I use Hydrocal, a powdered plaster that is WAY less expensive than hard, mold-making silcone. It's 3-5 times harder than plaster of paris, and brushing on two coats of glaze gave me a very glossy surface that releases well. Hydrocal is a bit tricky to mix, but if you search for it on YouTube, there are two EXCELLENT how-to videos on the process. (Follow it to the letter!) Bags of 5 lbs. to 25 lbs. are widely available on amazon.com; my first 5-lb. bag cost me just $15. Hope this helps