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The first step is to find a rock with suitable strata.
(Strata - the lines or detail of the rock.) The idea is to find a rock where
the details aren't too big for your scale. For HO scale a lump of coal
makes some of the best molds. I also created some of my best molds from
layers of compacted dirt. The detail there was amazing. It really
helps to have rocks from the area where you are modeling. If you are
making a mold from a piece of limestone it will not look like granite when you
make a mold. Research the area you are modeling to find out what kinds of
rocks are common in the area. If possible, go to the area and find
samples. The more you have seen the area and the more color pictures you
have the better your created rocks will look. I model southwestern
Colorado. The types of rock there changes by elevation. With that in
mind the strata will change the higher I model and the color will change as
well.
Once a suitable original has been located you will paint several
layers of latex rubber onto to to create the mold. This rubber will not
stain or harm the original in any way. Woodland Scenics and several others
have the latex rubber product that you can get from your hobby shop. If
you are using a rock or something besides compacted dirt, wash the original to
remove any loose dirt. Using a CHEAP paint brush (the 12 for a dollar
stuff you give your kids) paint on a coat of the rubber to the original.
Allow this to dry completely. Once the first coat has dried paint
on a second and, again, allow it to dry completely.
For the third coat you will want something to reinforce the
mold. Some people use gauze from a first aid kit. I like using my
wife's runnered panty hose. She was going to throw them out anyway...waste
not...want not. If you don't have any on hand it's cheaper to buy the
gauze. Paint on a coat of rubber and then press the hose or gauze
into the rubber. Then cover with more rubber on top of the reinforcement
material. This will create one think coat that completely encloses the
reinforcement material. This will keep the mold from tearing as you flex
it around.
Once the third coat has dried completely you are ready to remove
the mold from the original. Wash any dirt from the mold and you are ready
to make mountains. Do not use cast resins in molds made this way.
The heat created by the resin's curing process tends to cook the mold.
This I have learned the hard way. I have several of these molds that I
created in the late 1980's that are still just as good as they were when I first
made them. Store the molds in a cool and preferable dark location when not
being used.
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