Gilding Size is an adhesive material used to adhere gold, silver, copper or metal leaf to any surface that is being gilded.  It is similar to varnish in consistancy, most are oil based too. Gilding Size is applied to the surface, and left until it is almost dry but still tacky, the metal leaf is then applied to the surface and once it dries a permanent bonds is created.

There are many different type of adhesives that can be used as gilding size, and their a several types of size specifclaly formulated to be used as gilding size. Some are differnet depending on the gilding method you will be doing such as; oil based gilding or water based gilding.