io On the Colours ohtahied from the metall'tc OxUei, 



riot baked, except what might be expeded to rcfiilt from 

 ttie briUiancy given to colours by fufion. 

 " Thefe colours may be employed iudifcriminately, eithef 

 previoufly fufed or not fufed. 



"Tn a great heat thcv in part difappear, or produce a dull 

 lirick red ground, which is not agreeable. 



The compoiition of them is the fame both for foft porce- 

 rainand.for glafs. They do not change on the latter ; but 

 on foft porcelain they difippear almolt entirely on the firrt 

 expofure to heat, and to make any thing remain they muft 

 be employed very deep. 



"This fmgular effeft mufl: be afcribed to the prefence of lead 

 in the crull or glazing. I afTured myfelf of this by a very 

 fimple experiment, I placed this colour on window glafs, 

 andj having expofed it to a ftrong baking, it did not change. 



I covered fevcral parts of it with minium ; and again ex- 

 j5ofing it to the fire, the colour was totally removed in the 

 places where the red oxide of lead had been applied. 



By petforming this operation on a larger fcale in clofe vef- 

 fels, a large quantity of oxygen gas was difengaged. 



It appears to me that this obfervation clearly proves the- 

 i6lioa of oxidated lead on glafs as a deltroyer of colour : it 

 is feen that it does not aft, as was believed, by burning the 

 combuftlble bodies, which might tarnift the glafs, but by 

 diflblving, difcolouring, or volatilizing with it the oxide of 

 iron, which might alter its tranfparency. 

 " ■ Yelknvs. 



Yellows are colours vi'hich require a great deal of care in 

 the fabrication on account of the lead which they contain, 

 and which, approaching fometimes to the metallic (tate, pro- 

 di,Vces on them black fpots. 



The yellows for hard and foft porcelain are the fame : they 

 are compofcd of the oxide of lead, white oxide of antimony, 

 and fand. 



Oxide of tin is fometimes mixed with them ; and when it 

 is required to have them livelier, and nearer the colour du 

 Jo7ui, red oxide of iron is added, the too great rcdnefs of 

 Hvhlch-is diflipated in the previous fufion to which thev are 

 expofed by the aolion of the lead contained in this yellow, 

 Thefe colours when once made never change : they difippear, 

 hovvever, almoft entirely when expofed to a porcelain heat. 

 '^ 'Thefe yellows cannot be applied to glafs : they are too 

 ppake and dirty. That employed by the old painters on glafs 

 lias, on the contrary, a beautiful tranfparency, is exceedingly 

 J)rilHant, and of a colour which approaches near to that of 

 gold. Tiie proccfles which they gave clearly fliowed that 



filver 



