358 Dr. Pearson’s Experiments and Obfervations 
with refpeft to colour. The fame difappointment has been alfb 
occafioned by ufing a r u 1 tv iron rod in calcining the mixture. 
The bone-afhes procured from the fal ammoniac and 
fpirit of hart's horn manufactories, frequently failed in pro- 
ducing a white powder; and fo didfometimes the bone-aflies, 
called prepared hart’s horn, fold by the druggifts. Even after 
a fine white coloured mafs had been made, if it was pulverized 
in an iron mortar that had extremely little calx upon its furface, 
or dirt, the powder was not white. 
The yellow coat and glaze on the fides of the crucible and 
furface of the calcined mixture of bone and antimony, in thefe 
experiments, is to be afcribed rather to the fufion of the clay 
of the crucible with the antimonial calx, than to the greater 
degree of fire in the part of the crucible in which it takes place ; 
or than to the calx of iron and fihceous earth of the veftel t 
becaufe the fame yellow coat and glazing are produced on the 
Wedgwood pyrometer pieces, which are placed in the middle 
of the charge, and where the degree of heat cannot be fo great 
as nearer the fide of the crucible, and yet a fnow-white mafs 
is produced between thefe clay pieces and the fides of the 
crucible. This eifeft of clay, in forming a yellow coat and 
glaze, is fliewn by the obfervation of what happens when the 
calcined mixture is put into a Wedgwood’s crucible, which is 
made of much purer clay than other veffels of this kind, and 
when it is fet in a larger Heffian crucible with the fpace be- 
twixt the two velTels filled with the fame calcined mixture. 
After expofure to a fufficient degree of fire, viz. about 120° of 
Wedgwood s icale, the infide and outfide of the inner cru- 
cible will be covered with a yellow vitreous coat and glaze as 
well as the infide of the outer crucible in con tail with the 
charge, while the reft of the matter within thefe veflels is of 
a fnowy 
