"How you should endeavor to copy and draw after as few masters as possible"

This is from Artists on Art from the XIV to the XX Century.

"When you have practiced drawing for a while as I have told you above, that is, on small panels, take pains and pleasure in constantly copying the best works that you can find done by the hand of great masters . . . And as you go on from day to day, it will be unnatural if you fail to pick up something of his style and of his mien. For if you set out to copy after one master today and after another one tomorrow, you will not acquire the style of either one or the other, and you will inevitably become fantastic, because each style will fatigue your mind . . . If you imitate the forms of a single artist through constant practice, your intelligence would have to be crude indeed for you not to get some nourishment from them. Then you will find, if nature has granted you any imagination at all, that you will eventually acquire a style individual to yourself, and it cannot help being good; because your hand and your mind, being always accustomed to gathering flowers, would ill know how to pluck thorns." --Cennino Cennini, CA 1372 - ?

Cennini wrote Il Libro Dell'arte - The Craftsman's Handbook - the definitive source of information about the technique of medieval artists. It contains an early description of egg tempera painting.