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.
