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I recently attempted a paper mache tunnell/mountain, and thought it was very dry but three days later it was moldy so I tossed it.

Now I want to attempt something with what the ligjtweight chalk like substance. I had a statue made of this that fell and broke some time ago. It weighed very little. When it broke, it was used for sidewalk chalk for a long time as it seemed to be chalk or very much made of calcium carbonate. What is this stuff called? Can it be made hollow with no underside using a cardboard form? How would I then remove the cardboard?

So I have an offset helix in place using some home made trestles on roadbed track, and thought I could just cover the trestles , up to the bottom side of the roadbed track with the chalk like substance. This would give me a perfect fingerprint for where the track should lie. I do not want to cement the track in place , instead I want it lying on the finished structure. Because it is an offset helix there is no track layer directly above another track layer where I want to do this.

I am looking for advice as to what the chalk like substance is called and how to best form the tunnell/mountain to the existing helix.
 

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I’m not exactly sure what the material is that you’re talking about. There is a light weight plaster that I’ve used that sounds similar. I think it’s brand name is hydrocal. I’ve used it for forming rock castings, but not for major structural forms. My recommendation is to use extruded foam insulation sheets to make your underlying mountain structure. It’s very light, yet strong. It can be shaped easily by a rasp. It‘s worked well for me on my layout. I used drywall mud over the top of the foam to smooth it out.

- Mark
 

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Yeah, you get a swing and a miss from me, too. It sounds like either a solid block of talc, or a lightweight Hydrocal plaster. While both of these are good for making fairly small structures or landforms, neither of them would have enough strength to make a major structural piece like a mountain / tunnel.

So, first things first: if you got mold growth, then the humidity in your train room is too high. Mold requires relative humidities above 65% to grow. Plaster will grow mold too. I'd get a dehumidifier in the room.

Anyway, there are a lot of options here. To give you a quick summary:
1) Hardshell. This technique involves making a framework of some kind (cardboard strips, window screen, wood lathes, etc) or using newspaper "pillows" to create the shape you want, These are covered with plaster cloth or paper towels, (or even used dryer sheets) dipped in plaster. When this is dry, most modelers will add a thin layer of plaster, drywall mud, or Sculptamold (a commercial art product made of plaster powder, powdered glue, and shredded newspaper) to make a smooth surface. Some people use " Ground Goop", a mix of Celluclay (another commercial art product), fine vermiculite, and glue, with paint and water added to get the desired color and texture).
2) Foam: not beadboard, but extruded insulating panels or florist's foam. Build it up in blocks / layers, then carve into the rough shape desired. Cover with plaster, Sculptamold, drywall mud, or Ground Goop.
3) A Woodland Scenics product called Shaper Sheet. Basically heavy metal foil with a napped fabric on one side, you form it into the shape you want (it is self-supporting), and cover the fabric with plaster or one of the other products listed above. It can be cut, if necessary, with ordinary scissors.

All of these have their advantages and disadvantages, but they all work well. Shaper Sheet can get expensive in large volumes, but it works really well. I made a tunnel out of it for my son's layout in about 15 minutes (I used my arm to create the arch of the bore), then another hour to build a 2' tall mountain over that corner of the layout.

Good luck!
 
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