56 A»t Olr GLAS8MAKING. 



[2-75 in.] thick, and 120 [47*2 in.] in diameter, was divided 

 by cracks from the centre to the circumference ; because the 

 adhesion of the glass to the sides of the crucible had pre- 

 vented it from contracting its dimensions, the clay, so 

 highly baked, not being susceptible of an equal diminution 

 of bulk in cooling. I preserve a piece of this, cut in the 

 form of a serre-papier, the transparency of which in so 

 great a thickness is remarkable, though the composition 

 was not prepared for being- colourless. 



Obs. III. Glass coloured red by copper. 



Red glass. Hitherto glass has been stained red, whether for church 



windows, or for imitating gems, only by combining in 

 different proportions, according to the tint desired, oxide of 

 gold by tin, oxide of manganese, and sulphuret of anti- 

 mony. Such are the compositions indicated by Fontanieu, 

 and in Loysel's Essay on Glassmakiuf. 



Glass stained Clouet has given a different process in his Inquiries into 



r.fl by iron, ^^e Composition of Enamels, which he was so kind as to 

 Communicate to me in mannscript some years ago, and 

 which I published in the Amials of Chemistry, for May, 

 1800. This process consists in fixing the colour of red 

 oxide of iron, by calcining a mixture of sulphate of .iroi> 

 and gulphate of alumine : but he precisely declares, that 

 we have no metallic oxide, which gives a red directly by 



fus^ioii Ihattiiis colour must be composed of different 



substances and that it is desirable to multiply experi- 

 ments with the new metals, which would perhaps furnish a 

 red, that is not to he produced directly or easily by any of the 

 metallic substances anciently known. He speaks of the ox- 

 ide of copptr only in the preparation of green enamel : and 



and by copper, though he sometimes obtained a tolerably fine red from it, 

 particularly by mixing with it oxide of iron, he says, that 

 this colour is very fugacious, and frequently disappears even 

 while the glass is making. 



Red glass An accident, that happened in 1783 at the plate glass 



b7a\wcddent"?""'*^''^*°'"y ^^ ^'- ^^'^>"' ^PP^^^'-^d to me to ascertain the 

 circumstances, in which we may hope to fix in glass the 

 colour of red oxide of copper ; and a direct experiment, 



made 



