Without going in detail into the art of enamelling, some observations may here be given which appear indispensable for an understanding of the manufacture of enamel colours. The enamels are glasses, but not every glass can be used for an enamel, since the constituents of the glass react with the substances by which the enamel is coloured, and with certain colouring matters quite different shades to the intended might be produced. Of equal importance with the composition of the glass used in the preparation of the enamel is the temperature at which the glass melts. Many colours bear but a low heat; they decompose at a somewhat higher temperature and produce quite the wrong colour. This is especially the case with the enamels coloured purple red by gold. The enamel colours are fixed by mixing them with an easily fusible glass (the flux), applying them to the object to be enamelled, whether metal, glass, or porcelain, and heating the painted article in muffles until the glass melts, and either dissolves the colouring matter or encloses it unaltered. According as the enamel melts as a whole, or the flux alone melts and encloses the colouring matter, so transparent or opaque enamels are produced. The former are really coloured glasses, the latter are glasses in which is enclosed the sintered colouring substance. Whilst enamels were formerly of importance only for artistic purposes, they have recently attained great industrial employment; not only is earthenware now covered with enamel, and thus made to resist the attack of chemical reagents, but boiler tubes are lined with enamel to prevent the formation of scale. The colours used for enamels must in all cases be metallic oxides; for yellow, silver and antimony oxides; for red, gold, copper oxide and ferric oxide; for blue, cobalt compounds; for green, copper oxide or chromium oxide. Two operations are required in the production of enamel colours, the preparation of the flux and its fusion with the real colouring substance. In most cases the latter operation takes place simultaneously with the fixation on the enamelled article, but the enamels may be melted and cast into lumps, which are then powdered. =White Enamels.=—These are always ordinary crystal glass, to which tin dioxide or potassium antimoniate has been added. Particular care must be taken that only pure raw materials are used, for almost unweighable quantities of iron compounds are sufficient to impart to the enamel a yellow tinge. In almost all cases the glass must be decolourised by the addition of pure pyrolusite (manganese dioxide), which at a red heat gives up a portion of its oxygen to the ferrous oxide contained in the glass, producing ferric oxide, which has much less colouring power. Too large a quantity of pyrolusite imparts a blue tinge to the enamel.