There are various recipes for casein. Some include lime or ammonia where I've included borax, but they all have the same goal - turning milk curds (known asquark) into something usable, either as a glue or a painting medium. Since borax is easily available from most groceries and since it doesn't have the smell involved that ammonia does, that's what were using here.

What you'll need -

1. A glass jar with a lid (a small pickle or olive jar will do just fine).
2. A mesh colander (yes, just like the one you use in the kitchen).
3. Spoons (I make sure I use old spoons for this - nothing I'll use for eating).
4. 2 cups of skim milk.
5. 1/4 cup of vinegar.
   
6. 1 teaspoon of borax (yes, that stuff in the laundry aisle).

Procedure:

1. Pour the milk into the glass jar. Either let it set until it's room temperature or warm it in the microwave for a minute or two. If you do microwave it, make sure it doesn't boil or get too hot - that will break down the proteins needed for your paint.
2. Add the vinegar to the milk and stir it well. Let this sit for about an hour or so. It doesn't need to be carefully watched, so there's no pressure. Read a book or go shopping or something. When you come back to it, you'll notice that the milk has curdled into a substance that resembles cottage cheese. This is good.
3. Strain the curds in the colander. Most recipes advise using cheesecloth or even a coffee filter to help with this, but I find that the colander by itself works just fine. When your curds are strained, rinse them with cold water so the vinegar is washed out. Then, return the curds to your glass jar. So far, so good.
4. Add the borax to the curds. You could even mix the borax with a little water first and then add it. The purpose of adding the borax is to dissolve the curds so that you aren't trying to paint with cottage cheese. You could do that. I've done it - mashing the curds into a paste - but it's not the best or easiest way to do things.
5. Stir your curd and borax mixture periodically for about an hour. By the end of this process, you should have a cup or so of something that looks like Elmer's Glue or heavy cream.

When not in use, put the lid on the jar and refrigerate what's left. Refrigerated, your casein binder will last about ten days. This is a natural product which isn't loaded with preservatives, so don't be disappointed that it doesn't "keep" forever. If you find that you've made way too much to use in that period of time, you can cut the recipe in half and you should be just fine.

You can use your casein just as it is as a glue if you want. To use it as a paint, you'll need to mix it with powdered pigments or another kind of paint, such as watercolour or gouache. For the best results, though, powdered pigments are the way to go. In a pinch, if you have a quality set of soft pastels on hand, you can use a knife to reduce part of a pastel stick to powder, and can mix your casein with that.

Casein paint is best when mixed fresh. I've tried to refrigerate a covered paint palette overnight, and have had mixed results.

The more casein binder you use with your pigment, the more translucent your paint will be. In general, casein paint is flat and opaque, like a poster paint. If you water it down, it becomes more transparent, like a watercolour. I find that I prefer to thin my casein paint by adding more binder to it so I can use it as a glaze. Glazing by adding more binder also tends to "lift" or disturb a previous layer of paintless than it does when you're glazing with paint thinned with water. This is similar to thinning oil paint with a few drops of linseed oil.

Earth Pigments has other recipes which use casein powder instead of milk, ammonia instead of borax, etc. You can check those out as alternatives to what I've described.