Colour earths are sometimes calcined at a high temperature in order to modify their structure and shade, the operation being accompanied, in some cases, by the destruction of organic admixtures and the expulsion of volatile constituents. An important feature of calcining is that it improves the covering power of many colours, especially heavy spar and certain ferric oxide pigments. This alteration is probably due to the heat causing the finest particles to cohere, and also to the expulsion of chemically-combined water, etc. The change of shade, which is often dependent on the degree and duration of the heating, is probably also connected with cohesion; but in many instances it is attributable to chemical modifications produced by the treatment; ferric hydroxide, for example, losing its water of hydration when heated and becoming transformed into ferric oxide. The details of the calcination process vary with the nature of the material, and will therefore be described, together with the appliances used, when we deal with the colours which require to be put through this treatment. MIXING AND IMPROVING It is very important that the maker of earth colours should always be able to turn out his products uniform in shade, and since the raw materials are liable to vary in character, and the composition of the earths from one and the same deposit is not invariable, the desired shade has to be obtained by mixing. For this purpose, standard samples must be prepared, for comparison in matching. [Illustration: FIG. 24.] Mixing is a highly important operation, on the proper performance of which oftentimes depends the sale of certain colours and the reputation of the maker. It may be effected in various ways, such as shovelling the ingredients together or by combining the work with grinding in edge-runner mills, ball mills, etc. Another method is the mixing barrel shown in Fig. 24, a strong cask mounted on an axial shaft driven by a motor, etc. The barrel is filled about two-thirds full of the materials to be mixed, and, after closing the feed door, is slowly rotated, since, if run at excessive speed, the contents are merely projected against the sides of the barrel by centrifugal force, and it can then be turned for hours without result. The mixing effect can be considerably increased by mounting the barrel so that the shaft is offset from the longitudinal axis of the barrel by an angle of about 30°, the contents being then moved from side to side at each revolution and thus more intimately intermixed by the twofold motion. [Illustration: FIG. 25.]