Gypsum and Plaster of Paris are closely related but not the same material. Gypsum is the raw mineral, a soft rock made of calcium sulfate with two water molecules bound into its crystal structure. Plaster of Paris is what you get when you heat gypsum to drive off most of that water, leaving a dry powder that can absorb water again and harden into a solid. The key difference comes down to water content: gypsum holds two water molecules per unit, while Plaster of Paris holds only half of one.
How Gypsum Becomes Plaster of Paris
The transformation is straightforward. When gypsum is heated to between 110°C and 180°C (about 230°F to 356°F), it loses roughly three-quarters of its chemically bound water and becomes a fine white powder called calcium sulfate hemihydrate. That powder is Plaster of Paris. The ideal temperature for complete conversion sits around 160°C. This heating process is called calcination, and it can happen in industrial kilns or, historically, in open-air fires.
The name “Plaster of Paris” comes from the large gypsum deposits that were extensively mined from the Montmartre district of Paris, where the material was produced for centuries.
What Happens When You Add Water Back
The reason Plaster of Paris is useful is that the process reverses. When you mix the powder with water, it reabsorbs that lost moisture and recrystallizes back into gypsum. Tiny needle-like crystals interlock as they grow, and within minutes the mixture stiffens into a rigid solid. This reaction releases heat, which is why a setting batch of plaster feels warm to the touch. The full chemical cycle is essentially: heat gypsum to make powder, add water to make solid gypsum again, but now in whatever shape you poured it into.
Setting typically begins within a few minutes and the material reaches working hardness in 20 to 45 minutes, depending on the water ratio and any additives. The final product has a density of about 1.6 grams per cubic centimeter and a hardness under 0.6 GPa under normal conditions, making it easy to carve, sand, or shape but too soft for structural load-bearing.
How They Differ in Practice
Raw gypsum is a mineral you can find in nature. It’s one of the softer minerals, rating just 2 on the Mohs hardness scale (you can scratch it with a fingernail). It forms in thick beds from evaporated seawater and is mined on every continent. In its natural state, gypsum is stable and doesn’t react with water because it’s already fully hydrated.
Plaster of Paris, by contrast, is a manufactured product. It’s chemically hungry for water and will absorb moisture from the air if left exposed. That reactivity is its defining feature and the reason it’s useful: you can mix it, pour it, mold it, and let it set into a hard copy of whatever shape you need. Once it sets, though, it has essentially become gypsum again, just in a new form with a dense interlocking crystal structure rather than the layered sheets found in the natural mineral.
Common Uses for Each
Gypsum’s biggest role is as the core ingredient in drywall (also called plasterboard or Sheetrock). It’s also added to cement as a setting regulator and used as a soil conditioner in agriculture, where it improves drainage and adds calcium and sulfur to the soil.
Plaster of Paris has a wider range of hands-on applications because of its ability to capture fine detail as it sets:
- Medicine and rehabilitation: Orthopedic casts for broken bones have traditionally been made from Plaster of Paris, though fiberglass casts are gradually replacing them. It’s also used to create molds of body segments for prosthetic and orthotic devices.
- Dentistry: Dental professionals use it to make precise models of teeth and oral tissues, preserving exact shapes and alignments for treatment planning.
- Construction: Interior plasterwork, decorative moldings, ceiling tiles, and cornices all use variations of the material.
- Art and craft: Sculpture casting, mold-making, and pottery rely on Plaster of Paris for its ability to reproduce fine surface detail.
Safety Considerations
Plaster of Paris powder can irritate your eyes, skin, and respiratory system. Breathing in gypsum dust repeatedly, especially during mixing or sanding, can cause coughing and irritation of the nose and throat. Working in a ventilated area and wearing a dust mask during mixing or sanding is a practical precaution.
The setting reaction also generates heat. For thin applications like craft molds, this warmth is barely noticeable. But thick layers or large volumes can get hot enough to cause discomfort, which is one reason medical professionals monitor temperature carefully when applying orthopedic casts directly against skin. Keeping the plaster layer at an appropriate thickness and using room-temperature water helps manage heat buildup.
The Short Version
Gypsum is the naturally occurring mineral. Plaster of Paris is gypsum that has been heated to remove most of its water, turning it into a reactive powder. Add water to that powder and it transforms back into solid gypsum in whatever shape you’ve poured it into. They’re the same chemical compound at different stages of hydration, which is why the two names often get used interchangeably even though they describe different points in the same cycle.