Plaster of Paris

Making Hills with Plaster of Paris

This technique goes back many years in the model railroad hobby.  Earlier, papier-mâché was used, laminations of paper soaked in wheat paste spread over a form of chicken wire or screen wire. But the wheat paste attracted insects and was soon replaced with the plaster of Paris approach.  The Woodland Scenics SubTerrain System utilizes some of the techniques described here.

There are different ways to construct the hills and valleys that make up the slice of geography you are creating. I admit to being a plaster of Paris person myself, since the materials are commonly available and clean up fairly quickly.  Paper towels soaked in plaster of Paris, as discussed later in this chapter, are laid over a form, usually balled up newspapers and masking tape, but screen wire can also be used.

You will often hear references to a material called Hydrocal®.  This product is a brand of gypsum-based plaster of Paris produced by U. S. Gypsum. It is stronger than ordinary plaster of Paris, but it may be more difficult to obtain.  Ordinary plaster of Paris can usually be purchased from a hardware store. If either material has been exposed to excessive humidity, it will not set properly. Some brands of plaster of Paris are packaged in containers similar to milk cartons, sealed against such moisture; these are preferable since they will stay fresh longer, even after opening.

After you have marked the general location of the hills with a pencil, you should put backing forms for retaining walls and tunnel portals into position.  These forms, cut out of Masonite or thin wood, are used to support not only such scenery details as tunnel portals, but also the edges of the plaster of Paris hills. The backing pieces allow you to shape the scenery contours without having to mask the detail parts. Forming the contours with either plaster of Paris is a messy process, so keeping pieces such as detailed retaining walls and tunnel portals out of the way is desirable.  When installing the backing, take care to allow proper clearance for the final detail. The backing should be glued or screwed to the layout board, and once in position, the backing can be used as a support for the scenery.  After the basic forms of the hills are established, add these detailed parts to the backing pieces.

The forms of the hills are made by balling up newspaper and holding it in place with masking tape. The newspaper provides the initial support for the scenery layers.  These layers are made by mixing up a solution of plaster of Paris and water.  Use inexpensive mixing containers such as a plastic mixing bowl and two cups for measuring. Mark one of the cups Dry and the other Water to help avoid unpleasant surprises. Following the instructions on the package, put measures of water first, then plaster of Paris, into the mixing bowl. Mix only enough for a few minutes of work, since large batches will set before much can be used. 

Mixing the plaster of Paris with water starts a chemical reaction; adding extra water after the reaction has begun does not slow it; but adding additional dry plaster of Paris accelerates this reaction.  After the plaster of Paris has been evenly dampened, paper towels are dipped into the mixture and laid over the balled up newspaper forms. Because of the weight of the wet towels, the newspaper will compress somewhat (something to keep in mind while setting the initial lumps of newspaper in place). Gradually, work your way around the layout, laying out segments of the plaster of Paris-saturated paper towels. In all, each section of the hills should have three layers of towels.  Once dried, you will notice that these forms are surprisingly strong. The completed form is self-supporting, and many cases, the newspapers can be removed, leaving a hollow shell. You will find that each layer can be applied after the earlier layer has had a few minutes to set up.  The whole process goes rather quickly. If you are planning to remove the balled up newspaper inside the hillside, wait overnight until the plaster of Paris has had time to dry thoroughly.

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