Golden Glass.

Scientific American 44, 24.7.1847

The following remarks regarding the making of colored glass, are worthy of attention, inasmuch as its manufacture is daily becoming of more importance to American glass manufacturers.

Not long after the time when the art of making the copper-red glass was lost, Kunkel appears to have discovered that gold melted with flint glass was capable of imparting to it a beautiful ruby color. As he derived much profit from the invention, he kept his method secret, and his successors have done the same to the present day. The art, however, has been practiced ever since for the purpose of imitating precious stones, &c., and the glass used to be sold at Birmingham, England, for a high price, undder the name of Jew's Glass. - The rose colored scent bottles, &c., now commonly made, are composed of plain glass, flashed or coated with a very thin layer of the glass in question. Numerous experiments have been made on this subject, and have been successful in producing glass of a fine crimson color. Dr Lewis states that he once produced a potful of glass of beautiful color, yet was never able to succeed a second time, though he took infinite pains, and tried a multitude of experiments with that view.One cause why so many persons have failed in the same attempt, is suspected to be in the fact that they have used too large a proportion of gold; for it is certain, that an additional quantity of gold beyond a known point, so far from deepening the color, actually destroys it altogether. Another cause probably is, that they have not employed a sufficient degree of heat in the fusion. It has been found that a degree of heat, judged sufficient to melt cast iron, is not strong enough to injure the color. It would appear that in order to receive the color, it is necessary that the glass should either contain a proportion of lead, or of some other metallic flux. Bismuth, zinc and antimony have been found to answer the purpose, but it has been attempted in vain to impart any tinge of this color to crown-glass alone.

Glass containing gold exhibits the same singular change of color on being exposed to a gentle heat. The former when taken from the crucible is generally of a pale rose-color, but sometimes colorless as water, and does not assume its ruby color 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 color, leaving the glass of a dingy brown, with a blue transparency like that of gold leaf. These changes of color 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, it is found that metallic gold gives the red color 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 oxygen must be expelled long before the glass has reached the melting point. It has long been known that silver yields its color to glass while in the metallic state, and everything leads one to suppose the case is the same as to gold.

There is still one other substance by means of which it is possible to give a red color to glass, and that is a compound of tin, chromic acid, and lime; but the trials fo not lead us to suppose that glass thus colored, will ever be brought into use.

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