One characteristic of gypsum and an important feature for us is that, when heated, the water content is lost in two steps. First of all, 3/4 of his water is discharged. The resulting calcium sulfate hemihydrate (CaSO4 • 0.5 H2O; mineral name bassanite) can be converted into gypsum by adding water in minutes.
In fact, this is the mysterious white powder that – erroneously – is sold in the shops as gypsum. If the gypsum is further heated, it will lose its entire water content and will form an anhydrous calcium sulfate crystallizing in a hexagonal system, the so-called screed gypsum.
Bassan is sold as stucco gypsum, used in the construction industry, as a powder. Mixed with plenty of water, it solidifies in minutes and turns into fine-grained gypsum.