---
title: "Alumina — density, melting point and properties"
description: "Alumina has a density of 3,900 kg/m³ (3,700–3,980). Melting point, strength, conductivity, what it is used for, and whether it floats, rusts or is magnetic."
url: https://atoneplace.net/material/alumina/
site: "At One Place"
---

# Alumina

**Density of alumina: 3,900 kg/m³ (3,700–3,980)**

3.90 g/cm³ · 243.5 lb/ft³

The workhorse technical ceramic: hard at Mohs 9, an excellent electrical insulator, and cheap because it is made from the same bauxite feedstock as aluminum. Its stiffness of 370 GPa is nearly twice steel's, but it has no ductility whatsoever, so it fails suddenly from whatever flaw happens to be largest — which is why ceramic strength is quoted as a statistical distribution and why the tensile figures here are half the flexural numbers usually advertised. Corundum, ruby and sapphire are the same compound as single crystals.

## Properties

- **Density:** 3,900 kg/m³ (3,700–3,980) — 3.90 g/cm³
- **Melting point:** 2,072 °C
- **Tensile strength:** 200–300 MPa
- **Young's modulus:** 370 GPa
- **Thermal conductivity:** 30 W/(m·K)
- **Hardness:** 9 Mohs
- **Thermal expansion:** 8.1 µm/(m·K)
- **Floats in water:** No
- **Magnetic:** No

## What it is used for

- Grinding wheels and abrasives
- Electronic substrates and insulators
- Wear-resistant liners and pump seals
- Ceramic body armour plate

## 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
- alumina: 3,900 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× |
| alumina ← | 3,900 kg/m³ | 3.90× |
| platinum | 21,450 kg/m³ | 21.45× |
| gold | 19,300 kg/m³ | 19.30× |
| tungsten | 19,250 kg/m³ | 19.25× |

## Does it rust?

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

## Is it magnetic?

Alumina is not a metal and has no ferromagnetic response.

## Common questions

### What is the density of alumina?

3,900 kg/m³ (3,700–3,980), or 3.90 g/cm³.

### Does alumina float?

No. At 3,900 kg/m³ it is denser than water.

### Is alumina magnetic?

Alumina is not a metal and has no ferromagnetic response.

## Related pages

- [Does alumina float?](https://atoneplace.net/does-it-float/alumina/)
- [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/)
- [Plaster](https://atoneplace.net/material/plaster/)

## Sources

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