The next time you see a fresco, look for the seams. Here is a close-up of one of Giotto’s frescoes in the Scrovegni Chapel in Padua.
Giotto painted an angel a day, sometimes two; but he also did the complete figure of Christ on the cross in one day. How can anyone know that?
A painter has to work fast when painting fresco—that is one of its peculiarities. He must finish his picture before the lime sets up, which will happen in only a few hours. That means he has to plan his work carefully: divide the wall or ceiling up into sections only as large as he can finish in a day.
He carefully trims the edges of his plaster each day; but as there is no way of blending the new day’s lime with the old, a seam between them is always visible; and so, by these seams, you may know how many days it took him to paint the fresco, and just what he painted every single day. Normally painters start painting at the top of the wall and from left to right, just as if they were writing on a page.