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
Einstein's Blackboard writing on the blackboard, although ephemeral in nature, is of historic interest because the equations displayed are taken from a model of the universe proposed by Einstein in May 1931 known as The Friedmann-Einstein universe. The last three lines on the blackboard are estimates of the density of matter in the universe "ρ", the radius of the universe "P" and the timespan "t" of the expansion of the universe respectively ("L.J." on the blackboard indicates "light years" in German). It has recently been shown that these estimates contain a systematic numerical error. The blackboard is considered a "mutant" object or