If the porcelain is to be painted, it must again be exposed to heat in the furnace. The colours used for the painting of it are all derived from metals; and many of them, though dull when applied, acquire considerable lustre by the action of the fire. The colours are always mixed with some kind of flux, such as a mixture of glass (204), borax (208), and nitre, melted together, and afterwards ground. Gum or oil of lavender is used for mixing up the colours. When the painting is finished, the pieces are exposed to a heat sufficient to melt the flux, and thus fix the colour. 119. _TRIPOLI is a kind of clay of yellowish grey, brown, or white colour, sometimes striped or spotted, and of an earthy texture._ _It feels harsh and dry to the touch; is soft, scarcely adheres to the tongue, and will not take a polish from the nail._ This substance obtained its name from having formerly been imported into Europe from Tripoli, on the north coast of Africa. It is, however, now found in several parts of Germany; and a granulated kind has been discovered in England. Tripoli is used for the polishing of metals and stones. For this purpose, it is mixed with sulphur, in the proportion of two parts of tripoli to one of sulphur. These are well rubbed together on a marble slab, and are applied to the stone or metal with a piece of leather. When tripoli is combined with red ironstone, it is used for the polishing of optical glasses. It is sometimes made into moulds, in which small metallic or glass figures and, medallions are cast; and a kind of tripoli is found near Burgos, in Spain, which is used as an ingredient in the manufacture of porcelain. In Derbyshire,, and several parts of Staffordshire, is dug a kind of tripoli which has the name of _rotten stone_. This is considered to be a produce of limestone shale, which has undergone a decomposition by exposure to the air and moisture. It is used for most of the same purposes as tripoli. CLAY SLATE FAMILY. 120. _CLAY SLATE, or ROOFING SLATE, is a kind of stone of foliated texture, and greyish, black, brown, green, or bluish colour._ _It breaks into splinters, does not adhere to the tongue, yields generally a clear sound when struck, and is nearly thrice as heavy as water._ Vast and extensive beds of slate occur in different parts of the world; and this mineral sometimes constitutes even a principal portion of mountains. In our own country there are many important quarries of it, particularly in Westmoreland, Yorkshire, Wales, and Derbyshire.