---
title: "Plaster — density, melting point and properties"
description: "Plaster has a density of 1,100 kg/m³ (900–1,300). Melting point, strength, conductivity, what it is used for, and whether it floats, rusts or is magnetic."
url: https://atoneplace.net/material/plaster/
site: "At One Place"
---

# Plaster

**Density of plaster: 1,100 kg/m³ (900–1,300)**

1.10 g/cm³ · 68.7 lb/ft³

Gypsum plaster sets by rehydration, not drying: the powder is calcium sulfate that has had its water baked out at about 150 °C, and adding water back rebuilds the crystal, which is why it warms as it goes off and why the reaction cannot be reversed by wetting it again. That same chemistry makes it fire-resistant — heat has to boil off the chemically bound water, holding the surface near 100 °C for as long as the water lasts. It has no melting point; heated far enough it simply dehydrates back to powder.

## Properties

- **Density:** 1,100 kg/m³ (900–1,300) — 1.10 g/cm³
- **Tensile strength:** 1.5–3 MPa
- **Young's modulus:** 5 GPa
- **Thermal conductivity:** 0.4 W/(m·K)
- **Floats in water:** No
- **Magnetic:** No

## What it is used for

- Wall and ceiling finishes
- Casting and mould making
- Orthopaedic casts and bandages
- Cornices and decorative mouldings

## How it compares

*Water is 1,000 kg/m³. Anything with a shorter bar than water floats in it.*

- air: 1.2 kg/m³
- expanded polystyrene foam: 20 kg/m³
- rigid polyurethane foam: 40 kg/m³
- Water — the float line: 1,000 kg/m³
- plaster: 1,100 kg/m³
- tungsten: 19,250 kg/m³
- gold: 19,300 kg/m³
- platinum: 21,450 kg/m³

*Water is 1,000 kg/m³. Anything below that floats.*

| Material | Density | Relative to water |
| --- | --- | --- |
| rigid polyurethane foam | 40 kg/m³ | 0.04× |
| expanded polystyrene foam | 20 kg/m³ | 0.02× |
| air | 1.2 kg/m³ | 0.00× |
| plaster ← | 1,100 kg/m³ | 1.10× |
| platinum | 21,450 kg/m³ | 21.45× |
| gold | 19,300 kg/m³ | 19.30× |
| tungsten | 19,250 kg/m³ | 19.25× |

## Does it rust?

Plaster contains no iron, so it cannot rust. Whether it degrades depends on the material, not on oxidation.

## Is it magnetic?

Plaster is not a metal and has no ferromagnetic response.

## Common questions

### What is the density of plaster?

1,100 kg/m³ (900–1,300), or 1.10 g/cm³.

### Does plaster float?

No. At 1,100 kg/m³ it is denser than water.

### Is plaster magnetic?

Plaster is not a metal and has no ferromagnetic response.

## Related pages

- [Does plaster float?](https://atoneplace.net/does-it-float/plaster/)
- [Concrete](https://atoneplace.net/material/concrete/)
- [Brick](https://atoneplace.net/material/brick/)
- [Gypsum board (drywall)](https://atoneplace.net/material/gypsum-board/)
- [Tungsten carbide](https://atoneplace.net/material/tungsten-carbide/)
- [Reinforced concrete](https://atoneplace.net/material/reinforced-concrete/)
- [Graphite](https://atoneplace.net/material/graphite/)
- [Porcelain](https://atoneplace.net/material/porcelain/)
- [Silicon carbide](https://atoneplace.net/material/silicon-carbide/)

## Sources

- Typical range compiled from manufacturer specifications — At One Place (/about/)