Q: I’m making models in clay that have lots of undercuts, and would like to take molds of them so I can make permanent sculptures in plaster or polyester resin. What is the cheapest thing I can use that won’t tear up the originals too badly?
A: You don’t mention what you mean by “clay”. Would that be potter’s clay (waterbase) or plastiline (oil-base)? Mold materials work differently depending on what they are applied to. Silicone compounds, in particular, are sensitive to the sulphur found in most plastiline. Urethane rubber is cheaper, and works quite well for most applications. Natural latex is the cheapest, but is very tedious to apply, does not last well, and undercuts are a particular problem due to its thinness. Generally one doen’t expect to preserve an unfired clay model used as a pattern for mold-making.
Offhand, I would say that moulage is the best bet for what you propose. This is a compound that melts in a double-boiler and is applied warm (which can be a problem if your model is plastiline). But it is gentle to most patterns, and can be melted down and reused, so it is fairly economical. While it is reasonably firm, its toughness can be enhanced by embedding gauze in the mold wall, though this makes recycling a bit harder. You can make quick shells using melted wax and cloth that will help support the somewhat floppy moulage once it is stripped off your pattern. There are other compounds that work the same way, such as hide glue and polyvinyl acetate, but moulage smells the best, and goes on the coolest. I know it works well with plaster. In any case, I would advise a small-scale experiment to test the reactions between the mold compound you decide on and the clay you used, and between the mold and the polyester resin (which is really nasty stuff, but that’s another issue).