Canvas is usually stretched across a wooden frame called a stretcher and maybe coated with gesso prior to being used to prevent oil paint from coming into direct contact with the canvas fibres which would eventually cause the canvas to decay. A traditional and flexible chalk gesso is composed of lead carbonate and linseed oil, applied over a rabbit skin glue ground; a variation using titanium white pigment and calcium carbonate is rather brittle and susceptible to cracking. As lead-based paint is poisonous, care has to be taken in using it. Various alternative and more flexible canvas primers are commercially available, the most popular being a synthetic latex paint composed of titanium dioxide and calcium carbonate, bound with a thermo-plastic emulsion.