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.
The trade names of the white earth colours include Cologne chalk, Bologna chalk, Briançon chalk, Champagne chalk, Blanc de Bougival, Blanc de Meudon, Spanish white, Blanc d’Orleans, Blanc de Troyes, etc. All are either more or less pure chalk, marl, or a fairly white clay, pipeclay—which is also used for making clay pipes and for removing grease spots.
Gypsum
The mineral known as gypsum, or alabaster, consists of calcium sulphate, or sulphate of lime, its composition being expressed by CaSO4 + 2H2O. In gypsum the crystalline structure is just discernible, whilst other varieties, such as the so-called “marine glass,” occur in considerable quantities as large, perfectly transparent masses. “Russian glass” consists of large, transparent lumps possessing the specific property of gypsum, viz. that of cleaving in two directions, in a high degree. Alabaster is composed of finely granular masses, which are either quite white, or else yellowish, or traversed by grey veins. This variety of gypsum is very abundant in central Italy, and the best blocks are employed for the production of works of art.
Ordinary gypsum, which frequently occurs in the vicinity of dolomitic limestones, is found in a great variety of colours, bluish-grey, yellowish or reddish tints being the most common. Pure white lumps, which are plentiful in some deposits, can be used as white pigment, the method of preparation being simple, viz. merely reducing the mass to powder. This is easily effected, the specific hardness of gypsum being only 2; and in many cases it is soft enough to scratch with the finger-nail.
If the original gypsum is white, the powder forms a dazzling white flour which, notwithstanding, is of comparatively little value as a pigment, on account of its low covering power. For this reason, powdered gypsum is chiefly used for making plaster of Paris (calcined gypsum) for plaster casts and stucco. Gypsum may also be employed to advantage for lightening various colours, since it is inert towards even the most delicate.