174 Mr. Couper on the Chemical Composition of Pottery. 



By mixing sandy clay and red clay together we gain an artificial 

 yellow clay, which is often employed. 



Yellow clay, as analysed by Mr. John Brown, was found to contain — 



Silica, 58*07 



Alumina,.... 27'38 



Protoxide of iron, 3*30 



Lime, -50 



Water, 1030 



Magnesia, a trace. 



99-55 



(7.) Fire Clay is also very abundant in this country, and occurs both 

 on the surface and several fathoms under ground. It is termed marl, and 

 is used principally in potteries for making saggars, or vessels for placing 

 the ware previous to burning, to protect them from the flame; and, 

 owing to its coarse particles, which cause the body to be very porous, is 

 well adapted for strong heats. Crucibles or large pots for glass works, in 

 which the glass is fused, are also made from fire clay, as well as bricks 

 known under the name of fire bricks. This clay was analysed by Mr. John 

 Brown, who obtained — 



Silica, 6616 



Alumina, 22*54 



Protoxide of iron, 531 



Lime, 142 



Magnesia, a trace. 



Water and Coal, 3-14 



98-57 

 (8.) Flint, as used in potteries, is first calcined, then water ground, in 

 which state it is used for mixing with clays, and is called slop flint ; but 

 for glazes, it is evaporated to dryness, and used in the dry state with 

 other articles which constitute the glaze. 



(9.) Cornish Stone or granite, is water ground, then evaporated to 

 dryness for mixing in glazes, and is used in the slop state for mixing with 

 clays. 



(10.) Plaster of Paris, or gypsum, which is employed in forming the 

 moulds in which certain kinds of pottery are cast, is a native sulphate of lime, 

 and is a very important article to the manufacturer of earthenware, owing 

 to its singular property of parting easily with the clay, by the application 

 of a slight heat. Plaster of Paris requires to be dried at a high tempera- 

 ture before using it ; but if it is over dried it will not set for making 

 moulds ; the drier the stucco the harder are the moulds that are made of 

 it, and they will stand more readily a greater degree of wear. Plaster of 

 Paris casts, as commonly prepared, cannot again be used for the same 

 purpose. 



