Depletion gilding

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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. [1] It is sometimes referred to as a "surface enrichment" process. [2]

Contents

Process

Most gilding methods are additive: 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.

In depletion gilding, 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. Since no gold is added, only an object made of an alloy that already contains 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 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 will react with either metal.

The object to be gilded is coated, immersed, or packed in a suitable acid or salt, and usually heated to speed the process. These chemicals then attack the metallic copper and silver in the object's surface, transforming it to various copper and silver compounds. 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 historically. Meanwhile, the relatively inert gold is left 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, 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: its cost and rarity, and its softness and denseness. 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 electrum. 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.

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 electroplating. Some individual artisans and small shops continue to practice it.

However, depletion gilding was widely used in antiquity. [3] 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. [4] Some ancient alloys, such as tumbaga, may have been developed specifically for use in depletion gilding. The technique was not known to be used by Anglo-Saxons until detailed examination with electron microscopes of treasures such as the Staffordshire Hoard revealed its use in the twenty-first century. [5] [6] [7]

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.

Related Research Articles

Alloy Mixture or metallic solid solution composed of two or more elements

An alloy is an admixture of metals, or a metal combined with one or more other elements. For example, combining the metallic elements gold and copper produces red gold, gold and silver becomes white gold, and silver combined with copper produces sterling silver. Combining iron with non-metallic carbon or silicon produces alloys called steel or silicon steel. The resulting mixture forms a substance with properties that often differ from those of the pure metals, such as increased strength or hardness. Unlike other substances that may contain metallic bases but do not behave as metals, such as aluminium oxide (sapphire), beryllium aluminium silicate (emerald) or sodium chloride (salt), an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, opacity, and luster. Alloys are used in a wide variety of applications, from the steel alloys, used in everything from buildings to automobiles to surgical tools, to exotic titanium alloys used in the aerospace industry, to beryllium-copper alloys for non-sparking tools. In some cases, a combination of metals may reduce the overall cost of the material while preserving important properties. In other cases, the combination of metals imparts synergistic properties to the constituent metal elements such as corrosion resistance or mechanical strength. Examples of alloys are steel, solder, brass, pewter, duralumin, bronze, and amalgams.

Metal Type of material

A metal is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typically malleable or ductile. A metal may be a chemical element such as iron; an alloy such as stainless steel; or a molecular compound such as polymeric sulfur nitride.

Electroplating Creation of protective or decorative metallic coating on other metal with electric current

Electroplating is a general name for processes that produce a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be coated acts as the cathode of an electrolytic cell; the electrolyte is a solution of a salt of the metal to be coated; and the anode is usually either a block of that metal, or of some inert conductive material. The current is provided by an external power supply.

Corrosion Gradual destruction of materials by chemical reaction with its environment

Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, carbonate or sulfide. It is the gradual destruction of materials by chemical and/or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and preventing corrosion.

Metalworking Process of making items from metal

Metalworking is the process of shaping and reshaping metals to create useful objects, parts, assemblies, and large scale structures. As a term it covers a wide and diverse range of processes, skills, and tools for producing objects on every scale: from huge ships, buildings, and bridges down to precise engine parts and delicate jewelry.

Gilding Covering object with layer of gold

Gilding is a decorative technique for applying a very thin coating of gold to solid surfaces such as metal, wood, porcelain, or stone. A gilded object is also described as "gilt". Where metal is gilded, the metal below was traditionally silver in the West, to make silver-gilt objects, but gilt-bronze is commonly used in China, and also called ormolu if it is Western. Methods of gilding include hand application and gluing, typically of gold leaf, chemical gilding, and electroplating, the last also called gold plating. Parcel-gilt objects are only gilded over part of their surfaces. This may mean that all of the inside, and none of the outside, of a chalice or similar vessel is gilded, or that patterns or images are made up by using a combination of gilt and ungilted areas.

Patina Change of objects surface through age and exposure

Patina is a thin layer that variously forms on the surface of copper, brass, bronze and similar metals, or certain stones, and wooden furniture, or any similar acquired change of a surface through age and exposure.

Repoussé and chasing Metalworking technique

Repoussé or repoussage is a metalworking technique in which a malleable metal is shaped by hammering from the reverse side to create a design in low relief. Chasing or embossing is a similar technique in which the piece is hammered on the front side, sinking the metal. The two techniques are often used in conjunction.

Flux (metallurgy)

In metallurgy, a flux is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining.

Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, to improve IR reflectivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish.

<i>Tumbaga</i> non-specific alloy of gold and copper used in pre-Columbian Mesoamerica

Tumbaga is the name for a non-specific alloy of gold and copper given by Spanish Conquistadors to metals composed of these elements found in widespread use in pre-Columbian Mesoamerica in North America and South America.

Gold plating

Gold plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver, by chemical or electrochemical plating. This article covers plating methods used in the modern electronics industry; for more traditional methods, often used for much larger objects, see gilding.

Selective leaching, also called dealloying, demetalification, parting and selective corrosion, is a corrosion type in some solid solution alloys, when in suitable conditions a component of the alloys is preferentially leached from the material. The less noble metal is removed from the alloy by a microscopic-scale galvanic corrosion mechanism. The most susceptible alloys are the ones containing metals with high distance between each other in the galvanic series, e.g. copper and zinc in brass. The elements most typically undergoing selective removal are zinc, aluminium, iron, cobalt, chromium, and others.

Arsenical bronze

Arsenical bronze is an alloy in which arsenic, as opposed to or in addition to tin or other constituent metals, is added to copper to make bronze. The use of arsenic with copper, either as the secondary constituent or with another component such as tin, results in a stronger final product and better casting behavior.

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 a surface of sterling silver to bring up a thin layer of fine silver. Then 24 carat gold foil is applied with heat and pressure—mechanical gilding—to produce a permanent diffusion bond.

Colored gold

Pure gold is slightly reddish yellow in colour, but coloured gold in various other colours can be produced.

Silver-gilt Silver gilded with gold 14kt

Silver-gilt or gilded/gilt silver, sometimes known in American English by the French term vermeil, is silver which has been gilded with gold. Most large objects made in goldsmithing that appear to be gold are actually silver-gilt; for example most sporting trophies and many crown jewels are silver-gilt objects.

Staffordshire Hoard Hoard of Anglo-Saxon gold and silver metalwork discovered in 2009

The Staffordshire Hoard is the largest hoard of Anglo-Saxon gold and silver metalwork yet found. It consists of almost 4,600 items and metal fragments, amounting to a total of 5.1 kg (11 lb) of gold, 1.4 kg (3 lb) of silver and some 3,500 pieces of garnet cloisonné jewellery.

Gold parting

Gold parting is the separating of gold from silver. Gold and silver are often extracted from the same ores and are chemically similar and therefore hard to separate. Over the centuries special means of separation have been invented.

Irogane is the term for a set of Japanese metals – forms of copper, and copper alloys – treated in niiro patination processes, traditionally used in sword-making, catches for sliding doors, and luxury highlights on larger objects, and in modern times, in jewellery. The alloys contain two to five metals. Some scholars believe that methods similar to those involved in irogane production may also have been used in ancient Egypt and the Roman world, as well as China and Tibet (dzne-ksim).

References

  1. ↑ Blakelock, E. S. (2015). "Never Judge A Gold Object by Its Surface Analysis: A Study of Surface Phenomena in a Selection of Gold Objects From the Staffordshire Hoard". Archaeometry. 58 (6): 912–929. doi:10.1111/arcm.12209.
  2. ↑ The Surface Enrichment of Carat Gold Alloys - Depletion Gilding. Mark Grimwade in Gold Technology, Issue 26; July 1999.
  3. ↑ David Bates; Robert Liddiard (18 June 2015). East Anglia and Its North Sea World in the Middle Ages. Boydell & Brewer Ltd. p. 246. ISBN   978-1-78327-036-1.
  4. ↑ Precious Metal Objects of the Middle Sican. Izumi Shamada and Jo Ann Griffin in Scientific American Special, Volume 15, Number 1, pages 80-89; 2005.
  5. ↑ Britain's greatest treasure hoard reveals how goldsmiths fooled the Anglo-Saxon world
  6. ↑ "Staffordshire hoard research reveals secret of Anglo-Saxon 'gold'". 16 October 2014.
  7. ↑ Blakelock, Eleanor. "Contextualising Metal-Detected Discoveries: Staffordshire Anglo-Saxon Hoard" (PDF). Barbican Research Associates. Retrieved 5 October 2016.
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