COLOURS OBTAINED FROM METALLIC OXIDES* 14^ 



was no difference between the flower before and after the 

 baking, except that brilliancy which colours naturally receive 

 by fli f ion. 



The colours may either be previoufly fufed or not at plea- 

 sure. 



In a violent fire they either partly difappear, or produce a dull How affected by 

 , i • i i n i , i- , • ii ii violent heat, 



and brtck-duit red colour, which is not at all agreeable. 



Their compofition is the fame either for tender porcelain or They are good 

 for glafs. They do not change on the latter, but on the for- tender ircelain • 

 mer they almoft intirely difappear by the firft fire, and they 

 mufi be laid on very heavily in order to have any part vifible. 



It is to the prefence of lead in their glaze that this iingular ^ fc m ^ ft be hea- 

 effect nnift be attributed. I have afcertained this by a very j atter# 

 fimple experiment. I placed this colour on window-glafs and 

 'fired it very ftrongly, and it did not change. 



I then covered fome parts of it with minium, and again ex- Experiments 5 

 pofed it to the fire. The colour totally difappeared in thofe 

 places where the red oxide of lead had been applied. 



When I performed this operation on a larger fcale in clofed 

 veffels, a large quantity of oxigengas was difengaged. 



This obfervation I think clearly proves the effect of oxided wl ? icn Aewthat 

 lead as a difcolourer of glafs : we fee that it does not operate, co i ours g j a f 8# 

 as has been fuppofed, by burning combuftible impurities in the 

 glafs, but by diffolving, difcolouring, or volatilizing the oxide of 

 iron which may affect its clearnefs. 



Concerning the Yellows, 



Yellows are colours that require much precaution in fabricat- Yellows are 

 ing, on account of the lead they contain ; which fometimes, for ." led h Y thc 

 by approaching to the metallic date, produces black fpots. and antimony 



The yellows of hard and tender porcelain are the fame. 

 They are compofed of oxide of lead, white oxide of antimony, 

 and fand. 



Oxide of tin is fometimes added ; and when it is required and fometimes 

 very lively and refembling the colour of the marigold, red ox- tin * 

 ide of iron is added, the very deep colour of which difappears 

 during the previous fufion they undergo, on account of the 

 lead contained in this yellow. When thefe colours are once They are nof 

 made, they do not change ; they difappear almoft intirely in chan g ca Me. 

 the porcelain fire, 



Theft 



