Chap. 13.] Potters Clay. 79 



and contains a very final! proportion of calcareous 

 earth, and fometimes of gypfurri* It is generally 

 fo ;.d in finall cakes of the hardnefs of chalk, like 

 which it marks white. Its colour is fnow white, and 

 when examined by the microfcope it is found to con- 

 fift of fmall tranfparent cryftals. It efferveices with 

 acids. 



II. PORCELAIN CLAY, argilla apyra, the kaolin of 

 the Chinefe. This is very refractory, and is with 

 great difficulty brought into perfect fufion. After it 

 has been fubmitted to the action of heat, it is of a folid 

 texture, and is fo hard as to ftrike fire with fteel. It 

 is found of an excellent quality in Japan, and likewifc 

 in different parts of Europe. In Sweden it is met with 

 in coal-pits, between the ilrata of coal. Common 

 pipe-clay belongs to this divifion, and differs from 

 porcelain clay only in being lefs pure, which prevents 

 its burning to a good colour. 



The efTential ingredient in all kinds of pottery is 

 clay or argillaceous earth, becaufe thefe earths are ca- 

 pable of being kneaded, and eafily receiving any form, 

 and of acquiring much foiidity and hardnefs by ex- 

 poftire to fire. Pottery is, in general, covered with a 

 glazing, or is made to undergo a vitrification at the 

 furface, without which it would be pervious to fluids. 

 Ordinary pottery is glazed with glafs of lead mixed with 

 metallic calces, or with fufible metallic earths. A 

 fine kind of pottery is made of white clays, or fuch as 

 whiten in the firej the furface of which is vitrified by 

 throwing into the furnace, when the ware is fufficiently 

 baked, fome common fait and faltpetre. The Eng- 

 lifli ftone ware is compofed of tobacco-pipe-clay and 

 ground flints. The uie of the flints is to give ftrength 

 Co the ware, fo that it may preferve its form during 



the 



