AT 
TAB. CCXXIV. 
SILEX Petuntse. 
Kaolin.’ 
Tuts substance (found at St. Stephen’s in Cornwall, &c.) 
has long obtained the name of Kaolin because of its re- 
semblance to the Earth so called in China, on account of 
its being used in Porcelain manufactories, and is by some 
considered as necessary to be used with the Petuntse for 
hardening the mixture for the better sort of Porcelain. It — 
is a kind of decomposed Granite, being an aggregate of 
Feldspar, Quartz, and Talc, and is, for some purposes, 
ground up together to make Crucibles, but it depends upon 
the proportion of the three ingredients to determine how it 
may best be used. If carefully separated and washed, the 
decomposing Feldspar, with what Talc and perhaps fine 
Quartz are among the washing, make the Porcelain Clay, 
(Feldspath argiliforme of Haiiy,) commonly so called, and 
the difference observed by Wedgewood, viz. of 60 parts of 
Clay and 10 of Quartz, depends upon circumstances that 
seldom allow any aggregate rock to be very regular. 
Authors differ in the analysis thus : 
Silica from Aas 52) ito 471 
Alumine . elon tO ail 
with occasionally a little Lime, Magnesia, Sulfate of Barytes, 
and perhaps Iron, which may sometimes give a redness 
more or less common in Feldspar. 
One of its characters is, to be infusible: this, however, 
may depend upon the proportion of the mixture. Some 
say it is infusible in a porcelain-heat; others say, nearly 
infusible in the greatest heat of a porcelain furnace. Ac- 
cording to Achard, Kirwan, v. 1. p. 58, determines that 
