The Plaster Cast: Gypsum That Sets Round a Broken Limb

A broken bone heals only if its ends are held still in line for weeks. Splints of wood and bandages stiffened with starch or egg white took hours or days to harden. Antonius Mathijsen, a Dutch army surgeon at the military hospital in Haarlem, rubbed plaster of Paris into coarse cotton bandages in 1851: soaked in water and wrapped round the limb, they could be moulded to it and set hard within minutes. He published the method in February 1852, and armies adopted it because it could be applied on the battlefield. The chemistry is simple: the hemihydrate takes up water and crystallises as gypsum. But the reaction gives off heat, and the water the mix needs to be workable is far more than the reaction uses. This rung works out the heat and the water, models a setting cast's temperature, and makes a cast round a bottle, measuring its temperature as it sets.