The preservation of antiquities

by Friedrich Rathgen

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.