Dental stones: the water-powder ratio determines accuracy more than the declared class

Gypsum is the cheapest material in the laboratory and the one that determines the accuracy of everything built upon it. A wrong model produces a wrong restoration, whatever the technology.

The ISO classification distinguishes five types. Types I and II are impression and preliminary model plasters; type III is model stone for removable prosthetics; type IV is die stone for precision models; type V has increased expansion.

The difference between types lies not in chemical composition — all are calcium sulphate hemihydrate — but in crystal shape, which determines how much water the mix requires.

Type III requires about 30 millilitres of water per 100 grams of powder, type IV about 22, type V still less. Less water means denser crystals, hence greater hardness and lower porosity.

From this follows the point that matters above all others: the water-powder ratio must be respected, and respected by weighing. Adding water to make the mix flow more easily reduces strength disproportionately to the amount added.

A model mixed by eye with a type IV can end up weaker than a correctly proportioned type III. One pays for the better material and obtains the poorer result.

Setting expansion is the characteristic usually regarded as a defect, whereas it is deliberate compensation. Gypsum expands during setting by a known amount, typically between 0.08 and 0.15 per cent for type IV.

That expansion offsets the contraction of impression materials and of casting alloys. Type V, expanding up to 0.30 per cent, exists for high-shrinkage alloys.

Using a type V where a type IV is called for produces slightly oversized dies, and crowns that come out uniformly wide. It is an error discovered only at try-in.

Vacuum mixing is no luxury: it eliminates air bubbles incorporated during mixing. A bubble on the die surface, right at the margin, produces a restoration that will not close — and the bubble is invisible until the cast is poured.

Working and setting times are also governed by water temperature. Warmer water accelerates setting, colder water slows it, allowing timing to be adjusted without additives.

Chemical accelerators and retarders alter expansion instead, and should be used only where the manufacturer specifies: they modify a parameter that had been calibrated.

In summary: choose the type by intended use and required expansion, respect the water-powder ratio by weighing rather than by eye, and use vacuum mixing to eliminate the most insidious defect — the one invisible until pouring.