Experiments with breakaway plaster

The main problem I face when practising the dark art of miniature pyrotechnics is that my models are, frankly, too small. Apart from the impact this has on the realism of miniatures at such small scales, any pyrotechnician will tell you that whilst creating large explosions is easy, making small ones is very difficult. Flames and smoke are hard to scale. However, I simply don't have the space in which to build large miniatures, and this is not a situation likely to change in the foreseeable future. At the same time, I am still trying to find the best techniques to achieve certain effects, and this is very much a process of trial and error. Consequently, my experiments so far are very hit & miss.

Take the exploding bunker as an example. The bunker miniature itself is barely 15cm high. In order to blow apart, the model - cast in rigid polyurethane foam resin - was cut up into small pieces and reassembled loosely, like a 3D jigsaw puzzle. However, being so small overall made it very difficult to cut up into small enough pieces, and so the chunks of bunker that fly apart are unrealistically large.

However, I am now working on a new approach. Much experimentation has resulted in a type of foamed plaster which, when cured, is quite brittle and crumbly. It consists simply of adding a small amount of bicarbonate of soda and vinegar to the liquid plaster. These two additional ingredients react and produce bubbles which aerate the plaster causing it to foam.

It can be cast in standard moulds, but because it's terrible at reproducing smooth or detailed surface textures it is necessary to first apply a thin layer of normal plaster to the inside of the mould to capture the relevant detail, and then fill it with the foamed plaster mix. Finally, it needs baking in the oven at around one hundred and ten celsius for a few hours to properly cure and dry.

Before or after baking, it doesn't really matter which, it is easy to carve out the cavities required for small hard charges, which are then cemented in with normal plaster. The result is that the charges are embedded in the heart of the castings.

My first experiment can be seen below - it utilises the moulds for the front of the bunker (the upright and the horizontal section, both of which conceal a doorway).

This is the upright section, and shows how the application of normal plaster first has captured the brick-work texture from the mould:
This is the horizontal section on top - foamed plaster was poured straight into the mould, hence the very rough texture:
In both images you can see the cavities carved out for the charges on the righthand side. This is what the castings look like with the charges installed and normal plaster added to cement them in place:
And this is the test video:


It was only semi-successful in that the timing of the fireball charge was off and it ignited too early (but I know why and can correct this in future) and the second charge in the upper section failed to go off at all. Nevertheless, the explosion of the upright section proves the technique can work spectacularly. Shot with a Casio EX-FH25 at 240fps.