Raw materials which are not amorphous, soft and clayey must first be crushed, an operation facilitated by heating to redness and quenching in cold water. Oftentimes the heating causes a change of colour and improves the covering power—a point to which reference will be made later on.
In the following description of the various raw materials, the chemical composition of the pure minerals will be given, together with an enumeration of the most common impurities.
(A) White Raw Materials and Pigmentary Earths
Limestone (Calcite, Limestone, Chalk)
The number of materials furnishing white earth colours is comparatively large, and these colours are particularly important, because, not only are they extensively used by themselves, but they also serve as adjuncts to other colours and for the production of special shades. The chief raw material for the preparation of white earth colours is the mineral calcite in its numerous modifications.
Calcite, or calc spar, occurs very frequently in Nature, and is one of the most highly diversified minerals known. In its purest state it appears as “double spar” (calcite), in the form of water-white crystals, which are very remarkable for certain optical properties. White marble is also a very pure variety of calcite, in which the individual crystals are very small. The various coloured marbles owe their appearance to certain admixtures of extraneous substances, chiefly metallic oxides.
No sharp line of demarcation separates marble from ordinary limestone, the difference between them really consisting only in the degree of fineness of grain; and all limestones which grind and polish well may be classed as marble. As is the case with marble, there are also limestones of various colours, grey being, however, the most common. This grey limestone forms huge mountain masses which, in Europe, follow for example, the Alpine chain on its northern and southern edges.
A few other examples of calcite may be mentioned which occur in certain localities and, in part, are still in course of formation. To these belong the stalactites and stalagmites, which sometimes consist of extremely pure calcite. They are formed by the action of water, containing carbonic acid in solution, which trickles through cracks and cavities in limestone rock and dissolves out calcium carbonate from the adjacent stone. On prolonged exposure to the air such water gives off its free carbonic acid again; and as the calcium carbonate is insoluble in pure water, it separates out in crystalline form. The masses formed in this way usually resemble icicles in shape, and the finest examples are to be found in the well-known stalactite grottoes at Krain, whilst the grotto at Adelsberg is renowned for its beautiful stalactites. Occasionally, stalactites have an opaque yellow or brownish tinge, which they owe to the presence of iron oxide.
A formation similar in its origin to stalactites is the so-called calc sinter and calcareous tuff. The former often occurs in cavities as irregular masses which, in some places, enclose large quantities of fossil animal bones, in which case they form “bone breccia” (crag breccia). Calcareous tuff is deposited from numerous springs, occasionally in very large quantities, enveloping plants and sometimes forming thick deposits in which the structure of the plants can be clearly recognised.
In some places a more or less pure white, extremely friable variety of calcite is met with under the name “mountain milk” or “mountain chalk” (earthy calcite), which seems to be a decomposition product, and consists of a mixture of arragonite and chalk. Arragonite—which will be referred to later—is completely identical, chemically, with calcite—both being composed of calcium carbonate—the sole difference being their crystalline form.