They are also often used as the basis for other make up SFX effects and for headpieces, or to cover up the hair when making face and head life casts.
Bald caps are easy to make, and the materials aren't too difficult to assemble. You will need:
A polystyrene, glass or fibreglass head (used for hat displays and similar).
Some cheap bath sponges
DVD marker pens
PVA Glue (if using a polystyrene head).
Without doubt the most important, and possibly most difficult to acquire is the head. Glass ones are the best but they are expensive. Mannequin fibreglass ones can sometimes be found at car boot sales or bankrupt stock, but you can use a polystyrene one perfectly well. These are cheap and readily available on the net. I got my mannequin head for nothing from a closing down sale, and the polystyrene one cost me a fiver with postage.
Step 1: Preparing your head
If you can only get a polystyrene head then you should consider preparing it for use. You can make bald caps using the head as is, but eventually the soft surface of the head will start to break down and lumps will get pulled out of it.
You need to give it A LOT of coats of PVA glue to seal and strengthen it. I took 3 weeks and 25 thin coats of PVA, allowing each one to dry in between. This gives a really tough and non-porous 'skin' to the head so I can use it for all sorts of work. Obviously it won't take heat or serious chemicals which will both melt it.
Once you have a sealed head it is advisable to mark it. This will allow for good feathered edges to your bald cap. Decide if you want an all over cap (which must be cut back), or if you are happy that the ears can be left out.
Mark the head using the DVD pens (I use different colours for each edge) such that the first line covers the entire area of the cap, the second is about 1/4" (six millimetres) inside the first, the third is 1/4" inside that and the fourth 1/4" inside that. See the picture for a visual guide.
Your head is now ready.