Time For Ground Goop!

Posted on November 11th, 2012, by Michael Rountree

Lately I have been working on a big mountain scene with a tall steel trestle, and I had gotten as far as roughing in the mountain shape using expanding foam over a wire mesh. The mountain shape needed some refinement so I have used some additional Great Stuff to build it up in areas, and I’ve used my knife to carve away other areas (mostly removing the crown off of blobs). But the “ground goop” or finished plaster layer will smooth over things and give a final contour to the land. Here is how I go about making my topsoil.

The basic mix is of Cell-U-Clay and drywall mud or joint compound, both of which mix in water to whatever consistency you prefer. There’s no chemistry or curing going on here, so exact ratios are not required. I mix up a small batch at a time, using a plastic cup and an old metal spoon – metal is critical, because it gets rather thick and when I’ve tried to use plastic spoons, they tend to snap in half.

There’s a lot of air in the dry papier-mache, so after wetting that first batch I added some more. I also keep an old squirt bottle with plain water in it, so that I can easily add water if needed.

In general, I mix about as much joint compound as I do Cell-U-Clay, but this ratio can be adjusted for various texture effects. My bucket of joint compound is perhaps 6 or 7 years old now, and you can see that it is drying out a bit, cracking here and there, but none of that matters since it will all get workable again when water is added.

Here you can see the proportions I used for this batch. Why mix the two? Well, Cell-U-Clay on its own tends to be lumpy and not as easy to trowel around, the joint compound thins this out and makes it a bit more workable. Joint compound on its own isn’t very strong, but the fibers in the Cell-U-Clay act as a reinforcing binder.

I mix this together to a thickness that clings to the spoon, though it is not necessary to do so. If I am going to be carving rocks, I’ll make it thicker still, but if I’m doing gently sloping terrain I might go thinner. In this case, because I am going to be working around the bridge footings to bury them, I want the thickness for greater control when placing it.

We’re not ready for application yet! Next come the color and texture elements. First, for basic color, I use tempera paints (also sold as poster paint), not only because it is cheap but also because it remains workable after it dries, just by re-wetting it. It takes a good amount of the paint, don’t be stingy with it! I prefer to have intrinsic color in my plaster so that, when I drill into it to set a tree trunk for instance, I don’t end up with little ant hills of white. Instead, I just get “dirt” that can be worked back into the scenery by wetting it.

Incidentally, I mix all of this up primarily with an up and down plunging motion, rather than swirling or stirring, so that I get it blended with the material at the bottom of the cup. And because it is so thick, you can get something a workout mixing it all up! After blending in the paints (about twice as much brown as black in this case), here is what it looks like. It looks a lot like… well, let’s be generous and call it chocolate cake mix!

In addition to basic coloring, I might choose to add texture elements too. Especially if I am going to be sculpting rocks, I want the right mix of texture elements, such as the following. First up is real dirt, in this case some of the clay soil from my yard. This tub contains the remnants after I screened and sifted it for usable fine dirt for scenery topping.

Another thing I use, mostly for rock faces, is scoopable kitty litter. One big container is all you’ll ever need, unless you actually own a cat!

When you go buy the kitty litter, though, you’ll need to make a fool of yourself in the store. I say that because you’ll want to visually inspect it to ensure that you get one that does not include any blue or green scented granules, which are rather common. Also be sure that you get scoopable or clumping litter; this is formed of a type of clay that will “dissolve” in water. I didn’t add a lot to this batch, just enough to show you what mine looks like.

Another possible color agent is dye, or even india ink. I had a bit of Rit Dark Brown and added it to darken things up slightly.

You’ll notice that it all dries a lot lighter than it looks when wet, so try for a deep chocolate brown when you’re mixing. Of course, soils in different places come in different colors so you might need to adjust things for your terrain. This batch will dry to a dusty grey, more grey than brown, but that’s okay, we’ll be adding a final sprinkling to give it good color in just a bit.

You can see that I just scoop it out with the spoon, and use this as a trowel to spread it around. The bottom of the spoon is a good shape to push the mass of goop into place, the top is good for holding a quantity of goop, and the edge is good for impressing strata lines when doing rocks. I hope to add to this thread something showing the sculpting of rocks using the spoon, but for now we’re just spreading soil.

This shot shows just how much coverage I got out of that one cup’s worth. Due to the uneven blobs of foam below, the ground goop is thicker in some places than others, and since I’ve blended it to a stiff viscosity I can do some careful shaping.

I pushed it up tight to the footings, trying to be careful not to discolor them. However, if I do get a bit sloppy, I can always clean up the footings later, by using a stiff paintbrush soaked in clean water to scrub off the offending goop.

You may have noticed the tool marks left behind by the spoon, leaving the ground non-smooth. That’s okay, this ground goop takes a long time to dry out and we will have plenty of time tomorrow or maybe even the next day to pat these down flat with fingertips.

The other benefit of the long working time is that now we can do some tricks, such as working in some rocks. Scuplting rocks in wet ground goop is easy enough, but how’s about I show you an even easier way to get good-looking rocks into your scenery?

This method is, in fact, as “Dumb as a Box of Rocks“!

This collection of talus is just something I collected on one of my forays, several years ago; you might be able to find something like this in your back yard, or if you have a place to go hiking you could take along a container. Just keep your eyes open, every now and then pick up a stone at your feet and examine it for potential – you want one good face that has some texture and detail to it. I spread this all out in a box so that I can find the pieces I want to use, as I’ll basically be pressing these into place like a tile mosaic.

Here you can see that I have embedded a fair number of larger-sized rocks. I sprinkle the finer stuff all over and gently pat that into place, too. This gives a top layer of real dirt, which will be held in place due to the moisture in the goop. Later, it will be held in place due to the scenic cement or glue used to adhere ground foam or other materials. My goal is to produce terrain that looks realistic, if one were to imagine that all vegetation had vanished. But I won’t invest as much detail in the parts of this scene where the trees will more fully cover everything. Here at the foot of the bridge, though, there will only be small bushes in places, scattered.

So that’s the method – now, I just have several more batches to mix up and apply! I don’t worry too much about getting an exact color match, since ground foam and static grass and other topping elements tend to mask the changes in hue between batches. Plus, I think the variation introduces some randomness that is more natural.

I’ll get to work on some rock sculpting so I can show you that as well. Now, back down to the train room!

Tags: ,

Leave a Comment