Q: How does one go about making sculpture in “cast marble” or “bonded marble?”
A: These are not materials per se, but rather marketing terms for an effect that can be created using various materials. Its advantage is the ability to be cast in a mold rather than needing to be carved like real marble.
It is most commonly made from polyester resin, with some whiting (calcium carbonate) thrown in as a filler. Different resin systems can also be used, such as epoxy or polyurethane, as well as different fillers, like silica flour or glass microballoons. Test a small quantity of your specific ingredients in the appropriate proportions before you mix a huge batch, to make sure the resin is actually compatible with whatever filler you decide to use.
Be warned that all these resins are more or less toxic, and take appropriate precautions—or better yet, avoid them altogether. After all, they don’t compare to the real thing; when you use this stuff you are making sculpture in plastic, not stone. You don’t say what you’re trying to make, nor where it will have to exist, but the closest thing to real marble you can pour is a mixture of white portland cement and crushed marble. The composition of the cement is essentially the same as that of limestone, first cousin to marble, and the two materials will erode at about the same rate. This makes it possible to polish the casting after it has cured, using the same methods as a stone sculptor, revealing the particles of stone. The late San Francisco sculptor Benjamino Bufano used this adaptation of the terrazzo technique quite a bit, and the pieces he created this way have weathered the decades fairly successfully outdoors.
Another material that will tolerate outdoor exposure is called “Forton MG”. It is a gypsum (plaster of Paris) based system that can be extended with aggregates to simulate stone fairly well. It is less toxic than the resin-based systems, and easier to work with than concrete. For indoor use, white Hydrocal works okay and is even easier and cheaper. You can get this, and most of the other materials mentioned above, at Douglas and Sturgess in San Francisco (1-888-ART-STUF) or a sculptor’s supply closer to you. But if you really want a marble sculpture; you should start with a piece of stone and carve it. Making sculpture in plastic, concrete, or plaster and calling it “[something]-marble” is confusing at best; deceptive at worst.