You cannot using the heat vulcanized materials that have been discussed on this list lately,,, BUT is a no problem issue making RTV (Room Temperature Vulcanizing) mold materials. These are liquids, use a catalyst to “make them go off or harden or set”. These RTV materials are of different types of rubber, urethane, silicon (both tin and platinum cured) as well as others. I use tin cured silicons for large sculpture pieces as the platinum cured materials are about double in price. Also the tin cured (they last an easy 10 years) have a greater range of hardness choices and are often most of the time, more tear resistant. Also platinum cured materials are FAY MORE easily contaminated than tin cured if one is using both types (I have separate areas I work in, my mixing tools are separate, any mold frames are separate, etc. My passed wife used heat vulcanized molds until we met and I started doing her bronze molds/castings and from then on we made molds directly from wax masters. She was happy not to have to make oversized pieces to account for the shrink in heat vulcanized molds that is often a problem.
If interested, I can make molds for you from either material, the cost of a ring mold is about $60.00 in tin and $90.00 in platinum. Also with either type of silicon, waxes pretty much release without any release agents. There are also silicon heat cured materials if you want to try that direction…. but these cannot be used to make molds from wax as they go int a vulcanizer just as the “regular” heat vulcanized materials do.
http://MLCE.net if you want to see the bronzes Cynthia (Thomas) did that I did the foundry work on.