Plaster is one of my favourite media because of its versatility. You can cast it, carve it, stick blocks of it together and, if you don't want the finished article to be made from plaster, it's the ideal substance to use for a master when you want to make a latex mould.
Plaster comes in a variety of types. The main differences between them being the coarseness of the particles, strength, and setting time. Setting times are generally short such that you have a working time of somewhere between 5 and 15 minutes.
For model making purposes, fine casting plaster is probably best and this can be bought 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 so if you're going to be using a lot of plaster it makes sense to track down a large bag. If you do go to a builders' merchant be sure to get casting plaster (they don't ALL stock it) and not the stuff for spreading on walls.
When buying plaster, consider the length of time it will take you to use the quantity you are considering. Plaster needs to be stored away from dampness but will age even if stored in ideal conditions. Old plaster sets faster than new plaster so you won't really want it hanging around, especially in a bag that's been opened. My favourite casting plaster for small, model terrain type projects is a product called Keramin that I can obtain from Hobby's in either 1kg or 5kg bags. However I usually just get a 1kg bag so I don't have it hanging around for ages.
Your first consideration should be how you are going to deal with any mess. Yes, I know you don't plan to spill anything, but how are you going to handle it if/when you do?
When casting small items I generally set up in the kitchen, on days when I've got the place to myself. With plenty of water and paper towels in reach, a wipe clean floor and a couple of sheets of newspaper on the wipe clean surfaces I'm ready for anything. I don't think I've every had a major spill but that's no reason to get cocky and skimp on the precautions. Don't wear your best suit either.
TIP: Have a large bin handy which a) has an open top and b) has a waterproof bin liner inside. When you find yourself needing to dispose of a handful of wet, plastery paper towels you don't want to be messing about trying to open the swing lid of your 'normal' kitchen bin.
You should also bear in mind that washing plaster off tools or your hands into the sink is a VERY bad idea. You might get away with it for years but why risk it? Plaster is best removed from tools while it is still wet - just wipe it onto a paper towel and drop it in the bin. You shouldn't actually get any plaster on your hands but if you do, wipe it off while it's wet. Any that's left can be cracked off by wiggling your hands over the bin after it's dried.
Be aware that fresh plaster will set more quickly if it comes into contact with plaster that has already set. Thus it is essential 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 off with a plastic scouring pad.
DO NOT wash plaster down or into drains. Plaster sets by a chemical process and will happily set in the U-bend. Even set plaster can build up and cause blockages in drains.
Perhaps the most important thing to know about mixing plaster is that you should put water in a bowl and add the plaster to that. You can NOT do it by adding water to plaster powder. It just does not work.
A ratio of 2 parts plaster to 1 part water will generally produce a mixture of suitable consistency for casting but read the instructions and do a few test. The instructions on the Keramin that I like to use suggests 3:1 but I find this too thick with the small moulds that I am generally using. On the other hand 2:1 is too runny and I find that somewhere in the middle i.e. 5:2, is just about perfect.
Given that we need to add the water first you can't use a measuring cup to add water to your mixing bowl and then start dunking the wet cup into the dry plaster. If you don't have two suitable measuring cups here's a tip:
You should aim to mix a quantity that can be used in this time.
Decide how much water you are going to need (more about this in a minute) and find an appropriate measuring cup. Use the cup to put the required amount of plaster into a larger 'cup' and weigh it on the kitchen scales. Armed with that information I can mix batches of plaster by adding one measuring cup of water to your mixing bowl and weighing out the appropriate amount of plaster into a the larger container that never gets wet, and sprinkling it into the mixing bowl.
Note that the first time you do this you do this you will find yourself looking at the measures of plaster and the single measure of water and thinking "I'm going to need more water". If you are as stupid as me you will find yourself thinking the same thing every time you do this and will never cease to be amazed as how so much powder gets absorbed into so little water.
A final thing to note is that when you mix plaster you need to stir it, not whisk it. You should endeavour to release trapped air, not stir more in.
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 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.
Plaster that has been mixed for casting should have a consistency like pouring cream. Remember that we're pouring it, not troweling it onto a wall.
There's an art to pouring plaster:
Work quickly but do not rush.
If you pour the plaster in a thin stream over the lip of your mixing container, this will help to release trapped air.
Be aware that the plaster in your mixing bowl will begin to settle immediately that you stop stiring so take a couple of breaks from pouring to give it another gentle stir.
Think about where you are pouring. Depending upon the shape being filled it may be appropriate to pour directly into the middle of the mould or to pour such that the plaster runs down the side. Consider where air is most likely to get trapped and pour accordingly.
Don't fill any given cavity in one go. If you fill it in one go then any air trapped at the bottom has the full depth of plaster to rise though. If you fill it a bit at a time you will get less air in the mould.
You'll see lots of suggestions for getting air out of moulds ranging from poking about with cocktail sticks and brushes, through the somewhat anti-social technique of banging on the work surface, to the use of vibrating tables and vacuum cabinets. Useful though though these techniques may be, they should not be employed to make up for sloppy mixing and/or pouring technique.
No matter how careful you are, you'll probably end up with a bubble or two that needs filling. You may also need to close up some gaps if you're constructing (more about this later).
As already mentioned, plaster adheres well to itself so big gaps. Small gaps and air holes are probably better dealt with using ready mixed Polyfilla. I particularly like the stuff that comes in tube because I can squeeze out a the kind of amount that I might use in say 5 minutes, use it, and repeat the process.
The image to the left shows a piece with a lot of air bubbles. I wouldn't normally try to salvage a piece that with this bad however those blocks were part of a set of blocks that I bought on eBay and I had to use them. The image to the right shows they same blocks (yes, really) with Polyfilla applied.
The crucial thing is to wet the plaster first (a brushload of water is enough) to prevent the dry plaster from sucking all of the moisture out of the Polyfilla as soon as it comes into contact with it. This buys you some working time and I find that the ideal tool at this kind of scale is a cocktail stick.
Pieces of set plaster can be stuck together and the adhesion of wet plaster to that which has already set is very good. Thus 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. Really small pieces of plaster such as Hirst Arts' blocks can be joined with PVA glue.
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.
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.