If this precipitated chalk be moulded into prisms for sale, the blocks are laid on one of their broad sides until firm enough to turn over on to one of the narrow faces, slabs of gypsum being used as the supporting material, in order to ensure uniform drying. The gypsum absorbs water with avidity and thus dries the prisms evenly. A defect of these prisms is their great fragility; but their strength may be improved by mixing a little very weak solution of dextrin to the mass after the last washing-water has been completely removed. In drying, the dextrin binds the material of the prisms sufficiently to keep them from breaking except under the influence of a fair degree of force. CALCAREOUS MARL As already mentioned, calcium carbonate rarely occurs in a perfectly pure condition in Nature; and chalk, also, is frequently contaminated by other minerals. A variety of limestone occurring as extensive deposits in many places is that in which calcium carbonate is associated with clay. Sometimes the clay predominates, and the mineral is then known as marl, being really a clay contaminated with chalk. If, on the other hand, the chalk forms the chief constituent, the mineral is termed calcareous marl. Calcareous marls are used in much the same way as limestone, some modification, however, being necessitated by the presence of the clay. Although limestone containing a certain amount of clay can be burned in the kiln, it yields an inferior lime that is of little use to the builder owing to its low binding power. Marl of a certain composition finds an important application in the manufacture of hydraulic lime or cement. The only kind of marl suitable for pigment is that containing clay with very little colour; and this is of somewhat rare occurrence, because most marls contain sufficient ferric oxide to give them a yellow shade. Marl that is fairly free from ferric oxide, however, can very well be used as pigment; and many white pigments sold as “chalk” are really finely ground marl. In accordance with the general practice, in the colour industry, of giving colours a great variety of names, and suppressing the real names, which, so far as the artificially prepared colours are concerned, should bear some reference to their chemical composition, numerous white earth colours bear fancy names, though really consisting of chalk, lime (generally marl), or white clay. In France, where both chalk and clay are of frequent occurrence--the soil of Champagne, for instance, being all chalky--the manufacture of the white earth colours is extensively practised, and a large number are put on the market, usually named after the place of origin, and consisting of either calcium carbonate or marl.