Pumice stone — sometimes called ‘floating stone’ — is a lightweight, porous natural stone containing so many trapped air bubbles that it floats on water. It forms when volcanic magma cools rapidly and the gases dissolved in the molten rock become ‘frozen’ into a sponge-like structure.
Properties
- Density — 250–900 kg/m³ (lighter than water when pores are closed)
- Open or closed pores — determines whether the stone floats or fills with water
- Insulating — air pockets give a low thermal conductivity
- Fire resistant — volcanic origin, unaffected by fire
- Easy to work — can be cut with an ordinary hand saw
Types of pumice-like stone
| Name | Origin | Use |
|---|---|---|
| Pumice | Foamy volcanic glass | Abrasive, lightweight concrete |
| Bims | Eifel region, Germany | Bims concrete blocks, insulation |
| Tuff | Volcanic ash | Building stone, facing |
| Lava stone | Volcanic lava | Paving, oven linings |
Applications
Pumice and bims are used for:
- Lightweight concrete — aggregate for lighter structural concrete
- Pumice blocks — for internal walls and non-load-bearing partitions
- Chimneys and flues — thanks to high heat resistance
- Loose-fill insulation — poured into floors and roofs
- Decorative paving — as a rough, ornamental element
Historical and cultural
In antiquity, pumice was already used for Roman vault construction precisely because lightweight stones reduced the load on supporting walls. The Pantheon in Rome has a dome in which lighter pumice blocks are placed higher up while heavier stones sit lower down. This clever weight gradient enabled a clear span of nearly 44 metres.
Difference from other lightweight stones
- Aerated concrete — factory-made foamed concrete, not natural
- Lava concrete — concrete with lava aggregate, heavier than pumice concrete
- Expanded clay — kiln-fired clay pellets
- Pumice / bims — natural volcanic product
Related terms
- Bims
- Lightweight concrete
- Tuff
- Aerated concrete
- Volcanic rock
- Natural stone
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