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.