Gypsum (Industrial)

What gypsum (Industrial) actually is

Mineralogically, gypsum (Industrial) is hydrated calcium sulphate, calcined to plaster of Paris and stucco, and the basis of the entire wallboard industry — CaSO4.2H2O, calcined to the hemihydrate CaSO4.0.5H2O, crystallising in the monoclinic system.

The colour is caused by colourless to white; clay and iron give grey, buff and pink. That is worth knowing because it determines what heating, light and time can and cannot do to the stone.

A refractometer reads 1.520-1.530, and the specific gravity sits near 2.30-2.37; cleavage is perfect in one direction, two others distinct - selenite splits into flexible sheets.

Where gypsum (Industrial) comes from

The recognised sources are San Marcos Island, Baja California Sur, Mexico - one of the largest gypsum quarries in the world, Milford Station, Nova Scotia, Canada, Sorbas, Almeria and Gallocanta, Spain, Naica and Chihuahua, Mexico (the giant selenite crystals of the Cueva de los Cristales) and Iran, Thailand, Turkiye, and the Paris Basin, France (hence 'plaster of Paris').

Origin affects price in this material, so we name the locality on every quote rather than the country. "African" or "Asian" tells a buyer nothing.

How gypsum (Industrial) is treated

Calcined at 120-180 C to the hemihydrate, which rehydrates and sets when mixed with water - a fully reversible chemistry that is also why gypsum board fails in floods. Uses: wallboard and plaster, cement set retarder (about 5% of every bag of Portland cement), agricultural soil conditioner for sodic soils, moulds and dental casts, and art and sculpture. SYNTHETIC GYPSUM MATTERS: flue-gas desulphurisation gypsum from coal power stations supplies a large share of wallboard feedstock in the USA and Europe, and as coal plants close that supply is shrinking, pushing demand back to natural quarries. Selenite and satin spar are the ornamental varieties; both are water-soluble and cannot be worn or washed.

None of this is a problem when it is said out loud. It becomes one the moment it is left out.

What gypsum (Industrial) is confused with

The usual confusions are Anhydrite CaSO4 (the dehydrated form), Calcite and limestone, Alabaster (massive fine-grained gypsum, carved) and Synthetic FGD gypsum.

The separation is rarely a matter of opinion: refractive index 1.520-1.530 and specific gravity 2.30-2.37 settle most of it in a minute, and magnification settles the rest.

What gypsum (Industrial) is used for

Gypsum (Industrial) is bought by heavy industry, architecture and interiors, carving and sculpture, mineral collecting and decorative objects — and each of those trades wants a different form of the same material.

We quote each application in the unit that trade actually buys in, by the tonne, by the square metre or by the block.

Durability and care

Hardness 2 - the Mohs 2 reference mineral; cleavage perfect in one direction, two others distinct - selenite splits into flexible sheets. That combination decides what it can safely be set into.

Below about 5.5 a stone will be scratched by ordinary household dust, which is largely quartz. That does not make it unusable — it makes it a pendant, an earring or an object rather than a ring worn every day.

Warm water, mild soap and a soft brush. Avoid ultrasonic cleaners on anything fracture-filled, stabilised, dyed or visibly included.

What gypsum (Industrial) costs

Indicative pricing for gypsum (Industrial) is $8-20 per tonne for crude rock; calcined plaster $80-200 per tonne; selenite and satin spar specimens $2-50.

That is a range, not a quote. Colour, size, clarity, cut and origin move the figure a long way inside it, and the only honest price is one given against a specific stone. Send us what you need and we will price it.

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