Making Rocks

A canyon requires rocks, and so rocks I've been making.

Molds

For the steep cuttings near the track I made a latex mold of a rock in the garden. This was a fairly smooth-faced rock and so hopefully will represent where the railroad has blasted and excavated away the rock.

After applying 3 layers of latex to the rock I peeled off the mold and prepared it for casting. To cast it I supported the mold on some card and spooned in some plaster. From the same batch of plaster I applied some to the vertical polystyrene surface where it was to go.

I kept a careful eye on the plaster, and as soon as it was no longer liquid (but still quite soft), I transported it to the site. Using the cardboard to support the mold I bent it to shape and pressed it onto the wall. I left the mold in place till it was dry, then peeled it off.

This worked very well for getting the texture and grain of rock onto a large area reasonably quickly. I repeated this again to extend the wall towards the tunnel.

What I forgot was that merely repeating the exact same section of rock would look very predictable, so while the plaster was still slightly soft I carved some additional cracks into it. Were I to do it again I would just rotate the mold 180 degrees.

Tunnel Portals

One thing that struck me while looking at photos of the Thompson River Canyon was the clearly visible lines on the tunnels from the boxing used during casting. To replicate this I found some grooved styrene, meant to represent the side of a building. However by using this as a mold surface I would get the reverse, with fine raised grooves over the surface of my casting.

So I made up some carefully measured molds and mixed up a high strength mix of plaster (PVA glue, water, and Hydrocal).

The bits of each side of the tunnel broke off while releasing the casting but these were easily reattached, and some balsa braces were added to strengthen it.

Rocks

That just leaves rocks. I tried using some left over castings, but they didn't do a very good job of replicating the long spines of rock that the real thing features. So I decided to try hand carving rocks. It sounded daunting, but I had a go and was pleased with the results. I soon developed a good rhythm.

1. Mix up 4 teaspoons of plaster.
2. Spoon onto polystyrene and roughly shape.
3. Wait till it is just firm enough that brushing the surface with a toothpick leaves no marks. Should take 5-10 minutes.
4. Attack it with a craft knife.

By digging the tip of the knife just into the surface and then flicking up, it rips a chunk of plaster off, leaving a rough rocky texture behind. This is the trick. By using the natural properties of the plaster I don't have to carve in all the texture, just the rough shape.

Here is a before and after shot of my first rock.

I soon got the knack of it and learned how to get the best results. The 4 teaspoons of plaster mentioned above is about the limit of what I can manage. More than this and I don't have time to carve it before it sets.

At the end of the afternoon I'd covered one of of the scene.

I added in the tunnel portal and some boxing to make the liner, carefully checking clearances with my two largest wagons.

Today it was more of the same. I soon got into a good rhythm and a few hours later I had most of the scene covered. The black crosshatching shows areas that still need some rocks.

Plenty of work still to go. The next step will be adding texture between all the rocks, but before I do that I should stain the rocks.