J 8 On the Colours ohtahted from the metallic OxuleS, 



compofttion of the colours employed on clafs are, in gencrafj 

 the ianie as thole applied to porcelain. Neither of them diflfcr 

 but in their proportions; but there are a great number of 

 enamel or porcelain colours which cannot be applied toglafs, 

 ■where they are deprived of the white ground which ferves to 

 give them nplicf. When feeu bv refraftion, ieveral of them 

 entirely change their tone, and aflunie a dirty tint, which can 

 be of no ufe : we fliall make them known when we come to 

 treat of colours in particular. Thofe which can be em- 

 ployed on this body change fometimes in baking, and ac- 

 quire a great tranfparency. In general they are not beau- 

 tiful ; but, when placed between the light and the eye, they 

 then fcem to anfwer the only objeft that can be propofed in 

 ])ainting on glafs. 



The baking plates of painted glafs prcfents more difficul- 

 ties than might be fuppofed. Care mult be taken not to dis- 

 figure the piece, or alter the colours : all the works we have 

 read recommend a bed of gypfum. This method has fome- 

 times fucceedcd ; but the glafs, for the moft part, becomes 

 white and full of cracks. It appears that glafs too alkaline 

 (and alkalies arc the inoft common in white glaf^) fuffers it- 

 felf to be attacked warm by the fulphuric acid of the fulphatc 

 of lime. We were eafily able to bake pieces of glafs mucli 

 larger than ihofe before painted, by placing them on very 

 ilraight plates of earth or of foft porcelain. 



Of Colours in particular. 



After cotlefting the general phsenomena exhibited by each 

 clafs of vitrifiable colours, confidercd in regard to the body 

 on which they are applied, I muft make known the moft 

 intercfting particular pha^nomena exhibited by each prin- 

 cipal kind of colours employed on foft porcelain and glafs in 

 a porcelain furnace. 



Of Reds, Purplts, and I^lolcfs, made from Gold. 



Carmine red is obtained by the purple precipitate of Caflius: 

 it is mixed with about fix parts of its flux; and this mixture 

 is employed dirc(!:t:ly, witliout being fufcd. It is then of a 

 dirty violet, but by l)aking it accjuirts a beautiful red carmine 

 colour : it i?, however, exceedingly delicate ; a little too much 

 ieat and carbonaceous vapours eafily fpoil it. On this ac- 

 count it is mure beauuful when baked with charcoal than 

 with wood. 



This colour and the purple, which is very little different, 

 as well as all the Oiade? obtained from it, by mixing h with 

 f.uiifcr colourf, rsally thange on all porcelain and m every 



hsQci. 



