What Is Perlite Made Of? Volcanic Glass, Popped Like Corn
Perlite is volcanic glass heated to 900°C until its trapped water flashes to steam and expands it. That is why it is white, light and permanent.
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Volcanic glass, popped like corn. Crushed rock heated to about 900°C until the water trapped inside flashes to steam and blows it up to many times its size.
How it is made
- Mined as perlite rock — a glassy volcanic deposit
- Crushed and graded to the required particle size
- Heated to about 900°C in a furnace
- The trapped water flashes to steam, and the softened glass expands into bubbles
- Cooled, producing closed-cell white granules up to twenty times the original volume
That is the entire process. No binders, no coatings, no chemical treatment — which is why it is accepted under most organic growing standards.
Perlite against vermiculite
| Perlite | Vermiculite | |
|---|---|---|
| Raw material | Volcanic glass | Expanded mica |
| Structure | Closed cells | Layered, spongelike |
| Does what | Holds air, drains | Holds water and nutrients |
| Appearance | White, hard, crunchy | Brown-gold, soft, flaky |
| Use it for | Cuttings, succulents, wet mixes | Seed starting, moisture lovers |
| CEC (nutrient holding) | Negligible | Significant |
The one-line version: perlite for drainage, vermiculite for retention. A mix that stays soggy wants perlite; a seed tray that dries out by lunchtime wants vermiculite.
The asbestos question belongs to vermiculite, not perlite
Perlite has no asbestos association at all. The concern people half-remember relates to a single historic US vermiculite mine whose deposit was contaminated with asbestos, which was closed in 1990.
Modern vermiculite is tested, and perlite was never part of that story.
The dust, which is a real consideration
Perlite is inert, but dry perlite throws a fine silica-bearing dust that irritates eyes, nose and throat.
- Dampen the bag before opening and pouring — a splash of water settles it
- Wear a mask when mixing large volumes
- Mix outdoors or in a ventilated space
- Rinse it if you are using it alone for cuttings
What it actually does in a mix
- Creates permanent air space so roots get oxygen
- Prevents compaction as organic matter breaks down around it
- Improves drainage without reducing total water held at the surface
- Lightens heavy mixes, which matters for large containers
- Lasts. It will still be doing the job in ten years
The usual proportion is 10 to 30 per cent by volume for houseplants, and up to half for succulents, cacti and cuttings. Snake plants and similar want the high end — see repotting a snake plant for what that mix looks like.
Common questions
What is perlite made of?
Is perlite natural or man-made?
Does perlite break down in soil?
Is perlite safe to handle?
What is the difference between perlite and vermiculite?
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