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The technique of decorating a metal surface by use of acids. The usual process is to cover the design on the surface with an acid-resisting substance and then to immerse the piece in acid that eats away (corrodes) the uncovered portions; but for decoration of fine lines, the entire piece is covered with the acid-resisting substance and the design is scratched through it with a sharp tool and then the piece is immersed in the acid. The covering is often a type of wax or varnish, and the usual acids for metalwork are nitric acid for silver and copper and aqua regia for gold and platinum.
The process is sometimes used in modern times in lieu of engraving or chasing. It is similar to the process of etching glassware and ceramic ware, except that their hydrofluoric acid is used. Sometimes tautologically called 'acid etching'.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson
Etching refers to the technique of creating art on the surface of glass by applying acidic, caustic, or abrasive substances. Traditionally this was done after the glass was blown or cast.
In the 1920s a new mould-etch process was invented, in which art was etched directly into the mould, so that each cast piece emerged from the mould with the image already on the surface of the glass. This reduced manufacturing costs and, combined with a wider use of colored glass, led to cheap glassware in the 1930s, which later became known as Depression glass. As the types of acids used in this process are extremely hazardous, abrasive methods have gained popularity.
Typically, "glass etching cream" available from art supply stores consists of fluoride compounds, such as sodium fluoride and hydrogen fluoride. The fluoridation of the glass (which is a network covalent solid of silicon dioxide molecules) causes the characteristic rough, opaque qualities of frosted glass.