Bronze resin, also known as cold cast bronze, is made from a mixture of bronze powder combined with polyester, epoxy or other resin, making the materials for creating sculptures much cheaper than bronze.
There are also fewer, less complex stages involved in the casting of bronze resin, which makes the whole process much simpler and quicker than using the ‘lost wax method’ employed in the hot casting method. Ultimately bronze resin closely resembles the appearance of hot cast bronze at a fraction of the price.
Bronze resin sculptures will last a long time if looked after. However, although strong, they do not have the ultimate strength and durability of bronze. They are generally frost proof but not shatter proof. In extreme weather conditions, when water may pool in areas of the sculpture and then freeze, they are susceptible to the effects of expansion and contraction which may result in damage such as shattering.
The cold cast method is not just limited to bronze; many finely ground materials such as zinc, copper, iron and stone such as marble can also be used to create a bonded casting.
HOW ARE BRONZE RESIN (COLD CAST BRONZE) SCULPTURES MADE
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 will be made.
STEP 3 CASTING THE SCULPTURE - THE FINAL ‘POSITIVE’
The next step is to coat the inside of the silicone rubber mould by partly filling it with a liquid mixture of bronze powder and resin, rotating the mould to ensure an even coverage and to remove any air bubbles. Once hardened, this layer can be thickened by adding more of this mixture in additional pours
A solid cast can be achieved by adding a filler material such as resin or resin combined with stone dust, to give the sculpture more strength. Alternatively, the first coat may be backed up by laminating it with resin and fibre glass. With both approaches, the sculpture may require reinforcing with an internal metal structure.
STEP 4 FINISHING PROCESS
The final sculpture is then finished by hand, in order to hide any seam-lines left where the two halves of the mould came together and to reinstate any marks or texture lost during the casting process, often using grinders and possibly infilling with a bronze powder and resin slurry.
The bronze finish is brought to life by carefully polishing the surface of the sculpture, being careful not to over-polish which could result in the thin layer of bronze being worn away. Following this process, a protective layer of wax may be applied.
Due to real bronze powder being used on the surface of the sculpture, it is possible to colour (patinate) the sculpture to help mimic the effect of foundry hot cast ‘lost wax’ bronzes. Various chemicals are brushed or sprayed onto the sculpture to achieve the patina, to create very subtle effects or dramatic colour transformations.
The Step 2 ‘negative’ rubber mould is used for casting all the bronze resin sculptures, whether it is an Open Edition (Unlimited Run) or a Limited Edition (Limited Run), making it a much more economical process.
In the case of a cold cast Limited Edition, the ‘negative’ mould should be destroyed once the chosen number of the Limited Edition has been cast e.g. with a Limited Edition of 50, then 50 sculptures will be created from the master rubber mould. Once the final 50th cold cast bronze has been completed, the master rubber mould is destroyed to ensure the integrity of the limited edition.