Introduction
Plaster materials, or gypsum products, are the main materials for working models. They come from the mineral gypsum, calcium sulfate dihydrate (CaSO₄·2H₂O). The whole technology rests on one reversible change: heat removes part of the crystal water, and mixing with water puts it back.
Manufacture
- Beta-hemihydrate: firing in open kilns at 110 to 120 °C at normal pressure gives spongy crystals.
- Alpha-hemihydrate: heating in an autoclave at about 125 °C under 1.3 atm for 6 hours gives regular, dense crystals. Adding a 30% calcium chloride solution improves their density.
- Synthetic alpha-hemihydrate: a by-product of phosphoric acid production, and free of impurities.
Setting reaction
Setting works because hemihydrate is about 4.5 times more soluble than dihydrate. Hemihydrate dissolves until the water holds more calcium sulfate than dihydrate can stay dissolved with, so dihydrate crystallizes out and more hemihydrate dissolves.
Less than half of the mixing water is needed for the reaction. The rest stays between the crystals and later evaporates, leaving pores. This is why more water gives a weaker product.