The following is a collection of information I've gathered on casein as a paint and medium over the past few years. I don't claim to be a scholar on the subject, but have used it extensively in some of my professional works for exhibit and commercial clients. If any information here sounds unclear or doesn't make sense to you, please chime in.
The word ďcaseinĒ refers to the protein or curd precipitated from milk or soy. You can often find it listed on food labels as the milk ingredient. The gluten serves as a rather strong binder that mixes with water, and is water-resistant when dry, but not exactly waterproof; although, it may be varnished with a formaldehyde solution for better permanency. It gets harder as it ages, much like egg tempera. Because the binder comes from milk, it is a very ancient medium for painting. Itís still often used for painting wood furniture, and was commonly used on theatre sets before the introduction of acrylics. The fact that it can have a limited shelf life caused it to fall somewhat out of favor. Itís a very popular medium for illustrators since it doesnít reflect light and therefore photographs very well.
In addition to being an excellent water based paint in it's own right, it can also be used as a size or primer, although not as strong and more absorbent than hide glue. It also behaves as an oil and water emulsion, but reportedly causes the oil to yellow over time. It adheres very well to porous material like paper and wood, but dries somewhat brittle so itís not recommended on a flexible surface like stretched canvas. It also works well on stone, and is often used for murals.
Shiva is perhaps the best known artistís brand of casein, but Pelican also sells it (Plaka.) The interesting feature in these commercial brands is that they never spoil. I have tubes of Shivas that are years old, and it's still very usable. Shiva also sells a clear casein emulsion and shellac based varnish. Their color selection is limited, but you can make your own using pigments mixed with the emulsion. I like their paint overall, but sometimes it can be rather watery.
There is a brand called Iddings, now owned by Rosco, that is made for theatre painting, and it comes in large cans (quart & pints,) or 1 ounce sample jars. These jars are a good deal, @ $1 each compared to @$6 price of Shiva, but they mixture will spoil after a few months. There are a few online stores that carry it, or check theatre supply companies.
Other places sell casein powder where you just add water and/or pigment to make only what you need. Without too much trouble you can even make your own directly from milk. Casein can mix with gouache or watercolors to extend your color choices, but be aware that some of those colors are not lightfast or permanent. Gouache is fairly easy to make too.
If youíve ever used gouache, youíll find casein handles much the same way, especially in how fast it dries. Commercial brands of gouache usually have chalk added as an extender that causes it to dry to a paler shade, which also happens to casein paint when it dries due to its milk base. The ďstrokeĒ of the opaque paint is rather short due to the way the binder pulls and drying rate, but it can be watered as a wash and still holds very well. The body of the paint is such that fine details are easy to paint. It also dries to a matte finish. There are differences to gouache, however, outside of the obvious binder component. Mainly, gouache can always be re-activated easily with water, but casein is more water-resistant when dry and makes a tougher film. It can also make thicker brushstrokes (impasto.) Like egg tempera, casein is a good choice for underpainting oils.
Tips and notes:
When painting over a dried paint layer of casein, itís a good idea to use as little water as possible or else the bottom layer will mix in. You can glaze this way or paint over a toned underpainting. Although thereís really no ďfat over leanĒ rule here as with oils since it dries so quickly, if the paint is applied too thick it tends to cake up, which makes it unmanageable, and will crack over time, and paint layers underneath can absorb some of the binder on top. As such, itís better if top layers have more binder in them. The best result comes from not layering it too thick. If making corrections, instead of painting over an area you donít like, I recommend just wiping it off with a damp cloth.
To blend a media like this that dries quickly, I find itís best to mix your values on your palette first, and then blend them together on the surface. I like it when the brushstrokes show, so I prefer to do as little blending as possible. The overall surface size I use is not that large, about 18 inches maximum but usually around 1 foot. To paint large strokes of opaque paint youíd have to mix up a lot, which would dry too quickly. The brushes I use are rather small, except for washes. You can re-wet your paint dabs somewhat on your palette with a drop of water, but after a while it becomes unmanageable. I typically only squeeze out small dabs of 2-4 colors for an isolated area, which is a method Iíve also adopted for oils, and works well for me.
When the paint dries the color value shifts a notch brighter, especially for diluted mixes. This makes repainting over a dried area difficult if youíre trying to match the previous value. You need to either guess at it with a darker shade, or repaint that whole section with the new value. It takes some practice to get that right, and can affect your typical painting method if youíre expecting to rework areas later.
Another compatible emulsion is methylcellulose, which is made from wood pulp. The main advantage it has is it never spoils. Itís also a good oil and water emulsion. Sinopia sells 1 pound bags of this called Tylose. A little bit goes a very long way. 1 tablespoon made 4 ounces thatís lasted me over a year through several paintings.
Always use distilled water when painting since the bacteria in tap water can accelerate the spoilage of casein.
The recipes that include ammonia (instead of lime or borax) need to be used in a properly ventilated area. Shiva paints have a strong odor to them that I suspect is ammonia; although, thereís no warning about that on the label. The odor takes a while to dissipate as the paint dries so use with caution. After drying for a few hours the odor is gone. Iíve used borax, which is easier to find than ammonia carbonate, and it works quite well.
Soft brushes, including synthetics, work best for glazing, but even stiff hogs-hair can be used for textures or scumbling.
Shiva ďSigna-seinĒ, http://www.richesonart.com
Real Milk paint, http://www.realmilkpaint.com/
Iddings Deep Color, http://www.rosco.com/us/scenic/iddings_deep.asp
Kama Pigment (check the demo section,) http://www.kamapigments.com/
More info, http://www.angelfire.com/yt/modot/painting.html
Hopefully you found this informative and helpful. Feel free to contribute your own experiences or questions.