The patterns upon ordinary porcelain, which are chiefly in blue, in consequence of the facility of applying cobalt, are generally first printed off upon paper, which is attached to the plate or other article while in the state of biscuit ; the color adheres permanently to the surface when heat is properly applied : other mineral colors, such as the oxides of chrome and manganese, are also occa sionally employed in the same way.
The manufacture of porcelain is a more refined branch of art ; the mate rials are selected with the greatest cau tion, it being necessary that the com pound should remain perfectly white af ter exposure to heat ; it is also required that it should endure a very high tempe rature without fusing,and at the same time acquire a semivitreous texture, and a pe culiar degree of translucency and tough ness. These qualities are united in some of the oriental porcelain, or China, and in some of the old Dresden ; but they are rarely found coexistent in that of modem European manufacture. Some of the French and English porcelain, es pecially that made at Sevres and Wor cester, is extremely white, and duly translucent ; but it is more apt to crack by sudden changes of temperature ; more brittle, and consequently requires to be formed into thicker and heavier vessels ; and more fusible than the finest porcelains of Japan and China.
The colors employed in painting porce lain are the same metallic oxides used for coloring glass, and in all the more delicate patterns they are laid on with a camel-hair pencil, and generally previ ously mixed with a little oil of turpen tine. Where several colors are used, they often require various temperatures for their perfection ; in which ease those that bear the highest heat are first ap plied, and subsequently those which are brought out at lower temperatures. This art of painting on porcelain, or in enamel, is of the most delicate descrip tion : much experience and skill are re quired in it, and with every care there are frequent failures; hence it Is attend ed with considerable expense. The gild ing of porcelain is generally performed by applying finely divided gold mixed with gum-water and borax ; upon the application of heat the gum burns off, and the borax vitrifying upon the surface causes the gold firmly to adhere ; it is afterwards burnished.
In the manufacture of various kinds of pottery employed in the chemical labor atory, and especially in regard to cruci bles, many difficulties occur ; and many requisites are necessary, which cannot be united in the same vessel. To the late Mr. Wedgewood we are indebted for vast improvements in this as well as in other branches of the art. Crucibles composed of one part of pure clay mixed with about three parts of coarse and pure sand, slowly dried and annealed, re sist a very high temperature without fu sion, and generally retain metallic sub stances ; but where the metals are suf fered to oxidize, there are few which do not act upon any earthen vessel, and some cause its rapid fusion, as the ox ides of lead, bismuth, &c. Where sa
line fluxes are used, the best crucibles will always suffer ; but platinum may oft en be employed in these cases, and the chemist is thus enabled to combat many difficulties which were nearly insur mountable before this metal was thus ap plied. Whenever silica and alumina are blended as in the mixture of clay and sand, the compound softens, and the ves sel loses its shape when exposed to along continued white heat, and this is the case with the Hessian crucibles : conse quently, the most refractory of all ves sels are those made entirely of clay, coarsely-powdered burned clay being used as a substitute for the sand. Such a compound resists the action of saline fluxes longer than any other, and is therefore used for the pots in glass fur naces. A Hessian crucible lined with purer clay is rendered much more reten tive ; and a thin china cup, or other dense porcelain, resists the action of sa line matters in fusion for a considerable time. Plumbago is a very good material for crucibles, and applicable to many purposes ; when mixed with clay it forms a very difficultly fusible compound, and is protected from the action of the air at high temperatures : it is well cal culated for small table furnaces.
There are three kind of glazes in gene ral use—one for the common pipe clay ware, another for the finer kind, and a third for the ornamental kind. The common glaze is composed of fifty-three parts white lead, sixteen parts Oornish •tone, thirty-six of ground flints, four of flint glass. These compositions are ground with water into a thin paste. Another is twenty parts of flint glass, six of flints, two ot nitre, and one of bo rax. This is mixed together, and twenty parts of it are ground with twenty-six parts of feldspar, twenty of white lead, six of ground flint, four of chalk, nine of oxide of tin and a small quantity of the oxide of cobalt.
Another glaze consists of twenty parts of flint glass, six of flints, two of nitre, one of borax. These must be calcined to gether, and to twelve parts of it add forty parts of white lead, thirty-six of feld spar, eight of flints, six of flint glass, then grind the whole together into a paste. These substances make a glaze which is not easily acted upon by vege table acids, and is very hard. The oxide of tin and borax is said to produce a good common glaze, not dangerous like lead for cooking vessels. In glazing earthen ware the smallest possible quantity of lead should be used, but a glaze can be made of ground glass and borax to an swer any purpose, for what is a glaze but a glass surface ?