Gold is one of the oldest metals known to humankind. It is the most malleable and ductile of all metals. Gold applied as a thin strip, square leaf, or plating interacts with light with intensity unmatched by other metals. 

Gold is very soft in its purified state. Cold working has no effect on the stiffness of gold as it does with most other metals. Only when alloyed with other metals, usually copper and silver, does gold improve hardness. Gold is also very dense and has good conductivity of electricity and heat. It is a powerful infrared reflector, thus it stays cool and is used where surfaces must reflect intense heat.

Gold is highly corrosion-resistant. It will not be affected by normal exposure to the environment. That is why it stays so shiny and reflective. Sulfur, the usurper of copper and silver, has no effect on gold, nor do chlorine and carbon dioxide. Water will not corrode gold, and most organic salts have little effect. It can also be hammered thin to the point at which it is partially transparent, 0.01 mm thick.

Gold Gilding used on the Cathedral of The Immaculate Conception
Gold Gilding used on the Cathedral of The Immaculate Conception
Gilt bronze used on a sculpture by Walter de Maria
Gilt bronze used on a sculpture by Walter de Maria

Gilding Gold

One of the most common ways of applying gold to architecture is through gilded metal. Gilding, an ancient skill, involves applying a thin layer of gold foil in the form of strips or small squares onto the surface of another material.

There are a wide range of gilding applications whereby a thin layer of gold is applied to a substrate material. The original ancient process used mercury as an amalgam, or mixture of the two metals. In this technique, mercury would be removed as a vapor by heating the substrate, leaving the gold behind to adhere to the surface.

Because mercury vapor is a very toxic substance, today, gilding processes include vacuum deposition, electroplating, sputtering, as well as several other application methods. Generally, on exterior architectural and ornamental gilding, the base material is usually metal. Copper or copper alloys are the metal of choice but lead-coated copper, cast iron, and tin have also been gilded.

Deterioration of Gold Surfaces

The discoloration or tarnish seen on pure gold is the product of the underlying metal corroding, not the gold itself. The gilding is porous, and if a separating barrier between the gold and the base metal has been breached, the underlying metal will corrode if moisture is present. If the gilding layer is not pure gold, it will develop a slight oxide or tarnish as well.

Gold is very noble, and most other metals will be sacrificial when in proximity to gold. Gold has a very high reduction potential. In equilibrium, it will cause most other metals to undergo galvanic attack.

Gold is often gilded onto other metals in one of its many alloy forms. As just stated, gold typically does not tarnish or corrode, but the underlying metal does via the porosity of the gilding.

Colors of Gold Alloys

The color of gold alloys depends on the mix of three elements: gold, silver, and copper. Increase the silver content in relation to gold and the color changes from yellow to yellow-green and then to white with a yellow tint. Copper, on the other hand, changes the gold alloy progressively redder in color. Gold and silver are miscible into each other; that is, molten gold and silver will dissolve completely into each other and form a homogeneous solid material. Gold and copper will as well. This will occur under all ratios of the two metals. However, silver and copper will not; they are miscible only within a very tight range of ratios of the two. Mixing all three metals will create an alloy that is much harder and less malleable than alloys of gold-silver or gold-copper. Color depends on the ratio of these three elements:

Electrodeposition of Gold

Plating of gold by electrodeposition is another means of applying a thin layer of gold onto the surface of a metal. Electrodeposition can be selectively applied by using resistive material. In selective electrodeposition, a portion of the surface is plated. The gold coating is very thin with a very fine grain. Thus, it matches closely the underlying surface being plated and provides a deep reflective luster. The color obtained is the very yellow color of 24-carat gold.

Texturing of Gold Surfaces

Adding texture to the surface prior to applying the gilding or electroplating can enhance the reflective nature of the gold finish. Texture can be induced into the sealing layer of acrylic or epoxy; or it can also be developed in the base metal. By adding texture to the surface, the intensity of the reflection can be enhanced by the scattering effect of the roughened surface. Light-scattering brings out the color of the metal. Even a slight coarseness will enhance the appearance of the gilding and further brighten the surface, particularly when viewed from a low angle of incidence. The drawback is, the coarse surface collects and holds dirt, requiring more frequent cleaning.