Mr. Coupeu on the Chemical Composition of Pottery. 177 



tin. Chinese Porcelain. 



•operior. inferior. 



Silica, 72-96 7104 68-96 



Alumina and protoxide of iron, 24*78 22*46 29*24 



Lime, 104 3*82 1*60 



Alcali and Loss, 122 2*68 — 



100* 100* 99*80 

 Specific gravity, 2*419 2*314 2*314 



VI. Earthenware is a very porous and less compact body than china 

 or porcelain, owing to its containing little or no alkali, which is the great 

 difference between these bodies. I had a piece of ware manufactured, 

 resembling in appearance porcelain, as regards its porosity and compact- 

 ness, slightly transparent, and capable of standing a very strong and 

 sudden heat ; it was produced by mixing soda to the extent of 3J per 

 cent, in a little clay prepared for the common white body, and was then 

 fired in the biscuit kiln. The clay employed having been previously well 

 dried, so as to weigh it without water, the proportional quantity of soda 

 requisite was then calculated and weighed out ; the clay was again mixed 

 with water along with the soda ; it was then formed into capsules, which, 

 after being fired, and then broken, presented the appearance of a vitrified 

 or fused body. 



(1.) The common white ware, or earthenware, is made from flint, 

 Cornish stone, China clay, and blue clay, and does not require such a high 

 temperature in burning as the porcelain does. By analysis of a piece of 

 white ware, manufactured in this city, it was found to contain — 



Silica, 68*55 



Alumina and protoxide of iron, 29*13 



Lime, 1*24 



Magnesia, a trace. 



98*92 

 x Specific gravity, 2*36 



Coloured ware is also manufactured from tho same substances, but 

 mixed with a colouring agent which stains the body. 



(2.) The toqua, or blue coloured ware, is coloured by cobalt, chrome, 

 and oxide of zinc. 



(3.) The sage, or greenish blue coloured ware, by nickel and cobalt. 



(4.) The drab, or buff coloured ware, by chromate of iron, or nickel. 



(5.) The body for the cane, or yellow coloured ware, is produced by a 

 mixture of sandy clay and common red clay, the same as is used for red 

 bricks, but is generally produced from the natural yellow clay found in 

 particular localities. 



(6.) The last mentioned body is also employed for making Rockingham 

 ware, which only varies from the cane ware by possessing a different glaze. 



(7.) The common black ware body is made from the red clay alone. 



