If the painter in tempera required chiefly patience and delicacy, the painter in fresco must have resolution and audacity. He must calculate each day’s work exactly, and a whole day’s work could be spoiled by a single slip of the hand in the tired evening hour. For fresco, the working sketch was roughly copied in outline on a plaster wall. Then any part selected for a day’s work was covered with a new coat of fine plaster. The effaced part of the design must be rapidly redrawn on the wet ground. Then the colors were laid on from their little pots, and only the sound mineral colors which resist lime could be employed. The vehicle was simply water. The colors were sucked deep into the wet plaster, and united with it to form a surface as durable as the wall itself. Generally the colors were merely divided into three values,—light, pure colors, and dark. Everything was kept clear, rather flat, and blond, highly simple and beautifully decorative. One of the later painters, Cennino Cennini (active about 1400), tells us that a single head was a day’s work for a good _frescante_. The touch had to be sure, for a mis-stroke meant scraping the wet plaster off, relaying it, and starting all over again. The fresco painter accordingly needed discipline and method. Nothing could be farther from modern inspirational methods. Where everything was systematized and calculated in advance, you will see it was quite safe for a master to entrust his designs to pupils who knew his wishes. Every fresco when dry was more or less retouched in tempera, but the best artists did this sparingly, knowing that the retouches would soon blacken badly or flake off.