Etching is done by hydrogen fluoride, the powerful corrosive acid obtained by heating spar or cryolite with sulphuric acid. Glass was thus etched by Schwankhart, of Nuremberg, about 1860, and the acid itself was obtained and investigated by Scheele a few years later. The glass is covered with some substance, such as wax or pitch, upon which the acid will not act, and the required lines are scratched with a needle through the wax to the surface of the glass. The whole is covered with a solution of hydrogen fluoride, or exposed to acid vapor, when the parts unprotected by wax are eaten away more deeply the longer they are exposed to the action.