J06 COLOURS OBTAINED FROM METALLIC OXIDES. 



come diluted by the mixture with the glaze, and the firft fire 

 changes a painting apparently finifhed, into a very flight 

 Iketch. 

 And the oxide The oxide of lead contained in the glaze is a more powerful 

 mically on them" cau ^ e °f tne great changes thefe colours undergo. Its deftruc- 

 tive action is principally exercifed on the reds of iron, and is 

 very remarkable. I {hall relate fome experiments that parti- 

 cularly prove this. 

 Thefe colours It has already been (hewn that the two principal caufes of 

 fcarcely V change-*h e cnan g e which colours on enamel and tender porcelain un- 

 able, except dergo, do not relate to the compofition of thefe colours, but 



thTare fa^T* intirel y to the nature of the g Iafs u P on which the y are placed. 



•«pon. The aflertiqn that the colours of porcelain are fubject to confi- 



derable change, relates to the colours of foft porcelain, (porce- 

 lain tendre) a fpecies of ware now almofl totally abandoned. 



The paintings It follows from what I have faid, that the paintings of 



before mention- p 0rce ] a j n re quire to be feveral times re-touched and burned, 

 ed require much f n . 



touching up. in order to poflefs the neceffary ftrength. Though thefe paint- 

 ings have always a certain fofinefs, they are conftantly more 

 brilliant, and never fubjeel; to the inconvenience offcalingoff. 

 Hard porcelain ; The hard porcelain, according to the divifion I have laid 

 ^uV 8 ^" 0t clown, is the fecond fpecies of ground or excipient for the me- 

 tallic colours. It is known that the bafe of this porcelain is a 

 very white argil, called kaolin, mixed with a filiceous and 

 calcareous folvent, and the glaze of which is nothing but feld 

 fpar fufed without an atom of lead. 

 or of Saxony, This porcelain, which is that of Saxony, is of much later ' 

 receives two ^ ^ e a ^ Sevres than the foft or tender. The colours employed 

 viz. ontheglaze a re of two kinds, the firft ufed for reprefenting different ob- 

 or painting co- je<5ts, are baked with a very inferior fire to that required for 

 ours> the baking of the porcelain itielf. They are very numerous 



and varied. 

 or beneath it 5 The others, which require to be fufed at as great a heat as 

 or ground co- ^t f or baking the porcelain, are laid on the general fur- 

 face. They are much lefs numerous. 

 The colours for The colours for painting are made up very nearly of the 

 painting are fame materials as thofe for tender porcelain ; they only con- 

 nC fo y tie 6 fo^ tain more flux. This flux is compofed of the glafs of lead, 

 porcelain; but (called rocaille) and of borax. I have not yet met with any 

 contain more work tha( . treats of ^ compo f lt i on u f e an d effects of thefe 



colours. In fad, it has no where been aflerted in print, that 



all 



