Gypsum and glue
Weigh 10 grams of rabbit glue in grains or ground, and put it for a few hours to swell in 180 cc of water (for very large canvases 30 grams of glue and 540 cc of water), it is good to add half a teaspoon of anti-fermentative, this will help preserve the unused glue for much longer.
Heat the mixture in a bain-marie, when it is melted remove both containers from the heat and slowly add some sifted gilder’s gypsum until you see the gypsum powder floating on the surface, then continue adding gypsum until all the glue is completely saturated.
Mix well with a brush and add up to 90 cc of hot water to make the mixture less dense and honey to make it more elastic. If you want to give a color to the preparation, pigments or dyes, such as walnut mordant, diluted in water can be added.
Mix well and spread the mixture on the canvas with a brush, when the layer is almost dry (lighter patches are formed) you can give a second coat, depending on how much the texture of the canvas must remain visible.
Spread a coat of rabbit glue over the entire dry preparation. This last layer can be avoided if the painting is to be done in egg tempera and also when a watercolor drawing and a tempera sketch are required. On this you can follow a painting with soft resins, or soft and hard resins such as coppale, to avoid having a too absorbent base.
In order not to undergo loosening over time, the canvas must be damaged (part of the fibers is removed) and prepared with a coat of rabbit glue before being treated, to perform this operation you need a very strong temporary stretcher on which to stretch the canvas and wet it with very hot water at least once or twice before being able to spread the hot glue on the newly re-tensioned cloth.
It is possible to use a commercial canvas treated with gypsum and give it a coat of gypsum and glue preparation. It is preferable that the operation is preceded by a layer of glue and also, in the case of canvases prepared with acrylic plaster, a scraping of the surface with fine sandpaper.
Panels and tablets
This preparation is also perfect for wood panels: in this case the glue can be stronger, meaning that the quantity of water must be reduced compared to that of the glue: for 10 grams of rabbit glue, about 140 cc of water can be used (dilution 1:14 instead of 1:18).
Usually no dyes are added to the compound and it is left white.
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