Purple red is obtained by means of gold; formerly purple of Cassius was exclusively employed for this purpose, but an equally bright red can be obtained from gold chloride. The colour obtained from gold will bear only a very low heat; it must be mixed with a very fusible glass, brought on to the article to be enamelled, and heated just sufficiently to melt the mass. The following mixture is used for gold red and other delicate colours: 3 parts of calcined borax, 3 parts of quartz sand and 1 part of chalk. A very small quantity of gold is sufficient to produce a deep red, the amount of gold preparation used for a pale red or a medium purple must be carefully weighed. =Blue Enamels= are always coloured by cobalt oxide. Any cobalt compound can be used, since at a red heat the silica of the glass displaces other acids, and produces cobalt silicate. It is most simple to take cobalt nitrate. This salt is readily obtained completely pure, and absolute purity is of great importance, for only thus is a pure blue produced. Cobalt oxide produces a more or less deep blue when fused with varying quantities of glass. In order to produce shades similar to turquoise and forget-me-not, a white enamel must be used beneath the blue, or bone ash must be mixed with the blue enamel; this produces a paler colour, which is also opaque. =Green Enamel= is coloured by copper oxide or chromium oxide. In the former case 1 part of copper oxide is used to 30 to 50 parts of glass, according to the depth of shade required. The enamel made with copper oxide alone has never a pure green colour; it exhibits a blue tinge. A pure green is obtained by adding a very small amount of ferric oxide, which produces a yellow and compensates for the blue shade, so that a pure green is formed. Chromium oxide produces a beautiful emerald green without any addition. The enamel may be exposed to very high temperatures without injury. Green enamels may also be made by mixing blue cobalt enamel with a yellow enamel: an unexceptional shade results, but this method is seldom used, since green enamels are obtained in a simpler manner from copper and chromium oxides. =Violet Enamel.=—Manganese dioxide is used exclusively to produce violet enamel; an extremely small quantity is sufficient to colour a considerable quantity of glass. A pure violet shade is only obtained when very pure manganese dioxide is used; the artificially prepared substance should be used to produce the finest violet: the cost is considerably greater than that of the mineral pyrolusite, but equally good results can rarely be obtained from pyrolusite. =Black Enamel.=—When a large quantity of ferric oxide, copper oxide or cobalt oxide is fused with a glass, a deep black enamel is obtained. Generally mixtures of these oxides are used; experience has shown that a much deeper black is thus obtained than from any one alone.