Gesso, while the name for a preparation for canvas, is also a substance used in gilding. Gilding gesso is made of slaked plaster, white lead (or replacement), organic glue such as fish glue, and sugar. A small amount of colouring can be added to aid in applying the gesso – so it is easy to see where the gesso has been applied.
Once prepared, the gesso can be used immediately, or moulded into small cent sized tablets, which will set extremely hard, and can be mixed with distilled water when required.
When gilding with gesso, it is applied while very wet, and “spooned” onto the surface to be gilded. The aim is to create a puddle of gesso in the shape of the design to be gilded, which, when dry, will be raised above the surface of the paper or vellum.
Gesso allows gilding to be given a very high polish, and the raised surface provided by the gesso allows light to catch the gold.
Gesso is very long lasting when applied properly. Gilding from the great manuscripts such as the Book of Kells was done with gesso, and has in some cases lasted for over a millenium.
Gesso is available from specialty art shops or calligraphy sources. My preferred source (in Australia) is “Will’s Quills” – a mail order calligraphy shop based in Sydney.