Size—the adhesive for gold leaf—is formulated for specific properties. Viscosity (thickness) is one critical property. Size that is too thick does not spread evenly or allow the gold to seat properly. Size that is too thin runs, pools, and does not provide enough substance for proper bonding.
Surface tension is equally important. Surface tension affects how the size wets the surface and how the gold leaf conforms to the sized area. Traditional sizes are formulated to have surface tension properties that allow the leaf to settle evenly. If surface tension is wrong, the leaf may curl, bunch, or not adhere uniformly.
The chemistry of size is therefore not arbitrary. Each size formula—whether traditional animal glue, modern water-based, or synthetic Kölner products—has been developed to provide specific viscosity and surface tension characteristics. When you mix or thin size, you change these properties. This is why strict adherence to size preparation (water ratios, aging time, etc.) is important. The size is a engineered fluid, not just a simple adhesive.