Saturday, July 16, 2016

About Traditional Gesso: Part Two

Smoothing away pinholes in gesso

In our last installment of the water gilding process we discussed an overview of preparing gesso, its function in sealing the grain of wood and allowing the gold leaf to be burnished. It is also of great importance to understand the relationship between the strength of the binder (usually RSG - rabbit skin glue) and its relationship to the filler (whiting - calcium carbonate). If the glue strength goes much beyond the 10% solution, the possibility of crack mechanisms in the gesso increases; likewise, if the glue strength is too weak, gesso delamination from the wood surface could occur.

Once the proper gesso mixture has been prepared and at least 5-7 coats have been applied, the gesso is allowed to sit overnight to thoroughly dry. It is then ready for surface treatment to provide a smooth surface.

There are different ways to smooth gesso, the most common method today being sanding. I like using 3M Tri-M-Ite sandpaper as it holds up well and cuts through the gesso efficiently. Primarily, I use either 220 or 280 grit, depending on the surface and how aggressive the paper needs to be. As a rule of thumb I recommend using as fine a grit as possible while still being able to effectively smooth the surface. If the finer 280 grit works well there is no need to use the heavier coarse 220 grit which will leave heavier sand marks which will later need to be removed. If starting with the heavier 220 grit, follow-up with a light sanding of the 280 grit and a light, final dry polish with either 600 or 1,000 grit wet or dry paper.
There are other techniques that may be used to smooth gesso. Cennino Cennini discusses in his 15th century treatise Il Libro dell'Arte (The Craftsman's Handbook) the use of 'little hooks' and a spatula to scrape gesso smooth as well as the use of water polishing with a damp rag. The use of a method called re-cutting, prevalent during 17th century France and Italy, is still used today.

Other treatments to gesso for aesthetic affect include incising and punchwork which create texture and often elaborate surface decoration as a compliment to the gilded surface.

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