Wednesday, July 13, 2016

About Traditional Gesso: Part One

Traditional Gesso is the combination of a protein binder (RSG), distilled water, and Calcium Carbonate - also known as whiting, or chalk - as a bulking agent. It is not to be confused with the oil-based or acrylic material sold today for use on canvases in preparation of painting.

Preparing a chair for gilding with gesso
The traditional gesso recipe used in water gilding performs two functions: filling the grain of the wood and providing a smooth, hard surface that allows the gold leaf to be burnished to a brilliant lustre. 

The preparation of all of the recipes used in water gilding are quite precise including that used to make gesso. One very important consideration is the strength of the RSG formula which is a 10% solution: 9 parts water to 1 part dry RSG, either in pebble or granular form. If the RSG solution is prepared too strong there is risk in developing crack mechanisms in the gesso layer. Likewise, if the RSG solution is too weak, one faces the possibility of delamination of gesso from the wood substrate.

Numerous layers of warm gesso are painted, stippled, or sprayed onto the wood surface. Each layer is allowed to dry only long enough so the application of the next layer does not disturb the previous layer. Anywhere from 5-20 coats may be applied, depending in part upon the object and the desired effect. The concept, which dates back to the early Egyptians, is that the wood is intended to appear as solid gold, an effect created largely by the use of gesso as a ground for gilding.

There are a variety of approaches used throughout the world in the successful preparation of gesso and although recipes and methods may seem at first glance to differ, the basic principles of glue strength and the ratios of RSG to Whiting are quite similar when analyzed. Logically, they would ultimately need to be since the relationship between the binder and the particles of calcium carbonate are subject to the same chemical principles.

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