Mercury silvering or fire gilding is a silvering technique for applying a thin layer of precious metal such as silver or gold (mercury gilding) to a base metal object. The process was invented during the Middle Ages and is documented in Vannoccio Biringuccio's 1540 book De la pirotechnia. An amalgam of mercury and the precious metal is prepared and applied to the object which is then heated, sometimes in oil, vaporizing most of the mercury. The technique is dangerous since mercury is highly toxic, especially in its vapor phase. Mercury silvering can be detected through a variety of methods.