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The series of changes are particularly well illustrated by the Egyptian grave of Meten[4], the stones from which are now in the Royal Museum in Berlin. The three illustrations here given show: (1) an undecayed block of limestone, (2) a block with pitted surface, and (3) a block the surface of which was formerly covered with hieroglyphics, but which is now totally destroyed by flaking. The blocks of the latter kind were found in the lowest layer, or lowest but one, while those blocks which were above were the best preserved. As the amount of salt present scarcely varied, these specimens offer a striking illustration of the greater influence of moisture in the deeper soil than at the higher levels.

Fig. 1.

Limestone block, surface well preserved.

Fig. 2.

Limestone block with pitted surface.

Fig. 3.

Limestone block showing destruction of surface.

Baked clay, particularly that of Egyptian ostraca (i.e. fragments of pottery showing inscriptions), exhibits similar changes, as is shown in the accompanying illustrations. The surface of some fragments is found to be almost completely covered with a layer of salt, which, apart from impurities of clay and dust and remains of the black lettering, consists of almost pure sodium chloride; only a trace of magnesium sulphate being found on analysis.

In contrast with this very loose superficial incrustation, the inner portions of the ostracon contained considerable quantities of sulphates. Figure 4 represents a fragment with granular efflorescences of sodium chloride, and also fine needles of magnesium sulphate[5]. As a general rule the amount of salt is small compared with the bulk of clay or limestone: thus it was found by titration that three separate fragments contained 0·13, 0·20, and 0·48% calculated as sodium chloride, and in one series the average of 16 fragments was 0·13%. But the percentage of sodium chloride has often been found higher, more especially in larger objects of baked clay, being in one instance as high as 2·3%. The disintegration of the surface is due to the mechanical action of moisture which results in the scaling off of portions of the surface. This does not however exclude a chemical action of the salts upon the clay, especially when this has been only slightly baked. Thus by merely washing such fragments in cold distilled water, not only sodium and magnesium compounds but also those of aluminium and calcium may be removed. The soft powdery patches, which some limestones show instead of scales, are also evidences of chemical action; thus in one case a cuneiform tablet[6] of dolomitic stone showed decomposition at those spots where the salt was firmly deposited as an incrustation, and here the stone, elsewhere smooth and hard, was found, on washing away the salt, to be soft and porous.

Fig. 4.

Potsherd showing saline efflorescence of sodium chloride and magnesium sulphate.

Although, as has been already remarked, sodium chloride generally constitutes the bulk of the salts present, and only in rare cases, as I have for instance shown in an Egyptian Fayence and in several Greek clay vases, is the amount of sulphates greater, yet there are in collections clay objects (Fig. 5) covered with needles of sodium nitrate[7] (Chili saltpetre) where the nitric acid has been contributed by the decomposition of organic substances; and here the presence of nitrates proves inimical to antiquities just in the same way as a coating of limewash may be seen to be destroyed by the so-called wall-saltpetre[8].

Fig. 5.

Pottery showing efflorescence of sodium nitrate.