Frame in White (Gesso)The heretic comes from my "unusual" techniques and statements, and I also don't take my recipes and methods quite as seriously as some, but I'm involved with hide glues daily as a part of my profession, and I have been around for awhile. Lazy, too, so lazy that I often work very hard at trying to avoid work. Cennini was an obsessive-compulsive, whose recipes are incredibly over wrought, by the way. Heresy.
(Edited several times)
Everybody has a different recipe for gesso, and I think I've read them all, they all vary, so ultimately you have to make your own way. Ratios are, to me weird, because glue is a dry product measured by weight, and water is by volume. Ratio? Also, there are different strengths of glue. The designation, RSG, is not rabbit skin glue, but hide glue, of a high strength. (Here's a ratio: rabbits to beef ... ) This is not particularly important because you need to do this by feel. Recipes are a starting point, and like some of the best cooking, subject to modification.
Start with 40-60 grams by weight of your dry glue material, to 20 ounces of water by volume. Soak, (length of soak depends on the granule size, or if you are using leaf or sheet glue. It needs to be wet, completely.) heat to 140-150 degrees, and then allow to gel, and cool. It should be a gelled material, firm enough to push with a finger, but soft enough that it can be fissured by pressing two fingers on to the gel and spreading them. Adjust your mixture to achieve that. Add whiting, French chalk, precipitated calcium carbonate, whatever. For a 20 oz. mix, I use 2 of the small tuna cans of precipitated calcium carbonate, by volume. (Very scientific, ehh?) The PCC is very even in texture, and is the same chemistry as chalk, etc. I usually let my mix sit, for at least a day, refrigerated if it is hot. This allows the PCC to absorb the liquid, and everything smooths out. My first coat is this mix, brush scrubbed into the surface, and licked off, so there is no real surface coating; very thin. It is absorbed into the surface of the wood. Then I spray subsequent coats. Part of the reason for using my gesso mix as the sizing coat, is I can very clearly see that I have coverage, and I'm lazy. Once one has a feel for the mixture, it can be adjusted for different batches of glue, and for different properties, such as how hard it is to sand and carve. For frames, that require hand chasing and sanding I tend to a softer gesso. For panels, where I can use a power sander, a little harder is fine.
I also mix and heat my gesso in quart type canning jars, using a small microwave to heat the mixture. I use a quick read thermometer to learn how much time and at what power level on the microwave, aiming for 140 - 150 degrees. (electric glue pots, heating glue all day long, are set for approximately 150 degrees, and whatever you call it, it's hide glue, same stuff chemically)
Temperature is a touchy subject, some saying heating a mix to 140-150 degrees will ruin the mix, or when it touches the cooler surface of the substrate, will create pinholes. Actually, by the time it gets from the container, on the brush, and to the application, it has cooled considerably. I think pinholes come from not doing the first coat scrubbed and then "licked" off, vigorously. Licking it off is the important part. Spraying, even hot gesso, is cool by the time it reaches the substrate.
Additives: Uniodized ( I don't know that the iodine would matter, but it's that lazy thing) table salt can be added as a gel suppressant, to retard the setting of the gesso, letting air bubbles rise and escape, and also leveling the surface. Other gel suppressants are urea, used to make liquid hide glue, and large mammal urine. This is why some ancient recipes call for urinating in the mix on a dark and stormy night under an old oak tree. I'll just use salt, about a tablespoon (the palm of my hand) for my 20 ounce mix. I also add a "splash" of alcohol to the mix as a wetting agent, and sometimes to the framer, as a calming agent. This probably helps with pinholes, and helps the framer stop gyrating. I have some quart cans of solvents with squirt nozzles on them, so I'll count to 10 as I'm adding alcohol to the 20 oz. mix. Very scientific.
As with the salt, plain, I will often use distilled water, especially for bole, or filtered water for gesso. This falls under the heading of "materials safety", in that I eliminate things that might cause problems, such as all the additives in tap water, the iodine, etc..
So there, the lazy heretics guide to gesso.
An earlier post about pinholes.