“This forms what is known as crude raw ultramarine. It contains about 15 per cent. of sulphate of soda, which must be removed before the colour is fit for sale. “For this purpose it is washed with hot water in large tubs, after which it is ground in wet mills to an impalpable powder, and allowed to stand for about an hour in a large tub, in order to remove the coarsest particles and dirt which are sure to be present. It is then removed to another tub, where it settles for four or five hours, and from this it passes to others, where it stands for various lengths of time, increasing, of course, as the powder to be settled becomes finer, the last settling occupying three or four weeks, and producing the strongest quality that can be obtained--that is to say, it will bear mixing with more of a reducing medium, such as mineral white, than would a former settling for the mixture in each case to be of the same depth of colour. “The water, after the final settling, still contains about 5 per cent. of ultramarine. This would take five or six months to settle, and as this time could not generally be given to it, it is precipitated with lime water, which has a sort of coagulating influence upon the particles, which can then be removed by filtration. It is a curious thing that this last quality is quite different from the one preceding it, being very inferior in both colour and strength. “After settling, all the various qualities are dried in kilns, and sifted through fine brass wire sieves by means of a fan, which breaks up the lumps and forces the particles through the meshes of the sieve, which must be very close--about 100 to the inch--in order that the ultramarine may be perfectly smooth and free from lumps or grit of any sort. When finished, it should be in the form of an impalpable powder--the finer qualities so fine, indeed, as to feel almost _buttery_ when rubbed between the fingers. After this process the different qualities and shades are mixed to certain standards, and are then ready for sale. “The uses of ultramarine in the arts and manufactures are very numerous and important. The most important, from the point of view of quantity, is the manufacture of ‘square blue’ for washing purposes. In the preparation of this article the ultramarine is generally mixed with bicarbonate of soda and some glutinous material, to help it to retain its shape, and is then pressed into the well-known form of small square or oblong blocks.