Bronze is an alloy of copper and tin, made by heating the two metals together and allowing them to cool. Bronze is more expensive than silver since it requires more complex processing in its manufacture.
The high cost of bronze sculptures is also due to the labour intensive casting process involved in creating a bronze sculpture. Using the ancient ‘lost wax’ method, it necessitates an extremely high level of skill, judgement and experience, employed over a number of stages.
The easiest way to understand the 'lost wax' method is to imagine a series of alternating ‘positive’ forms and ‘negative’ moulds. For example, a ‘positive’ form would be a foot; if pressed into damp sand and removed it would leave a ‘negative’ impression of the foot in the sand.
THE LOST WAX METHOD
STEP 1
CREATE THE ORIGINAL SCULPTURE - THE ‘POSITIVE’
The artist sculpts an original model usually in clay, plaster, stone, resin, wood or metal. This is the first ‘positive’ form.
STEP 2
MAKE A ‘NEGATIVE’ MOULD
A mould is made of this original ‘positive’ form by coating it in silicone rubber which is supported by a casing of plaster.
The rubber is meticulously painted onto one half of the original sculpture. The plaster shell is applied to the rubber once the rubber has hardened. This process is repeated for the other half of the original sculpture, resulting in a two part rubber mould encased in plaster; the two halves of the mould can be easily separated and pulled away from the artists’ original without causing damage.
This completed empty rubber mould is a ‘negative’ of the artists original sculpture (the ‘positive’). It is the new master from which all copies in the edition will be made.
STEP 3
MAKE A HOLLOW WAX REPLICA – THE SECOND ‘POSITIVE’
The next step is to create a duplicate of the original sculpture, this time in wax, which becomes the second ‘positive’ form.
To do this, the interior of the rubber mould is either painted with molten wax or the halves of the rubber mould are put together and molten wax is poured into the rubber mould and then poured out again once the interior of the mould is evenly coated with wax.
More molten wax is poured into the assembled mould several times until an even thickness of approximately ¼” of wax is achieved and the excess wax is poured out. Once the wax is cooled, the rubber mould is removed to reveal the wax replica. The rubber mould is then stored safely ready to make another wax replica for the edition.
This process results in the production of a wax replica (the second ‘positive’) of the original sculpture (the first ‘positive’). At this point the artist will usually inspect the wax model and correct any imperfections, which can take many hours, in order to ensure the wax replica is a faithful reproduction of the original sculpture. At this stage, a unique edition number is incised into the wax of the replica.
For each sculpture cast in the edition, a separate wax replica is made. For example, in an edition of 25 sculptures, 25 separate wax replicas will be made from the master rubber mould. Once the final 25th bronze has been completed, the master rubber mould is destroyed to ensure the integrity of the limited edition.
STEP 4
CREATE A SECOND ‘NEGATIVE’ MOULD
The wax replica is next sprued and gated. This means attaching a wax funnel (pouring cup) and wax rods, known as sprues, to the wax replica. These will provide a path of entry for the liquid molten metal and allow trapped air to escape, ensuring an even flow of metal to all parts of the final mould during the pouring process.
The wax replica complete with funnel and sprues is next dipped into a heat resistant liquid clay solution (slurry) and then coated with a very fine heat resistant sand which will provide detail to the mould. Once dry this process is repeated 6 to 12 times using coarser sand to give the mould strength. Each layer has to be dry before the next application of clay slurry and sand, until the mould is approximately ¼” thick. This strong ceramic/sand shell (the investment) over the wax replica ‘positive’ will form the second ‘negative’ mould.
STEP 5
LOSING THE WAX & POURING THE BRONZE
Once the ceramic and sand shell covering the wax replica is completely dry it is turned upside down and fired in a kiln at 1500-1800 degrees Fahrenheit to make it strong. It is during the firing process that the wax replica melts and pours out of the ceramic shell/mould, hence the term ‘lost wax’.
This heating process strengthens the now empty ceramic/sand mould (the ‘second negative’) which is then placed in a sand pit with the pouring cup facing upper most.
Molten bronze is poured into the cup of the mould and down through the sprues, filling the empty cavities.
STEP 6
FINISHING THE FINAL ‘POSITIVE’ SCULPTURE
As the bronze cools it solidifies and the clay/sand mould is broken open and discarded. This leaves the final ‘positive’ bronze sculpture with the bronze rods created by the sprues attached (see image).
The bronze sprue rods are then cut off, and the surface texture where they have been removed is recreated. Any remnants of the ceramic/sand mould that are left are sand blasted away. If a sculpture has been cast in several parts, they are now welded together. The surface of the bronze is reworked to resemble the surface of the original artists’ sculpture, a term known as metal chasing, which may take many hours of work.
STEP 7:
PATINATION
The final process is to colour the surface of the bronze to create the patina. This mimics the oxidisation process that occurs naturally over many years, resulting in the bronze changing colour. Modern processes can quickly create very subtle effects or dramatic colour transformations. Various chemicals are brushed or sprayed onto the bronze and heat may also be applied to achieve the patina.
Finally, after all these complex steps, the bronze sculpture is complete.