Lyndsie Selwyn
Abstract
This paper summarizes information about the corrosion chemistry of gold, gold alloys, and gilded silver and copper. It is intended as a review to provide conservators of gold or gilded objects with relevant information about corrosion and corrosion prevention. Through the ages, the attraction of gold has always been its excellent chemical stability; in the natural environment, gold does not, in general, corrode. Section one covers the physical and chemical properties of pure gold, including the special conditions under which gold does corrode. The next section deals with porosity in a gilding layer and the conditions responsible for the corrosion of the underlying (substrate) metal. Typical corrosion problems of two common substrate metals, copper and silver, are addressed. Section three describes the corrosion of ternary alloys of gold, silver, and copper. Such a ternary alloy may be present in the gilding layer, which is rarely pure gold, or in the metal under the gilding. The last section briefly deals with corrosion prevention.
Gold chemistry
Gold (Au) is often called the noblest of noble metals because of its remarkable resistance to corrosion. Gold chemistry depends on whether the corrosive medium is non-complexing or complexing. In a non-complexing environment, gold remains in the elemental state. In oxidizing media containing complexing agents, such as cyanides (CN–) or chlorides (Cl–), gold dissolves to form stable complexes, usually with gold in the +1 or +3 oxidation states. Stable gold (I) complexes form when gold reacts with alkaline cyanide or acidic thiourea solutions. Such complexes act as intermediates in gold recovery processes. An example of a stable gold (III) complex is [AuCl4]–, formed when gold reacts with aqua regia (3 parts hydrochloric acid and 1 part nitric acid). This complex can react with ammonia or ammonium salts to form explosive gold compounds (‘fulminating gold’). The complex can also be reduced, using acidic tin (II) chloride solutions, to colloidal gold. Under certain conditions, colloidal gold has a purple color (‘purple of Cassius’). A sensitive spot test that conservators can use to test for gold is based on the formation of colloidal gold.
Corrosion of metal beneath gold
Gold finishes, if pore-free, protect the substrate metal from corrosion. If the finish is not pore-free, the underlying metal is exposed to the environment and so is more likely to corrode. For example, the mercury gilding process, which is based on applying a mercury-gold amalgam to a metal followed by heating to remove the mercury, produces a porous gold layer. The metal corrosion rate through pores in the gold is often accelerated relative to the rate in the absence of a gold layer. This is a galvanic effect caused by the noble (cathodic) gold layer in contact with an active (anodic) substrate.
This section focuses on the corrosion of two common substrate metals, copper (Cu) and silver (Ag). Under atmospheric conditions, copper and silver react with reduced sulfur-containing gases, mainly hydrogen sulfide (H2S) and carbonyl sulfide (COS), to form copper sulfide (Cu2S) and silver sulfide (Ag2S), respectively. These sulfides grow through pores and produce dark spots that can spread over the gold surface. Copper sulfide spreads at a slower rate than silver sulfide. Copper is also susceptible to attack by volatile organic acids such as acetic acid. Under burial conditions, gilded objects can be exposed to water containing dissolved oxygen, carbon dioxide, and chlorides. Such conditions result in chlorargyrite (AgCl, also called cerargyrite or horn silver) forming on exposed silver, and cuprite (Cu2O), nantokite (CuCl), and basic copper compounds (e.g., malachite, Cu2(CO3)(OH)2) forming on exposed copper. Nantokite plays a key role in bronze disease.
Corrosion of gold alloys
Although pure gold does not corrode in the natural environment, gold alloys, such as tumbaga alloys or natural electrums, do corrode. Tumbaga alloys are gold-copper alloys from South America containing from between 90 and 10 weight percent copper. Such alloys can be gilded through a depletion gilding process that selectively corrodes (removes) the alloying constituents from the outer surfaces leaving only the gold. Natural electrums are gold-silver alloys containing high percentages of silver. This section describes the tarnish and corrosion properties of ternary mixtures of gold, silver, and copper, and discusses the importance of microstructural effects on corrosion. Tarnishing or more severe forms of corrosion, such as stress-corrosion cracking, depend primarily on the gold content. Alloys with greater than 50 atomic percent gold are usually corrosion resistant.
Corrosion depends on the alloy microstructure. The ternary phase diagram for Au-Ag-Cu mixtures is rich in microstructural features and includes single-phase regions, two-phase regions, and regions of ordered structure (e.g., Cu3Au, CuAu, and CuAu3). Moreover, the alloy microstructure depends not only on the alloy composition, but also on the heat treatment. For example, a rapidly cooled alloy may have a homogeneous, single-phase structure and be more resistant to tarnishing than the same alloy slowly cooled and having a two-phase structure. (The two-phase microstructure enhances tarnish reactions through microscopic galvanic cells.) On the other hand, the two-phase microstructure is thought to inhibit cracking. As a result, stress-corrosion cracking usually occurs in single-phase alloys.
Corrosion prevention
Almost all methods of corrosion prevention are based on common sense; keep harmful materials away from the metal surface. Water is one essential ingredient in a corrosive environment. Other factors include particulate matter, especially those containing chloride ions or sulphate ions, and pollutant gases, particularly hydrogen sulfide. Simple corrosion prevention methods include using silica gel to keep artifacts dry and using activated carbon to remove harmful gases. If storage or display conditions are not suitable for protecting an artifact, then coatings such as wax or lacquers (lncralac, Acryloid B-72, or nitrocellulose lacquers) are recommended for polished surfaces.