Manchester Memoirs, Vol. xlv. (1900), No. 1. 3 



so far as the glaze itself is concerned, no cases of plumbism 

 can arise in their manufacture. 



A very different set of conditions regulates the manu- 

 facture of the greater portion of the pottery produced in 

 England. English earthenware is made from clay mix- 

 tures possessing great plasticity. A working mass is 

 made containing 50 to 60 per cent, of various native clays, 

 with variable proportions of ground siliceous and felspathic 

 minerals. In English china, the materials used for hard- 

 paste porcelain are mixed with a large proportion of bone- 

 ash, so that the mixture will vitrify at a much lower 

 temperature. With a vessel made of either earthenware 

 or bone china, the highest temperature at which it can be 

 fired up, and yet retain its shape, is far below either the 

 melting point of felspar or the temperature of the salt- 

 glazing kiln, and in effect English potters are limited to the 

 use of glazes which will melt perfectly, and flow easily and 

 evenly over the surface of the pottery, at temperatures 

 ranging from 1000^ to iioo^C. In order to obtain glazes 

 suitable for this lower range of temperatures it is necessary 

 to combine the felspar, which still forms the basis of the 

 glaze, with silicates of the alkalies and alkaline earths, or 

 with their borates. Glazes can be made in this way, per- 

 fectly free from lead, and melting at the required 

 temperature, but in actual work on the commercial scale, 

 they are very partially successful, as they are subject to 

 serious defects, which render their extended use, in the 

 present state of our knowledge, impossible. It will be 

 readily understood that one of the conditions governing 

 the employment of any substance or process commer- 

 cially, is the certainty of its results. The making of 

 pottery is in any case attended with great risk from 

 many causes, of which the chief is the impossibility of 

 controlling the temperature of every part of the kiln to 



