Photos by the author
Making rock outcroppings from plaster castings and rubber molds is not a new technique in model railroading. Nothing short of some really artistic hand carving can duplicate the myriad of facets and angles that occur in rocks, especially granite. But by taking a rubber mold from a real rock (coal works great for this) and then filling it with plaster can produce quite respectable results. However, the usual way of producing boulders, i.e., breaking up the dry plaster castings, does not often produce convincing loose rocks because the back side of the plaster casting is flat (see Photo 1) and devoid of rock detail. Of course, you could always make sure to glue the boulder onto the layout flat side down. But if you are trying to replicate a riprap embankment or a boulder field, checking each individual rock can be tedious or impossible. And using real rocks or pebbles in conjunction with plaster rock castings can be frustrating, especially when trying to match color and texture. So the best method is to have plaster outcrops and plaster boulders.
To make plaster boulders that are 3D and eliminate the smooth, flat side, just sandwich the wet plaster between two (or more) rubber rock molds! Any fast-setting plaster can be used. I prefer Hydrocal because it can reproduce most of the tiny cracks in a good rubber mold, and it sets very fast, so you can make lots of boulders in an afternoon or evening. If possible, try to use the same plaster as you use in your traditional rock casting methods.
Begin the process by selecting a bottom mold. You can either use your own molds or buy commercial ones. You need a mold that will lay flat. Then choose a top mold that roughly exhibits the same pattern as the bottom mold, e.g., dont mix a granite mold with a shale mold. You can use several smaller top molds to equal the size of the bottom mold.
Next, mix up some plaster. You want to make a mix that is thick, but still pourable. For Hydrocal, I use a three-to-two ratio, i.e., three parts of plaster to two parts of water. Only mix enough plaster to fill the area in the bottom mold that will be covered with the top mold(s). You can precolor the plaster if you want. I tend to color the boulders after they are completely dry. Always add plaster to water, not vice-versa, to prevent premature setting. I use my hands to thoroughly mix the plaster. Plaster will dry out your skin, but shouldnt pose a health risk. Use gloves if you are sensitive to dry skin.
After thoroughly mixing the dry plaster with water, immediately pour the mix into the bottom mold (Photo 2). Then wait a few minutes until the plaster just starts to become firm. If you are using Hydrocal, this time is about three minutes. Arrange the top molds so that they are handy and in the correct placement order as shown in Photo 3. Then press the top mold(s) into the setting plaster. Press down hard enough to cause some of the plaster around the edges to ooze out (see Photo 4).
Continue to press down until you begin to feel the plaster hardening. For Hydrocal, this is about seven to ten minutes. The plaster should be real firm, but not completely set. While waiting for the plaster to harden, remove and discard as much of the ooze as you can from around the edges of the molds. It should be easy to remove by just picking at it with your fingers. The plaster has set enough when you can take a piece of discard and snap it cleanly in your fingers. When that happens, with one hand under the bottom mold and one on the top, flex the mold sandwich several times to break up the still-setting plaster as illustrated in Photo 5. This step will fracture the plaster into hundreds of different sized pieces. If you cannot flex the casting sandwich, you waited too long.
Let the plaster set for another few minutes after the breaking process. Peel apart the rubber molds and let the plaster boulders fall out (Photo 6). Let the boulders dry for an additional ten to 15 minutes without handling them.
There will be some boulders that will have unnatural air pockets like the one shown in Photo 7. Unless you are modeling ropy lava, these bubbles just dont look natural and need to be removed. Use a carving tool (like a small wood chisel) to chip out these holes (Photo 8). If the boulder has too many dents, just discard it. Try not to handle the boulders any more than necessary or the crisp edges will round off.
Next let the boulders fully cure. For Hydrocal, a period of three or four days is usually sufficient. If you are able to feel any moisture in the boulders, then they are not yet ready to use. When they are dry, color or stain them using your favorite methods. Since these rocks are made from the same material as your rock castings, matching the colors of boulders and rock outcroppings is easy. Just be sure to color the boulders before gluing them to the layout or the glue will seal the plaster and cause uneven coloring. Photo 9 shows a boulder field on my layout.
By making a rubber mold sandwich with plaster as the stuffing, and then coloring the rocks that form, you can make some very effective boulders for your layout. The above technique works best to duplicate rocks like granite, but it can reproduce other rock types. Casting boulders is easy and fun!