Gilt bronzes in the 18th and 19th centuries: history, technique and the principal bronziers

Gilding is a technique intended to enhance works of art, objects of worship or precious objects by covering them with gold. It has been known since Antiquity, notably in Egypt. This metal, the only one at the time with the particular property of not oxidizing, was a symbol of immortality, and therefore of the divine.

Thus we find wooden statuettes, and objects in metal or stone, whose surface, in whole or in part, has been covered with a very thin gold leaf, so as not to obscure the details of the support.

This practice has evolved over the centuries, and at the beginning of the 21st century a great many objects are still gilded. Over time, two other gilding processes appeared.

Leaf gilding

Leaf gilding, the oldest process, is still practised, notably for low-relief supports that do not take well to other processes, such as wood (framing mirrors or paintings), wrought iron, commemorative plaques, etc., as practised by the Ateliers Ardeco, the Ateliers Gohard, the Atelier Léonie, the Atelier Mariotti, the Atelier de dorure Palomares and the Atelier Huber.

But also roofs (the Dôme des Invalides), sculptures and other decorative schemes (the Opéra Garnier in Paris).

Gold being a highly ductile metal, hammering makes it possible to obtain very thin (a few micrometres), pliable leaves without breaking the grain of the metal. A gold leaf is laid flat on a support, then picked up with a wide brush called a “palette”, to which it clings through static electricity. The gold leaf is then laid down on the surface to be gilded, sometimes prepared with egg white to ensure adhesion. Burnishing gives the final appearance. In gilding à la française, the polishing is carried out with an agate stone shaped like a wolf’s tooth, which was used for this operation in the 18th century.

Mercury gilding

This process consists of applying to the support, perfectly cleaned with acids, a liquid amalgam — gold having the particular property of dissolving in mercury. The object is then heated, which causes the mercury to evaporate, leaving the gold alone in the smallest details of the support2.

This process gives a very solid and durable gilding, but can be applied only to objects of small size, for reasons of handling, and able to withstand the ordeal of fire. These are most often bronzes for works of art or furniture, or other metals.

The tone of the gilding, doré, surdoré, très surdoré d’or moulu, is achieved by making several passes (three, even four or more), and thus by increasing the thickness of the metal deposit, and consequently its resistance over time. The finish can be achieved through various recipes:

• scratch-brushing (grattebossage), to modify locally the contrast between a matte area and a bright area, in order to bring out the modelling,
• burnishing, which consists of pressing the layer of gold into the pores of the support, using a tool called a burnisher, made of a hard stone set in a handle: haematite or agate.

It is a process that gives gilding of great quality and durability, and it was applied to most gilt-bronze sculptures from the Renaissance onwards. It is today virtually abandoned, since its use releases mercury vapours that are highly toxic and harmful to the craftsman’s health (mercury poisoning) and to the environment4. This process is also called or moulu, or “ormolu” in English.

It was used in particular, in 1819, by the French sculptor-bronzier Pierre Philippe THOMIRE to execute the bronze ornamentation of the Empire-style “Demidoff Vase”, veneered in malachite with ormolu, 1.715 metres high. The vase had been commissioned by Count Nicolas Demidoff to furnish the Villa San Donato, his residence near Florence.

Depletion gilding, also called gilding by removal or by stripping, is a process used on gold alloys (electrum, tumbaga, shakudo). The operation consists, by means of an acid attack, of removing all the metals making up the alloy with the exception of the gold. The result, on the surface of the alloy object, is a thin film composed of pure gold.

This process was widely used by the pre-Columbian civilizations of Mesoamerica, who used tumbaga to fashion artefacts, essentially of a religious nature.

Electroplated gilding

This is the most technologically recent process. It uses Galvani’s discovery, which consists of immersing two metal electrodes in a salt bath, forming an electric cell. If a current is passed through, metal molecules are made to move from one electrode to the other, from the anode to the cathode. This process is called electroplating.

The object, first made conductive with plumbago (lead dust) if it was not already, is immersed in a conductive bath and serves as the cathode. The anode is made of stainless steel or platinum.

This process was mastered in the 19th century by the goldsmith Christofle, whose reputation rests on the gilding of the immense, well-known statue crowning Notre-Dame de la Garde in Marseille, executed under Napoleon III.

This process is currently the most widespread, on an industrial scale, notably in electronics. It allows an even gilding whose thickness can be controlled. As far as art bronzes are concerned, it has the drawback of tarnishing a little, and of having a somewhat cold metallic lustre, not always pleasing, most often matte.

Chemical gilding

Gold salts can decompose on contact with cupreous metals — copper, bronze, brass, nickel silver — depositing a thin layer of gold. A fair number of formulas for gilding on bronze rely on this effect.

– Bath gilding. 100 g of potassium cyanide and 20 g of auric chloride (AuCl3) are dissolved in two litres of distilled water. The pieces to be treated are dipped in the bath, heated to 80 °C.

– Gilders’ formula. This is the same preparation as the previous one, fixed in the form of a paste to be rubbed onto cupreous objects, which can be composed as follows: 1 g of gold chloride, 3 g of potassium cyanide, 5 g of Spanish white, 0.25 g of potassium tartrate in very fine powder, dissolved in 5 g of distilled water.

These two products are extremely dangerous: they are violent poisons owing to their cyanide content. The following recipe, less effective, is also less aggressive.

– Non-toxic formula. This formulation relies on the preparation of a potassium gold salt. Auric chloride is dissolved in a very small quantity of aqua regia and a cloth is soaked in the yellow solution obtained. When the cloth is burned, a potassium gold salt is obtained, contained in the residual ash. A bright layer of gold is deposited on a cupreous object rubbed with this ash, using a cloth soaked in 90° alcohol.

These three processes allow only very thin layers of gold to be deposited, since the decomposition reaction of the gold salt stops as soon as there is no longer any contact with the copper.

Vacuum gilding

New techniques have reformed the traditional technology of gold coating in comparison with electroplated gilding. The ion-deposition process produces coatings just as magnificent and sumptuous, but more resistant to corrosion and friction, and their effect is, so to speak, unalterable, which is very important and enhancing for luxury decorative objects.