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41 ' s CLAY FAMILY. \ is named biscuit porcelain by workmen * A fourth ope ration is the covering the surface of the biscuit with a varnish or enamel, which must be applied exactly over all the points of the surface, and incorporated with the paste, without cracking or flying. This enamel is com posed of pure white quartz, white porcelain, and cal cined crystals of gypsum, and sometimes principally of felspar: these substances are ground with the greatest care, then diffused through water, and formed into a paste. When we use it, it must be diluted in water, so as to give it considerable liquidity, and we then plunge into it the biscuit porcelain. The porcelain is now ex- pose to eat, sufficient to melt the enamel or covering) and then it constitutes white porcelain ; and in this state it may be applied to every purpose. If the porcelain is to be painted, it must again be exposed to heat in the furnace. The colours used are all derived from metals; and many of them, though dull when applied, acquire a consider able lustre by the action of the fire. The colours arc mixed with a flux, which varies in the different manu factories : in some, a mixture of glass, borax, and nitre, is employed; this mixture is melted in a crucible, and the glass is afterwards ground, and incorporated with the colour. Gum, or oil of lavender, is used as a vehicle, when we wish to lay it on the porcelain. When the painting is finished, the ware is exposed to a heat suffi cient to melt the flux containing the colour. The beautiful purple colours on porcelain, are from pxide of gold, called powder or precipitate of Cassius; the violet * Figures, and generally all porcelain articles which are neither to be painted nor exposed to water, have no occasion for any covering; they arc tlten sold in the state of biscuit.
