[identity profile] pharminatrix.livejournal.com 2007-10-23 08:11 pm (UTC)(link)
His choice of the color yellow is especially interesting, given the nature of pre-synthetic yellow pigments:

On the Character of a Yellow Called Orpiment:
A color known as orpiment is yellow. This color is an artificial one. It is made by alchemy, and is really poisonous. And in color it is a handsome yellow more closely resembling gold than any other color. It is not good for use on a wall, either in fresco or with temperas, because it turns black on exposure to the air. It is very good for painting on shields and lances. A mixture of some of this color with Bagdad indigo gives a green color for grasses and foliage. Its tempera calls for nothing but size. Sparrowhawks are physicked with this color against a certain illness which affects them. And this color is, to start with, the most refractory color to work up that there is in our profession. And so, when you want to work it up, put the amount you want onto your stone; and, with the one which you hold in your hand, proceed to coax it, little by little, so as to squeeze it from one stone to the other, mixing in a little of the glass of a broken goblet, because the powder of the glass attracts the orpiment to the roughness of the stone. When you have got it powdered, put some clear water on it, and work it up as much as you can; for if you were to work it for ten years, it would constantly become more perfect. Beware of soiling your mouth with it, lest you suffer personal injury.

On the Character of a Yellow which is Called Realgar:
A yellow color known as realgar is yellow. This color is really poisonous. We do not use it, except sometimes on panel. There is no keeping company with it. When you want to work it up, adopt those measures which I have taught you for the other colors. It wants to be ground a great deal with clear water. And look out for yourself.

Cennino Cennini, Il Libro dell' Arte