Fig. 22. Babylonian clay cone before treatment.
Fig. 23. Babylonian clay cone after treatment—firing, treatment with hydrochloric acid and steeping.
After thoroughly drying the tablets first in the air, then in the drying oven at a temperature of 212°F., supported on glass rings, it is well to impregnate them. This can be best carried out by placing them, while still warm, into melted paraffin wax, and raising the temperature to about 250°F. [120°C.]. The wax is allowed to cool to about 160°F. [70°C.], when the tablet is removed upon a broad band of gauze, any excess of wax is drained off, and the object is wiped with a soft cloth. The benzine-varnish mixture or zapon may also be used for impregnation. If heating in the muffle to Seger cone 022 is insufficient to allow of the removal of the incrustations, or if the condition of the clay does not warrant soaking in water or acid, the object must be again placed in the muffle and fired to Seger cone 010 [950°C.; Watkin, No. 13, 1742°F.], and, if softening occurs upon the application of water or acid after exposure to this temperature, recourse must be had to a third heating to Seger cone 05 [1050°C.; Watkin, No. 18, 1922°F.]. Higher temperatures than this are not advisable, for the lime, sodium chloride, and other salts found in some Babylonian tablets may partially fuse. During firing therefore the appearance of the object must be carefully watched, and the temperature lowered at once by reducing the gas-supply, if signs of fusion are noticed.
Additional Methods of Impregnation. If clay objects have a smooth surface, it is, according to the “Merkbuch[106],” advisable to impregnate them with Belmontyl oil[107], for varnish in the course of drying gives a lacquered appearance to the surface. According to the same authority the surface of glazed vessels can be restored by impregnating them several times with a mixture of poppy seed oil and benzine [20 grammes clarified poppy seed oil in 270 gr. benzine, i.e. 1 in 131⁄2], and by subsequently brushing them first with soft, then with harder brushes. There are, however, many other substances used in different collections for impregnation, a few of which are subjoined.
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.
(e) Fayence.
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]).