Gilding is the process of covering an item with a thin layer of gold. The application of other types of metal leaf may also be referred to by the same name, though gold and silver are the most common. Gilding has been used since ancient times, particularly in the arts of East and South Asia and the Middle East. It has long signified an ostentatious display of wealth, though it has become less expensive in the modern era as a result of less time-consuming application techniques. Imitation gold leaf also reduces the cost of the process and is often used for decoration today.
There are two basic methods of gilding: mechanical and chemical. Mechanical gilding, the only type used in the ancient era, begins with gold leaf, which is made by hammering gold into paper-thin sheets between layers of parchment. The gold leaf used in the ancient era was typically thicker than today's gold leaf and has consequently survived the centuries quite well.
Different types of mechanical gilding are used for different materials. A metal surface must be painstakingly prepared by cleaning, scrubbing, heating, and soaking in an acidic apricot solution. Bronze must be prepared with mercury in order to receive gold leaf, then heated, while iron and steel must be etched and heated just short of red-hot before the gold leaf is applied. After the gold is applied to a metal surface, it must be burnished, or polished, with an agate stone.
Water gilding, which uses a layer of gesso and a layer of bole to make the gold leaf adhere, is traditionally used for wood surfaces. The gold leaf must be brushed on with a gilder's tip before burnishing in this type. A fourth mechanical type, oil gilding, uses an adhesive oil primer to gild the walls of a building. No burnishing is necessary in this process.
There are a few different forms of chemical gilding as well, all of which use gold in a chemical compound at some point in the process. Cold gilding, used on a silver surface, consists of dipping a linen cloth into a liquid solution of gold in aqua regia, burning the cloth, and rubbing the ashes onto the silver.
Wet gilding uses a solution of chloride, gold and ether. The mixture is manipulated until the ether absorbs the gold out of the acid, and the ether can then be painted onto an iron or steel surface. As the ether evaporates, only gold is left. This form must be finished by heating and burnishing.
Fire gilding, also called wash gilding, begins with an amalgam of gold, or a mixture of gold with mercury. If this method is used on a wrought metal surface, a primer of pure mercury must be used. A plain metal surface must be prepared with nitric acid.
After the amalgam is applied, it must be heated so that the mercury evaporates and leaves the gold behind. Overheating will ruin the project. Next, the gold must be scrubbed smooth and coated with gilding wax, and the wax must be burned off. Finally, the surface is coated with a mixture of nitre or other salts with water or weak ammonia. Fire gilding, traditionally used for buttons, produces beautiful results, but presents a significant danger to workers because of the high amounts of mercury involved.