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.
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).
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.
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.
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.