Glaire is a form of adhesive used in gold tooling for bookbinding. Unlike adhesives used in other forms of gilding, glaire is not tacky when the gold leaf is applied. It only becomes adhesive when it is heated by a tool.
The traditional way to make glaire is to beat the white of an egg with salt, then leave it to stand overnight. The salt preserves the albumen (for a while, anyway), and the beating breaks up the structure of the egg white.
Many bookbinding suppliers sell glaire in crystal form, which is reconsitituted with distilled water, vinegar or skimmed milk. Like fresh glaire, glaire made from crystals goes rotten within a few days.
A shellac-based glaire formula was invented in the early Twentieth century. It is sold ready-mixed by bookbinding suppliers, and lasts for several months without going off.