A couple of things for clarification. Fire or mercury gilding .Used to gold plate metal prior to the introduction pf electroplating in the 1840s. Gold is dissolved in mercury to form an amalgam which is applied to the metal . The mercury is then heated leaving the gold stuck to the base metal. Because the temperature is similar to that needed to blue high carbon steel it could be used to good decorative effect on sword blades and high quality armour. Yes , the fumes are very toxic .
Japaning is an entirely different process commonly used in the eighteenth century as a decorative protective finish for iron plate or copper items. Sometimes called Pontypool enamel after Pontypool in Wales which specialised in this process. Linseed oil is boiled down to a sticky paste and dissolved in turpentine together with pigments . Applied to the object and baked at a low temperature, usually involving several coats . The resulting finish is hard and very durable and could be overpainted often in imitation of Japanese lacquer. Often seen on military items such as hat boxes or trunks in a crude imitation of tortoiseshell
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