"You promised to explain how to use the flat RTV mold of a stone wall to make a curved liner to match my tunnel portal. I'm still waiting."

Oh. Sorry, young Cobweasel. Here is a method you may use to curve thin castings. It works either with the face on the inside (as in the underside of arches or concave retaining walls) or with the face on the outside (as in convex retaining walls, bridge supports, and castle towers). For both the castings themselves and to weld the parts together, we will need more of the same material we used to cast the initial tunnel portal: Either the brown, fast-setting grout (Quickset) or the white gypsum cement (Hydrocal).

The only power tool you'll need for the project is a saber saw. The only convenient high-tech hand tool would be a rubber bucket. Once again, let the pictures tell the story. Is the projector ready, Uncle Russ? Will someone please get the lights...?

1. First we will cast the curved ceiling of the tunnel. We begin with our old friend, the basic, flat RTV wall mold. We take advantage of its flexibility by bending it over a curved form. Cut 3 or 4 curved-top boards to support it (face-side out) and screw them to a base. Be sure the width of the boards and diameter of the top equals the width of the tunnel minus twice the thickness of the rubber mold.

2. Drape the mold over the form and clamp, tie, or tape it solidly. Then spread newspapers all around. It may help to tilt the assembly first to one side then to the other as you apply cement by the handful. Give it plenty of time to cure completely and even to dry out just a little to be sure it is at its maximum strength. If it should break at all it would be when you remove it from the mold. Unscrew and remove the support boards carefully.

Finally, divide one more flat slab in half horizontally and make a casting of each half. The two pieces will be the tunnel sides.

2. For the two side wings, cut a stick to fit diagonally across the mold. The example in the photo is stepped. You should paint any wood that touches cement with a coat of fast-drying sealer (for example, Kilz). Rapid-setting grout sticks more tenaciously than portland or gypsum cements, so brush everything with a soap solution. Position the wing castings against the tunnel portal and "weld" them together with a few handsful of freshly mixed casting material. Pile it extra thickly in the back where it won't show. Have enough patience to let it cure completely.

3. The tunnel's straight walls are "welded" to the ceiling arch in the same way as the wings join the portal face. With a longer tunnel, you could join more liner segments together but, in practice, you would never see them; a wooden box would work as well.

4. One last step before final assembly: Fill any crack visible behind the "weld" using the same cement. After setting in place on the layout, but before back-filling with dirt, "weld" the liner to the portal to prevent separation.

CONCLUSION

If you were to make only one RTV mold for most of your cast accessories, a shallow pan mold of a stone wall surface would be the easiest and the most basic. Out of just one mold can come a variety of shapes you could use separately or in assemblies. The ultimate use could be for casting inexpensive plaster or gypsum "lumber" to build other, three-dimensional models such as piers, buttresses, bridges, viaducts, overpasses, and structure walls. You could cast everything in stone. After all, a good quality mortar mix costs only about five cents a pound.

SOAP: A MOLD RELEASE FOR ALL REASONS

It is cheap, environmentally safe, and easily washes off castings.

Dissolve one cup of Ivory Snow in two cups of hot water. Let stand until cool. Heat to melting point in microwave before brushing on. It is liquid when hot but becomes a protective gel when cold, separating the mold from the cement, and helping in mold clean-up between castings.