Where do I start?
You probably already decided whether you want to cast your wall in plaster, gypsum cement (such as Hydrocal), or portland cement (mortar mix) and how you will color it. None of that matters to the mold. Only the number of castings matters. For now, let's assume you will cast more than two copies. You'll need a pattern, a permanent flexible mold, and a mold box, as in Figure 1.

What can you tell me about the pattern?
You will want it suitable for the style and period of your layout. Then you should decide what material to represent. The easiest way to make such a surface is with generic textured sheets. Noch and Plastruct offer a variety of brick, cement block, stone block, random stone, wood plant, or form-poured concrete sheets. If you prefer to duplicate a specific era or style, and you have the patience, try building your own wall with little bricks or blocks of plaster, or cut yourself scale timbers, or embed pea gravel or 1/2-inch crushed rock in a layer of soupy plaster. Modeling and sculpting clay, even sugar cubes, will work. Remember you'll only use the pattern once, to make the mold. It deserves the utmost in accuracy but can use the most temporary of materials.

Carving lines in a slab of wet plaster or clay gives an unrealistic hollow face to each block, so for that method, wait for the plaster to harden or use a wire loop to remove the clay instead of pushing it upwards. Whatever you use, you will want the pattern to be 1/2- to 3/4-inch thick. That will be the thickness of the final casting. Since I found the Noch brand of textured sheets best complements my own European style layout, I standardized on Noch's panel width for all molds and mold boxes. They are 15 inches square. Yours may be any size to suit your needs, but 15 x 15 x 3/4 inches is a workable size for portland cement castings.

And for the flexible mold?
The two best choices are room temperature curing (RTV) silicone ($7.50 per pound) and urethane ($2.60 per pound).

Permanent heat-curing vinyl chloride plastisol ($2.40 per pound) and a family of re-usable hot melt thermoplastics ($4.50 to $5.00 per pound) exists for someone equipped to handle them. For our occasional hobby use, we must weigh the hazards and difficulties of melting or curing them safely at 350 degrees Fahrenheit. Besides, the hot plastics shrink as they cool and can cause a loss of detail and dimensional accuracy.

I avoid listing the latex mold materials of natural and neoprene rubber here. They require special skills, and possibly one of you readers will submit an article on their use.

Later I will describe how to use the less expensive seaweed gels, Alginate and Moulage, for casting only a couple of parts at a time, such as a pair of tunnel portals.

Okay, so my choices are down to silicone or urethane. And the winner is....
Hold on. You forgot to say whether you have more time or more money. If cost is no factor, go with silicone. It is reasonably tough, it will reproduce the finest detail even down to the fingerprints you leave on the pattern, and it sticks to absolutely nothing except itself and its first cousin, glass. Without the need for messy mold releases, your fine detail remains and the surfaces of the pattern and castings suffer no oil or grease contamination.

Urethane, on the other hand, bonds forever to everything it touches. It requires the extra step and nuisance of mold releases. But since urethane rubber costs only a third as much as silicone and is more tear resistant, I use it exclusively for all my masonry casting molds.

How much will a mold cost?
Now you're about to see why I mentioned the costs per pound. A flat, 15 inch square mold, such as the one in Figure 2, will weigh 8 to 10 pounds and will cost between $20 and $25 in urethane. In silicone it would cost between $60 and $75. By the way, those prices are all for 50 pound quantities from industrial sources. If you buy a pound at a time from a craft shop, you may expect the price to double or triple. As an extreme example, my own latest mold for a 50 foot, double track viaduct section required 45 pounds of urethane. Re-usable hot melts would certainly have saved money, but I have trouble destroying molds just for the rubber.

On to mold releases, please.
The first batch of urethane resin and curing agent you stir together will look to you as thick as cold honey and you may wonder how it will reproduce the surface detail of your pattern. Don't worry. While its viscosity may be high, its surface tension is more like that of gasoline. That means it will penetrate the most microscopic pores and cracks in your pattern until it reaches the gel stage (after two to four hours). When it gels, it permanently locks the pattern and mold together. It also will leak out of even the smallest joint in the mold box as you watch helplessly.

If the pattern has any porosity at all, seal it initially with two or more coats of a fast drying lacquer sealer. The next layer may be either a couple of absolutely void-free coats of polyvinyl alcohol (PVA) you should allow to dry hard and pull down tightly onto the surface, or a coat of thick, sticky grease-like mold release. The PVA gives the best surface but I prefer to sacrifice some detail and use the grease. It assures a 95-percent good release of the cured polyurethane rubber mold from its pattern and mold box.

As for the box itself, if you will only use it once to hold the liquid RTV, you need only seal it well with lacquer. But if it must support a floppy or three-dimensional mold, it deserves tight joints and a good primer/gloss enamel finish. If you build it so you may unscrew and remove one side, or if you hinge the sides, you'll be able to pull out the mold more easily.

What if, after all that, I used too little mold release and the rubber bonds everything together?
Do what any other skilled, mature craftsman would do. Cry.

After I have a flat rubber mold for a retaining wall, can I do anything else with it?
Now is when the fun really starts! First, bend it, as in Figure 3, for a curved wall. Or cast a plaster or Hydrocal slab and cut it with a saber saw for tunnel portals (Figures 4 and 5), sticking on arch bricks with a little fresh plaster. As Figure 6 suggests, if you combine simple elements, the potential is endless!

Go for it! Blow $10.50 on a hundred pound sack of casting plaster and start playing in earnest. By the next issue you should be ready for Advanced Patterns And Molds.

Where may I find more information and buy the materials?
First, call the publisher of the best book I have ever seen on the subject: Thurston James, The Prop Builder's Molding & Casting Handbook, Betterway Publications, Inc., P.O. Box 219, Crozet, VA 22932, (804)823-5661. Price in paperback, $19.95.

Then look in your Yellow Pages under Plastics-Raw Materials or call Hastings Plastics in Santa Monica, California (310)829-3449. They give good advice, sell in small quantities, and ship anywhere.