The hero panel has been re-gessoed! Hopefully it'll dry well, and will be sanded smooth next weekend. I tried a little something different with the process this time, ordering Italian rabbit skin glue to mix the gesso. It's an interesting product; darker thatn the stuff I'd been using, but lightens considerably when hydrated and melted. I applied the gesso with a wide brush and smoothed it with a thin, flexible scraper so that hopefully sanding will be minimized.
Once it's dry and smooth, I'll need to seal and prime the surface. In cross sections of Leonardo's work, it appears as though there's not a sealer between the gesso and the white lead layer. On my test panel, I tried applying a thin coat of white lead directly to the smoothed gesso surface. The walnut oil in the paint seems to have penetrated the gesso layer and stained it darker. I'm also thinking that it may compromise some of the gesso's strength. So, even though there doesn't seem to be any evidence of it in the magnified cross sections I've seen, I'm planning on making a dilute solution of white shellac (which is really CLEAR shellac) and denatured alcohol.I'll brush that over the gesso surface to seal it and protect it from the oil layers that'll be applied over it.
The lead white on the test panel is drying slowly, of course. It's a thin layer that I applied with a wide brush, then used a fan blender to smooth so that it's practically free of brush strokes. SInce Leonardo's technique was to use many thin colored glazes of paint to achieve the illusion of depth on his paintings, the underlying surfaces have to be as perfectly smooth as possible. Every step requires more research and intense focus.
The oil penetration on the test panel means I'm researching possible reformulations of the white lead priming to make a highly pigmented, thin and somewhat porous paint for priming the hero panel once it's smooth. I'll be having to grind more lead white paint soon, so I'll likely try some experiments with additives to the paint to give it the desired opacity, thinness and absorbency. One possible configuration will be to mix lead white pigment with the powdered gesso mixture I've been collecting as I sand the panels. I'm not sure what properties this will lend the primer, but that's the nature of experimentation, no?