How did fresco artists transfer a full-scale drawing onto fresh plaster?

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Charles Sprague Pearce: Religion — Mural Detail of *Religion*, mural in lunette from the Family and Education series by Charles Sprague Pearce, 1897; in the Library of Congress, Thomas Jefferson Building, Washington, D.C.

In the process of buon fresco, artists transfer a full-scale preparatory drawing, known as a cartoon, onto the wall using a tracing.1

After applying the final, smooth coat of plaster—called the intonaco—to a section of the wall that can be completed in a single session (a giornata), the artist places the tracing against the fresh, damp surface. By carefully aligning the tracing with adjacent sections of the wall, the artist then traces the pertinent contours and interior lines onto the plaster. This leaves a faint but accurate guide that allows the painter to apply water-based pigments before the plaster dries and sets.1