Q: I know there is a process to make plaster objects look like bronze. How is it done?
A: The first thing is to prime the plaster item with shellac, first using a light coat cut with an equal part of denatured alcohol, letting that dry, then applying a full-strength coat. This seals the plaster and prevents it from sucking the volatile components out of the subsequent layers of paint, interfering with their film formation. After the shellac is dry, put on an oil or lacquer based gold paint, either by brushing or spraying. There are various types of gold paint available, which vary widely in their effects. Then cover the whole thing with a dark brown colored oil paint, and wipe it off the high spots while it is still wet. This process can be repeated several times, until the “antiquing” seems about right. Optionally, one can go for a “verdigris” effect, using shades of green oil paint over the brown once the brown has dried, rubbing it off the high spots just a little. Paste wax can be used over the surface for protection and glossiness.
There are other versions of this process which depend on coatings containing copper or bronze powder. These can be treated with chemicals to obtain an actual patina, either greens or shades of brown. Another approach is to add atomized copper or brass to certain formulations of gypsum cement, in sufficient concentration to make the surface somewhat metallic. This can then be patinated in a similar way. Or, alternatively, one can paint the plaster item with a conductive solution, then electroplate it with copper. This can then be treated with chemicals to achieve various color effects.