On the Art of Glass-Pai?iting. 461 



with a very thin layer of the glass in question. I have myself 

 made numerous experiments on this subject, and have been 

 completely, and at last uniformly, successful, in producing 

 glass of a fine crimson colour. One cause why so many per- 

 sons have failed in the same attempt*, I suspect is that they 

 have used too large a proportion of gold ; for it is a fact, that 

 an additional dose of gold, beyond a certain point, far from 

 deepening the colour, actually destroys it altogether. An- 

 other cause probably is, that they have not employed a suffi- 

 cient degree of heat in the fusion, I have found that a de- 

 gree of heat, which I judged sufficient to melt cast-iron, is 

 not strong enough to injure the colour. It would appear, 

 that in order to receive the colour, it is necessary that the 

 glass should contain a proportion either of lead, or of some 

 other metallic glass. I have found bismuth, zinc, and anti- 

 mony to answer the purpose, but have in vain attempted to 

 impart any tinge of this colour to crown-glass alone. 



Glass containing gold exhibits the same singular change of 

 colour on being exposed to a gentle heat, as has been already 

 noticed with respect to glass containing copper. The former 

 when taken from the crucible is generally of a pale rose-colour, 

 but sometimes colourless as water, and does not assume its 

 ruby colour till it has been exposed to a low red-heat, either 

 under a muffle or in the lamp. Great care must be taken in 

 this operation, for a slight excess of fire destroys the colour, 

 leaving the glass of a dingy brown, but with a blue transpa- 

 rency like that of gold-leaf. These changes of colour have 

 been vaguely attributed to change of oxygenation in the gold; 

 but it is obviously impossible that mere exposure to a gentle 

 heat can effect any chemical change in the interior of a solid 

 mass of glass, which has already undergone a heat far more 

 intense. In fact I have found that metallic gold gives the red 

 colour as well as the oxide, and it appears scarcely to admit 

 of a doubt, that in a metal so easily reduced, the whole of the 

 oxywen must be expelled long before the glass has reached its 

 meltino- point. It has long been known that silver yields its 

 colour to glass while in the metallic state, and everything 

 leads one to suppose that the case is the same as to gold. 



There is still one other substance by means of which I find 

 it is possible to give a red colour to glass, and that is a com- 

 pound of tin, chromic acid, and lime; but my trials do not 

 lead me to suppose that glass thus coloured will ever be 

 broutfht into use. 



• Dr. Lewis states tliat he once produced a polfull of glass of beautiful 

 colour, yet was never able to succeed a second time, though he took in- 

 finite pains, and tried a multitude of experiments with that view.— Com- 

 vicrcc (if /Iris, p. 177- 



