The tin dioxide used in the glaze is made by direct oxidation of tin in air. It has been found that tin burns much more easily in air when it is mixed with lead; 20 to 40 parts of tin are melted with 100 parts of lead, and the alloy heated in shallow vessels in the air; it takes fire at a certain temperature, and it is only necessary to remove continually the layer of oxide in order to oxidise the whole of the metal within a short space of time. The mixture of oxides obtained in this manner is freed from particles of metal by grinding and levigation; when it is fused with the glass, the lead oxide enters into combination, whilst the tin oxide is embedded in the colourless glass. When paste enamels are required the mass is melted in shallow crucibles, poured into water and broken up to a coarse powder, which is again fused. In some cases this operation must be repeated several times to obtain a quite homogeneous product, for tin dioxide is very heavy and sinks to the bottom of the fluid glass; this is readily seen if the fused enamel is allowed to cool in the crucible. When the crucible is broken the lower portions of the fused mass are dense white, whilst the upper are merely milky white. By repeatedly fusing the mass it is endeavoured to obtain it uniform. In fusing the enamel care must be taken that it is heated in a vessel from which the fire gases are completely excluded, since the smallest quantity of coal or of ferruginous ashes coming into contact with the melted mass would injure its colour. If the melting point of the enamel is too high it can be lowered by the addition of a small quantity of pure quartz sand. A fine white enamel is obtained by using litharge or red lead. The mixture contains 60 parts of quartz sand, 30 of alum, 35 of common salt and 100 of red lead. It is advisable to add a small quantity of finely-powdered talc to the sand. In consequence of the considerable proportion of alumina contained in this enamel it is difficult to melt, and can be heated to very high temperatures without injury to the shade. When antimony oxide is used a glass free from lead must be taken; lead glass does not give a pure white with antimony oxide. A very good white enamel is obtained by fusing 3 parts of crystal glass with 1 part of sodium antimoniate. =Coloured Enamels= owe their colour to metallic oxides. Enamels coloured by metallic oxides, in which the oxygen is firmly united, can be fired without great precaution, but if they contain oxides easily decomposed, great care is required to obtain a fine colour. In enamelling metals a white under-glaze is generally used beneath the coloured enamel; it consists of a refractory white enamel. By this means it is more easy to obtain pure colours.