Scientific American 9, 20.11.1847
(Concluded from our last.)
The process must have been to melt side by side in the glass house a pot of plain and a pot of red glass; then the workmen, by dipping his rod first into the plain and then into the red glass pot, obtained a white lump of plain glass covered with a coating of red, which, by dexterous management in blowing and whirling, he extended into a plate, exhibiting on the surface a very thin stratum of the desired color. In thus state the glass came into the hands of the glass-painter, and answers most of its purposes, except when the subject required the representation of white or other colors on a red ground; in this case it became necessary to employ a machine like the lapidary's wheel, partially to grind away the colored surface till the white substratum appeared.
The material employed by the old glass-makers to tinge their glass red was the protoxide of copper, but on the discontinuance of the art of glass-printing, the dependant manufacture of red glass of course ceased, and all knowledge of the art become so entirely extinct, that the notion generally prevailed that the color in question was derived from gold. It is not a little remarkable that the knowledge of the coppered should have been so entirely lost, though printed receipts have always existed detailing the whole process. Baptista Porta (born about 1540) gives a receipt in his Magia Naturalis, noticing at the same time the difficulty of success. Several receipts are found in the compilations of Neri Merret and Kunckel, from whence they have been copied into our Encyclopaedias. None of these receipts, however, state to what purposes the red glass was applied, nor do they make any mention of flashing. The difficulty of the art consists in the prone nss of the copper to pass from the state of protoxide into that of peroxide, in which latter state it tinges glass green. In order to preserve it in the state of protoxide these receipts prescribe various deoxygenating substances to be stirred into the melted glass, such as Smith's clinkers, tartar, soot, rotten wood, and cinnibar.
One curious circumstance deserves to be noticed, which is, that glass containing copper when removed from the melting pot sometimes only exhibits a faint-greenish tinge yet in this state nothing more than simple exposure to a gentle heat is requisite to throw out a brilliant red. This change of color is very remarkable, as it is obvious that no change of oxygenation can possibly take place during the recuission.
The art of tinging glass by protoxide of copper and flashing it on crown glass, has of late years been revived by the Tyne Company of England, at Chiosy in France, and in Suabia in Germany, and in I827 the Academy of Arts at Berlin gave a premium for an imperfect receipt To what extent do modern glass makers make use of these new glasses, we are ignorant; the specimens we have seen were so strongly colored as to be in parts almost opaque, but this is a defect, which might no doubt, be easily remedied.