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• Working With Plaster
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Working With Plaster

Working With Plaster

by Andy Slater

Plaster is perhaps my favourite media because of its versatility. You can cast it, carve it, stick blocks of it together (with more plaster) and, if you don't want the finished article to be made from plaster, it's dead easy to make a latex mould (see article elsewhere on this site) from the plaster original.

Buying

Plaster comes in a variety of types. The main differences between them being the coarseness of the particles and the setting time. Setting times are generally short such that you have a working time of somewhere between 5 and 15 minutes. You should aim to mix a quantity that can be used in this time. As already mentioned, chunks of set plaster can be glued using wet plaster and the adhesion between set and wet plaster is very good. This means that you can create large objects that need more plaster than you can apply in the working time by building up layers of plaster or sticking smaller components together.

For modelling purposes, fine casting plaster is probably best and this can be bough in anything from 1kg packs at craft shops to 25kg bags at builders' merchants. Because of the economies of scale you'll probably find that the 1kg bag and the 25kg bag cost about the same. Obviously if you're going to be using a lot of plaster it makes sense to track down a large bag. Bear in mind however that, depending on your purpose, 1kg will go quite a long way and with a 25kg you have the problem of storing whatever is left over. Obviously it needs to be stored away from any dampness but even so it will age and old plaster sets faster than new plaster so you won't really want it hanging around for years.

Mixing

The correct way to mix plaster is to add the plaster to water. A ratio of 2 parts plaster to 1 part water will generally produce a mixture of suitable consistency although it may make sense to make a stiffer or runnier mix depending upon the intended use.

It is worth being aware that fresh plaster will set more quickly if it comes into contact with plaster that has already set. It is essential therefore that the tools used for mixing are thoroughly cleaned after each mix. Whatever you do though, don't clean them in the sink because the plaster will block the drains. My usual approach is to mix the plaster using a lollypop stick or large plastic spatula (depending upon how much I'm mixing) and mix it in any plastic container from a yoghurt pot to a polythene bucket. At the smaller end of this range, the mixing tools are thrown away after each mix while at the larger end they are simply left to dry. When the plaster is dry, flexing the plastic spoon and bucket will cause the remnants of plaster left in/on them to crack away and it can then be tipped into the dustbin. Any remaining plaster can be rubbed of with a plastic scouring pad.

Casting

Plaster will take on a very high degree of surface detail when cast in a suitable mould and is an excellent material to use particularly if you want to carve modifications afterwards.

Casting is usually done in a latex or rubber mould. More solid moulds can be used but it is better to use a flexible mould even if this needs to be enclosed in a support casing (see article on making latex moulds for more details) to stop it distorting under the weight of the plaster. The reason for this is that when the plaster is poured into the mould there will be a tendency for air bubbles to get trapped. This can be overcome by filling the mould only about a third full and then flexing and tweaking the mould to dislodge any bubbles. When the entire inner surface of the mould has been coated in this manner the mould is topped up.

Obviously for the purpose of casting a slightly runnier mix than 'normal' is required.

Constructing

One of the beauties of plaster is that new, wet plaster will adhere very well to plaster which has already set. This allows objects to be built up in layers or by sticking cast shapes together with more wet plaster.

It is important to realise that once set, plaster can be thoroughly wetted with no adverse effects. The reason that this is so important is that if you apply wet plaster to dry plaster, the moisture will be drawn out of the wet plaster making it unworkable within seconds. It will also lead to weakened plaster because the setting of plaster is a chemical process rather than one of the material simply drying out. The material should remain wet while it sets or else its strength will be reduced. The point is that if you are applying wet plaster to plaster that has already set, the set plaster should be thoroughly wetted first.

Another approach to construction which is used for large objects and results in a much lighter object and uses far less plaster, is to apply the plaster to a former made from chicken wire. After the wire structure has been created, pieces of cloth or paper are dipped into wet plaster and applied to the wire surface. The paper/cloth is there to fill the gaps in the wire mesh, and after this initial layer, further layers of plaster can be applied without this reinforcement.

Carving

However you create your 'lump of plaster' it can then be cut and carved with all manner of tools. This is best done when the plaster is completely dry. In this state, the carving will result in lots of messy white powder but damp plaster will clog carving tools within seconds and the mess will be much harder to clean up.

When sculpting large pieces, I usually begin with a tenon saw to lop of any large chunks followed by a surform to get the thing roughly into shape. I then progress through various files, chisels, knives and grades of sandpaper until I get what I want.

Once again, one of the reasons that I like plaster so much is that if a mistake is made with the carving you can simply build the area back up with wet plaster and start again.

One other thing worth mentioning is that if you are working with a hollow structure built on chicken wire and your carving takes you down to the wire, the solution is to crush the object inwards at that point and fill the resulting hollow with wet plaster. You can then re-carve the area to the required shape.