Just got back home from my favorite hobby store, got myself a cold drink and tried to recover from a mild case of sticker shock. The kits for molding rock surfaces seem to me to be rather pricey. My method is to flatten out a large chunk of modelers clay, select the rock I want to dulicate, then either simply press that rock into the clay surface as far as needed to copy the rock surface, or roll the rock along the clay surface to create larger surfaces. Then using the materail of your choice, I use ordinary patching plaster, mix and pour the plaster into the mold and let it cure. Once cured the plaster cast can be painted to suit your needs.
Most people in here who make their own molds use liquid latex.
The problem with your method is that you will lose a lot of fine detail when the clay sticks to the rock and then after casting you will have to spend a considerable amount of time getting the clay out of the crevices in the casting. I agree with the previous post - use liquid latex. It gives very good molds and they can be re-used.
Joe
Just my opinion, I think I paid $6.00 a piece for rock molds and they are still in good shape after making hundreds of pieces. A lot easier, less time consuming, and the woodland scenics molds are flexable and durable and last a long time. All I have to do is clean them with water between casts. And if I want just a part of the mold it’s no problem, I don’t have to fill the mold just to get the face piece I need.
Unless you are going to carve the rock faces dirrectly into styrofoam, the rubber rock molds will turn out to be the least expensive of a large mountain. I have learned to like carveing foam, but mst still like the mold approach.
I have had a set of Color Rite rock molds for about 20 years; they have outlasted 3 layouts. I like these better than the Woodland Scenics molds bescuse they are more flexible and better for molding directly on the hardshell surface. The current list price is $5.50 each, and that is good value for money.
This is a Walthers search page on rock molds:
A number of years ago I made a set of rock molds using a two-part flexible mold material. They were made by painting the mold material on the surface of large pieces of coal. After the second or third coat of mold material was painted on, I embedded some cheese cloth in the mold by laying it on the surface and painting two more coats of the mold material over it. This was done to strengthen the mold and prevent it from tearing. The microstructure of the coal surface has the appearance of a granite or limestone rock face. These days you might have a problem finding a source of large pieces of coal since most coal that is used now is of the small variety. It might be easier and more economical to just purchase some of the commercially available molds that have been mentioned in the above posts.
Bob