Opaque watercolor is one of the oldest media known. The mixing of glue with the powdered color is known as distemper and most names like poster color, designers’ colors, gouache, are really distemper.
The addition of oil and egg yolk, casein, make the paint easier to handle, allowing for a great deal of overpainting. They are usually referred to as tempera colors in this state. They have emulsions as binders.
Egg tempera and casein can be bought in tubes or pots; also gouache, which has little oil in it, and is beautiful to use. Egg tempera can easily be made by the addition of a little egg yolk to either pure powder color or with any tube paint other than casein or oil.
The method of making up egg tempera is simple. Separate the yolk from the white of the egg and carefully pierce the yolk so that the liquid it contains runs freely into a bottle. You can discard the skin. Then add a few drops of linseed oil and a few drops of formaldehyde. Shake briskly and it is now ready to add to your powder or your tube color. The formaldehyde will stop the egg yolk from going bad. This mixture will keep for quite a few months. Keep the container well corked.
If in doubt you can buy the ready mixed egg tempera color in tubes from most artists’ supply stores.
To get the most out of this beautiful medium, it is better to paint with egg tempera on a specially prepared ground of size and white, called gesso.
You will need gilder’s whiting, and any good glue that is not made with fish bones, as fish glues are inclined to attract moisture. Rabbit-skin glue, parchment size or vegetable size will do. After mixing, add a few drops of linseed oil.
Parchment and rabbit-skin glue come in sheets that have to be soaked before using. Soak overnight. When soft melt the size in warm water in a clean basin. Add oil. Add the whiting to this, carefully mixing all the time. The proportion of size to whiting is two parts of size to three of whiting. To thin size add water; to thin gesso add size. Keep the final mixture fairly thin and warm.
You cannot paint too thickly with poster color, and if you are not careful when overpainting,the underpaint may work up. These two disadvantages are overcome in casein, so that if you want to work thickly it is better to use casein. I sometimes fix a color that I am going to overpaint with fixative. This seems to hold the paint well enough for even the most vigorous overpainting. If you use poster color too thickly it tends to crack and flake off.