The examination should extend, on the one hand, to the natural percentage of moisture, and, on the other, to the purity of the material. The water content is determined by thoroughly drying a weighed sample, bearing, however, in mind the fact that pigmentary earths of a clayey nature vary in water content according to the time of year, besides changing in accordance with the weather when the won material is stored in the open. The purity can only be ascertained by an examination in which a sample of the soft, clayey material is crushed and passed through a narrow-mesh gauze sieve, the amount of the coarse particles--sand, small stones, etc.--remaining on the sieve being determined. A more accurate method, of course, is to separate the true pigmentary earth from the gangue by levigation. For this purpose, a weighed quantity of the crushed, air-dry sample is placed in a relatively narrow glass vessel and thoroughly mixed with water, the turbid supernatant liquid being poured off after a short interval. The residue is repeatedly treated in the same way, until no more fine particles remain in suspension, the residue then consisting of impurities, or gangue. Of course, the washings can be collected, the suspended matter allowed to settle, and finally weighed in an air-dry condition. By this means an approximate idea of the yield of earth colour can be obtained at the same time. 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.