Hey there folks,
Quick (maybe) question... So I've been unsuccessful finding any local supplier of Ultracal-30. Any SFX people I know order it online and pay an arm and leg for shipping because they operate out of shops that can afford that. Total cost isn't too bad I guess, but shipping costs about as much as the product itself. I have trouble justifying that to myself.
The best I can find locally has been Hydrocal C1201
The other option I have, which is what John Mathews used for the puppets in "Frog and Toad Are Friends", is casting resin
What you are looking for is what is known as dental stone or dental plaster, i.e. a high-density, high strength plaster that takes fine detailing. There are a number of different makes. I wouldn't worry about getting the specific grade of Ultracal. I've used Crystacal R, Herculite, Prestia and others.
Here is a page from my local supplier http://www.industrialplasters.com/cat/plasters/dental-plasters They also have a very useful brochure, where you can compare the properties of different plasters - scroll down the page and click on brochure.
The issue about fine details cracking is mostly relevant where you have e.g. a mouth cavity (protuberance on the mould) which needs supporting. Wire inserted at the time of casting should be enough to ensure this is OK. You could also use a small stick of wood, but be careful it doesn't float out.
Use fibre reinforcement after you have covered the sculpt adequately, not in the first layer of plaster, or you will get fibres protruding through the surface of the mould. They are there for holding the mould together, but don't worry if you forget them. When plaster moulds are baked for foam latex, they can become quite brittle, and for this the reinforcement is necessary. A cold process such as silicone does not stress the mould nearly as much. But if you drop the mould and it does not have fibres, then it may well come apart!
Coming from the UK i had a similar problem of buying this stuff without paying huge prices for the postage. I found (like Simon said above) that there are variations on the same sort of plaster which work brilliantly, and for the last 2 years have bought Crystacal R from a UK ebay shop which is very reasonably priced.
Also, I tend to seek out any free/cheap-as-chips alternatives to things, and one is using cut up bits of hessian (burlap in the US) instead of the fibreglass. Someone may be able to tell me differently, but it seems to make the mould as strong as fibreglass would, and you can often get it for free (asking at your local greengrocers etc).
Thanks for the tips and info everyone! Just the words "Dental Plaster" alone was enough to open up my options for ordering material online. There's so many different types and brands. I'm also able to purchase smaller quantities than 50 lb bags.
I'm going to do what Nick suggested and just use the dental plaster for my initial detail coat, and use the Hydrocal with some fiberglass or burlap for the outside "mother-mold" portion.
Thanks a bunch!
Hemp fiber also works quite well for a fiber reinforcement.