The ancients were acquainted with the means of staining glass by the employment of oxide of copper; it is mentioned by Neri and Kunckel, in their works. The art was however so completely lost at the close of the last century, that it was generally believed that glass was always stained red by means of Cassius purple. It was not until 1828 that M. Engelhardt, of Zinsweiler, succeeded in staining glass red by means of a mixture of equal parts of oxide of copper and protoxide of tin : this process was tried with success in the glass manufactory at Hoffurmgsthal, Silesia.
The protoxide of tin is now done away with, and the compound employed is nearly the same as that mentioned by Neri, but more simple. It is composed of a mixture of copper scales (which are almost entirely composed of oxide), and oxide of tin (zinnasche) obtained by the oxidation of that metal in a state of fusion in contact with the air, to which a small quantity of iron filings is sometimes added, when a scarlet tint is required to be produced. Should the color by accident disappear, it may be brought out by again bringing the copper into the state of oxide ; this is done by introducing into the vessel a small quantity of tin or iron scale. It will of course be understood that the glass to be operated upon must not contain saltpetre, nor any other oxidizing substance.
Glass stained by means of oxide of copper is of a very deep color, and can only be worked in thin sheets, and by covering it with a thick colorless glass (plate glass.)