Rabbit skin glue is a traditional organic sizing material used for centuries to prepare canvas for oil priming. Conservation scientists caution painters that rabbit skin glue absorbs atmospheric moisture on damp days and swells, gives off moisture on dry days and shrinks. This movement of the size layer can cause aged oil paintings to crack according to the Smithsonian Conservation Lab. Still, it is widely used by traditional oil painters to size linen or cotton canvas prior to applying an oil ground. This size acts as a barrier to protect the fabric from deterioration from the oils in the primer. Rabbit skin glue is also mixed with titanium white and precipitated chalk to create a dry gesso which is an ideal surface for encaustic and egg tempera paintings. Gilders also used rabbit skin glue to prepare bole. Rabbit skin glue is a dry granular powder and must be dissolved in warm water.