If, on the other hand, the acid is in excess, the clay is obtained free from all constituents soluble in the acid. The purified clay must then be freed from the calcium chloride, formed by dissolving the lime, by a thorough washing, since the clay would otherwise always remain moist on account of the hygroscopic properties of the chloride in question. Moreover, any small residuum of free acid would constitute a drawback on the clay being mixed with other colours. Calcium chloride is very soluble in water, and therefore can be completely removed from the clay by washing. The purified clay is left to settle down as completely as possible, and after drawing the liquid off from the sediment, the latter is suffused with pure water and left to settle once more. As a rule, two such washings will cleanse the clay of calcium chloride and free acid sufficiently to render the product suitable for any purpose. When large quantities of clay have to be treated in this manner, considerable amounts of calcium chloride solution will be obtained, which can be advantageously utilised for the production of precipitated chalk, all that is necessary being to collect the liquor in a large tank and treat it with a small quantity of slaked lime, to transform the surplus free acid into calcium chloride and precipitate the ferric oxide present in solution. At the end of a few days the liquor in the tank will consist of a very pure solution of calcium chloride which will furnish an excellent precipitated chalk when treated in the manner already described under that heading. BARYTES, OR HEAVY SPAR This mineral--chemically, barium sulphate, BaSO{4}--occurs native, as extensive deposits, in many places--England, Bohemia, Saxony, Styria, etc. It sometimes forms handsome tabular crystals, but more frequently compact masses, which may be pure white, grey yellow, etc., in colour, and are distinguished by high specific gravity (usually 4·3–4·7), to which the mineral owes its name. This high density also limits the application of the mineral, and it cannot be used as a pigment, in the true sense of the term, being only suitable as an adjunct to artificially prepared colours.