CaSO4 – calcium sulfate – can be either an odorless white powder or a colorless crystalline solid. It exists in various forms and states of hydration – it can form crystals together with 2 or 0.5 molecules of water. These forms are known as gypsum and plaster of Paris. It can also exist in an anhydrous state, with no molecules of water at all. Blackboard chalk is in fact 60% gypsum, but people often simply refer to it as "chalk." Many cultures, including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire, and medieval England, used a white variety of gypsum known as alabaster in sculpture. Nowadays, calcium sulfate is used for a variety of purposes including building materials, as a desiccant, as an impression material in dentistry, in medicine as a tablet excipient, and as a coagulant in producing tofu.