If you remember from way back when we were looking at ancient Egypt, ‘medium’ is the word we artists use when we talk about the substance that makes marks—ink, paint, crayon, pencil, pastel, chalk. Every medium needs 2 parts: pigment and binder. The pigment is the color. You get pigment from vegetable, animal, or mineral sources. The binder is what holds the pigment together and makes it stick to a surface. Liquid medium needs a third part: solvent.
Here’s how this new paint came to be invented—
Let’s go back to the Middle Ages. Mediæval painters used to bind pigment with egg yolk—seriously!—and water is the solvent. You have to paint methodically with egg tempera because the brush strokes dry quickly. It’s difficult to blend 2 colors together wet-on-wet. So, many egg tempera paintings are zillions of tiny brush strokes.
You crack the egg and separate the yolk from the white. Save the white for making transparent glazes. Put the yolk in a dish—carefully!—and prick its delicate skin with the point of a knife to get binder for pigment. Watch this guy make egg tempera: https://www.youtube.com/watch?v=5GCK5Y1rFEw
Watch this guy paint skin tones in egg tempera. As I said, zillions of tiny brush strokes:
Back to the beginning of The Western Civ User’s Guide to Reading & Writing.