Mercury gilding - Apollo Art & Antiques

What is mercury gilding?

Mercury gilding, also known as fire gilding, involves mixing pure gold together with liquid mercury to form a paste-like amalgam. This amalgam is then brushed onto the surface of a bronze object. Once the item has been covered with the amalgam, it is heated in a furnace until the mercury vaporizes, leaving the gold strongly bonded to the surface of the object, even in the finest details of the object. As a final step, the freshly mercury gilt object is burnished or polished using an agate tool. This gives a bright, high purity gold finish that is both beautiful and durable. Mercury gilding has been applied to most gilt bronze objects since the Renaissance. It is now almost abandoned, because its implementation releases mercury vapors that are very toxic and harmful to the health of the craftsman and the environment.

Is mercury gilding the same as ormolu?

Yes, both terms refer to the same process. The word ormolu derives from the French “or moulu” which means “ground gold” since one first has to grind the gold to a powder before being mixed with the mercury to form the gold-mercury amalgam that is used in the gilding process.

The manufacturing of mercury gilded bronzes

In this article we will describe, in more detail, the steps involved in the manufacturing of mercury gilded bronzes.

Bronze casting

The process of manufacturing mercury gilded bronze begins with the creation of a wax model, often sculpted by a skilled artist. This model is then coated with a heat-resistant material to form a mold. Bronze is an alloy of copper and tin, with additional elements such as zinc or lead sometimes included to achieve specific properties. The proportions of these metals are carefully measured to ensure the desired characteristics of the final bronze. The alloy is then melted at high temperatures and the molten bronze is poured into the mold, taking the shape of the original wax model. Once the bronze cools and solidifies, the mold is removed, leaving behind the rough bronze cast.

Cleaning of the bronze cast

Once the bronze objects are cast and polished, the surface is meticulously cleaned to remove any impurities or oxidation that may have formed during the casting process. This preparation is crucial to achieving a smooth and even gilded surface.

Chasing of the bronze

The bronze casting undergoes a process known as chasing, also referred to as chiseling, where artisans refine the surface details with fine chisels and smooth out imperfections.

Mercury gilding of the bronze object

The next step involves preparing the mercury amalgam, which is a mixture of gold and mercury. Gold is typically alloyed with other metals like silver or copper to create a suitable material for gilding. This gold alloy is then ground into a fine powder and combined with mercury to form a paste-like amalgam. The proportion of gold to mercury is carefully controlled to achieve the desired color and thickness of the gilded layer.

Next, the bronze surface is meticulously cleaned and a layer of adhesive, often made from a solution of animal glue or Arabic gum, is brushed onto the clean bronze. This adhesive ensures a secure attachment of the gold layer to the bronze. Then the mercury amalgam is carefully applied to the bronze surface using a brush. The craftsman must work swiftly and precisely, as the amalgam has a limited working time before it solidifies. The mercury reacts with the bronze, forming a thin layer of gold-mercury alloy on the surface.

Firing

Once the gilding is complete, the object is heated to evaporate the mercury, leaving behind a layer of pure gold on the bronze surface. This process is often referred to as “firing”, hence the term “fire gilding” to refer to mercury gilding. The temperature must be carefully controlled to ensure the mercury is completely removed without damaging the gold layer or the underlying bronze.

The tone of the gilding is achieved by carrying out several passes (three, even four or more), and therefore by increasing the thickness of the metal deposit, and consequently the resistance over time.

Burnishing of the mercury gilding

After the firing process, the gilded bronze is polished to achieve the desired luster and finish. Skilled craftsmen may use various tools and techniques, such as burnishing with agate stones or brushes, to enhance the brilliance of the gold layer and create a smooth, reflective surface.

Conclusion

In conclusion, the manufacturing of mercury gilded bronzes is a meticulous and intricate art form that combines traditional bronze casting with the unique properties of mercury. The result is a stunning and durable gilded finish that has adorned sculptures and artworks for centuries.

While mercury gilded bronze was once a widely practiced and admired technique, it has fallen out of favor due to the health risks associated with mercury exposure. Nonetheless, the historical significance of mercury gilding persists, and surviving examples of ormolu objects serve as testaments to the craftsmanship and artistry of past generations.

Literature

Diderot & D’Alembert, “Encyclopédie”, 1751-1777.
Alfred Roseleur, “Guide pratique du doreur, de l’argenteur et du galvanoplaste”, Paris, 1873.
Tardy, “La pendule française”, Paris, 1975.