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.”