Cold casting refers to casting a mixture of urethane resins and metal powder, such as bronze, in a mold, resulting in a sculpture or ornament that resembles a solid metal object. It differs from casting molten metal, which requires a high heat level and is usually done in a foundry in that the metal is never heated and is rendered fluid by its mixture with urethane. Cold casting has a cost advantage over foundry casting. It can also be done in a home shop or studio without specialized equipment.
Treat the molds with a release agent to prevent the urethane from sticking to the mold. Commercial release agents are available in a spray form. Use a brush to work the release agent into any crevices or details of the mold. Allow to dry according to label instructions before continuing.
Mix the urethane and metal mixture according to instructions. The urethane usually comes in two parts which begin to set after mixing. Most products call for the mixture of one of the resins with the metal powder prior to adding the second part of the resin. The ratio of urethane to metal powder can be varied for artistic preference, although Sculpt.com recommends a ratio of 100 grams of the first part of the urethane to 900 grams of bronze powder. After thoroughly mixing, add 100 grams of the other part of the urethane.
Pour the mixture into the mold. The mold is rotated and turned, known as slush casting, so the bronze and urethane mix adheres to all portions of the mold. The intent is to create a hollow cast sculpture within the mold.
Add urethane mixture, without the bronze powder, to the interior of the sculpture. This produces a cost saving by reducing the amount of metal powder necessary for the statue or sculpture. Allow the sculpture to dry as recommended by the urethane manufacturer.
Remove the mold from the sculpture. Use steel wool to polish produce a metallic sheen. Clear finishes or colored patinas can be added as desired.
Add some lead beads to the urethane mix used in the center of the sculpture. This adds some weight to the piece giving it more of the impression of a solid cast bronze sculpture.