Procure several thick, clear pieces of crown-glass, and immerse them in melted wax, so that each may receive a complete coating, or pour over them a solution of wax in benzine. When perfectly cold draw on them, with a fine steel point, flowers, trees, houses, portraits, etc. Whatever parts of the drawing are intended to be corroded with the acid should be perfectly free from the least particle of wax. When all these drawings are finished the pieces of glass must be immersed one by one in a square leaden box or receiver where they are to be submitted to the action of hydrofluoric acid gas, made by acting on powdered fluor-spar by concentrated sulphuric acid.
When the glasses are sufficiently corroded they are to be taken out, and the wax is to be removed by first dipping them in warm and then in hot water, or by washing with turpentine or benzine. Various colors may be applied to the corroded parts of the glass, whereby a very fine painting may be executed. In the same manner sentences and initials of names may be etched on wineglasses, tumblers, etc.
Glass may also be etched by immersing it in liquid hydrofluoric acid, after having been coated with wax and drawn on, as in the last method. There is this difference, however, in the use of the liquid and the gas, that the former renders the etching transparent, whilst that produced by the gas is quite opaque.
In this method the potassa of the glass is set free, whilst the silex or sand is acted on, consequently no vessel of glass can ever be employed with safety to contain this acid in a liquid state, as it would soon be corroded into holes. It is, therefore, generally preserved in leaden bottles, on which it has no power to act.
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