Gypsum plaster

Gypsum is calcium sulfate with two water molecules built into its crystal. Heated to around 150 °C it gives up three quarters of that water and becomes the hemihydrate — plaster of Paris. Mixed with water it takes it back and recrystallises into an interlocking mass of gypsum needles, and it has set.

The cycle is reversible, which is unusual and useful: plaster is one of very few building materials that can be reduced back to its raw material and reused.

And the retained water is why gypsum products resist fire. In a fire the plaster gives that water up again, and doing so absorbs a great deal of heat and holds the surface near 100 °C until it has all gone. A plasterboard partition buys a measured number of minutes for exactly that reason, and it is chemistry rather than the board being hard to burn.

Gypsum rock is crushed and calcined in a kettle or rotary kiln. Setting takes minutes and is controlled with retarders, which is what separates a casting plaster from a wall plaster from a dental one.

It expands very slightly on setting — a fraction of a per cent — which is why it takes a fine mould impression, and why it is the material of casts, moulds and dental models rather than something that shrinks away from the detail.

Plasterboard, which is a gypsum core between paper liners and is the internal wall surface of most modern buildings in the English-speaking world. Wall and ceiling plaster. Cornices and decorative mouldings.

Outside construction: casts for broken limbs, dental models, sculpture and pattern moulds, and the moulds that slip-cast ceramics are made in — where the plaster's porosity draws water out of the clay slip and is the reason the process works.

Used in Egypt from before 3000 BC and in Anatolia earlier still; some of the oldest plastered surfaces known are Neolithic. Plaster of Paris takes its name from the gypsum of Montmartre, worked from the medieval period and mandated for use on Parisian timber buildings after fires — an early fire regulation resting on exactly the property described above.

Plasterboard is an American invention of the 1890s and became dominant after the Second World War, because it replaced a skilled wet trade with a dry one.

Calcining gypsum takes far less energy than making cement or lime, because it is driving off water rather than decomposing a carbonate — no carbon dioxide comes out of the rock.

It is genuinely recyclable and increasingly is: clean plasterboard waste is reprocessed into new board. Its disposal is the problem, and a specific one. In a landfill, gypsum in anaerobic wet conditions is reduced by bacteria to hydrogen sulfide, which is toxic and smells, so gypsum waste is banned from mixed landfill in many jurisdictions and must be segregated.