Why Does Baking Mud Make Stone?

Step 1 of 5

Drying and firing are not the same thing.

A sun-dried mud brick goes soft again in rain, because only the water left. A fired brick does not, because the grains themselves have changed.

Step 2 of 5

Clay is made of tiny flat plates.

Wet clay works because those plates slide over each other on a film of water. That is why it can be squeezed into any shape you like.

Step 3 of 5

The first heat drives the water out for good.

Some water is chemically locked inside the plates. Above about 500°C it leaves, and the plates collapse into something that can never take it back.

Step 4 of 5

Hotter still, the edges begin to melt.

Around 900 to 1,100°C the grains soften just at their contact points and weld to each other. The brick becomes one piece rather than a heap of grains.

Step 5 of 5

Too hot and it slumps.

Push much past the fusing point and enough of it melts that the brick sags out of shape. Brickmaking is a narrow window, held carefully.

Fired bricks from Mohenjo-daro are about 4,500 years old and still hold their shape and size. Sun-dried bricks from the same era survive only where it never rains.