Bronze art: a bit of history and some technical information

Any bronze artwork, whatever the alloy used, must have the following, indelible, visible marks on the metal:

This choice is made, once and for all, by the artist before the manufacturing of the first piece.

According to the applicable rules, a piece of art designated as “original” can be manufactured in no more than 12 copies, even if the composition or colour of the alloy varies from one piece to the other.

Out of these original copies, four of them called “épreuves d’artistes” (artist’s proof sheets) must bear the following numbers, in roman numerals: EA I/IV - EA II/IV - EA III/IV - EA IV/IV. The artist can not always sell these copies. The other eight copies must bear the following numbers, in arabic numerals: 1/8, 2/8, 3/8, 4/8, 5/8, 6/8, 7/8 and 8/8.

Before the first founding, the artist can choose to manufacture fewer copies than 12. When the chosen quantity is reached, it can in no way be exceeded.

When the artist decides, as soon as the first founding, to create his or her pieces of art in multiple copies, these artworks will bear a consecutive number which also mentions the total number of copies planned (1/100, 2/100, 3/100 for instance or 1/300, 2/300, etc.)

As it is the case with originals, once the chosen quantity is reached, it can in no way be exceeded.

When a sculpture is cast in one copy only, for instance from a wax directly created by the artist, the artwork will bear the mention "PU" (for “pièce unique”, unique specimen) or 1/1. This particular sculpture can not be manufactured another time.

Catherine and Philippe Boisseau manufacture their sculptures in 12 copies or fewer with the dimensions of the original model. They accept no new series with other dimensions than those of the original model.

The artist can create an original model from various materials: plaster, clay, wood, metal, stone... the list is never all-comprehensive.

If the model is in clay, it must dry for a month or so and it is then baked for two days at a temperature from 980°C.

A mould is made of the original model or sculpture. The rigid outer moulds contain the softer inner mould, in silicone, which is the exact negative of the original model. At first, the inner mould looks like a pasty liquid: one adds a small quantity of catalyst so that it can solidify.

Moulds are often in two or more pieces, and a shim with keys is placed between the various parts during construction so that the mould can be put back together accurately.

Once the mould is finished, molten wax is poured into it until the desired thickness is reached. The mould is then turned upside down and the exceeding, unhardened, wax runs out.

The wax model must be filled with some refractory material. This core, made of plaster, crushed firebricks, sand and water, is poured into the wax mould. It is important that it contains no air. It is put to dry in a vertical position until the mixture hardens.

Once dry, the hollow wax copy of the original model is carefully removed from the mould. The wax parts are assembled into a whole. This wax resembles the original sculpted clay and is checked for accuracy before the process continues. If need be, the hollow wax copy is then “chased” by the artist: a heated metal tool is used to rub out the marks that show the parting line where the pieces of the mould came together and to correct any detail, particularly the faces. The wax now looks like the finished piece and the artist can sign the copy when the signature must be engraved on the wax.

The wax sculpture is dipped several times into a vat of porcelain until a thick shell of porcelain has built up. The porcelain is left to dry and then fired to harden it into a negative mould. During this process, wax heats up, melts and runs out through the resin paths specially designed to enable the molten casting material to flow and air to escape. The melted wax cannot be recovered and reused because it contains impurities, hence the name of “lost wax”. The oven is gradually heated up until it reaches a temperature of 700°C. According to their size, moulds are baked from 48 to 72 hours.

Bronze contains mostly copper (80%); the rest is a mixture of tin and zinc. Melting temperature is around 1300°C. Once the bronze is ready to run out, the founder puts refractory moulds on a sand ditch. The metal in the melting pot is poured into the moulds.

When the mould has cooled down, the shell is hammered, releasing the rough casting. Paths are cut down, bronze is cleaned and the core is removed.

This is the finishing step: with a chisel, a drill and other tools, the artist sharpens and finalizes every shape, every detail. Eyes, hair locks, heads and thin paws are corrected with a chisel. Finally, foundry’s stamp, signature and number are engraved.

A patineur’s workshop is not unlike an alchemist’s laboratory. Oxides, acids and sulphurs are applied with a brush on the heated bronze. When the bronze has cooled, wax is applied to seal the patina. Patina gives a bronze its particular aspect.

The Bronze Age took place between 2300 and 800 B.C., putting an end to the Neolithic Age. Bronze Age started in South-East Asia, where copper and tin – the main components of bronze - abound. In Europe, the Bronze Age only started between 2300 and 1800 B.C. Very soon, craftsmen attained mastery in bronze casting. Bronze played a key role in exchanges between humans, who worked in bronze for basic tools (axes, weapons, coins) as well as for art. When Iron Age came, around 100 B.C., bronze has ever since flourished in art. Today, bronze is still considered a precious, if not luxurious, material.