Friday, October 8, 2010

Gesso Formula

I started off the blog thinking that perhaps getting too bogged down in what I thought were the more mundane details of the project would cause people to tune out. But, Kathy had asked about some specifics, so I thought I'd address that request in a regular post.

My gesso formula was based on measurements given in Ralph Mayer's book, The Artist's Handbook of Materials and Techniques. I used in my final formula, 2 3/4 oz. of Italian rabbit skin glue, from Natural Pigments, and 1 qt. of water. I put the glue and water in a double boiler and let it hydrate overnight. The next day, I melted the glue over low heat on the stovetop and added an equal volume of Italian gesso (calcium sulfate), also from Natural Pigments. I added the gesso by sifting it slowly into the melted glue mixture and letting it wet out before adding the next bit. Once the gesso was in the pot and all hydrated, I stirred carefully with a small whisk to blend it all together. That created the gesso grosso, which was laid on the panel in a somewhat thicker layer and smoothed with an offset spatula or a wooden slice (see Daniel Thompson's book The Practice of Tempera Painting referenced below). For gesso sottile, I added a bit less gesso so that the mixture was like heavy cream. The gesso sottile was brushed on and then smoothed with a spatula. Each coat was allowed to dry to the point of having no surface sheen, then the next layer added. I put on 5 layers of gesso sottile. Oh, and all the gesso was strained through a double layer of cheesecloth before application, and kept warm in the double boiler between coats. The finished gesso surface is probably a little harder than it needs to be, but I thought it would be preferable to err on the harder side than make the gesso too soft.

My reference for gesso application was mostly from Daniel V. Thompson's great book on egg tempera painting, The Practice of Tempera Painting, Materials and Methods.

Once the gesso was totally dry (I gave it a week), I sanded the panel smooth and scraped the surface (with a steel scraper from Natural Pigments) to a slight sheen. Then it was sealed with a very dilute mixture of denatured alcohol and clear shellac, and was ready to prime. For the priming, I used white lead ground in walnut oil to a fairly thick paste, and thinned it to a nice brushable consistency with gum turpentine. The priming was applied with a wide sable brush, then smoothed with a fan blender. The surface dried practically free of brush strokes. Some authorities suggest letting the panel season for 6 months to a year before beginning work, and others say there's no empirical evidence to support that practice. I opted to start with no wait period. Because I'm impatient like that.

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