Engraving on GLASS. Professor Beckmann has proved, that so early as the year 1670 the art of etching upon glass was discovered by Henry Schwanhard, son of George Schwanhard, who was a celebrated glass-cutter, patronized by the emperor Ferdinand III. about the middle of the last century. At the time of his death, 1667, the father practised his art at Prague and Ratibon. Whether the son followed the same business at the same towns, or removed to Nuremberg, is not very evident; but in the year above mentioned, some aqua-regia (nitro-muriatic acid) having accidentally fallen on his spectacles, he was surprised to find the glass corroded by it, and become quite soft. He thus, it is said, found himself in possession of a liquid by which he could etch writing and figures upon plates of glass.
But it is probable, as Beckmann seems to think, that he had discovered the fluoric acid itself; for in the year 1725 there appeared in a periodical work the following receipt for making a powerful acid, by which figures of every kind can be etched upon glass.
"When the spiritus nitri per distillationem has passed into the recipient, ply it with a strong fire, and when well dephlegmated, pour it, as it corrodes ordinary glass, into a Weldenberg flask. Then throw into it a pulverized green Bohemian emerald, otherwise called hesphorus, (which, when reduced to powder, and heated, emits in the dark a green light), and place it in warm sand for 24 hours. Take a piece of glass well cleaned, and freed from all grease by means of a ley; put a border of wax round it, about an inch in height, and cover it all over with the above acid. The longer you let it stand so much the better; and at the end of some time the glass will be corroded, and the figures which have been traced out with sulphur and varnish will appear as if raised above the pane of glass."
That the Bohemian emerald or hesphorus mentioned in this receipt is green sparry fluor, cannot, says the professor, be doubted; and he seems to have as little doubt of the receipt itself having passed from Schwanhard and his scholars to the periodical work of 1725, from which it was inferred in the Oekonomische Encyclopedie of Krunitz. This supposition certainly acquires a considerable degree of probability from the similarity of Schwanhard's method of etching to that which is here recommended, and which is so different from what is now followed. At present, the glass is covered with a varnish either of iinglass dissolved in water, or of turpentine oil mixed with a little white lead, through which the figures to be etched are traced as on copper; but Schwanhard, when he had drawn his figures, covered them with varnish, and then by his liquid corroded the glass around them. His figures, therefore, when the varnish was removed, remained smooth and clear, appearing raised from a dim or dark ground; and M. Beckmann, who persuaded some ingenious artists to make trial of this ancient method of etching, declares, that such figures have a much better effect than those which are cut into the glass.