The paste of these vases is similar to terracotta in its general characteristics, such as the constitution of the mixture of which it is composed; it is in general very delicate, but deeper in tone and finer in texture than that of the terracottas. Brongniart has described it as “tender, easily scratched or cut with a knife, remarkably fine and homogeneous, but of loose texture”[739]; but it would be more accurate to say that it varies in one respect, being sometimes so hard that cutting or scratching has no effect upon it. When broken it exhibits a dull opaque colour, varying from red to yellow and yellow to grey. On being struck it gives forth a dull metallic sound; it is exceedingly porous, and easily allows water to ooze through. The surface was protected by a fine, thin alkaline glaze, which is semi-transparent, enhancing the colours with which the vase was painted, like the varnish of a picture. It is this glaze which forms the special distinction of the Greek painted vases and renders them, in contradistinction to common pottery or earthenware, the counterpart of the medieval faïences or majolicas, or the finer porcelain of the present day. As to the chemical composition of the paste, it would seem that hitherto investigations have been confined to vases of Italian origin, but probably those found on Greek soil would yield similar results. The principal ingredients are clay, silicic acid, and iron oxide, with slight admixtures of carbonate of lime and magnesia. The principal results of previous investigations have been tabulated by Blümner,[740] and yield the following average result (chiefly from analyses of vases from Southern Italy):— Silicic acid 52 to 60 parts. Clay earth 13 to 19 parts. Chalk 5 to 10 parts. Magnesia 1 to 3 parts. Iron oxide 12 to 19 parts. The largest proportion of clay found in any one vase was 27 parts; there was also one instance given of 24 parts of iron oxide. The variations in tone of the clay of Greek vases are very marked. The usual colour is an ochre varying from yellowish-white to brownish-red, the mean being a sort of orange. These variations were apparently regulated by the amount of iron oxide employed. It has been noted by Jahn[741] that vases were sometimes moulded “double,” _i.e._ turned on the wheel in two different thicknesses of clay, the finer and ruddier forming the exterior surface for decoration.