Depletion gilding
Encyclopedia
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.
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.
Like other gilding processes, depletion gilding provides a way to produce the appearance of pure gold without its disadvantages. Most obviously, the cost and rarity of gold limits the use of the pure metal, especially in large objects. In addition, pure gold is both extremely soft and very dense, two factors that limit its utility. By producing a layer of gold over a layer of copper or other metal, objects can be made that are lighter, sturdier, and cheaper while still appearing to be nearly pure gold.
. Certain chemicals, such as oxalic acid
, attack copper but do not affect either silver or gold. Using such a chemical, it is possible to remove only the copper in an alloy, leaving both silver and the gold behind. Thus, if the original object is composed of copper, silver, and gold, it can be given a gold surface by removing both silver and copper, or an electrum surface by removing only the copper.
Likewise, with an appropriate chemical, a layer of nearly pure silver can be produced on an object made of copper and silver. For instance, sterling silver
can be depleted - 'depletion silvering' - to produce a fine silver surface, perhaps as preamble to application of gold, as in the Keum-boo
technique.
However, in the majority of cases depletion gilding is in fact used to produce a gold finish, rather than one of electrum or silver.
. Some individual artisans and small shops continue to practice it.
However, depletion gilding was widely used in antiquity. While it requires skill to execute it well, the process itself is technologically simple, and uses materials that are readily available to most ancient civilizations. Some form of depletion gilding has been used by nearly every culture that developed metalworking. The South American Sican
culture in particular developed depletion gilding to a high art. Some ancient alloys, such as tumbaga
, may have been developed specifically for use in depletion gilding.
Certain cultures are thought to have attached mythical or spiritual significance to the process. Gold was considered sacred in many early civilizations and was highly valued in nearly all of them, and anything relating to it had the potential to take on cultural importance. Moreover, the ability to turn what appeared to be an object made of copper into what seemed to be pure gold would be very impressive. There is some speculation that depletion gilding may have contributed to the concepts of alchemy
, a major goal of which was to physically transform one metal into another.
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
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.
Process
Most gildingGilding
The term gilding covers a number of decorative techniques for applying fine gold leaf or powder to solid surfaces such as wood, stone, or metal to give a thin coating of gold. A gilded object is described as "gilt"...
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
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
and/or silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
. Unlike gold, both copper and silver readily react with a variety of chemicals. For example, nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
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.
Like other gilding processes, depletion gilding provides a way to produce the appearance of pure gold without its disadvantages. Most obviously, the cost and rarity of gold limits the use of the pure metal, especially in large objects. In addition, pure gold is both extremely soft and very dense, two factors that limit its utility. By producing a layer of gold over a layer of copper or other metal, objects can be made that are lighter, sturdier, and cheaper while still appearing to be nearly pure gold.
Variations
The term depletion gilding usually refers to the production of a layer of gold. However, it can also be used to produce a layer that is an alloy of gold and silver, sometimes referred to as electrumElectrum
Electrum is a naturally occurring alloy of gold and silver, with trace amounts of copper and other metals. It has also been produced artificially. The ancient Greeks called it 'gold' or 'white gold', as opposed to 'refined gold'. Its color ranges from pale to bright yellow, depending on the...
. Certain chemicals, such as oxalic acid
Oxalic acid
Oxalic acid is an organic compound with the formula H2C2O4. This colourless solid is a dicarboxylic acid. In terms of acid strength, it is about 3,000 times stronger than acetic acid. Oxalic acid is a reducing agent and its conjugate base, known as oxalate , is a chelating agent for metal cations...
, attack copper but do not affect either silver or gold. Using such a chemical, it is possible to remove only the copper in an alloy, leaving both silver and the gold behind. Thus, if the original object is composed of copper, silver, and gold, it can be given a gold surface by removing both silver and copper, or an electrum surface by removing only the copper.
Likewise, with an appropriate chemical, a layer of nearly pure silver can be produced on an object made of copper and silver. For instance, sterling silver
Sterling silver
Sterling silver is an alloy of silver containing 92.5% by mass of silver and 7.5% by mass of other metals, usually copper. The sterling silver standard has a minimum millesimal fineness of 925....
can be depleted - 'depletion silvering' - to produce a fine silver surface, perhaps as preamble to application of gold, as in the Keum-boo
Keum-boo
Keum-boo is an ancient Korean gilding technique used to apply thin sheets of gold to silver, to make silver-gilt. Traditionally, this technique is accomplished by first depleting the surface of sterling silver to bring up a thin layer of fine silver...
technique.
However, in the majority of cases depletion gilding is in fact used to produce a gold finish, rather than one of electrum or silver.
Applications
Depletion gilding is a decorative process, with no significant industrial applications. It is not widely used in modern times, having been superseded by processes more suited to mass production, such as electroplatingElectroplating
Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
. Some individual artisans and small shops continue to practice it.
However, depletion gilding was widely used in antiquity. While it requires skill to execute it well, the process itself is technologically simple, and uses materials that are readily available to most ancient civilizations. Some form of depletion gilding has been used by nearly every culture that developed metalworking. The South American Sican
Sican
Sican may refer to:*The Sican culture in what is now Peru*Sican language*The Sicani, a people of ancient Sicily...
culture in particular developed depletion gilding to a high art. Some ancient alloys, such as tumbaga
Tumbaga
Tumbaga was the name given by Spaniards to a non-specific alloy of gold and copper which they found in widespread use in pre-Columbian Mesoamerica and South America.-Composition and properties:...
, may have been developed specifically for use in depletion gilding.
Certain cultures are thought to have attached mythical or spiritual significance to the process. Gold was considered sacred in many early civilizations and was highly valued in nearly all of them, and anything relating to it had the potential to take on cultural importance. Moreover, the ability to turn what appeared to be an object made of copper into what seemed to be pure gold would be very impressive. There is some speculation that depletion gilding may have contributed to the concepts of alchemy
Alchemy
Alchemy is an influential philosophical tradition whose early practitioners’ claims to profound powers were known from antiquity. The defining objectives of alchemy are varied; these include the creation of the fabled philosopher's stone possessing powers including the capability of turning base...
, a major goal of which was to physically transform one metal into another.
External links
- The Ganoksin Project
- The Surface Enrichment of Carat Gold Alloys - Depletion Gilding (also used as a reference)