Gesso is traditionally plaster or chalk mixed with animal glue. It creates a ground that is both absorbent and responsive. When size (the adhesive for gold) is applied to gesso, the gesso absorbs moisture at a controlled rate. This absorption is the mechanism that allows the gold to settle and bond properly. The gesso also gives the surface slight texture—microscopically rough—that helps the gold conform and adhere.
The color of the gesso also matters. White gesso reflects light through the gold, creating brightness. Tinted gesso creates different optical effects. The thickness of the gesso layer affects how the gold sits on it. Too thin and the ground may not have enough structure to support burnishing. Too thick and the gold may not bond properly.
Gesso is also slightly yielding. When you burnish gold, you are not just polishing—you are gently pressing the gold into the gesso, creating a mechanical lock. That is why proper gesso is essential. Modern acrylic gesso is convenient but behaves differently than traditional gesso. For work that demands precision and longevity, traditional gesso remains the standard.