed only 28·9 grammes, was steeped in 11⁄2 litres of distilled water, and gave the following result for every 100 c.c. used:

Water changed after 2 days required 3·6 c.c. silver solution.
Water changed after 3 days required 0·4 c.c. silver solution.
Water changed after 4 days required 0·0 c.c. silver solution.

The steeping was, therefore, in reality complete after five days, and, as the steeping water was thoroughly mixed before the withdrawal of the 100 c.c., the total quantity of sodium chloride contained in the figure can be calculated as follows:

For the 15th part (viz. 100 c.c.) of the water 3·6 + 0·4, i.e. 4·0 c.c., of decinormal silver solution were used, which is equivalent to 15 × 4·0, i.e. 60 c.c., of silver solution for the whole quantity. Now 1 c.c. of this decinormal solution corresponds to 0·00584 gramme of sodium chloride; the water therefore contained 60 × 0·00584 gr., or 0·35 gr. sodium chloride. Thus the figure contained altogether 11⁄5% of sodium chloride.

In addition to the chlorine compounds, there was also a considerable quantity of sulphates, the presence and disappearance of which were tested by adding to a few cubic centimetres of the water a dilute solution of barium nitrate or of barium chloride[100]. The soluble barium salts give with sulphates a white precipitate or cloudiness of insoluble barium sulphate. If therefore on the addition of a solution of barium nitrate no cloudiness appears, even after some time, it may be concluded that sulphates are no longer present in the water. When the ostraca have been washed and dried, it is often possible to make the script more distinct by varnishing them over with a varnish-benzine mixture (1:6).

It is advisable to subject friable objects of earthenware to the process of impregnation (cp. the impregnation of unbaked clay, p. [81]).

The Removal of Incrustations. Incrustations of earth or lime can be easily removed if the earthenware has been well baked, but trial must first be made with a drop of dilute hydrochloric acid, whether the earthenware itself is not attacked by the acid. The specimen is then placed upon a glass ring or suspended in water containing 2% of hydrochloric acid[ [101]. This mixture, which must be renewed every 24 hours, will remove incrustations which it would be difficult to remove by mechanical means, while crystals of gypsum of considerable size, which are often found on clay tablets of Assyrian origin, are easily dissolved in from two to four days.

Figures [16] to [21] represent two Assyrian tablets which have been cleaned by myself in this manner. It will be seen that the cuneiform characters, which before treatment were almost invisible, are now distinctly legible.

Fig. 16. and Fig. 17.
Assyrian clay tablet with incrustations. Before and after treatment.

Fig. 18., Fig. 19., Fig. 20. and Fig. 21.
Assyrian clay tablet before and after treatment.

After this treatment with acidulated water the acid must itself be removed by careful washing in pure water. Here too a solution of silver nitrate will serve as a test, for, so long as any chlorine, and therefore any hydrochloric acid, is present in the water, a white precipitate or cloudiness is produced.

The method of titration with yellow potassium chromate is not applicable here, for the free acid prevents the appearance of the red precipitate. The steeping must therefore be continued in distilled water until the addition of silver nitrate no longer produces any cloudiness.

Baked earthenware which shows colouring, or which has incised lines filled with substances containing lime, must not be steeped in acidulated water, nor will ostraca bearing inscriptions in iron ink stand this treatment; these are, however, fortunately rare: in fact amongst several thousand fragments few have shown incrustations of lime or gypsum. Should any such be found a cautious attempt should be made to remove the incrustations by some mechanical means. Rhousopulos[102] carries out the cleaning of Lecythoi[103] and clay vases, which are painted in water-colours and which have a thin white incrustation, by dipping them into a 5% solution of pure hydrochloric acid. As soon as the colours show the least sign of running, or if an efflorescence makes its appearance, the vase is immediately removed and allowed to dry. It is then dipped into distilled water and allowed to dry a second time. Impregnation is not necessary.

“If the treatment is otherwise successful, but an earthy layer remains upon the colour, the spots which are thus affected are lightly touched with the finger whilst the object is still in the liquid. Rubbing, or any sort of mechanical attack, is absolutely out of the question.”

This process evidently requires the greatest care and constant attention.

(d) Slightly Baked Or Unbaked Clay.

Impregnation. If upon examination it is found that a drop of water softens the clay, the same line of treatment must be followed as in the case of limestones which exhibit a similar condition (see p. [73]), i.e. they must be subjected to the process of impregnation[104]. As the colour of the clay objects is yellow-brown or red-brown, the varnish benzine mixture will be the most suitable application for the purpose. A considerable number of sun-dried Assyrian clay tablets treated in this manner have given good results, and have undergone no change during the last five years, in fact they may now even be laid in water without crumbling.

In the case of slightly baked or unbaked Babylonian clay tablets the method formerly employed was merely to remove deposits of lime, clay, gypsum, etc., by lifting or scraping them away with pointed or wedge-shaped tools, for the soft clay would not stand treatment with water, still less with 2% hydrochloric acid. The difficulty in avoiding damage to the clay surface, when removing the deposit, makes this method both tedious and risky. Warming to 200-300°C. in a drying oven, or in an iron box embedded in sand, seldom aids the removal of incrustations; moreover, this treatment has no hardening effect upon the clay, and thus does not facilitate the removal of the injurious salts by soaking. A further expedient therefore remains, that of heating the clay to higher temperature, whereby it is fully baked and rendered capable of resisting subsequent treatment with water or 2% hydrochloric acid. At the Royal Museum this firing is done in muffle furnaces[105], the smaller of which has a capacity of about one cubic foot, and is heated by six and twelve Bunsen burners. The temperature is regulated in the same way as in porcelain manufacture by the use of Seger’s cones[105], which are placed in the muffle, where they can be seen through the observation aperture. To avoid cracking the heating must be gradual, the gas-supply being very gradually increased. The firing must at first be adjusted to cone 022 [590°C.; Watkin, No. 1, 1094°F.]; the gas is then turned off and the furnace allowed to cool as slowly as possible. To effect this the damper is closed and all openings into the muffle are made up with fire clay. The clay tablet is removed when quite cold (usually in 18-24 hours), and, as a rule, much of the incrustation can then be removed by means of a soft brush. Should the removal prove difficult, and a preliminary trial have shown that it will bear the treatment, the removal of the deposits will be assisted by soaking for two or three days in water. Should the tablet prove capable of bearing treatment with 2% hydrochloric acid it may remain in the acid for 12 to 18 hours. If necessary the acid may be renewed once; it must then be thoroughly removed by steeping in ordinary water and finally in distilled water, until the wash-water is free from chlorides. After steeping, the tablets will be found somewhat softened and occasionally coated with a slimy growth of algae, care must therefore be used in changing or taking them from the water. The best way to handle them is to place the fingers of the two hands under the tablet.

Fig. 22. Babylonian clay cone before treatment.

Fig. 23. Babylonian clay cone after treatment—firing, treatment with hydrochloric acid and steeping.

After thoroughly drying the tablets first in the air, then in the drying oven at a temperature of 212°F., supported on glass rings, it is well to impregnate them. This can be best carried out by placing them, while still warm, into melted paraffin wax, and raising the temperature to about 250°F. [120°C.]. The wax is allowed to cool to about 160°F. [70°C.], when the tablet is removed upon a broad band of gauze, any excess of wax is drained off, and the object is wiped with a soft cloth. The benzine-varnish mixture or zapon may also be used for impregnation. If heating in the muffle to Seger cone 022 is insufficient to allow of the removal of the incrustations, or if the condition of the clay does not warrant soaking in water or acid, the object must be again placed in the muffle and fired to Seger cone 010 [950°C.; Watkin, No. 13, 1742°F.], and, if softening occurs upon the application of water or acid after exposure to this temperature, recourse must be had to a third heating to Seger cone 05 [1050°C.; Watkin, No. 18, 1922°F.]. Higher temperatures than this are not advisable, for the lime, sodium chloride, and other salts found in some Babylonian tablets may partially fuse. During firing therefore the appearance of the object must be carefully watched, and the temperature lowered at once by reducing the gas-supply, if signs of fusion are noticed.

Additional Methods of Impregnation. If clay objects have a smooth surface, it is, according to the “Merkbuch[106],” advisable to impregnate them with Belmontyl oil[107], for varnish in the course of drying gives a lacquered appearance to the surface. According to the same authority the surface of glazed vessels can be restored by impregnating them several times with a mixture of poppy seed oil and benzine [20 grammes clarified poppy seed oil in 270 gr. benzine, i.e. 1 in 131⁄2], and by subsequently brushing them first with soft, then with harder brushes. There are, however, many other substances used in different collections for impregnation, a few of which are subjoined.

In the Museum at Vienna friable clay objects are laid for two or three minutes in a dilute solution of warm size, and when dry are brushed over with a solution of shellac; size alone, or a solution of shellac alone, is frequently used for impregnation, or to give a coating. In the Museum at Wiesbaden thin specimens are impregnated with a solution of white of egg, brittle objects with dilute fish glue, while for hard objects a solution of shellac or melted shellac is used.

(e) Fayence.

I have been able to wash out the sulphates from several Egyptian fayence figures in spite of the glaze, the fissures in which allowed the water to penetrate into the interior. The process of steeping, which was necessarily somewhat prolonged, was tested from time to time by the barium nitrate test (vide p. [77]).

(f) Objects of Stucco and Nile-mud.

These are rare, and in almost all cases contain salts. As, however, they will not bear steeping, they must be preserved by means of impregnation only. The varnish-benzine mixture should be used for this purpose.

(g) Sandstone and Granite.

These scarcely need any special preservative process, but Kessler’s fluates are useful for the impregnation of weathered sandstones which are exposed to the open air (see p. [ 72]).

They can be cleaned by washing with warm water, while calcareous incrustations may be removed by hydrochloric acid. A thick coating of oil paint was successfully removed from an Egyptian statue of sandstone by placing it in an alcoholic solution of soda. Oil colours and similar substances may often be removed with ease and completeness from stone, plaster, wood, etc., by placing the objects in air-tight vessels together with a vessel containing alcohol. The alcohol vaporises, even at the ordinary room-temperature, and causes a softening of the paint. The time required for the treatment depends upon its age and hardness.

Appendix.
Cement for Earthenware. Restorations.

To fix together pieces of broken pottery good Cologne glue is useful, but it has the disadvantage that it can only be used when warm. For this reason it is better to use liquid fish-glue [Syndeticon], which may, if necessary, be thinned with a little vinegar. Fire-clay dust in waterglass is used in the Museum at Breslau. A thick ropy solution of shellac[108] may also be mentioned, for the use of which the opposing surfaces must be first moistened with alcohol.

Gum arabic and dextrin should not be used, for objects thus cemented readily fall to pieces unless kept in perfectly dry rooms. This, however, may also be said of earthenware which contains salts, if cemented with glue or fish-glue. Previous steeping would obviate this difficulty.

Chalk, plaster of Paris, brick-dust, or fire-clay dust are often added to the fish-glue, dextrin, etc. Without giving additional strength to the cement, these substances may be of use in filling up small gaps between the fragments to be cemented.

For filling up larger gaps the “Merkbuch[109]” recommends stone cement, for the preparation of which it gives the following prescription:

“Mix 500 grammes of Cologne glue with three sheets of strong white blotting-paper, or four sheets of white tissue paper, shredded as small as possible, and boil until it becomes thick, stirring the whole into a perfectly smooth pulp. Let it boil thoroughly, and while stirring continually, and working with a stout wooden rod, add 21⁄2 kilogrammes of very finely sifted dry purified whiting. After working this mixture thoroughly, add 80 grammes of linseed oil, which must be also thoroughly worked in. To preserve the glue add 50 grammes of Venetian turpentine. This stone cement will take any shade of colour if mixed with lamp-black or coloured earths.”

(h) Iron.

The various methods for the preservation of iron objects which have been or are still in use may be divided into two groups. To the one group belong those methods in which the objects are preserved with their coating of rust, or with the rust that has penetrated them; to the other group belong those in which the removal of rust precedes preservation. The former methods must be applied when the iron has been completely converted into rust or when the rust has only left a small metallic core. These methods may of course be used also for all iron antiquities.

The methods of the second group can be applied to those objects only which still retain a strong metallic core, in which case the objects regain the more or less grey or white surface of fresh unoxidized iron. These methods are at present little known, and therefore but little used, for owners and the general public are still accustomed to see in the covering of rust the evidence of antiquity with which they are loth to part.

In addition to these methods, there are others which are of an intermediate kind, either special or a combination of methods from both these groups.

(1) Methods of preserving Objects of Iron without removal of the Rust.

Impregnation. The earliest processes, which are to some extent still in use in some collections, are simple impregnation methods, in which the object is either painted once or more with the impregnating medium by means of a brush, or is placed directly in the medium itself. In either case the penetrating power of the solution used is directly proportional to its fluidity.

The following media may be used for the purpose:

(1) Warm size.

(2) Warm isinglass solution.

(3) Solution of waterglass.

(4) Solution of shellac in alcohol.

(5) Rubber solution in carbon bisulphide. The mass after swelling is dissolved in benzine[110].

(6) Copal varnish diluted with turpentine.

(7) Copal varnish mixed with linseed oil[111].

(8) Linseed oil.

(9) Linseed varnish.

(10) Linseed varnish mixed with an equal quantity of petroleum.

(11) Bees’-wax dissolved in turpentine.

(12) Bees’-wax dissolved in benzine.

(13) Petroleum.

(14) Vaseline.

(15) Melted paraffin.

(16) Oleate of lead: 100 grammes of olive oil, 100 gr. of lead oxide, and 100 gr. of water are boiled until all the water has evaporated and the mass has become grey. The mass is extracted by shaking it with alcohol, and the residue is dissolved in absolute ether, in the proportion of 100 gr. of ether to 5 gr. of the substance. Before use it should be diluted with a little ether[112].

(17) Speerschneider’s mixture. This consists of 8 parts of rape oil, 1 part of bees’-wax, 1 part of pine resin, and 2 parts of benzene[113].

(18) Collodion, or the mixture used in the Museum at Donaueschingen, which consists of 30 grammes of collodion, 2 gr. of camphor, and 1 gr. of oxalic ether.

In addition to these materials, there are other mixtures of resin, varnishes, and bees’-wax, with their appropriate solvents, but they do not possess any special advantages as impregnating solutions.

After treatment with size or isinglass, iron objects may be given when dry a coating of linseed oil, linseed varnish, solution of shellac, etc.

The materials numbered 7 to 10 in the above list should be applied warm to enable the viscid fluids to penetrate the rust, for the more readily the solution enters the object the better is the result obtained. Apart from the fact that they are easily ignited at a high temperature, they must not be heated beyond 230°F. [110°C.], otherwise objects which consist largely of rust will fall to pieces[114].

In the process of impregnation a twofold result is aimed at, viz. to prevent the rust from crumbling, and to exclude air from the specimen. The application of heated linseed oil or linseed varnish is founded upon the supposition that these substances enter into a chemical combination with ferric oxide to form a stable compound; this is, however, disputed by some modern authorities[115]. Neutral substances offer a safer method for the exclusion of air, and of these melted paraffin is undoubtedly the best. The paraffin must be quite pure and free from stearine, as can be ascertained from the melting point; thus pure paraffin melts at 130°-150°F. [55°-65°C.], stearine at 160°F. [70°C.]. Paraffin with a melting-point higher than 65°C. should be looked upon with suspicion.

In many collections the objects are heated before impregnation with media which are insoluble in water, or they are exposed to the air for six to twelve months after excavation. This latter proceeding is, however, certainly inadvisable if the iron contains chlorine, and if this is the case not one of these methods produces satisfactory results.

On the other hand, as almost all the iron antiquities which do not contain chlorine compounds may be treated by the methods of the second group, simple and direct impregnation is passing more and more out of use. Before impregnation all soluble substances, especially chlorine compounds, must be removed by steeping.

(2) Preservation by Steeping and Subsequent Impregnation.

Krause’s Method. The water used for steeping should be preferably lukewarm, and should be changed every twenty-four hours. It is even better, at least for the first time, to lay the object in water and then raise it to boiling-point, a measure which will allow the more ready penetration of the water. As in the case of limestones, earthenware, etc. (p. [59]), care must be taken to place the objects as near to the surface of the water as is possible. Small objects may be put in glass jars, large ones in wooden troughs, tin vessels, or wooden boxes lined with zinc or lead. Any little excrescences on the iron, which are frequently filled with ferrous chloride, should be punctured to give the water unimpeded and more speedy access. Crumbling objects should be held together by tightly wrapping them in muslin. Curators must decide for themselves how far means such as files, chisels, or small hammers may be used to remove the rust or earthy material conglomerated by rust.

Although much recommended, the method of adding soda or lime water to remove the chlorine as soluble sodium chloride or calcium chloride is, in our opinion, inadvisable. Both these substances precipitate the iron from the ferrous chloride or ferric chloride (which are soluble in water) as insoluble hydroxide of iron, which more or less closes the interstices, and thus impedes the access of water to the interior.

The process of steeping can here again be controlled by the use of the silver solution (p. [62]), for if there no longer appears any or only very little cloudiness the steeping may be considered complete. The length of time required for steeping depends upon the thickness of the rust and the porosity or existence of cracks in it, and if the objects are of considerable size, it may extend over several weeks.

After steeping the object should either be dried in the open air, and later on a warm stove, or be placed for a few days in alcohol to remove the water, after which the rapid evaporation of the alcohol will quickly dry it. The steeping of iron objects in warm alcohol has been recommended[116], but if their size is considerable the method is an expensive one. This method has the advantage that the alcohol penetrates the rust sooner than does water, and also prevents oxidation, which may be actually produced by the water. It may perhaps be advisable to dilute the alcohol, the usual strength of which is 95% to 96%, with about an equal volume of water, for some salts are not readily soluble in pure alcohol. When dry the object is warmed for a few hours in a mixture of equal parts of good linseed varnish and petroleum. The petroleum serves to dilute the varnish, which can thus more quickly permeate the entire mass of iron and rust. On account of the inflammable nature of the mixture the warming should be done over a water-bath. For small objects a cylinder made of ordinary tin-plate, measuring from 6 to 10 inches [15 to 25 cm.] in diameter and 6 in. [15 cm.] in height, may be used. To increase stability the lower half should be of a smaller diameter, and fitted into an iron tripod. The same end is attained by soldering a ring round the middle of the cylinder, which will rest on the ring of the tripod. The cover consists of a number of copper rings gradually diminishing in diameter, which fit closely into one another, thus enabling porcelain vessels of various sizes to be used. For larger objects, such as swords, two long rectangular troughs (Fig. [24]) of stronger plate should be used. The following sizes will probably be found useful: one about 40 inches [100 cm.] long by 4 inches [10 cm.] broad, and 4 inches [10 cm.] deep, and the other slightly larger. Handles should be fixed at the upper edges. Three iron bars 1 inch [21⁄2 cm.] thick and 4 inches [10 cm.] in length are laid across the bottom of the larger trough, on which the smaller is placed. The space between the two vessels is filled with water to a depth of 2 inches [6 cm.]. The trough is warmed on a stove, or better, where gas can be had, by means of a number of Bunsen burners fitted with rose or ring burners, over which the trough may be supported upon tripods. While heating care must be taken that the water does not boil over, which can be easily avoided by regulating the gas supply. As the water evaporates further quantities should be added as required. After simmering for about two hours, the objects should be removed and allowed to drain; they should then be placed on a tripod, or on glass rings, on the warm stove in cold weather, to accelerate the evaporation of the petroleum and the setting of the varnish. In summer drying chambers may be used; these are sold by dealers in physical and chemical apparatus, or can be made at little cost by a tinsmith.

Fig. 24. Water-bath. 1⁄15 nat. size.

If the objects have been steeped in pure alcohol, or at least towards the end of the treatment in three changes of alcohol, so that all the water is replaced by alcohol, they may be dipped directly without drying in the varnish mixture, for the alcohol evaporates in the varnish bath which is at a temperature of 194°-203°F. [90°-95°C.]. As the varnish hardens, the iron thus treated acquires a glazed surface; other means of impregnation may therefore appear preferable, e.g. a solution of gum-dammar or melted paraffin. For impregnation with the dammar solution the object must first be dried, and the air-pump used in the way described on p. [ 68]. On account of the inflammable nature of the benzine heat must not be applied, nor indeed is it necessary.

When impregnating with pure paraffin[117], specimens may be lightly wiped with a cloth, but need not be dried. The paraffin may be heated to 212°-248°F. [100°-120°C.] without danger, so long as it is kept from direct contact with the flame. A thermometer should be used, and, as soon as the paraffin has melted at a temperature of about 60°C., the object should be placed in it by means of tongs. When the temperature has risen above 212°F. [100°C.] the water is converted into steam, and causes a brisk ebullition of the melted paraffin. The quantity of paraffin used should, therefore, be such that its level remains at the least 2 inches [5 cm.] below the upper edge of the vessel.

When the bubbles have ceased to rise, thus showing that all the water is expelled, the paraffin should be allowed to cool to a temperature of 180°-190°F. [80°-90°C.]. The iron should be taken out with tongs, and the liquid allowed to run off. It should then be wrapped, while still at 80°C., in soft blotting-paper or in a piece of old linen to absorb the superfluous paraffin. If the surface of the object is very uneven, or if there are deep cracks or holes in which the paraffin can collect, it will, when cold, form a white mass, and should therefore, while still warm and fluid, be soaked up with filter paper, or distributed evenly by means of suitable brushes. The superfluous paraffin may also be absorbed by putting the object in dry sawdust; any sawdust which remains attached can be removed when cold with benzine, or it may be scraped from the spots where it has collected with a knife or spatula. Any spots where the iron may have become exposed may be covered with a thin coat of paraffin dissolved in benzine.

Ekhoff’s Method[118]. The objects are laid for two or three months in water which is changed every two or three days, a small quantity of quicklime being added[119]. After this steeping, and after some of the rust has been removed mechanically, the object is lightly dried and put into heavy petroleum of sp. gr. 0·85 to 0·95, which is then heated up to 220°F. [105°C.]. A thermometer should be used to ensure this temperature. This temperature being higher than the boiling-point of water, the water contained in the object evaporates and causes the petroleum to bubble, as in the method previously described. When all the water has been replaced by the petroleum the bubbling ceases. After the fluid has somewhat cooled down, the iron is taken out and is allowed to remain for about an hour in sawdust, which absorbs the superfluous oil. Finally, while gently warming the object over a warm, but not too hot, stove, it is coated over with a mixture of 1 part of bees’-wax and 2 parts of turpentine, or better with paraffin dissolved in benzine. Heavy petroleum, which we have found by experience to be a suitable material, is preferable to varnish in so far as the iron is impregnated by a neutral substance which is practically liquid paraffin, but has the disadvantage of being highly inflammable and of being difficult to obtain at so high a specific gravity.

Straberger’s Method. This method, for the description of which I am indebted to Herr Straberger, has proved effective in the preservation of a number of iron antiquities in the Museum at Linz on the Danube. Even iron objects, which had been in bad condition and had undoubtedly contained chlorine, have after treatment by this method shown no signs of change, while the dull black surface has an agreeable appearance.

Straberger places the newly-excavated objects immediately into linseed oil to prevent the access of air. After remaining in the oil for some time they are taken out, wrapped in cloths saturated with linseed oil, and removed packed in sawdust. Upon arrival they are unwrapped and put into water, to which a small quantity of soda is added to remove the oil more easily. The water is frequently changed, and the objects are meanwhile cleaned mechanically with emery paper and hard brushes. Any blisters are removed by the aid of a small hammer and chisel. After steeping they are dried and smoked over a candle flame which is allowed to play over the whole surface. The soot is then rubbed off with a cloth or soft brush. Objects with a smooth surface may be rubbed with india-rubber. The preservative action of this proceeding depends upon the fact that during the smoking, in addition to the soot, oily products of combustion are deposited from the candle flame, which prevent the access of air and moisture to the iron.

“Objects which are much decayed or cracked should, when cleaned and thoroughly dry, be again placed into linseed oil which has been slightly warmed and should remain therein for a few days before being smoked. Upon removal from this second oil bath they should be lightly wiped and dried over a moderately warm stove or in the sun. Patience is necessary, and nothing further should be done until the oil has entirely dried in the fine cracks and crevices and firmly binds the mass. The oil crust on the surface is then loosened by soaking in a strong soda solution and wiped off, after which the object is dried, smoked over the candle flame, and the soot wiped or brushed off with a soft brush. The smoking and wiping may be repeated if necessary.”

Herr Straberger states that his treatment has been successful when impregnation with isinglass and coating with shellac has failed.

The methods of Hartwich and Jacobi hold an intermediate place between the above methods and those which will be subsequently explained. With the former they have this in common that they do not call for the entire removal of the rust and that they require the use of linseed oil; on the other hand their application presupposes the existence of a strong metallic core, otherwise when the rust is removed they will show merely a skeleton of the original object. The existence of a sufficiently substantial metallic core can be easily ascertained from the weight, for an object, which consists solely or in great part of the oxide of a metal, is much lighter than one of the same size which is largely metal. The ring also affords a test, for an iron object, of which the greatest part is metallic iron, gives a clearer note when struck than one which is chiefly rust. A still more certain test is the use of a file or a drill (comp. page [107]).

Hartwich’s Method[120]. This method is intended for objects of an especially large size, the hard oxide coating of which does not allow satisfactory steeping. Hartwich heats the object to redness, allows it to cool slowly, and then scrapes off the outer layer which has been rendered friable by this treatment. The subsequent procedure is that of Krause’s method, viz. warming in linseed varnish.

Jacobi’s Method. The method of preservation of iron antiquities used in the Saalburg Museum at Homburg is described by Jacobi as follows: The object is heated in the fire of a forge, which causes the chief part of the rust to flake off, while any rust which still adheres is removed when cold by water and brushing. The object is again held in the flame with tongs and heated (smaller objects may be placed on an iron plate); and during the heating is quickly taken out three or four times and each time brushed over with linseed oil. Most of the linseed oil is thus burnt and the deposition of carbon gives to the iron a black colour, while the oil which has been partially burnt or hardened by the heat produces a slight lustre. This process, as carried out at Homburg by a locksmith, is that which blacksmiths ordinarily use to blacken iron objects and to protect them from rust. The preservation has proved permanent, and only in rare cases has it been found necessary to repeat the process. These good results are probably due to the fact that the antiquities of iron preserved in that Museum are for the most part found in good condition, having very little rust and certainly containing only a very small amount of chlorine. Iron articles which contain chlorine but which still have a good metal core, after washing, drying, and a cautious preliminary application of heat, are ready for treatment by Jacobi’s method.

Inlaid Iron Objects require especially cautious treatment. Although I have not had any personal experience in the treatment of objects of this kind, good results have been obtained in several Museums, especially in that at Mainz.

The following quotation from the “Merkbuch” (p. 75) describes the method which is applied at Mainz, where it probably originated:

“Objects of this kind which are likely to have been originally inlaid with silver, gold, copper or brass, as is frequently the case with objects of the Merovingian period, are not placed in alcohol after the steeping, but are warmed and dipped three or four times into a hot dilute solution of isinglass. The heating is necessary, otherwise the isinglass will set on the surface and will not penetrate into the interior. When the object has been dried and the isinglass has set, the layer of rust which covers the inlaid ornaments is scraped off with a graving tool, and any spongy hollow parts are filled up with a paste made of iron rust and isinglass, before the inlaid work is cleaned. During the scraping the object is held in the left hand on a little wooden board covered with plush or thick chamois leather, to which it is fixed as firmly as is necessary by means of a vice. In scraping special care must be taken that the graving tool follows the lines of the designs, for in scraping across the design it may slip under the flat silver thread and raise it out of its place. When the ornamentation has been completely laid bare, it is rubbed with emery cloth and then polished with a brush and fine emery powder. The piece is then dipped into a solution of gum-dammar, and, when the surface is dry, emery is again used to remove the varnish, which gives the silver a slightly yellow colour. The object is then protected from the influence of air and moisture by the transparent retouching varnish of Sohnée frères (Paris).”

A modification of Krefting’s method (p. [108]) has proved eminently successful in the treatment of iron objects inlaid with silver. Krause[121] recommends that the article be placed, with the inlaid surface downwards, for 24 hours in a mixture of

10 grammes of 40% acetic acid,
10 grammes of ammonium chloride,
70 grammes of distilled water,
10 grammes of aluminium powder.

It is then removed from the bath, carefully brushed and washed, and, if the inlaid work is not yet cleaned, is replaced in the bath. This is repeated until the inlaid work is completely exposed. Spots of ferroso-ferric oxide which are difficult to remove may be ground away by an emery wheel, care being taken that the inlaid surface is held against the lower side of the wheel (which must be rotated in the reverse direction) so that it is always in sight.

All the methods of this group, which have been applied to many articles in various Museums, exhibit one inherent defect, for any rust which remains after treatment may cause the continued oxidation of the iron. The effects of this action of rust are, I believe, extremely small, and it must at the same time be admitted that iron antiquities, even if they have been well steeped and afterwards impregnated, do not always remain in a permanent and sound state of preservation. If in such a case the well-known small watery bubbles should make their appearance, the steeping has undoubtedly been insufficient. This evil can be remedied by gradually heating the object to redness to destroy the impregnating material, and by a careful repetition of the steeping and impregnation.

(3) Preservation of Iron Antiquities by Removal of the Rust.

Steffensen’s Method (Copenhagen). The objects are carefully heated over a flame and are then laid in dilute sulphuric acid. The sulphuric acid dissolves a certain amount of the iron, and it is found by experience that the chemical action is strongest at those spots where any rust remains, and that this is detached by the hydrogen which is produced. When the cleaning is sufficient, the iron is laid in a dilute soda solution to neutralise the acid, and is afterwards well washed with water and dried in an oven. When dry the iron is brushed over with a solution of bees’-wax (or better of paraffin) in benzine, the evaporation of which leaves a protective coating of bees’-wax or paraffin.

Blell’s Method. The method proposed by Blell and applied by him to many of the objects in his collection is distinct from that described above, although in its earlier stages the principle is the same. The following quotation is taken from the description of his method which the author read before the Antiquarian Society[122] at Königsberg:

“If a specimen is found to have a sufficiently strong core of iron it should be heated in the furnace to bright redness and then dipped into water. The expansion of the iron caused by the heat and the subsequent contraction caused by the sudden cooling thoroughly loosens the layer of rust. Large iron objects with a strong and firmly attached incrustation of rust will require a repetition of the process. By this means not only is the rust converted into a red powder which is easily rubbed off, but the object itself is rendered more suitable for the subsequent treatment. At the same time the heating process removes any coating of oil, fat, etc., which may have remained from previous attempts at preservation, and which would interfere with the further stages of the process. Smaller or delicate specimens should be treated in the flame of a spirit-lamp, but special care must be taken that there is sufficient iron present. Sword blades and other tools and weapons with sharp edges should be heated only, for the sudden cooling may cause cracks in the cutting edges.”

To complete the removal of the incrustation of rust which has been loosened by the heating process, or by the heating and sudden cooling, the object should be placed

“in a well-stirred mixture composed of one part by weight of sulphuric acid in nine parts of water. Bubbles of hydrogen will immediately rise and the rust will begin to separate. In freshly prepared acid objects which are not very rusty will be freed from rust after four to six hours, those covered with a deeper layer of rust in about twelve hours, but several days, or even weeks, may be necessary. The duration of the process depends upon the strength of the acid and the character of the rust, viz. whether it is thick and solid, or thin and porous, and whether the iron is of a soft, or of a hard character.

When first making use of this method it is advisable to use dilute acid and to take out the objects several times in the course of the day and examine them, while during the night they should be taken out of the acid and placed in soft water[123].

For the acid bath and for rinsing it will be found convenient to have two pairs of wooden troughs having the following internal measurements:

(1) An internal length of 10 inches [25 cm.] by 71⁄2 inches [19 cm.] in breadth and 43⁄4 inches [12 cm.] in depth, which will be useful for the larger number of objects.

(2) For long narrow objects, e.g. sword-blades, and long spear-heads, the internal measurements should be 40 inches [100 cm.] long by 4 inches [10 cm.] broad and 3 inches [8 cm.] deep.

Small fragile objects are most satisfactorily treated in glass vessels or glazed earthen pots or vases.

The acid must have free access to all parts of the object; if a sword, for example, lies flat upon the bottom, the under-surface apparently remains unacted upon by the acid. This should be remedied by the use of a couple of small wooden supports.

Frequent rubbing with a cloth and forge scale[124] or coarse sand greatly helps in removing the rust, but gentler treatment is required for the smaller and more fragile objects. The rust is often very firmly attached in some portions of the object, and in this case those areas which have been already freed from rust should be coated over with lard, which is free from salt, to protect them from further action of the acid, while the pockets of rust are alternately treated with acid and graving tools. No particle of rust should be allowed to remain, for sooner or later it will begin to spread, whatever precautions may be taken.

The action of the acid becomes less effective if it has been used for several objects. A little fresh acid should then be added. The more active the sulphuric acid, the brighter will be the grey colour of the iron after the rust has been removed. If old acid has been used the iron will be of a dirty grey colour, and should then be placed into fresh acid for a short time until it assumes a clear light grey colour.

The third part of the process begins with the removal of the iron from the acid bath and has as its object the removal of every trace of the acid, otherwise the rust will very quickly return and cover the whole surface. The object is therefore immediately and repeatedly rinsed in soft water and carefully dried; the cheapest material for this purpose is cotton waste, but ordinary linen-cloth must be used for objects with jagged edges, for the threads will catch in the notches and hinder the drying. This should be done without delay, or a change of the colour from light grey to yellow will betoken a new formation of rust. Articles showing a very complicated construction, which are however rare from the Iron Age, should be packed in perfectly dry hot pinewood sawdust, while those which are still more difficult to dry, for example, coats of chain-mail, after thorough rinsing, should be immediately put into a pan with melted lard, free from salt, and boiled until the cessation of bubbling shows that all the water has been driven off by evaporation.

They are then rubbed dry or are laid in hot sawdust, after which they are brushed over with melted lard and placed in this condition for at least half an hour in a moderately hot cupboard until the fat has penetrated into the finest pores of the iron. That this has really taken place may be proved by the use of a file.

When by this means all trace of sulphuric acid has been removed the fourth stage of the process is reached, viz. the removal of the grease from the surface and the subsequent application of some preparation to prevent the access of air and moisture. Most of the grease is removed by placing the objects in a warm place on blotting-paper. Any grease still remaining on the surface can be entirely removed with a cloth or paint-brush by means of benzine. If no restoration or repair is required nothing more is necessary than to apply the protecting solution.”

A white varnish has much to recommend it from its protective power, but as it gives to iron an unsatisfactory gloss, it is preferable to use a solution of bees’-wax in benzine.

Having made use of Blell’s method in a number of cases I have a few suggestions and modifications to offer. The heating should be carried out carefully and gradually, lest the sudden conversion of the moisture in the rust into steam should cause small explosions which would scatter pieces of rust. There is no danger of this if the objects are heated in an oven; they should not therefore be heated in an open flame. For smaller objects I use a box six inches [15 centimetres] square, of strong tin-plate loosely covered with an iron lid, or with a piece of asbestos sheet; but if the objects are large, e.g. swords, spearheads, etc., I heat them on a strong piece of tin-plate bent round to form a channel, and covered with a long piece of asbestos sheet, the edges of which are bent over the edges of the channel, to retain the heat as much as possible.

It is advisable, in my experience, to use the sulphuric acid well diluted, e.g. in the proportion of 1 to 20, and to renew it several times if necessary. In mixing concentrated sulphuric acid with water great caution is required on account of the evolution of heat. The acid should be poured in a thin stream into the water, but not vice versâ, and the mixture should be constantly stirred with a glass rod. If a glass vessel is used for the mixing, it must not be too thick lest the heat should cause it to break, but the larger the proportion of water to the sulphuric acid, the less considerable will be the rise of temperature.

For boring out rust spots which have eaten deeply into the iron a dental drill can be used with success, and a great variety of drills and milling cutters can be obtained. The rinsing, which Blell carries out by moving the object to and fro close under the surface in a vessel full of water, may be sufficient for thin iron objects, such as swords, knives, spear-heads, and similar objects. Larger specimens should be freed from the acid by putting them into a still more dilute solution, and, when necessary, by steeping for a short time in water. It may also be advisable to put the objects into dilute soda solution to neutralize the sulphuric acid, but this does not do away with the necessity for steeping in water. The brown coating of rust which may possibly follow the steeping can be removed by the use of steel-wire brushes, which can now be made of such fine wire that their softness almost equals that of a moderately soft tooth-brush. Brass-wire brushes should not be used, on account of the yellow colour which they give to the iron. I always put the objects directly after steeping into clean fat heated to 250°F. [120°C.], for brushing over with fat and warming in a stove often caused a slight tarnish to cover the surface. I have also used paraffin wax instead of fat.

For the method of restoring iron antiquities and of filling up large gaps, the reader should refer to Blell’s detailed account; it will here suffice to quote his statement that a mixture of iron filings with tin filings can be used for this purpose. These are melted and applied by the aid of a blowpipe.

The accompanying illustrations represent iron antiquities which have been treated by Blell’s method: the sword (Fig. [25]) proved, after reduction, from its two ridges to be a scramasax; on the spear-head (Fig. [26]) treatment revealed a small copper ring at the most constricted part, while the fibula, which previously had been a mass of rust, now shows the spiral which had been totally disguised.

Fig. 25. Iron sword treated by Blell’s method.

Fig. 26. Iron spear-head treated by Blell’s method.

Fig. 27. Iron fibula treated by Blell’s method.

Krefting’s Method. The electro-chemical method of Krefting was originally published in “Aarsberetning fra Foreningen till Norske Fortidsmindesmaerkers Bevaring,” 1892 (p. 51), but in “Finska Fornminnesföreningens Tidskrift[125]” there is a translation into German by H. Appelgren, and an additional series of observations and experiments by him. His remarks are equally applicable to Blell’s method, and the following extracts and quotations from this paper give Krefting’s method of procedure and the circumstances under which it should be applied.

Small fragile objects such as fibulae, thin clasps and bracelets or those which are much eaten away by rust, are not suitable for this mode of treatment, thus:

“A knife which is much corroded, and which when taken out of the earth shows a distinctive form (for example, that of the Early Iron Age), may lose so much by the application of the electric current that every distinct sign of its original character is destroyed. The characteristic edges of a spear-head or of an axe of the late Iron Age, or the equally characteristic point of an iron sword, may, if the rust has eaten deeply into them, be unrecognisable when removed from the electrolytic bath. A sword, the hilt of which is inlaid with copper wire or is plated with silver or gold, or the blade inlaid with inscriptions in gold, silver, or copper, may be totally destroyed by incautious treatment; for the ornamentation, if undermined by rust, may be detached with the rust from the underlying iron.”

On the other hand, objects of sound metallic iron covered with an incrustation of rust about 1⁄25 inch [1 millimètre] in thickness may be easily cleaned in this manner, but if on using a file the metal does not appear at all, or only at a depth of 1⁄8 inch [3 millimètres], great caution must be used. If there is reason to believe that there is gold or silver inlaid work undermined by rust, Appelgren recommends that the object should, as a preliminary, be laid in clean water, which should be renewed every day. After some time, three weeks at the most, sufficient rust will have been cleared away by carefully brushing with a steel brush to lay bare the ornamentation, at least in part, and it can then be ascertained whether there is any rust underneath which would, if Krefting’s method were used, cause the ornamentation to be detached.

The line of treatment is as follows: The metallic iron core is laid bare by filing in several places. The specimen is then wrapped with strips of zinc in such a way that the zinc is in actual contact with the bare metal (Fig. [28]). The whole is then placed into a 5% solution of caustic soda[126]. Appelgren uses a solution of 31⁄ 2-41⁄2 lbs. [11⁄2-2 kilogrammes] of caustic soda in 2 gallons [10 litres] of water. The rust is cleared away by voltaic action; the iron forms the negative pole, the zinc the positive of a voltaic cell, in which the water is resolved into its constituents, viz. oxygen and hydrogen. At the negative pole, i.e. the iron, the hydrogen rises up in small bubbles and acts in part by mechanically detaching the rust as in Blell’s method, in part also by the chemical conversion of the rust into metallic iron, or into a compound which contains a smaller quantity of oxygen than does ordinary rust. The oxygen combines with the zinc to form zinc oxide, which is dissolved in the soda solution. The process is usually completed in 24 hours[ [127]. The black powder which is loosely attached to the iron is best rubbed off with wet sand and fine wire brushes. Any hard pieces of black stable rust (Edelrost), magnetic oxide of iron, which have not yielded to the electric current should be removed by means of a small chisel. After rinsing the object thoroughly in water, it should be placed in melted paraffin at 240°F. [115°C.], which will expel every trace of moisture. On removal the melted paraffin should be allowed to drain off, and thus leave when cold a protective covering upon the iron[128].

The following points should be observed in the application of the method. Vessels of glass or glazed earthenware should be used for the reduction, while long swords can be put into tall glass cylinders or into wooden troughs, the interior of which must be coated over with paraffin. The soda solution must be kept in a closed glass bottle[129]. It should be diluted with water until the specific gravity, as shown by the hydrometer, is 1·06; the mixture will then contain about 5 per cent. of caustic soda. During the reduction process the mixture frequently assumes a brownish colour as the result of the presence of organic matter associated with the rust. On account of the dissolved zinc which it contains it cannot be used a second time, unless regenerated by boiling with quicklime. The solution is, however, so cheap that this is scarcely worth the trouble.

The objects should be handled with metal tongs, and should not be touched with the hand until they have at least been dipped or rinsed in water, for the soda solution has an injurious effect upon the skin. A basin containing vinegar, dilute hydrochloric or sulphuric acid should always be at hand into which the fingers should be quickly dipped if they have been in contact with the caustic soda. These materials will serve also for cleaning the vessels used in the reduction process.

The zinc strips should be 1⁄4 to 1⁄3 inch [1⁄2 cm. to 1 cm.] in breadth, and should be cut out of a piece of sheet zinc of moderate thickness, but of sufficient pliability.

Any firmly fixed rust may be removed by mechanical means, such as the graver, drill, etc., as has been previously mentioned. If in rinsing a slight layer of oxide appears, although this is rare, it should be brushed off with a steel-wire brush.

If one portion only of a specimen requires reduction (the other portion having, for example, remains of wood attached, and therefore being unsuitable for reduction), that portion only should be wrapped with the zinc and immersed in the solution.

The results obtained by Krefting's preservation-process are quite as surprising as those which are afforded by Blell’s method. Figure [29], taken from Appelgren’s work, shows the lower portion of a spear-head before and after treatment, by which it became apparent that the whole socket was plated with silver, with two engraved and gilded animal figures. Fig. [34] represents a piece of a sword, on which an inscription was brought to light by the reduction process.

Fig. 28. Krefting’s method. Iron spear-head wrapped with strips of zinc.

Fig. 29.
Iron spear-head before and after treatment by Krefting’s method.

Fig. 30. Iron pin from “Danes’ Graves,” Yorks. [Cp. Yorks. Phil. Soc. Report, 1897.]

Fig. 31. The same after treatment by Krefting’s method, still showing chalky accretions.

Fig. 32. Iron object from Lamel Hill[130], York. It appears to have been originally rivetted to wood or leather.

Fig. 33. After treatment by Krefting’s method.

Fig. 34. Piece of iron sword-blade showing inscription, after treatment by Krefting’s method.

Hartwich’s Reduction Method[131]. This method is only applicable to small objects, because it necessitates the subjection of the objects to red-heat in a glass tube in a current of hydrogen. By these means the hydrogen combines with the oxygen of the oxides, which are thus reduced to metallic iron. Owing to the explosive nature of a mixture of hydrogen and air, this process should only be carried out by one who is conversant with chemical methods, for results which are equally good can be obtained at less expense by Krefting’s method. For Hartwich’s method a strong core of metal is essential, for although objects which are entirely oxidized may be thus reduced, the result will be the formation of a more or less loose iron powder which is frequently in such a fine state of division that by union with the oxygen of the air, in consequence of the great amount of surface presented, it becomes red-hot with the formation of ferric oxide as a combustion product.

It is advisable to apply a combination of Blell’s or Krefting’s method with one of the first group (under certain conditions) to such iron objects as are found, during the process of preservation, to be penetrated by black stable rust to such a degree that the complete removal would only leave a kind of iron skeleton. Fig. [35] represents such an iron dagger-sheath[ [132], the dark spots upon it being rust. After heating and cooling down and a short treatment with acid the removal of the rust was proceeded with mechanically, but was not completed. The object was then well steeped, and when dry was warmed in the varnish-petroleum mixture[133].

Fig. 35. Iron dagger-sheath after treatment by a combination of Blell’s and Krefting’s methods.

Iron objects, the size of which is inconsiderable, such as arrow heads, small rings, etc., can be very quickly reduced, if they still have a well-preserved core, by heating them for a short time in molten potassium cyanide[134]. The cyanide may be melted in a porcelain crucible supported by wire gauze on a tripod over a good-sized Bunsen burner, and the object introduced by the aid of tongs. The reaction is accompanied by vigorous effervescence and is soon complete. It is then taken out and dropped into cold water. By repeatedly boiling in fresh quantities of water it is thoroughly cleansed, then treated with paraffin wax, or the water may be expelled by alcohol. It is then dried, and finally impregnated with zapon. If the cyanide treatment is insufficient, any remaining rust may be removed by drills or other suitable tools. Hitherto this method has only been applied to a small number of objects, but there is no doubt that its use may be largely extended. Owing to the poisonous nature of the cyanide this method should be left to those who possess chemical knowledge. The disadvantage of the process lies in the difficulty of fusing large quantities of the potassium cyanide[135].

(4) Preservation of Medieval Iron Objects.

A complete treatise on this subject would be beyond the limits of a handbook, the following observations, therefore, will be sufficient for our purpose. The rust spots on objects of this kind are frequently only superficial and can be removed either mechanically by rubbing with pumice or emery, etc., or chemically by a concentrated solution of sodium sulphide[136]. To prepare this, sodium sulphide is dissolved in water, or flowers of sulphur are boiled in a solution of caustic soda. If the object is too large for immersion, the solution may be applied with a brush, and if the layer of rust is thick, the application must be repeated. After treatment the object must be rinsed in water and dried.

Small articles can be freed from rust by immersion in strong fuming nitric acid[137], for strong acid dissolves the rust only, while it induces in the iron the so-called “passive[138]” condition in which it is not acted upon even by dilute acids, and can be safely washed in water. When thoroughly cleaned, the most suitable protective is some neutral substance such as paraffin wax, vaseline, or paraffin dissolved in benzine, but any of the numerous forms of oil or fat may be used.

(i) Bronze and Copper[139].

Well-preserved bronzes with a stable patina, such as the highly esteemed glossy stable or “edel” patina, or that which, although not glossy, covers the bronze with a rough and often crystalline coating, should not be interfered with. Such bronzes as need treatment should be subjected either to simple cleaning or to some appropriate method of preservation.

The Cleaning of Bronzes. Bronzes, the metallic substance of which is more or less intact, while the surface is hidden under earthy or sandy material cemented together by copper compounds, may be cleaned either by mechanical or chemical means. When the materials forming the incrustation are more firmly cemented together than they are to the material beneath (which often still retains a polished surface), a small hammer may be used, but more adherent portions require the use of small chisels, which can be made to order in different shapes or sizes. I have used with advantage hammers with striking surfaces like those shown in Fig. [36]. The two on the right are rounded so that they touch the object at one point or on a line only. The process may be facilitated by the use of Springer’s method. A warm thick solution of glue should be spread upon the incrustation covering the bronze. As the glue dries and becomes cool it scales off, carrying with it some portion at least of the crust, thus leaving the metal clean. That part of the glue which remains can then be readily detached by gentle strokes with a hammer. The eyes should be protected when using the hammer, whether on the incrustation or on the glue.

Fig. 36. Hammer heads, natural size.

Other Methods. Since metallic oxides are scarcely, if at all, soluble in water, washing with water, even when a brush is used, will remove only earth or soil which is loosely attached. Compounds containing oxygen or oxygen and chlorine are, however, more or less soluble in ammonia, and, if they are thin and not too compact, after immersion for some time can be removed with a brush. Thick compact layers are loosened with difficulty.

Immersion in 2-5% hydrochloric acid acts more effectively, while sulphuric acid, nitric acid, and concentrated acetic acid have the same action. The frequent use of these reagents is, however, strongly to be deprecated, for it is impossible to remove the acid by simple washing with water after the incrustation has been removed. The bronze should be washed and placed in a very dilute soda solution or in dilute ammonia, after which it should be again well washed with distilled water. As has been explained in [Part I.], it is to chlorine compounds that the destruction of bronzes is chiefly due, and these are actually produced by the hydrochloric acid treatment. If the bronzes are not thoroughly washed, and this is no easy matter, sooner or later efflorescences will make their appearance, and the process of preservation must be repeated if the destructive action is to be arrested.

Various attempts have been made to remove the incrustation by raising the bronze to a red heat. This process is not recommended; for not only does it give to the bronze an unpleasant appearance, but it detaches any inlaid metal (gold or silver) or enamel which may be present.

In conclusion, it may be stated that, although the process is slow and laborious, the best results are obtained by careful removal of incrustations by mechanical means.

Preservation of Bronze and Copper Objects.

(A.) Methods of Impregnation. The impregnation of bronzes, as of the majority of antiquities, has for some time been carried out by the use of solutions similar to those already enumerated for iron. These are applied directly or after the specimen has been either steeped in water or treated with dilute acids. This latter treatment, as has been already stated, is to be avoided, and if used all acid must be washed out before the object is dried. Steeping in water is of little use, because compounds containing oxygen or chlorine are often insoluble in water, which will at most only wash off loosely attached dirt or earthy material. The impregnation process may therefore be applied directly, and this should be done in all cases in which the surface is much corroded, warty (Figs. [7] and [8]), or cracked (Figs. [37] and [38]), or in which there is little or no core of metal. Impregnation is also the only means of preservation when the formation of oxides has raised inlaid metals or enamel in such a way that the removal of the oxides would detach them. The “Merkbuch[140]” recommends poppy seed oil and benzine mixture (p. [70]) or the gum-dammar solution. To obtain thorough impregnation this should be carried out by extraction of the air, as has been already recommended in the case of limestone (p. [68]). The object must also be perfectly dry, which may be insured either by exposure to moderate heat or by keeping it for some time over anhydrous calcium chloride[141]. The object is placed under a glass bell jar, the edges of which are smeared with vaseline to ensure contact with the glass plate upon which it rests. The calcium chloride should be placed in an open glass vessel, beneath the bronze, but care must be taken that they are not in actual contact.

Fig. 37. Osiris showing cracking and destructive patina.

Fig. 38. Boeotian bridle with cracking patina.

Immersion of bronzes in paraffin wax at 240°F. [115°-120°C.] gives results which are as good, if not better, than those obtained by the use of solutions.

Should efflorescences make their appearances upon bronzes which have been impregnated, their further spread may often be successfully prevented by smearing fish-glue on the parts affected. Fish-glue, however, has not proved a satisfactory material for the complete impregnation or coating of bronzes which are in the last stages of decay.

(B.) Preservation by Reduction. It has been previously explained (pp. [28] et seq.) that the efflorescences upon bronze known as creeping or malignant patina which may in time cause the complete destruction of the metal are due to the action of sodium chloride. It is found upon all Egyptian bronzes and upon those from some other localities.

The metal, especially the copper, is converted into the so-called basic chloride. In the reduction processes an attempt is made to reduce these compounds again to metal, while the chlorine thus liberated forms chemical compounds, which may be subsequently washed out with water. There are two methods which effect this reduction, viz., that of Finkener (Berlin) and that of Krefting. The principle of both is electrolytic, and both bring about the complete removal of the patina and the restoration of a clean metallic surface.

To complete this portion of the subject a third method may be mentioned, viz., reduction by heat in a stream of hydrogen. This method[142] is, however, only applicable to small objects.

Finkener’s Method. Care must be taken when examining the bronze that the metallic-looking mixture of cuprous oxide with other copper compounds is not mistaken for metallic copper. When it has been ascertained that the bronze still has a good metallic core, and that any inlaid metals which may be present rest on the metal itself and not upon a crust of oxide, a platinum wire should be tightly wound round it. This should be connected by an insulated copper wire to the zinc or negative pole of the first of 3 or 4 Daniell cells, or, better, of two accumulators arranged in series. The object should then be immersed in a 2% aqueous solution of potassium cyanide. In the same solution, as near as possible to the bronze without actual contact, should be placed a piece of platinum foil connected first by an emerging platinum wire, and then by an insulated copper wire to the positive pole. The potassium cyanide completes the electric circuit and electrolysis takes place, whereby the water is split up into its constituents. The oxygen appears in small bubbles upon the platinum foil, but the hydrogen does not immediately make its appearance at the other pole, for, by combination with the chlorine and oxygen contained in the bronzes, free hydrochloric acid and water are formed. The hydrochloric acid in turn acts upon the potassium cyanide to form potassium chloride and hydrocyanic acid, both of which substances are dissolved in the water of the bath. The hydrocyanic acid can often be recognised in the room by its characteristic smell of bitter almonds. The process may be expressed by the following equations (neglecting the water produced by the oxygen of the oxide, which is of no importance in the process):

CuCl₂ + 2H = Cu + 2HCl,
HCl + KCN = KCl + HCN.

Although the chief portion of the potassium chloride and hydrocyanic acid are dissolved in the bath, the remaining traces of these substances must be removed by very carefully washing the bronze in water, after which it should be dried, and if necessary finally subjected to impregnation.

Some further observations may be made in connection with the practical application of this process.

Of course, other primary batteries may be used instead of the Daniell cells, but these latter may be specially recommended for the ease with which they can be procured and for the steadiness of their action. Information concerning the method of filling and using them may be obtained at any shop where they are sold. The copper wire and platinum wire should not be too thin, but must be at least from 1 to 2 mm. in thickness: they should be fastened together by binding-screws, and care must be taken that both the wire ends and the screws have clean surfaces. Glass vessels or glass cylinders are most suitable because the process of reduction can be watched, but large objects will of course require glazed earthenware baths. If wooden boxes are used they must be coated inside with paraffin wax. The strength of the cyanide solution should be 2%. Having a large number of reductions to carry out, I keep a 20% stock solution in a large bottle, one part of which is diluted with nine parts of water when required for use. Potassium cyanide is, as is well known, a strong poison, and care should therefore be taken to prevent access to any sore or cut on the hands; this can be done by the use of india-rubber finger stalls or gloves.

If the bronze object is neither too large nor too heavy it may be suspended in the bath by looping the platinum wire over the edge of the vessel. It is a convenient plan to use different coloured wires to distinguish the negative and positive poles of the battery, but should any doubt arise as to which wire should be connected with the bronze or which with the platinum, the following test will readily decide the question. Moisten a small piece of white filter paper with a drop of a solution of potassium iodide[143], and touch the two conducting wires with it simultaneously: a brown spot will be seen on the paper at the point of contact with one of the wires; this is the positive wire, and must therefore be connected with the platinum. If the current is passing through the cyanide bath and the bronze, bubbles of gas will appear upon the platinum foil, or the products of the decomposition of the potassium cyanide may change the colour of the bath near the platinum to yellow or brown, while at the same time cloudy streaks under the bronze will show where the potassium chloride and hydrocyanic acid, resulting from the reduction of the copper compounds, are meeting with the cyanide of the bath. If the platinum wire is not firmly fixed round the bronze, hydrogen may be formed upon it, and should this occur the wire should be drawn tighter.

Whilst the reduction is going on it is advisable to renew the potassium cyanide at least once, or even several times, if large and greatly oxidized bronzes are under treatment, for otherwise all the potassium cyanide may be consumed by the changes in progress; this can be ascertained with certainty by a smell of chlorine. When the bath requires renewal the bronze may be taken out with a pair of metal tongs, or if too large, two strong copper wires should be passed underneath it, the ends of which are wound round a strong glass rod or wooden stick. The bronze should then be well rinsed or brushed with a soft brush before it is put into the fresh bath.

Bronzes are frequently met with which are much deformed by an earthy or sandy layer cemented by oxide. These incrustations can be partly removed by a preliminary treatment with dilute hydrochloric acid, but the bronze must be afterwards carefully rinsed with water or even steeped to prevent unnecessary decomposition of the cyanide by the acid. Before reduction it is useful to secure thorough penetration by placing the vessel containing the solution and the bronze under a bell glass attached to an air pump, as has been previously explained (p. [68]).

During the process of reduction small whitish-green crystalline needles often collect on the platinum foil, but although in large numbers they are so minute that it has not been possible hitherto to determine their composition; they seem to contain copper and cyanogen. After some time the platinum becomes covered with a whitish-green or brownish deposit, which should be removed by rinsing in water and brushing; if this should not succeed the platinum must be dipped in hydrochloric acid, rinsed with water, and rubbed with fine sand. The glass vessel may be cleaned in the same way.

The reduction is complete when all the chlorine, previously combined with the metal, has combined with the hydrogen produced by the electrolysis of the water. There being no further chlorine with which the hydrogen produced by the continued action of the current may unite, the completion of the process is marked by the appearance of bubbles of that gas upon the surface of the bronze. The bubbles which rise from beneath often mark out the outlines of the object upon the surface of the bath.

Before the bronze is washed it should be placed in a fresh cyanide bath, but of 1% strength only. For large and especially for thick objects, this bath must be renewed several times, so as to allow the washing process to begin in the bath itself whilst the current is still passing through it. Care should also be taken that every side of the object in turn faces the platinum foil for some time, for if one side remains turned toward the platinum throughout the process, it will sometimes assume the red tint of copper, while the rest of the bronze retains a somewhat dark colour.

When finally removed from the reducing bath, after the black metallic powder has been thoroughly cleaned off with water and a soft brush, the object should be suspended for a short time in water at the ordinary temperature, or so fixed that there is a good depth of water beneath it; it should then be washed in hot water. When the bronze is first placed in water, whether hot or lukewarm, small bubbles of hydrogen will continue to rise for some time, while at the same time a whitish, or sometimes grey, gelatinous precipitate, consisting of a hydrated oxide of tin[144], will often fall from it. The grey colour is caused by the admixture of small particles of lead or copper.

At first I renew the water two or three times a day, then once in twenty-four hours, and finally at longer intervals, using distilled water throughout for small objects, but for larger specimens for the final washings only. For the earlier washings at any rate I use warm water. Cyanides as well as chlorides give a white precipitate with silver nitrate; this reagent will therefore serve to indicate the progress of the operation. If at the end of a fortnight in the case of small bronzes, or in three to six weeks for large objects, the water shows no cloudiness, or if upon the addition of yellow potassium chromate it instantly assumes a red colour (p. [62]), the steeping may be considered complete. Some Egyptian bronzes, especially those which contain a large proportion of lead, after steeping exhibit a whitish crystalline coating of lead carbonate or small hemispherical groups of crystals scattered over the surface of the metal, especially where the pores are large; when dry these can easily be removed.

An extended experience points to the conclusion that bronzes should be dried at once, and as quickly as possible. They should be wiped with soft cloths and then dried in a drying chamber or upon glass or metal rings on a stove. A simple form of drying chamber can be made with copper or iron plate of sufficient thickness, with a loose lid provided with a hole fitted with a cork, through which a thermometer passes. This can be heated over a Bunsen burner, but the temperature should not exceed 230°F. [110°C.]. Small objects may be freed from water by immersion in alcohol for twenty-four hours before drying.

The completion of the process may be gauged by the yellowish or reddish yellow colour which the bronzes should assume when they have been dried and wiped with a cloth or brushed; brushes made of the finest steel wire may be used for this purpose. A bright colour is but rarely seen on bronzes which contain lead. Egyptian bronzes frequently contain as much as 20% of lead, and such bronzes have nearly always a dull-grey or blackish appearance. A similar colour is seen on bronzes which contain no lead, but which are very porous, and are in an advanced state of decomposition. In such cases the finely divided particles of reduced metal are retained upon the rough surface of the bronze, and as all metals, when sufficiently finely divided, form a blackish powder without any metallic lustre, the whole object then appears almost black. It is difficult, and in many cases impossible, to remove this dust, especially that retained in the pores. Metal dust is injurious to the lungs, and if recourse is had to brushing, an efficient extractor for the removal of the dust-filled air is required[145]; but brushing and the use of bellows in addition frequently prove insufficient. Washing the objects with benzine is more effectual, but a trustworthy method of giving the bronze a better appearance is to place it into melted paraffin wax[146] at 250°to 285°F. [120° to 140°C.]. Yet the use of paraffin wax should be avoided if possible, for in spite of the most careful washing blue efflorescences may sometimes appear upon thick bronzes in the course of a year. If this should happen they must be washed out at once, and the bronze can again be submitted to the cyanide-reduction process. If however paraffin wax had been applied an attempt would have to be made to remove it by immersing the bronze in benzine or a mixture of ether and alcohol, or by heating, before the reduction process could be repeated.

Fig. 39. Bronze bull showing warty patina.

Fig. 40. The same after reduction by Finkener’s method[147].

There is no doubt that these bright-blue efflorescences are the result of an incomplete reduction, which in many cases can scarcely be remedied, for it is often impossible thoroughly to wash objects of great thickness. Thin bronzes, bronze plate, and copper plate remain free from efflorescences. Moreover, many bronzes, especially Egyptian ones, have a hard, non-metallic core, which in the casting has been partly fused or at least hard-burnt, and resists the effects of the washing.

Fig. 41. Bronze axe-blade before treatment by Finkener’s method (Aeg. 13203).

Fig. 42. The same side after treatment.

Fig. 43. Reverse side of axe-blade after treatment.

It is occasionally found that a bronze cannot stand the process of reduction, either because there is only a thin layer of metal over a stout core, or because the metal is permeated with cuprous oxide, which when tested with a file has a metallic appearance. The bronze must therefore be continually watched whilst it is in the cyanide bath, and if necessary should be taken out even before the reduction is complete. This should be done if large pieces or large quantities of a powdery precipitate fall from the bronze, or if it is found that a needle readily pierces the oxidized layer. A specimen of this kind must be taken from the bath, carefully steeped, dried, and impregnated[148].

It is not to be expected that bronzes which are in an advanced state of decomposition (e.g. Figs. [9]-[12]) can be so transformed by reduction as to appear as they did when they left the artist’s hand. For, although the decomposed oxidized layer is now reduced to metal, this no longer forms a coherent mass, but a loose powder, which, being deprived of its essential constituents, chlorine, oxygen and carbonic acid, no longer retains its coherency, but falls to the bottom. Only in the interior and in the pores is the reduced metal retained.

In addition to the preservation of articles by the removal of the injurious chlorine compounds (as is also the case with Blell’s and with Krefting’s method for iron antiquities), the process may result in the discovery of inlaid work, inscriptions or ornamentation, the presence of which was not suspected. The accompanying illustrations (Figs. [39] and [40]) show bronzes before and after the preservation process, while the axe-blade shown in Figs. [41]-[43] illustrates equally clearly the advantages which accrue from the treatment. Not less striking is the result of the treatment in the case of the dagger-sheath shown in Figs. [44] and [45] by which the design was discovered.

Reference may here be made to a case described elsewhere[149], in which reduction proved that what had been thought a single bronze object consisted in reality of two pieces which did not belong to each other, but were fitted together by means of a bottle cork of modern date! In another instance a bronze was found upon reduction to be brazed with a hard solder containing zinc, which was thus quite inconsistent with the age ascribed to the object.

Fig. 44. and Fig. 45.
Dagger sheath before and after treatment by Finkener’s method.

A short digression may be here made in order to discuss the question whether the composition of the bronzes undergoes any alteration. Three analyses[150] of Egyptian bronzes before and after reduction by Finkener’s method show that the change in composition is so slight as to be immaterial. It is of course obvious that greater differences will be seen in the results of the analyses before and after reduction of bronzes which are in an advanced state of oxidation, for in this case chlorine, oxygen, water, and carbonic acid constitute an appreciable proportion of the total weight. But even in these cases the analysis made after the reduction shows very slight variation from that of the original metal.

		Osiris		Osiris		Ibis
		Before	After	Before	After	Before	After
				Reduction
Tin		2·16	2·27	4·30	4·21	8·66	8·46
Copper		77·83	77·45	79·66	79·74	88·53	88·75
Lead		19·23	19·86	15·51	15·58	1·69	1·95
Iron		0·12	0·14	0·28	0·24	0·21	0·20
Nickel & Cobalt		0·29	0·24	0·20	0·17	0·30	0·29
Arsenic		0·17	0·23	0·17	present	0·32	present
Antimony		trace	trace	trace	—	0·20	present

The two latter bronzes were tested qualitatively only for arsenic and antimony, and when the three objects were washed the hydrated tin-oxide described on p. [130] was only found in the case of the Ibis. In this connection it should not be forgotten that slight differences in the quantities may be due to errors in the analysis as well as to a want of homogeneity in the alloy.

Krefting’s Method. This method is similar to that used for the reduction of iron (see page [108]). The layer of oxidized material is removed in several places by filing, hammering, or rubbing with emery cloth until the metal is exposed. The object is then wrapped round with strips of zinc, and placed in a 5% solution of caustic soda. The hydrochloric acid produced in the process of reduction acts upon the soda to form sodium chloride. Here too the greatest care must be taken that the steeping is sufficient.

Personally I prefer Finkener’s method, for potassium cyanide is more easily washed out than soda, and also, although poisonous, is less caustic.

Krefting’s method however has proved of considerable success in some cases, notably in the treatment of some 40-50,000 Roman copper coins at the Berlin Museum. These were, with few exceptions, covered with a crystalline layer resembling green malachite or blue azurite and were quite illegible. Various unsatisfactory attempts were made to clean them with ammonia, with warm and cold acids of different kinds, with acid and iron nails, and by electric current both in an acid solution and in a solution of potassium cyanide. The following method finally proved satisfactory[151]:

Krefting’s Method Applied to Oxidized Copper Coins.

“A thin plate of zinc with a bright metallic surface is perforated with a brad-awl, having a diameter of from 2 to 5 mm., until there are about 50 or 60 holes in each square metre. This is placed with the sharp edges of the holes uppermost on a row of glass rings (or crystallizing dishes will serve the purpose) 20 mm. in height resting upon the bottom of a large glass vessel. The coins, which in this case were 20 mm. in diameter, were then placed on the zinc plate, so that 7 or 8 of them occupy a space of 1 square decimetre. Another similarly perforated plate is laid upon them, and upon this more coins are arranged in the same way, and so on until there are six or eight double layers. A perforated zinc plate is then placed on the top with the sharp edges of the holes turned downwards, and over this a few zinc plates which have been previously used. The whole pile is surmounted with weights or stones resting upon glass rings or inverted glass dishes in order to press the sharp edges of the holes into the closest possible contact with the coins. A 5% solution of caustic soda is then poured over the whole, the immediate result of which is an evolution of gas. The reduction of the coins is usually complete in fifteen to eighteen hours, after which they should be well washed. After several rinsings in cold water they are placed, about 1000 at a time, in a large vessel fitted with a perforated false bottom containing hot water, which should be renewed three or four times every day. After four days the coins are wiped with a cloth and thoroughly dried on a warm oven plate or in a drying chamber at a temperature of about 212°F. [100°C.]. They are then brushed with a bristle brush before a dust extractor, a procedure rendered necessary by the fine metallic dust from the coins, which then assume a light or dark brown colour such as is seen on copper coins which are in actual circulation. The practice of placing the coins whilst still wet into melted paraffin wax at 260°F. [120°-130°C.], which gives a dark appearance even to the brightest, has the disadvantage that the wax prevents the use of sealing-wax for taking impressions, and is therefore not recommended.

The reaction is analogous to that which occurs in the reduction of iron. The copper of the coin forms in the alkaline solution an electric couple with the zinc, and the hydrogen which forms at the copper end reduces the copper compounds covering the coins to metallic copper, and thereby loosens them, while the zinc oxide which is simultaneously formed is dissolved in the soda solution. In actual practice a part only of the zinc oxide is dissolved, while the remainder forms a white coating on the zinc[ [152]. Experience shows that a 4-5% solution is the most suitable for this method of reduction, which gives the most favourable results when these details are followed. If for example the zinc plate is laid immediately on the bottom of the glass trough, if the coins are laid too close together on the plate, or if there are more than 6 to 8 double layers in a trough, the process of reduction is often incomplete, and it is then necessary to treat the coins a second time. It is scarcely necessary to mention that larger coins must be placed at proportionately greater distances from each other.

The 40-50,000 coins which were thus treated had originally been tinned, but the tin only remained at a few places. When the coins were washed immediately after the reduction, this tin could still be clearly distinguished, but on further washing, drying, and brushing, it ceased to be visible on account of the dark colour imparted to it by the finely powdered copper. In one or two cases lead appeared on the surface of the coin, but was easily removed by mechanical means.”

Fig. 46. Roman coins before treatment.

Fig. 47. Roman coins after treatment by Krefting’s method.

Cleaning Copper Coins by Melted Lead.

Although the results obtained by this method are less satisfactory than those produced by the preceding, it has the advantage of simplicity[153].

“Using a pair of tongs, dip the coins one by one into melted lead until the crackling, which begins at once, has ceased, which occurs in from 3 to 10 seconds. The hand should be protected with a glove from the spluttering molten lead. The coin is then thrown into cold water, cleaned, and placed until the next day in hot milk. It may be necessary to repeat the process when the coin has become cold. By this method an olive colour is imparted to the coin which many antiquaries prefer to dark brown, but personally I prefer Krefting’s method because it renders the inscription and designs far more distinct. A coin which after the treatment with melted lead has remained so covered with cupric oxide as to be still illegible can seldom be improved by a repetition of the treatment, whereas had the zinc treatment been applied in the first instance the result would probably have been satisfactory. This conclusion seems to be justified by the extremely small percentage of coins which, in my experience, have remained illegible after the treatment by electrical methods.”

(C.) Preservation of Bronzes by the Exclusion of Air.

In those cases in which the advanced state of decomposition renders the reduction process either inapplicable or at any rate inadvisable, or in which the decay is not likely to be arrested by impregnation, a further method of preservation remains, viz. the complete exclusion of air and moisture.

If air is completely freed from moisture the oxygen can no longer act in conjunction with the copper chloride upon the still intact metal (see page [29] et seq.), and the condition of the bronze will consequently remain unchanged.

A bronze, for example, which shows much decay should be placed after impregnation under a hermetically sealed bell glass, and beneath or near it should be placed some dehydrating agent, of which anhydrous calcium chloride is the most suitable (see note, p. [123]). To exclude the air completely the bell glass should have a projecting ground edge, which should be smeared with vaseline or grease and pressed firmly upon a thick well polished glass plate. The dehydrating agent may be placed in a glass vessel or dish in such a way as to be unseen, or it may be covered with two or three thicknesses of dark gauze or with black cardboard laid loosely over it. If an object is too large for a bell glass, or if several objects are to be exhibited together, a square plate-glass case with iron framework, made air-tight with putty, may be used as shown in the illustration (Fig. [48]). The lower part, containing calcium chloride, is partitioned off by a perforated plate covered with black gauze[154]. A hygrometer was placed behind the head, the indicator of which has remained at zero since it was first fixed several years ago, and the bronze has not hitherto shown any sign of change, although the inlaid gold is in parts raised from the metal by a light-green oxychloride. The cost of these cases is considerable, but for valuable objects this should not be considered. In the place of calcium chloride, sticks or lumps of caustic soda may be used with advantage, for this substance absorbs both moisture and carbonic acid.

Fig. 48. Method of mounting objects in air-tight cases.

This method of preservation is of course applicable not only to decomposed bronzes but to all valuable antiquities, whatever the material may be.

Appendix.
Methods of Bringing out Worn Lettering upon Coins.

These methods are founded upon the fact that the sunken areas of the coin are, by the pressure of the die in stamping, rendered denser than the raised portions, such as the inscription. The earliest method is that published by Brewster, reported by Süpke[155]. The coins when cleaned are placed upon red-hot iron, which causes the oxidation of the entire surface of the coin. The thin film of oxides varies in colour according to the duration and the intensity of the heat. The oxidation of the letters of the inscription differs from that of the surrounding parts, and is recognisable by a difference in colour. Drude[ [156], treating more especially of silver coins, remarks that the inscription is rendered legible by heating them to redness over a Bunsen-burner. It then, according to “Prometheus[157],” when viewed in a dark room, appears dark on a bright ground, especially if the coin has been previously polished and then roughened again by slightly etching it with acid. In conclusion, the method of Roux[158] may be quoted:

“The smooth-worn and polished coin is placed in a solution of copper sulphate or of some other metallic salt, and suspended between the electrodes of one or more cells of a battery (any other form of continuous current will serve the purpose). If the current is weak, the electrodes must be near to the coin. The stronger the current the more rapidly the impression appears. On the side which faces the anode or positive plate the impression is metallic; on the other side, after gently wiping off the less firmly adherent part of the oxide, the impression appears in grey lines. These markings can be fixed by varnishing them with a thin alcoholic solution of shellac. To render the impression legible on both sides, the coin should be placed upon the four upturned feet of an insulating stand. The larger the coin the deeper must be the layer of solution above and below the coin. The depth below should be equal to the radius of the coin.

This can perhaps be most conveniently carried out by placing that electrode in immediate contact with the coin which upon immersion in the solution becomes tarnished with the metal, i.e. the cathode or negative pole. Other portions which it is not intended to treat should be first covered with varnish.

The striking success of this method is due to the fact that that portion of the metal which has been compressed by the stamp is a better electrical conductor than the rest; no success could therefore be expected from the use of this process for the restoration of such objects as worn engraved copper-plates, etc.”

(j) Silver.

Preservative treatment of silver is scarcely necessary (cp. pp. [49]-[52]), except in those cases in which the silver is alloyed with a large percentage of copper, and which show efflorescences similar to those which appear upon bronzes containing chlorine. Electrolytic reduction will be found to be the most suitable method of treatment in such cases. To treat silver coins they should be placed in contact with iron nails in lemon juice. Instead of the citric acid, which is the active principle in this process, other diluted acids and other metals, e.g. zinc, may be employed. Flinders Petrie[159] has shown that the reduction can also be effected by a weak solution of common salt. Silver chloride is soluble in ammonia, and thin layers may be removed by the application of ammonia by means of a soft brush. Thorough rinsing with pure water, drying with soft cloths, and cautious warming are always essential.

An excellent reducing agent for single coins, the characters of which are rendered illegible by a layer of silver chloride, is molten potassium cyanide, or a mixture of this substance with sodium or potassium carbonate. In a short time the silver chloride is decomposed and removed from the smooth surface of the coin. After boiling out with water, steeping in alcohol, drying, and brushing with a soft brush, the coins may be coated with zapon. Coins treated in this way appear to be less brittle than those reduced by Krefting’s method. More troublesome but less dangerous, because potassium cyanide is not used, is the treatment of silver coins with a fused mixture of potassium and sodium carbonates. In this case the silver chloride is converted into silver carbonate, which is then decomposed with 50% acetic acid. Further treatment by washing, drying, and impregnation is carried out as previously described.

Silver which has become friable (p. [51]) can be rendered more compact by cautiously heating it to redness. It will however be advisable to entrust heating and mechanical treatment of objects which are much bent to some skilled silversmith, whose experience may prevent disaster. Silver objects which are largely converted into friable chloride, especially if they are much expanded, or if large portions have broken away in the process of removing the chloride, will hardly bear any other treatment than that of impregnation with gum-dammar solution or with paraffin wax. As silver chloride is easily fused such articles should not be subjected to heat.

Earthy matter can often be removed with a neutral soap and warm water, while calcareous accretions can be dissolved by a 2% solution of hydrochloric acid. Silver which has been blackened by silver sulphide may be laid in a warm 2% solution of potassium cyanide. All objects should be subsequently well washed with warm water.

(k) Lead and Tin.

Objects of pure lead and pure tin are rare. If much oxidized they should be washed with warm water, dried, and impregnated with a gum-dammar solution or with paraffin wax (pp. [70] and [91]). If in a good state of preservation they may be freed from any earthy or calcareous coating or from lead carbonate by the cautious use of very dilute nitric acid followed by steeping in water.

Ceresole[ [160] cleans oxidized leaden seals with 10% acetic acid, neutralises the acid with ammonia, and after five minutes in alcohol coats them thinly with wax. The seals are preserved between glass dishes (Petri dishes), the space between the dishes being filled with cement. I employ Krefting’s method for leaden medals, using either zinc and very dilute sulphuric acid, or zinc dust and caustic soda. Occasionally the zinc dust becomes firmly cemented by oxide to the surface of the lead, and, if this is the case, great care must be used in removing it. The washing process also requires care. A very efficacious method is to allow a stream of warm distilled water, from which the dissolved air has been driven off by boiling, to flow over the object in a porcelain dish. I now omit any impregnation with paraffin wax, and instead recommend removal of the water by alcohol, drying, and coating with zapon. To preserve the specimens after treatment, more especially from the injurious action of perspiration from the hands, they are placed between dishes of glass or of celluloid[161].

(l) Gold.

Objects of pure gold usually need only be cleaned with soap and water and a soft brush; lime may be removed by the application of a 2% solution of hydrochloric acid. A coating of silver chloride occurring on gold which contains a large percentage of silver may be removed by ammonia, or, in certain cases, by the alternate use of dilute hydrochloric acid and ammonia.

A layer of red ferric oxide (see p. [53]) is of frequent occurrence upon gold objects, and may be removed by warming the object in a stronger solution of hydrochloric acid, but soft brushes will often serve the same purpose. Pure gold being very soft, only the softest so-called “silver brushes” should be used, and all pressure or bending should be avoided. If friable the object should be carefully impregnated with a solution of gum-dammar (p. [70]).

(m) Glass and Enamel.

If covered with a film of dirt, or if when in a collection objects of glass or enamel undergo any alteration, they should be washed or steeped in lukewarm water. When dry they should be treated with pure olive oil or poppy-seed oil, which may be diluted with benzine. The oil helps to restore the lustre to the glass and to bring out the colour of the enamel. When thus treated the objects should be carefully protected from dust.

A decomposition of ancient glass when deposited in a museum has been hitherto only rarely observed, but allusion may be here made to the so-called ‘sweating’ of glass which is a question of considerable importance in Industrial-Art collections. In this case preservation is insured by washing with distilled water, drying, and coating with zapon. Further particulars may be obtained from the paper by Pazaurek[162].