Ultramarine is by far the most commonly used of the blue pigments. It is a chemical combination of silica, alumina, soda, and sulphur, but its exact chemical constitution is not known, the proportions of its ingredients varying somewhat with different makes. There are two principal varieties of ultramarine sold. One is known as sulphate ultramarine the other as soda ultramarine, from the materials used in the process of manufacture. In the first, silica, china clay, _sulphate of soda_, and coal are used; in the latter, silica, china clay, _carbonate of soda_, and sulphur are used. The sulphate ultramarine is distinguished by its very pale greenish blue colour, while the soda ultramarine is of a violet hue. Ultramarine is distinguished from other blues by the fact that acids completely decolorise it, with the evolution of sulphuretted hydrogen and the formation of a white precipitate of sulphate. The sulphate ultramarine is more easily decomposed by acids than the soda ultramarine, and some makes of the latter more easily than others. Alkalies and heat have no action on this pigment. Boiled in strong nitric acid, ultramarine is completely decolorised, a colourless solution being formed, and a gelatinous mass of silica being left as a residue. It is not as a rule necessary to make an analysis of the pigment; the above tests serve to distinguish it from other pigments. An assay of ultramarine should include the following points:--1st, colour or tint; 2nd, covering power or body; 3rd, acid resisting properties: this can be tested by making a very weak solution of sulphuric acid--about 4 oz. in 1000 oz. of water--and adding a little of this to the pigment contained in a glass, and noting how long it takes to bring about decolorisation; 4th, the power of resistance to the action of alum. When ultramarines are boiled with a solution of alum, they are more or less reddened thereby; those which are made with a large excess of silica are found to resist this action of alum better than those containing a normal quantity of this compound. Such ultramarines are preferred by the paper maker, who uses a large quantity of alum and sulphate of alumina in the sizing of his papers, and therefore he wants an ultramarine which shall not change in shade when used for tinting alumina sized papers. This point is easily tested. A solution of alum is made, and in a little of this a small quantity of ultramarine is boiled for a few minutes, and it is noted whether any change of shade occurs. If any sample is found to change much, that sample must be rejected for paper tinting, although it may be used by the painter or the laundress.