Depletion gilding

Depletion gilding is a method for producing a layer of nearly pure gold on an object made of gold alloy by removing the other metals from its surface. It is sometimes referred to as a "surface enrichment" process.

Most gilding methods are additive, that is, they deposit gold that was not there before onto the surface of an object. By contrast, depletion gilding is a subtractive process whereby material is removed to increase the purity of gold that is already present on an object's surface.

Essentially, depletion gilding produces a high-purity gold surface by removing everything that is not gold. More specifically, other metals are etched away from the surface of an object composed of a gold alloy by the use of acids or salts, often in combination with heat. Of course, since no gold is actually added, only an object made of an alloy that already contains at least some gold can be depletion gilded.

Depletion gilding relies on the fact that gold is highly resistant to oxidation or corrosion by most common chemicals, whereas many other metals are not. Depletion gilding is most often used to treat alloys of gold with copper and/or silver. Unlike gold, both copper and silver readily react with a variety of chemicals. For example, nitric acid is effective as an etching agent for both copper and silver. Under the proper circumstances, even ordinary table salt (sodium chloride) will react with either metal.

The object to be gilded is coated, immersed, or packed in a suitable acid or salt. These chemicals then attack the metallic copper and silver in the object's surface, transforming it to various copper and silver compounds. The object is usually heated to make the etching process more efficient. Regardless, the resulting copper and silver compounds can be removed from the object's surface by a number of processes. Washing, chemical leaching, heating, or even physical absorption by porous materials such as brick dust have all been used. However, the relatively inert gold remains behind, unaffected. The result is a thin layer of nearly pure gold on the surface of the original object.

There is no well-defined minimum gold content required to successfully depletion gild an object. However, as a practical matter, the less gold that is present, the more other material must be etched away to produce the desired surface appearance. In addition, the removal of the other metals usually leaves the surface covered with microscopic voids and pits. This can make the surface soft and "spongy" with a dull or matte appearance. This effect becomes more pronounced as more base metal is removed. For this reason, most depletion gilded objects are burnished to make their surfaces more durable and give them a more attractive polished finish.

...
Wikipedia