II. Preservation of Organic Substances.
(n) Bones, Horns, Ivory.
Many curators dry carefully and impregnate them with a gum-dammar solution or shellac; isinglass or glue are however preferable, for these aqueous solutions may be used for the treatment of damp objects, which could scarcely be dried without cracking. In order to permeate the object these solutions must be very dilute, and are most advantageously applied at a temperature of about 120°F. [50°C.]. The impregnation may also be effected in rarefied air under a bell glass (p. [68]). Friable bones and similar objects which might fall to pieces in the solution during impregnation should be bound with strips of gauze or with string before immersion; they are easily removed when cold. To prevent the formation of mould a small quantity of dissolved corrosive sublimate[163] is added to the glue, or when dry after impregnation the objects may be covered with a solution of shellac or resin. Impregnation is of very general application, and is frequently used for the preservation of fossil and pleistocene bones.
(o) Leather.
At Copenhagen the method used to render leather soft and pliable is to place it in train oil for an hour and then dry it with filter-paper. Lanoline may also be used with success[ [164]. Poppy-seed oil in benzine (p. [86]) is said to produce good results, but the “Merkbuch” recommends the preservation of leather in this condition in alcohol[165].
(p) Textile Fabrics, Hair.
Earth and soil may be removed by mechanical means, and, occasionally, careful washing may be possible. The objects should be dried and impregnated with a gum-dammar solution (p. [70]), poppy-seed oil (p. [86]), or a solution of india-rubber (p. [90]), or they may be preserved in alcohol (p. [159]). Some textile fabrics in the Copenhagen Museum owe their excellent state of preservation to Steffensen’s treatment, i.e. impregnation with a solution of india-rubber in turpentine with the addition of bees’-wax.
The following account of the treatment of textile fabrics from the Lake-Dwellings is due to Herr Heierli, of Zürich:
“The pieces as they were taken up were laid on the ground and thus slowly allowed to dry in the air. They were then placed between glass plates, the edges of which were pasted over with paper. Old pieces which had been dry for a long time, and which had become tender and friable, were laid on the ground and watered from time to time until they were soaked through; they were then treated in the manner already described.”
Egyptian textile fabrics preserved between glass plates often deposit a thin layer of salt on the glass, but this is easily wiped off (see p. [155]). It must first be ascertained by a previous trial in each case whether the salt can be removed by steeping in water or in alcohol and water.
Hair found in peat has always a dark-brown colour from impregnation with peaty matter. The method proposed by Bille Gram[ [166] for restoring the natural colour consists of repeated and alternate treatment with very dilute alkali solution and acid at about 120°F. [50°C.]. When the liquid ceases to show coloration the natural colour of the hair is restored.
(q) Feathers.
These do not require any treatment beyond protection against insects, which is attained by immersion in an alcoholic solution of corrosive sublimate, or by spraying with corrosive sublimate in either alcoholic or aqueous solution. Of course the poisonous nature of corrosive sublimate necessitates caution in its use and it should be always labelled as such.
The use of naphthalene is not always successful, and white scales of naphthalene are apt to make their appearance; nor does finely powdered pepper sprinkled on the feathers, either alone or mixed with finely powdered alum, give satisfactory results.
(r) Papyrus.
The method of cleaning and preserving papyrus in use in the Egyptian department of the Royal Museums at Berlin is as follows: Those pieces which are folded together or rolled are carefully straightened, and, if very friable, they are first placed between damp filter paper to render them uniformly pliable. Dust and dirt are removed with soft paint-brushes, crystals of salt which are often found[167] are picked off with forceps. Any growths of fungus are carefully scraped off with a knife. The papyrus thus prepared is then placed between two thick polished glass plates, the two opposing surfaces of which are covered with a very thin layer of vaseline. Air is frequently admitted to dry the papyrus, while the pressure of the glass plates tends to smooth it out, and after it has been so treated it is mounted between thin glass plates, the edges of which are pasted over with paper covered with an oil paint.
A papyrus preserved between glass plates often shows round the edges a whitish border about two millimetres in breadth, and on separation the glass plates show a slight film of the same white material on the surface which had been in contact with the papyrus. The formation of this film, which consists chiefly of common salt and is easily wiped off, may be prevented by previously washing the papyrus in distilled water, a proceeding which experience has shown to be harmless. As the papyrus will swim on the surface it should first be immersed in alcohol until soaked through; the process of steeping is then quite simple. The thinness of papyrus enables the steeping to be completed after 24 to 48 hours by two changes of the water, and care must be taken lest a too prolonged steeping should obliterate the lettering. The water assumes a yellowish or brown tint and the papyrus becomes somewhat lighter in colour on drying. Papyrus may also be preserved by zapon (see Appendix), but this method has no advantage over that of mounting between glass plates.
(s) Wood.
To preserve adequately articles of moist wood (and they are generally in this condition when first excavated), preliminary measures to prevent their drying in the air must be taken immediately after they are dug out of the earth. If found in water, as for instance articles from pile-dwellings, they should be conveyed in water; moist objects should be wrapped in several thicknesses of moist cloth, and the whole wrapped in gutta-percha membrane, or in a layer of moist moss. The cracks which arise in wooden objects which have become dried may frequently be closed up by laying them in lukewarm water.
As the earliest attempts at preservation were probably made upon wooden objects there is scarcely a collection in which a number of methods are not employed. One exception only is known to me, and here, after a plaster of Paris cast has been taken, the object is simply allowed to shrink. The methods proposed and carried out are so different and so numerous, especially as regards the liquid used for impregnation, and in such variety, that it is only necessary to deal with the most important. These may be divided into two classes, viz. dry and wet.
(1) Dry Preservation of Wood.
Moist or wet objects are placed in thin size or in a solution of isinglass till they are impregnated, after which they are dried gradually in a shady place. A solution of shellac, or varnish diluted with petroleum or benzine, is then put on with a brush.
Sometimes the objects are placed directly into a mixture of varnish and petroleum, or they are impregnated with melted paraffin. The former is preferable as a means of impregnation if there are cracks or holes, for the superfluous solution readily drips from the wood when it is taken out, while paraffin sets too soon to drain out of the cracks, and thus imparts an unnatural white appearance to the wood. Owing to the large size of the vessels which would be otherwise required, paraffin is only useful for small or medium-sized objects, but when making use of varnish one end of a large object[ [168] may be placed in the mixture while the solution is repeatedly poured over the object. After two or three days the opposite end should be placed in the solution. By repeating this process every part of the object will soon be thoroughly impregnated.
Objects of still greater size, such as a Viking’s ship, can only be preserved by painting the surface. In such cases it is advisable to begin with dilute varnish so as to allow the impregnating solution to penetrate as deeply as possible into the material, instead of merely forming a skin.
A solution of waterglass has in one instance been used for the preservation of a large boat, but the result is not satisfactory.
Leiner’s Method[169]. The wooden articles are laid in glycerine mixed with a small percentage of carbolic acid. The length of time during which they remain in the glycerine depends upon their size. When taken out they are lightly wiped and preserved without further treatment. If a growth of mould should occur it may be washed off.
Objects thus treated retain their moist condition and should therefore be very carefully protected from dust.
Speerschneider’s Method[170] (cp. p. [91]). Small specimens are heated for two hours in a mixture of
8 parts of rape-seed oil,
1 part of bees’-wax,
1 part of pine resin, and
2 parts of benzene.
Larger objects require a proportionately longer heating, but the mixture must not be allowed to actually boil. The moisture rises as steam and causes the solution to bubble. The bubbling however continues after the moisture has been driven off; great care must therefore be taken that the heating is not so prolonged as to cause the object to shrink. The highly inflammable nature of the mixture renders great caution necessary, and should it ignite, a lid, which should always be in readiness, should be put on the vessel. After impregnation the objects are wrapped in blotting-paper and laid in ashes for four days to prevent the access of air. The aim is doubtless to insure thorough absorption of the superfluous liquid which remains upon the object, which exposure to air would prevent by causing the mixture to set too rapidly. The same mixture can be used repeatedly, but each time two-thirds of the original quantity of benzene must be added.
Herbst’s Method[171]. The moist objects are boiled in a saturated solution of alum for two hours (hot water dissolves about 31⁄2 times its weight of alum), but if they are of some thickness the time must be proportionately longer. They are then taken out, and when the alum in crystallizing has made them more or less firm, the crystals adhering to the surface are washed off with warm water.
When thoroughly dry the wood is brushed over with hot linseed oil, which operation is repeated until no more oil is absorbed. A final thin coating of varnish or shellac is then given. According to Steffensen, the method followed at Copenhagen is to lay the objects in warm thin size for a quarter of an hour after impregnation with alum. This alum-method is there used for objects of oak, although the “Merkbuch” (p. 60) states that only the varnish-petroleum mixture should be used for impregnating this class of object.
(2) Preservation of Wooden Objects in Liquids.
The expense entailed by this method renders it applicable only to articles of small size.
The preservation of small objects in a flat vessel, the bottom of which is covered with glycerine, has the disadvantage that glycerine extracts organic substances and thus assumes a brown colour. If glycerine is used the object should undergo a thorough preliminary steeping, and the glycerine should be renewed until it remains colourless. Closed cylinders filled with glycerine or a mixture of glycerine and water are not convenient because wood nearly always floats in the liquid. This may be remedied however by the addition of alcohol.
Jenner’s Method[172]. When the objects have been thoroughly cleaned with water, pure alcohol, diluted with water until the specific gravity at 54°F. [12·5°C.] reaches 0·96, is poured over them. After six or eight weeks the alcohol is poured off and replaced by fresh alcohol of the same specific gravity. This alcohol is examined in a year’s time, and should always show a specific gravity of 0·96. The alcohol which has been poured off may be filtered, and if necessary decolourized by animal charcoal; when the specific gravity has been again raised to 0·96, by the addition of fresh alcohol, it may be used again.
The same process is applicable to textile fabrics, yarn, and leather.
Protection against Wood-worms, etc.
All the methods mentioned above will destroy insects and their larvae.
In cases in which it is either impossible or undesirable to use immersion or external application, as for instance in the treatment of objects of dry wood, the larvae may be destroyed by dropping petroleum, an aqueous solution of potassium arsenite, or corrosive sublimate, into the various small openings. This will also help to prevent further attacks.
If solutions are not applied insects may be destroyed by the vapour of carbon bi-sulphide or of crude benzene. These liquids, which are sufficiently volatile at the ordinary temperature, should be placed, together with the objects to be treated, in a closed box.
I have used a similar method for the destruction of wood-worms in Egyptian coffins. The coffin is placed in a large wooden box lined with tin plate. The lid, also lined with tin, is provided with projecting edges, to which strips of felt are glued. The weight of the lid by compressing the felt is sufficient to render the box air-tight. Six or eight glass vessels containing crude benzene are placed at the bottom of the chest and of the coffin itself. It need scarcely be added that the box must not be opened near a fire or light, as the vapour forms an explosive mixture with air; it is in fact advisable to have no light or fire in the room.
Insects can also be killed by naphthalene vapour, but as naphthalene is insufficiently volatile at ordinary temperatures, the method above described is more convenient[173].
Preservation and Cleaning of Coloured Wooden Objects.
For objects of this kind materials should not be used which, like varnish, tend to darken and so to damage the colours. Gum-dammar solution (page [70]) answers the purpose, but colourless collodion is better. Colours which are soluble in water (as is frequently the case with wooden objects from Egypt) cannot of course be cleaned with water, but benzine may be applied by means of soft cloths or brushes. Resinous or pitch-like substances may often be removed from coloured objects by turpentine mixed with benzine or ether.
A method of cleaning gilded or brightly coloured ecclesiastical figures which is used in the Breslau Museum is the application of a mixture of copaiba balsam and ammonia. This method is similar to that used to clean paintings[174], the action of the solution being that of a mild soap.
Antiquities which were originally uncoloured, but which have been subsequently painted, may be cleared of paint by means of a solution of caustic soda in water or alcohol.
(t) Amber.
After the mechanical removal of any adherent earth and dust, the specimen should be rubbed carefully backwards and forwards between the fingers covered with a soft woollen glove. Particles of soil should be picked out of any holes and indentations by using a strong horse hair[175]. It is then preserved by impregnation with a solution of shellac, poppy-seed oil, or isinglass (pp. [70] and [86]).
The following particulars of the method used in Messrs Stantien and Becker’s collection of amber have been supplied by Prof. Klebs:
“Amber is preserved best in distilled water: I add a very small quantity of glycerine and a still smaller amount of alcohol. A proportion of alcohol greater than 1% is injurious to the amber. A thick layer of gelatine containing glycerine is an excellent medium for the preservation of large objects if they are kept free from dust. This layer should be washed off and renewed every few years.”