Saturday, November 9, 2013


Making scenery: PLASTER CASTING

I had a go at making my own moulds out of latex, and the results have been rather pleasing. More recently, I have been using Gelflex too.

Latex mould, with plaster casting
First, you make a master of the thing you want to make lots of. You could make this out of almost anything, although I'd not recommend anything absorbent like card, because the latex can soak into that, and not peel off cleanly. I made my barricades out of lumps of modelling clay, with bits and pieces from my spares box stuck here and there. Build your master model on a flat surface with a lip around its edge. I found that the lid of an ice cream tub was perfect for this.
Next, get some latex, and paint it on. I bought my latex from a leather-working shop, where it was sold as glue. Use a cheap brush, and wash the brush immediately after use. If some of the latex does congeal on the brush, then you should still be able to peel it off with a bit of effort. Leave the latex for a while until it sets (may take a few hours), and then paint on another coat. You can get latex in a variety of viscosities. Go for the thicker end of the range, but not too thick. It should be liquid, not gunge. Too thick and it won't run into the detail on your model. Too thin and it will run straight off your model. One trick I found worked well was to paint on lots of latex, most off which ran off to form a puddle around the base of my model, and then to leave this for a while to thicken, and then to paint the thickened puddle over the model. This helps to build up the thickness of the latex layer. Another trick was to paint on a layer of latex, and then leave the model to dry upside down, with the ice cream tub lid supported at either end. This way, the latex congealed thickest on the peaks of the model, where usually it would run off and dry thinnest. I found that about a dozen layers was enough, but this depends on the thickness of the latex you're using, and the shape and size of your model.
Here you see three masters of rubble, to be used for adorning the outside corners of ruins. The latex is white where it is still wet, and yellow where it has dried. The three masters lie on an ice-cream tub lid, which has a flat surface in the middle, and a lip all around to catch run-off. I have painted the latex not just on the masters, but also around each master, to form a wide edge to the moulds. Drying latex does smell a bit of ammonia, so you may want to open a window, but I find that the smell doesn't take over the room.
I found when making moulds of masters that had flat vertical sides, that the latex ran off these sides so much that the moulds became very thin there, when nice and thick elsewhere. After a bit of use, they started to develop holes and leak. I found a rather neat solution to this problem. I washed a condom, and cut it into little strips, and used these as patches, which I simply glued on using thin latex. Condoms are made out of high-quality latex of an even thickness, and are very strong. When I made my next moulds with similar vertical sides, I added a layer of condom strips glued in with liquid latex when I first made the moulds, and these ones never leaked.
When the latex is dry and decently thick, you must decide whether to make a support for the mould. If the mould is small and simple then you could use it as it is without a support, in which case peel the latex off the master and use it. If it is large and will tend to flop out of shape, then before you peel it off the master, construct a wall around the mould (perhaps out of Lego, or perhaps just with sticky tape and card), and pour in plaster and allow it to set. This will form a perfectly fitting cup for your "glove" mould to sit in during casting. In this shot, you can just see the brick-pattern left by the Lego in the sides of the mould support.
Peel the latex off the master, and start casting. Mix your plaster reasonably thickly, support your mould by whatever means, and pour the mixture in. I experimented with inks and paints to colour the plaster, but found that these often weakened the result significantly, but others report that certain types of paint and plaster combine happily enough. The first casting will tell you a lot about your mould, and you will probably have to trim bits of it back, very carefully, to allow the plaster to run into all the detail. I found that the latex had run too far into the master in places, and that these excesses needed trimming.
The sort of plaster you use makes a big difference. I started with the cheapest, and this gave me a quick-setting mix, with decent detail, and a lightweight finished piece. The finished pieces were also rather weak, though. I tried with other sorts, and some are very different. The more expensive powder took longer to set, but set with significantly better detail and very much tougher. There was another problem, though: the good stuff separated in the mould, forming a thick layer of plaster at the bottom, and water at the top. This meant that if I poured plaster in to the top of the mould, that the top part of the mould (the bottom of the finished piece) didn't come out unless I went back to the mould before the plaster was dry, and topped it up with more powder. If I had made my master models on little raised platforms, then these platforms would have created a well in the finished mould, which would have been useful in solving this problem.
The latex moulds I made a few years ago went stiff. However, I found that by massaging them under warm water for a bit, they recovered well enough to use. Even so, I should store one good casting from each mould in case I ever need to make another mould of the same thing.
Put the castings somewhere warm to dry, then trim the bottoms flat with a scalpel, or file them flat with a rasp, or, as I did, a cheese grater. You could then paint them. I'd recommend first sealing the surface with undiluted PVA glue. Into this I mixed cheap acrylic paint, the effects of which you shall see below.

Rubble and barricades
There are five plaster castings in this shot. Two Esci British paratroopers walk amongst the wreckage in the street. Behind the officer with his pistol, is a pile of rubble with a few barrels in it. In front of him is a barricade. The master for this was made by pressing matchsticks (timber beams), cat litter (rubble), plastic and metal model boxes and doors and piles of bricks, into a lump of modelling clay. The castings for this barricade and the rubble was undercoated/primed with PVA mixed with grey paint. The bricks were then painted with thick terracotta paint, then dry brushed with light grey, and the whole thing varnished with polyurethane varnish mixed with black paint. You see here the result. Other piles include wagon wheels, cart fittings, oil drums, jerry cans, girders, and ammunition crates. One barricade was kept low-tech, so that it would serve for Napoleonic games, and another had all the World War Two stuff on it.

Stone walls
Some Germans search the fields for escaped POWs. Dog handlers and cavalryman by Irregulars, NCO by Combat Miniatures. The walls are a simple and obvious use for the plaster casting technique, and I have made a few pounds by selling castings to my fellow wargamers at the Tyneside Wargames Club. These walls have been given a very simple paint job. All I did was mix in black paint with the PVA. The pigment settles in the crevices and stays there, and one gets a nice variety of shades of grey. If I could be bothered, I'd add green to represent moss and the like.

Dragons' teeth anti-tank obstacles
This is why I started the plaster casting project. I was going to make some simple dragons' teeth, but stupidly did some research first. These are based on photographs of the "Seigfried Line" which defended Germany from France. The actual line was seven teeth deep, not four as here, but seven would have made an enormous thing on the gaming table, and since the ground scale is not literal in wargames, this seemed daft. Even so, each tile is getting on for five inches long, and the mould was difficult to support. Irregularities in the support of the mould led to a sort of undulating terrain in the finished product. Note that there are sections of concrete connecting the teeth. These ran deep into the earth, to make tunnelling under the line impractical.
The master models of the teeth themselves were made as follows. I cut four isosceles triangles out of plastic card, two much wider that the other pair. I glued these together to form a mould. I supported this mould with putty, and poured in a mix of moulding powder. I inserted a pin, head first, into the middle of the mix, and allowed the mix to dry with the pin sticking up in it. The pin served as a handle for pulling the shape out of the mould, after it had set. I then trimmed off the point of the pyramid with a scalpel. Using the same method and the same mould, I cast all the different sizes of dragons' teeth. The teeth are truncated rectangular-based pyramids.
The corner sections were made by taking an ordinary tile (which has fourteen teeth on it, in four columns: 4,3,4,3) and cutting it up into V-shaped sections. You can also see in this picture a destroyed section, made for games where engineers can blow things up. Again, I just converted a normal tile, rather than make a new mould.

Plaster casualty markers
Here we see castings used as casualty markers. I converted a standing figure into a lying one, mounted him on a base with a couple of cat litter rocks, and made a mould of him. Amazingly enough, all the detail of his entrenching tool and water bottle came out almost every time.
Gelflex is a soft rubbery substance I bought from my local model shop. It is a quicker way to make moulds than latex. I cut some of it up into lumps, put these in a small saucepan, and heated it on my cooker hob. All the while I stirred with a bit of wood, and wore safety goggles as the instructions advised. When the stuff had all melted, I poured it over the master, which I had surrounded with a retaining wall of Plasticine modelling clay. It bubbled for a bit, and then settled and cooled. I popped a lot of the bubbles before they congealed. The modelling clay gets very soft indeed from the heat of the molten Gelflex, and I found that with higher moulds (for pillars and the like) it collapsed into a messy puddle. I then made a surround using a sheet of pliable metal taken from a tube of tomato purée, which I taped together, and taped down to the work-top surface with masking tape. This worked fine.
There is no need to build up a mould layer by layer, as with latex, and the mould you get this way is a solid slab, which needs no supporting. I was delighted to discover that my first mould was a success, and had good detail and was easy to use. The original master, however, suffered greatly from the heat, and broke into many pieces. The molten Gelflex is a lot hotter than boiling water. Gelflex rubber is much softer and more flexible than most other mould materials, which means that I can take the castings out of the mould while they were still quite damp and weak, and this speeds production significantly.
In theory, you can reuse Gelflex again and again, although I wonder if I will ever be able to bring myself to destroy a good mould and melt it down to make a new one. Since it is so soft, I imagine that the moulds will wear out fairly quickly, but unless I start casting commercial quantities, I don't see this as a limitation. I'm only casting a few for my hobby.

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