Jonathan Thornton
Abstract
This paper explores the various surfaces that appear to be gilded, or did so originally, but without gold actually being present. While most of these surfaces and materials were intended by their makers to imitate gold, unintentional gold-colored corrosion products and corrosion layers giving rise to thin-layer interference phenomena may also be mistaken for gold. The history, structure and composition of both intentional and unintentional gold-like surfaces are described.
Gilding has often been imitated by the application of an orange-colored varnish on white metals including silver, lead, tin and polished iron and steel, as well as silver, tin and aluminum leafs. These coatings based on oils and oil-resin varnishes and various organic colorants, were sometimes called auripetrum in medieval and renaissance texts. Later recipes sometimes called changing varnishes and gold lacquers are usually based on spirit varnishes (predominantly shellac and sandarac) with added dyes and colored alcohol-soluble resins (gamboge, dragon’s blood, accroides). Less intensely colored coatings are used on brass and other copper alloys to more nearly match the color of gold Gold-colored coatings are still commercially produced.
Copper alloys, particularly those of copper and zinc (brass), were valued for their resemblance to gold, as indicated by a medieval term for brass; auricalcum. Names for brass alloys that most nearly resemble gold included Ormolu, prince’s metal or Prince Rupert’s metal, Mannheim gold, and Similor. In 1725 Pinchbeck, was invented by the jeweler Christopher Pinchbeck and became a synonym for any cheap imitation. Copper could also be “gilded” by being exposed to zinc fumes at a red heat.
Gold-colored brass alloys can be beaten into thin foils or leafs. The center of this industry from an early date was Germany; hence the common English terms Dutch (deutsch) gold, and schlagmetal. “Gold” powders were also milled from brass as directed by Theopbilus in the twelfth century. These powders were being exported from Germany as a commercial product by the seventeenth century and probably earlier. Commonly called bronze powders, these flake leafs are to this day one of the most common gold imitations.
Stannic sulfide in thin flakes was a gold imitation of considerable antiquity. Names for this material included purpurino, mosaic gold, aurum mosaicum and aurum musivum. European accounts of manufacture are common from the fifteenth century and usually involved heating mixtures of tin, mercury, sulfur and ammonium chloride. This material was probably prepared in China at an earlier date by heating tin, alum and ammonium chloride in a closed crucible One descriptive Chinese text dates from 300 AD In Europe Mosaic gold was commercially available as a “gold” powder into the 19th Century.
Optical interference phenomena can also give rise to gold colors. Some early formularies such as the twelfth Century Mappae Clavicula include recipes suggesting the intentional imitation of gold on silver by means of a thin-layer interference effect. Thin, unintentionally produced, silver sulfide layers on polished silver are easily mistaken for gilding. The thin magnetite layer on polished steel which produces the “light straw” color used by traditional smiths in judging the tempering beat can also be interpreted as gold. The intentional production of stable and brightly colored oxide layers on metals by means of direct electrical current is commonly called anodizing, and can be used to produce a spectral range of colors including gold. Anodized aluminum tumblers were popular in the 1950s and anodized titanium and niobium jewelry is currently fashionable. Gold and other metallic-sheen pigments are currently produced by thin layers of titanium and iron oxide on mica flakes. These mica based luster pigments are used extensively in the plastics and paint industries. They are also useful to conservators because of their stability.
Vapor deposition of metal films on a wide variety of substrates (including polymers, textiles, paper, and glass) is a common modem method for imitating gold and other metals. Coating is done in a vacuum and the coating metal is vaporized by an electrical arc, electron beam, or by the impact of magnetically accelerated ions (sputtering). Such vacuum metallizing is extensively employed in the auto, toy, and costume jewelry industries among others. Aluminum is the most commonly used coating metal, and is made to resemble gold by the application of colored polymer top-coats. Vapor deposition is also the method used to produce the mica-based luster pigments.