Once the initial scenery shapes are in place, they can be enhanced with the addition of rock castings. Look around you as you travel, and you will see outcroppings of rock in many places, especially where a railroad or highway has been placed through a hillside by means of a cut. The cut levels out steep grades, and occurs where it is cheaper to cut open the hillside rather than build a more expensive tunnel. Cuts also occur along the edges of hills to allow the railroad or highway to pass around it. Of course, exposed rock also occurs naturally, where water or wind erosion has stripped the softer soil away from the harder rock.
The use of rock castings adds an extra visual effect and, with practice, you will find them easy to use. The heart of the rock casting is the rock mold. You can make your own molds by using latex rubber, medical gauze, a parting agent, and any nearby friendly rock as a model. The rubber is used to capture the detail of the rock, the gauze reinforces the rubber, and the parting agent allows you to remove the new mold from the rock. I have never made my own rock castings, but those who have assure me that the process is simple and easy. Early on, I chose to buy commercially made rock molds made of the much more durable vinyl. Although the process is faster with several molds, one mold can do the job on many layouts.
To begin the casting process, mix up a small amount of plaster of Paris. It helps to have an atomizer on hand, filled with water. The mold is sprayed lightly with the water mist, and the plaster of Paris solution is poured into the mold. If the top of the plaster of Paris solution has clear water on it at any point, pour it off. Any water that appears on the top can also be wicked away with paper towels
Keep a casual eye on the mold as the plaster of Paris starts to set up, occasionally picking the mold up and flexing it gently. If the plaster of Paris is still liquid, it is not ready yet. When the plaster of Paris acts like cake batter and forms small cracks that open and close as you flex the mold, it is ready. If the plaster of Paris is rigid, then it is probably too late to use the casting, so dispose of it and start again.
Using your water bottle, dampen the area where the mold will be placed. This allows the rock casting to set up properly. If you apply the mold to a relatively dry plaster of Paris hill form, the moisture of the rock casting will be drawn out, causing the casting to be weak and crumbly.
The plaster of Paris-filled mold is applied to the hillside and allowed to gently follow the form of the hill. Any material that oozes out from the side of the mold can be spread with a finger. It is a good idea to leave the mold on for a bit, so that the casting will set up properly and also adhere to the hill form. After a while, gently peel the rock mold away from the casting. You may lose some small features when removing it, but most of the casting should stay in place. Any raw edges can have detail carved in them with a knife. While the plaster of Paris is relatively soft, you can also lightly carve strata of rock to help tie two castings together. Carving after the casting has set up is very difficult, so you need to work quickly. Clean the mold with water between pourings so that the next casting will have the same details.
If you are using two-part foam to produce scenery, you can use rock molds, too. The process is similar, with the mold first being treated with the parting agent rather than with water, and then being filled with the liquid foam. The casting is then applied, with the foam still liquid, to the hillsides made by the earlier applications of two-part foam over the Styrofoam® sub-base, forming a good bond. Once the foam has cured, the parting agent allows you to remove the mold from the casting.
As mentioned earlier, you can use just one mold to do your casting work. To avoid unnatural monotony, rotate the casting so that its pattern is not readily apparent. Castings can also overlap each other to provide one long, continuous rock outcropping.
Painting the Rock Castings
The plaster of Paris or foam has captured the texture of the rocks, but not the color. Now that you have placed the rock castings, it is time to make them look more realistic. You can use ordinary model paints, but please keep in mind that since very few things in nature are shiny, glossy model paints are out. Besides, it’s more fun to make a trip to an art supply store to buy acrylic tube paints.
To begin, you will probably want to buy tubes of the following colors: raw sienna, burnt sienna, burnt umber, raw umber, titanium white, and gray. A couple of inexpensive brushes are necessary, too, along with something called matte medium (matte is art talk for flat) and, if you will be representing water, gloss medium. The matte medium will be used later to attach the scenic ground cover to the hills you have made, as well as to attach the track ballast. You can substitute a white glue such as Elmer's, but it is glossy when it dries, rather than flat like the matte medium.
Start by spraying a thin wash of India ink on the rock castings. The wash is created by adding a small amount of India ink to a lot of water. India ink is very strong, so a little goes a very long way. Here again, an inexpensive atomizing bottle, like those that spray liquid window cleaners, is very helpful. Note how the thin wash of India ink brings out the details of the rock castings by accumulating in cracks and crevices, emphasizing the details. Experiment with different colors of paint to find the results you want. The general idea is to brush a small amount of the pigments onto the rock castings, and then use the water in the spray bottle and a brush to wash the color out into the castings. Allowing the white color of the basic casting and the India ink wash to show through the other pigments will make them look real, while too much pigment ultimately hides the details.
Once the colors have been applied to your satisfaction, lightly streak the white pigment over the high points of the rocks. The technique is called dry brushing, and a very light touch is called for here. The point of the brush is dipped into the white pigment, then brushed on a piece of scrap until most of the paint has been removed. By lightly passing the brush over the tips of the rocks, tiny flecks of white are left on the rock to highlight the details.