=Yellow Enamels= are coloured by silver, antimony oxide combined with litharge, or ferric oxide; from the latter a red can also be obtained. To produce a yellow enamel by means of silver the article is first enamelled white at a low temperature; silver oxide is then applied where required, and the article again heated. It then frequently happens that the surface has a metallic appearance owing to the reduction of a part of the silver oxide to metallic silver. When this coating is scraped off the enamel beneath is found to be coloured a fine yellow. The antimony yellow is obtained by mixing 1 part of antimony oxide, 1 part of alum and 1 to 3 parts of white lead, according to the depth of colour required. These finely-powdered materials are intimately mixed with 1 part of sal ammoniac, and heated in an open vessel with stirring until the yellow colour appears. The vapours of the ammonium chloride indicate the proper temperature for the operation. When this substance is completely volatilised the temperature should not be further raised, or the mixture would fuse. By means of ferric oxide a fine and durable yellow is produced; the quantity employed must not be too large, or a red colour will result. A very high temperature is required for burning in this colour. The alum used in making the yellow enamel colours serves to prevent the oxides from fusing. =Red Enamel.=—Ferric oxide is generally used. When a purple red is required it is obtained either directly from metallic gold or from purple of Cassius. Red iron enamel is made by slowly heating 20 to 25 parts of pure ferrous sulphate with 10 parts of aluminium sulphate until all the water of crystallisation is expelled, when the temperature is gradually raised to an intense heat. The shade depends upon the temperature: the higher it is the darker is the colour. Tests are taken from time to time of the mixture, and rapidly cooled. Hot ferric oxide is black, consequently the shade can only be judged with complete certainty in a cooled portion. Although the temperature required for the preparation of this colour is very high, yet it ought not to rise so far that the magnetic oxide is produced, a small quantity of which would cause the enamel to appear dirty red, since it gives to the flux a blackish green colour. The red ferric oxide colour should not be dissolved in the glass of the enamel, but should be embedded in it. Thus, when the enamel is burnt on, the temperature should not rise so high that ferric silicate is formed, otherwise a yellow or even completely black enamel results.