Cold Casting with Metal and Resins
The traditional bronze cast sculpture is expensive. The British firm of Alex Tiranti in 1959 developed a cold casting process to achieve the same looking result as a furnace produced article, at a fraction of the cost. The process consisted of coating the inside of a rubber mould with a metal filled gel coat of resin and then backing up or completely filling the mould with a resin containing a lower cost filler.
What Metals to use
Although bronze metal flake powders are often used to give coloured effects to resin castings, a true metal powder must be used to give a realistic metal cast effect in cold casting. Among the metals used in this process are Bronze, aluminium, copper, brass and iron usually of a 200 to 300 mesh size in irregular shaped particles (not flake).
These metals can be treated with chemical patina solutions to give true aged metal appearance. Being actual metals, colours will vary from batch to batch due to oxidation, so sufficient should be purchased to complete a particular cold casting job.
How much metal is required
The resin/metal mix should be as high as possible. Too little metal will give a very plastic appearance. An ideal metal to resin ration would be equal parts by volume. We add slightly more metal filler to make sure we have enough to yield a mix that is still pourable or brush able. The temperature of your working area is very important. At lower temperatures the resin mix takes longer to go off ad is much thicker than usual and so will not accept as much metal.
The usual amount of catalyst to be used with cold casting resins is 2 to 3% except with aluminium which usually only requires 1 to 2%.
Warning: Safety requirements
Always wear a face mask to prevent the inhalation of dust from the metal fillers. Aluminium powder can form dust clouds when being used which can form an explosive mix. Aluminium can also have a dangerous interaction with water. Always mix the resin with the catalyst first before adding the metal powder as the metal can react with the liquid hardener if used alone.
What colours are possible
Metal fillers can be mixed together to give colour variations. A lifeless bronze can be given a brighter look by adding a little copper. In reverse, a too striking copper can be toned down by adding a little bronze.
Project example: Cold Casting a Bust
What type of mould to use
Your bust mould may be made from any one of a number of rubbers and plasters. Silicone is highly recommended if you are going to make many cold castings from the same mould. Polyurethane is suitable for most uses, while plaster can be used for rigid articles with no undercuts. Latex is not suitable for metal/resin castings as it is not compatible with brass, bronze or copper, however it is fine for short run cold castings.
Your mould may be a one or two piece mould. If it is a plaster mould it should be completely dry. One coat of Release agent should be applied and let dry. Silicone moulds do not need a release agent. Polyurethane moulds should have the appropriate release agent applied.
How much metal to how much resin
The use of a clear casting resin is recommended as the resin colour affects the final outcome. It is important to use sufficient metal powder to give a truly metallic appearance to the final product. Too little metal and you will lose the metallic look or the metal will settle to the bottom of the mould. This is especially noticeable with low viscosity polyurethane resins. On the other hand too much metal is costly without adding the metallic effect. Regardless of which metal is used, the minimum amount of metal powder that can be used effectively is one part of metal powder to two parts of resin by volume. For a lot of our cold casting work we use equal parts of metal and resin by volume to produce a result that is indistinguishable from an actual metal article. We suggest that you experiment to determine the ratio of metal to resin that gives you the finished product that you require.
How to Mix
Stir the catalyst and resin together in a disposable paper cup using a wooden stirrer or tongue depressor. Then add the metal powder and stir till thoroughly mixed.
How to pour your mould
This mixture is then poured into the one-piece mould to yield a thickness of about 10-20mm thickness. This is achieved by placing a piece of laminated wood coated with mould release over the opening in the base of the mould after the resin/metal mixture has been poured into the mould, and then rotating the mould in all direction to ensure adequate coverage of the inner mould surface.
Pouring techniques
After 1 to 4 hours, when the bronze face coat is leathery, but has not yet hardened a low cost backup material can be poured into the mould to fill it. This can take many forms, from a silica filled resin, polyurethane foam, plaster, cement or iron loaded resin for weight. Another method would be to back up the bronze face coat with fibre glass and resin to yield a lightweight but very strong bronze faced cold casting.
De-moulding
When cured, de-mould and rub the surface with steel wool to bring up the highlights. Patina may be added with a black or dark oil based spray or brush on paint. Rub off the high spots with a cloth, leaving the paint in the texture and when it has dried, burnish with steel wool and wax if desired.
By Stan Alderson