As a pigment, chalk possesses many valuable properties. The organic structure of chalk gives it high covering power as a wash, a thin layer applied to a surface sufficing to mask the colour of the underlying ground completely. The lime in chalk being combined with carbonic acid, its basic properties are so extensively weakened that chalk can be mixed with even the most delicate colours without fear of their shade being affected. A coating of pure chalk paint on any surface will never change colour in the air; and on this account, chalk is extensively used both as an indoor wash and by wall-paper manufacturers. PRECIPITATED CHALK Many chemical processes furnish soluble salts of lime that constitute a by-product of little value. These salts, however, can be advantageously utilised for the preparation of an artificial chalk which is preferable to the native article in many respects. For instance, where large quantities of calcium chloride solution are available, and soda can be purchased at a sufficiently cheap rate, they can be converted into artificial chalk, because these two substances react on each other, forming, on the one hand, calcium carbonate, which is precipitated as a very delicate, insoluble powder, and on the other, sodium chloride, or common salt, which remains in solution, according to the equation:-- CaCl{2} + Na{2}CO{3} = CaCO{3} + NaCl. If, however, these solutions were mixed together in a crude state, the resulting product would be of only low value as a pigment, being of a yellow tinge and never pure white. This is due to the fact that the impure lime salts, being waste products from chemical works, frequently contain fairly large amounts of ferric oxide, and the soda also is often so high in that impurity that the colour of the precipitated chalk is considerably impaired. Fortunately, there is no difficulty in eliminating this ferric oxide by chemical means, and obtaining a product of superior colour to the best native chalk. This is effected by treating the perfectly neutral lime-salt solution with calcium carbonate, which causes the precipitation of the iron, a corresponding amount of lime passing into solution. In order to eliminate the ferric oxide from the lime-salt solution so completely that not even the most delicate chemical test known will be able to reveal any trace remaining, the solution is placed in a vat and stirred up with finely powdered chalk. If the solution contains any free acid, effervescence, due to the liberation of carbon dioxide, will take place; and in such event the addition of chalk is continued until the free acid is all neutralised, and the added chalk sinks to the bottom undissolved. The chalk should be in slight excess, so that a decided sediment is visible at the bottom of the liquid when at rest.