Rabbit Skin Glue
A strong glue made from animal parts, that is an ingredient in genuine gesso, is used for sealing (sizing) panels and canvas before priming and is used as sizing for papers. It stiffens canvas in preparation for gesso primer in oil painting. Also called hide glue.
For preparing canvas and panels the usual method is to soak the pellets or powder overnight, the next day heat in a double boiler and brush onto the canvas while still warm. Two coats are preferred to seal the canvas well, the first being scrubbed into the canvas to get well into the weave. Discard any left over as it does not re-heat well. Then prime the surface as normal.
Also see Gesso