There are two different methods for gilding on glass. One is called water gilding and is executed on un-blasted smooth glass, while the other method is called matte gilding, which is done on blasted glass. Water gilding produces a brilliant, shiny gold surface. For both techniques, one needs a substance that adheres the gold to the glass. In water gilding, this substance is gelatin, and in matte gilding, it is an oil-based size (glue).
-Ruth Dobbins, EtchMaster
