Calcium sulfate an unusual property: when mixed with water at normal (ambient) temperatures, it quickly reverts chemically to the preferred dihydrate form, while physically "setting" to form a rigid and relatively strong gypsum crystal lattice: This reaction is exothermic and is responsible for the ease with which gypsum can be cast into various shapes including sheets (for drywall), sticks (for blackboard chalk), and molds (to immobilize broken bones, or for metal casting). Mixed with polymers, it has been used as a bone repair cement. Small amounts of calcined gypsum are added to earth to create strong structures directly from cast earth, an
Liquid chalk However, some gyms require liquid chalk because it leaves less residue on gym equipment. Liquid chalk adheres to the hand better, reducing the need to re-chalk. Some liquid-chalk mixtures for climbing are made with magnesium carbonate, colophony, and ethanol or an alcohol that dissolves the colophony and quickly evaporates from the solution (as isopropyl alcohol or ethanol). Sometimes, resin or rosin is added to increase gripping properties or an additive for aroma is included because of the bad smell of spirit. Sports liquid chalk is sold in bottles. The user takes a small amount into their palms, spreading the chalk