The minerals constituting the raw materials for the preparation of the earth colours occur under very divergent conditions in Nature. Some of them, such as chalk, form immense deposits, even whole mountains, whilst in other cases, _e. g._ the blue ferruginous earths, the occurrence is connected with certain local conditions, and many are found only in isolated deposits, as pockets or beds. This last is the case, for instance, with the handsome brown iron pigments; and indeed the names by which they are known indicate that they are only found in well-defined localities, or that they are met with of special quality there. The brown earth colour known to all painters as Terra di Siena, is found at many other places as well as near Siena, but the product from that city acquired aforetime a special reputation for beauty, and therefore all similar earths, provided they are equal to that from Siena, also bear the same name in commerce. A number of raw materials for the preparation of earth colours are found, it is true, in many deposits, but their utilisation depends, in turn, on local conditions. For example, many copper mines contain, in addition to the other cupriferous minerals, those used, in the powdered state, as ultramarine or ultramarine green, and not infrequently lumps of mineral are found containing both blue and green together. However, it is only when these minerals occur in sufficient quantity to make the necessary sorting profitable that their manufacture into pigments can be regarded as practicable. Before commencing to work a deposit it is essential to make sure whether the raw material, or pigmentary earth, is actually suitable for the manufacture of earth colour. Even the general character of the material is important, those of soft, earthy consistency being much easier to treat, and the cost of preparation smaller, than if the raw material be hard, tough and crystalline. The extent and thickness of the deposit, and the ease with which it can be worked, also play an important, and even decisive part, since, other conditions being equal, it will not pay to erect a colour works unless the raw material is available in sufficient quantity and is cheap. Generally, the deposit is not homogeneous throughout, the mineral being purer in some places and more contaminated with gangue in others. The percentage of moisture also varies, and in short, a number of circumstances must be taken into consideration in forming a conclusion as to whether a deposit is workable or not. In order to arrive at a reliable opinion on all these conditions, sampling is indispensable. If the samples are of uniform character, they can be mixed together to make an average sample. But if they differ considerably in appearance, general character, proportion of gangue, etc., it is preferable to examine them separately, more especially when the area which each represents is large.