Mr. Colter on the Chemical Composition of Pottery. 



170 



metallic lustre, and is made from Cornish stone, flint, manganese, red lead, 

 and clay slip, the latter substance being a little clay mixed with water 

 until it becomes of the consistency of milk. 



(3.) The glaze for common black ware is made from the same materials, 

 in differont proportions, and has a brilliant black appearance. 



(4.) The glaze used for cane, or yellow coloured ware, is made from 

 flint, red load, and Cornish stone. 



(5.) The Egyptian ware owes its value to the beautiful and rich tinted 

 black glaze, made from flint, Cornish stone, red lead, and manganese, with 

 which it is covered. These four last mentioned glazes are made by stirring 

 the substances together with a certain quantity of water, and passing it 

 through a very fine sieve or search. Glazes do not require such a high 

 temperature to fuso them on the surface of the ware as the body does to 

 be burned. 



(6.) The glaze for salt glazed ware is common salt, which is thrown in 

 at the top of the kiln through a number of small apertures in the crown 

 of it, and diffuses itself through all parts of the kiln, giving the ware the 

 required glaze. The action that is supposed to take place when the salt 

 is thrown into the kiln, is owing to its decomposition ; the chlorine of the 

 salt combines with the hydrogen of the water, which is mechanically 

 lodged in the salt, forms muriatic acid gas, which passes off, while the 

 sodium, with the oxygen of the water, then unites with the silica in the 

 ware, forming a silicate of soda, which fuses on the surface. The salt is 

 not thrown in until the kiln has been raised to its greatest necessary 

 temperature. 



TABLE OF THE COMPOSITION OF CLAYS AND PORCELAIN WHEN 

 FREE FROM WATER. 



