Acid-etched glass |
Acid etching or «embossing» as it is sometimes called in the trade, involves principally the use of hydrofluoric acid, one of the few acids which will rapidly corrode glass. This acid, made by heating fluor spar (calcium fluoride) with sulphuric acid and condensing the resultant gas in water, was first chemically identified by a Swedish chemist in 1771, although it was certainly used several hundred years prior to this date. Stained glass workers used a substance called «strong water» to remove the surface of flashed glass (a sheet glass that has a thin layer of color on one side). During the 19th century the process of acid etching developed into a craft in its own right.
When acid etching was first developed, only the fumes of hydrofluoric acid were used. An acid–resistant paint, usually Brunswick black, was painted on the surface of plate glass in a decorative design. The glass was then exposed to the highly toxic fumes which rapidly frosted the unprotected areas. Finally the acid resist was removed to reveal the clear pattern on a frosted background.
The use of fumes to frost the glass was soon superseded by clear etching. In this technique the glass was painted in the same way but a wall of tallow (a form of animal fat) was placed around the edge of the glass like the rim of a tray. The surface could then be flooded with liquid acid. After about one hour, the acid was poured off, leaving the surface etched to a depth of about 0.5mm. Although clear, this finish was not as transparent as the original, and the glass was described as clear etched or «embossed». It was often silvered or gilded.
Clear etched glass was sometimes subjected to a further treatment known as «emboss and ground». Here the glass was ground by sprinkling emery powder and water on to the surface and rubbing it with a smaller sheet of plate glass. As a result, the raised areas of the sheet were ground and made obscure whereas the etched areas being lower were untouched by the abrasion and remained clear. This technique was widely used especially by sign writers who also gilded the etched areas.
Numerous other clear etching processes were employed to texture plate glass, the most common being «stipple embossing». In this process the etched surface was sprinkled with mica flakes during etching, creating a glistening stippled effect on the surface of the glass, like rain drops on a window pane.
Decorative acid etching techniques were also used on flashed glass to good effect.
In the early part of the 19th century, designer glass workers made their own hydrofluoric acid, but the pernicious nature of the process did not make the practice desirable. Soon it was possible to buy acid in gutta percha containers from manufacturing chemists. However, the process of etching remained a risky undertaking; hydrofluoric acid is an extremely dangerous chemical and there were no
One other etching agent was developed and exploited to the full during the 19th century. Known as «white acid», it was made by mixing hydrofluoric acid with an alkali, usually carbonate of soda (washing soda). This produced a «frosting acid» that imparted a dense white frost to the surface of glass. Subsequent etchings with dilute hydrofluoric acid produced a range of satin tones. This process is known in the trade as «French embossing» or «triple embossing» and was frequently employed in combination with brilliant cutting.
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| Image. Hand blown, sandblasted, acid etched glass. | |
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