CLAY MANUFACTURES. 681 
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99.40 
This analysis does not apply to the washed and strained clay. The 
sample analyzed was taken from the works of Frederick, Shenkle, 
Allen & Co., Liverpool. 
The theory of the glazing of pottery is very simple, but in its 
application lies the ex:ellence of one ware over another. There are 
certain substances that have the property of fusing under heat in the 
presence of free silica to a clear transparent silicate, of which glass is 
the type. To make a potters’ product useful it must have its tendency 
to absorb liquids removed, which is done by wetting thé biscuit-ware 
with a substance which will fuse to a clear glass with the silica of the 
clay, and give a smooth imperishable finish to the works. 
There is a large range of bodies chosen to do this work, notably 
the alkalies, the earths, borax, lead, ete. The glazes applied to yellow- 
ware are usually made as follows: Proportions of litharge (Pb O), flint 
(Si O,), spar (Potash or feldspar), Paris white (Ca Co,), are mixed up 
in a grinding vat, with a thin slip of pure clay. This last is necessary 
to keep the heavier bodies from settling out; all the elements being 
well incorporated, the mixture is transferred to the glazing trough 
and used ; heat of course on such a mixture, silica and lead, potash and 
lime with a little clay, would immediately produce a very fluid clear 
glass. Sometimes the carbonate of lead is used instead of the oxide, 
but the effect is the same. These are the main elements of a yellow- 
ware glaze; the special proportions used are kept secret by each potter, 
and even the constituents by many, though these may easily be de- 
termined by looking over the lists of bodies which would exercise the 
desired action. Each man believes his glaze better than his neighbor’s, 
and he has been obliged to adopt its different proportions to suit the 
different qualities of the clay. Some glazes will crack over the surface 
of the ware and greatly disfigure it, and a change of proportion or 
working is necessary to correct this tendency. Rockingham-ware is 
the same as yellow, except in color; the addition of a large amount of 
manganese oxide is made to the ordinary glaze, and the ware first given 
