Traditional water gilder’s use a gelatin mixture (size) that will facilitate a sturdy bond between the gold and the substrate.
Gelatin is made up of collagen from various animals. Most of the extractions come from the skin/hides and bones of animals such as cows and pigs. It is a colorless and translucent material that is brittle in its dry form. When it is dissolved in water and then cooled, it becomes gelatinous.
Gelatin comes in many forms: in a capsule, powder, granulated, or in diamond-cut sheets. We use the sheets. I simply break off a section and drop it into hot, tap water.
Refer to the water gilding post for more information.