Powder gypsum for the preparation of base materials for gouache gilding (priming).
Fine-grained powder calcium sulphate hydrate
Gypsum for gilders is used together with a mixture of rabbit-skin glue and water (called "colletta") for the creation of gypsum backgrounds for gouache gilding (priming). The first stage of water gilding consists in preparing the “colletta”: mix the rabbit-skin glue with water (1 part glue and 8 parts water) and let the mixture rest for about 12 hours so that the glue softens. Then place it in a bain-marie (hot water bath) for about 8 hours until it is blended well. It is important to make sure that the mixture does not come to a boil. After letting everything rest for another 2 or 3 hours, add part of the colletta to the Bologna gesso, which you have previously sifted, with an approximate proportion of 1 part of gesso and 3 parts colletta. The gesso should be added gradually and without stirring to prevent bubbles from forming once it has been spread and dried. The result is a dense, but semi-liquid compound that must be applied while still hot on the surface you are gilding. This procedure is also known as gessoing. The gesso and colletta mixture must be spread by using a brush, with several coats on a surface that is clean, smooth and free of dust or particles. After applying each coat, let the gesso dry and sand the surface with coarse-grained sandpaper in order to sand down any imperfections. Proceed until the surface is homogeneous and well polished. Once the last coat has been applied and it has dried, it is important to finish the surface with a scraper to recreate the sharp edges that need to be gilded, but be careful not to remove the gessoing. For the final sanding, we suggest using progressively finer and finer sandpapers in order to obtain a perfectly smooth surface.
Store in a cool, dry place in its own container, closed, and away from direct light and heat. The ideal temperature is 15-25°C with humidity under 50%.
Dilution percentage, application method and quantities applied may significantly affect the workability time. The most suitable application methods should be determined by experimentation on the basis of the surface being gilded and the desired result.