Water gilding is fundamentally a process that depends on moisture. Size is water-based. The leaf must bond through moisture interaction with the ground. But too much humidity and the size stays too wet too long. Too little humidity and the size dries too fast.
High humidity (above 60%) causes size to remain tacky longer, which extends open time but also makes the timing less predictable. The gold may not bond as firmly initially. Low humidity (below 40%) causes size to dry quickly, which narrows the working window but creates faster, firmer adhesion. Burnishing also responds to humidity. In dry conditions, the surface is harder and burnishes more sharply. In humid conditions, the surface is more yielding.
Gilders have traditionally worked by feel and experience, adjusting to seasonal humidity changes. In modern climate-controlled studios, you can monitor and maintain ideal humidity—typically 45-55% relative humidity. This consistency allows for more reliable, predictable work. For anyone working with water gilding, understanding humidity is as important as understanding technique. Environmental control is technical skill.