4«42 Mr. R. A. Couper on the Chemical Composition of 



been fired and then ground, is employed to render the body 

 still more open or porous, and also to give it a greater capa- 

 bility of standing sudden heats or colds. This ware is much 

 used in public works for chemical purposes : it is exposed to 

 the action of the flame during burning, whereas other kinds 

 of ware are protected by saggars from the flames. 



VII. The glaze vitrifies the surface of the body, rendering 

 it generally capable of withstanding acids. It is a very im- 

 portant point with the manufacturer to obtain a glaze which 

 will adhere to the body without crazing or peeling off, as he 

 may discover a good body, but not find a glaze to answer it, 

 since every glaze will not adhere to the same body, and hence 

 every manufacturer has a glaze of his own composition. 



1, The substances used in the preparation of the glaze for 

 white ware, are borax, china clay, flint, Cornish stone, Paris 

 white, and white lead. 



In preparing the glaze, a substance technically termed frett 

 is first made, consisting of borax, china clay, flint, Cornish 

 stone, and Paris white, which are fused together in a kiln, and 

 when ready allowed to flow into water, which shortens it, 

 owing to the water being mechanically lodged in it, and keeps 

 it from adhering to the bottom of the vessel, rendering it much 

 easier to pound. Frett is a beautiful glass, coloured by a little 

 iron, and is pounded and water-ground along with Cornish 

 stone, flint, and white lead : this constitutes the glaze for white 



ware. 



Analysis of Analysis of 

 white glaze. frett. 



Silica 43-66 55-98 



Lime ;........ -52 2-52 



Alumina and protoxide of iron 9-56 10-38 



Borax 20-08 3M2 



Carbonate of lime .... 10*88 



Carbonate of lead .... 15-19 



99-89 100-00 



Specific gravity 2*345 



A piece of earthenware was brought from America, having 

 been discovered several feet under ground, the glaze of which 

 was tested, and found to be composed of silica, iron, alumina, 

 lime, sulphate of lime and antimony, which was a beautiful 

 rich white glaze concealing a common red clay body. 



2. The glaze of Rockingham ware possesses a beautiful 

 brownish 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 consist- 

 ency of milk, 



3. The glaze for compapo blftck ware is made from the same 



