Gold leaf fails to adhere for specific reasons. The most common is incorrect sizing—either the size was applied too thickly, too thinly, or at the wrong moisture level. Size that is too wet causes the leaf to float. Size that is too dry causes the leaf to shatter and not bond. The window of correct application is narrow.
Surface preparation failures are the second most common cause. Inadequate gesso, improper sanding, contamination with dust or oils—all of these prevent proper adhesion. The gold may appear to stick initially but will flake off over time because the mechanical lock never formed.
Environmental factors also matter. Humidity changes affect size. Temperature changes affect adhesion dynamics. Working in wrong conditions—too dry, too cold, too hot—creates adhesion problems. Material compatibility also matters. Certain sizes work better with certain gesso formulations. Using incompatible materials creates failure.
The lesson is that gold leaf failure is diagnostic. It tells you what went wrong. Flaking indicates poor preparation or wrong moisture. Shattering indicates size that was too dry. Floating indicates size that was too wet. Understanding the failure mode tells you how to fix the process.