---
title: "What is a Blackboard chalk made of?"
description: "Mostly calcium sulfate — plaster of Paris — rather than the calcium carbonate the name suggests. Every component, the material it uses, and the constraint that made "
url: https://atoneplace.net/made-of/chalk/
site: "At One Place"
---

# What is a Blackboard chalk made of?

**What is a Blackboard chalk made of?: Mostly calcium sulfate — plaster of Paris — rather than the calcium carbonate the name suggests.**

Geological chalk is a soft limestone made of the plated skeletons of microscopic algae that settled on Cretaceous sea floors, and blackboard chalk was originally exactly that. Modern sticks are usually gypsum-based instead, because casting plaster gives a consistent hardness and a stick that writes without gouging the board. The distinction matters for anyone tempted to try classroom chalk in a garden or an aquarium, where the two minerals behave completely differently. Sidewalk chalk is the same gypsum in a fatter stick, and the "chalk" that gymnasts and climbers use is neither — it is magnesium carbonate, chosen because it absorbs sweat rather than because it makes a mark.

## Component by component

*Every material is a choice, and the third column is the constraint that drove it.*

| Part | Material | Why that material |
| --- | --- | --- |
| body | calcium sulfate hemihydrate (plaster of Paris), cast and dried | Plaster can be poured into a mould and sets in minutes, which is far cheaper than quarrying and cutting natural chalk. |
| traditional body | calcium carbonate, from natural chalk or limestone | It is softer and dustier; it survives in premium and dustless sticks, sometimes bound with a little clay. |
| colour | inorganic pigments for coloured sticks | Only mineral pigments survive being mixed into a wet, strongly alkaline plaster slurry. |
| dustless coating | binder or oil that makes particles clump | Dustless chalk is not dust-free; the particles are made heavy enough to fall on the tray instead of hanging in the air. |

## Other stationery and paper objects

- [Pencil](https://atoneplace.net/made-of/pencil/) — Graphite mixed with clay and fired, glued inside two grooved halves of softwood. No lead, ever.
- [Paper](https://atoneplace.net/made-of/paper/) — Cellulose fibres from wood pulp, plus up to a third mineral filler, sizing and brightener.
- [Ballpoint pen](https://atoneplace.net/made-of/ballpoint-pen/) — A tungsten carbide ball a millimetre across, rolling in a brass socket, fed by thick oil-based ink.
- [Glitter](https://atoneplace.net/made-of/glitter/) — Aluminium-coated polyester film, colour-coated and chopped into fragments — by definition a microplastic.
- [Crayon](https://atoneplace.net/made-of/crayon/) — Paraffin wax with pigment stirred in, poured hot into a mould and cooled. Almost nothing else.
- [Cardboard](https://atoneplace.net/made-of/cardboard/) — Two flat kraft paper liners glued either side of a fluted paper medium — the flutes do all the work.
- [Eraser](https://atoneplace.net/made-of/eraser/) — Either natural rubber with fine abrasive and oil, or plasticised PVC — two different ways of lifting graphite.
- [Playing card](https://atoneplace.net/made-of/playing-card/) — Two sheets of paper laminated around a black glue core, then coated in plastic — or solid PVC.
- [Postage stamp](https://atoneplace.net/made-of/postage-stamp/) — Coated paper with phosphor bands, printed in security inks, on either water-activated gum or a self-adhesive.

## Common questions

### What is a Blackboard chalk made of?

Mostly calcium sulfate — plaster of Paris — rather than the calcium carbonate the name suggests.

### What is the body of a Blackboard chalk made of?

Calcium sulfate hemihydrate (plaster of Paris), cast and dried. Plaster can be poured into a mould and sets in minutes, which is far cheaper than quarrying and cutting natural chalk.

### What is the traditional body of a Blackboard chalk made of?

Calcium carbonate, from natural chalk or limestone. It is softer and dustier; it survives in premium and dustless sticks, sometimes bound with a little clay.

## Related pages

- [Pencil](https://atoneplace.net/made-of/pencil/)
- [Paper](https://atoneplace.net/made-of/paper/)
- [Ballpoint pen](https://atoneplace.net/made-of/ballpoint-pen/)
- [Glitter](https://atoneplace.net/made-of/glitter/)
- [Crayon](https://atoneplace.net/made-of/crayon/)
- [Cardboard](https://atoneplace.net/made-of/cardboard/)
- [Eraser](https://atoneplace.net/made-of/eraser/)
- [Playing card](https://atoneplace.net/made-of/playing-card/)
- [Tennis ball](https://atoneplace.net/made-of/tennis-ball/)
- [Banknote](https://atoneplace.net/made-of/banknote/)
- [Golf ball](https://atoneplace.net/made-of/golf-ball/)
- [Glass](https://atoneplace.net/made-of/glass/)
- [Soccer ball](https://atoneplace.net/made-of/soccer-ball/)
- [Marshmallow](https://atoneplace.net/made-of/marshmallow/)

## Sources

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