"Do flexible molds have to last forever? Does a reusable and relatively inexpensive mold material exist, good enough for just a couple of castings?"

Well, young Cobweasel, how about seaweed? Seriously, moulage is an old, established mold material for theatrical use. (Moulage is French for "mold material".) While it costs about as much as urethane resin, it is re-meltable and usable practically forever. It is a flabby substance and tears easily. That lets you make only a few castings from each mold, depending on the casting material you use. Fortunately, most tunnels have only two portals so, if you want to cast them from cement, moulage would a good choice for such an application.

Moulage makes such excellent impressions and is so easy to work with, it becomes a cost effective alternative to RTV polymer mold materials for simple castings in quantities of half a dozen or so.

Let's see how to use it. Uncle Russ, is the projector ready? Lights please....

1. Every new casting starts with a pattern. You may cut your pattern from a plaster slab of a scale stone surface you previously cast in a flexible mold. We covered the process in the last issue. Draw the shape of your tunnel portal on the plaster.

2. Plaster (but not gypsum cement) cuts about the same as balsa wood. Discard the portal opening but save the arch part. Cut it into individual stones. An alternate method would be to leave off the arch. Instead, daub on a band of plaster and carve each stone by hand.

3. Put the cut stone pieces into a can with a lid and shake them vigorously until you have suitably "aged" them. I have shown them here in a temporary arrangement, according to size and spacing.

4. Nothing sticks to plaster better than plaster. Mix it in small batches to hold the arch stones in place and discard it when it starts to become stiff (in about ten minutes). You may have to moisten the slab and stones.

5. The plaster pattern is finished. For a moulage mold you may leave off the sealer or mold release. If you had made it from gypsum cement, you could stain or paint the pattern itself and use it outdoors after you made copies.

You could also have cast the tunnel portal directly with cement. In that case, you would start with the basic flat mold but block out the portal opening with a piece of wood of the correct shape. After the cement hardens (in about three or four days), remove it from its mold and knock out the wood center. Of course, such a casting would have no arch detail.

6. Melt the moulage over water in a double boiler until it looks like smooth, overcooked oatmeal. Be sure to stir it constantly to keep it from sticking. It will be almost smooth and ready to pour by the time it reaches 150 degrees Fahrenheit. If it is too thick, just add water. It is already 80-percent water.

Advice: For domestic harmony, use a special pan for the moulage and cook it in a lot of fresh air. Hot moulage gives off an aroma reminiscent of fertile pools at low tide on a warm summer's day.

Order from Kryolan Corporation, 132 Ninth Street, San Francisco, CA 94103-2603. Phone: 415-863-9684.

7. Hot moulage may be slightly lumpy but that has no effect on its ability to make excellent impressions. The center core reduces the amount of moulage you need. The weight keeps it from floating. Moulage sticks to nothing, including itself. Consider it more as a firm pudding than as a polymer.

8. When it cools, it comes out of the mold easily. But it will tear easily unless it has completely solidified. It starts to dry and shrink as soon as you pour it. To keep it for a few days, wrap it in a wet cloth, then seal it in plastic food wrap and keep it in the refrigerator. It smells less when cold.

9. Portland cement would take three or four days to cure enough to remove from the mold. Meanwhile the mold itself would shrink and tear. Solve that problem with rapid-setting grout (also known as Quickset). It hardens in less than an hour, allowing for a second pour before the mold shrinks appreciably. All that necessary haste makes for a project you finish quickly.

10. The first casting is out and the mold is ready for the second pour of grout. The highly reactive, hot grout will destroy some mold detail after the second use but the second casting itself will be fine. Recycle the mold by cutting it into bits and storing it back in its tub.

11. Molds of the reusable material, moulage, are excellent for a few masonry or gypsum cement castings with fine detail. Either rapid-setting grout or gypsum cements offer hard, weather resistant castings in an hour's time. In the next issue, I'll show how to complete your mountain scene using the basic flat RTV mold to cast the matching curved tunnel liner and its side wings.