Final answer:
Gum arabic, a water-soluble substance from acacia tree sap, is crucial in aquatint, where it's used as a resin to create tonal effects on a metal plate.
Step-by-step explanation:
The material derived from tree sap that is important to aquatint, a printmaking process that emulates the appearance of water-based media, is gum arabic. Gum arabic is a water-soluble compound made from the sap of the acacia tree. It is used in both aquatint and watercolor applications. For aquatint, resins, which can include substances like gum arabic, are sprinkled onto a metal plate and then heated to harden the powder before the plate is dipped in an acid bath to create tonal variation. Whereas in watercolor painting, gum arabic acts as a binder, mixing with pigment to allow for smooth application and adhesion to the paper.