How is leather made?

1. Leather starts as a hide, the skin of an animal such as a cow. First the hide is washed and soaked until it is clean and soft. But it will not stay that way.
2. So the hide goes into a big turning drum full of tanning liquid. The liquid soaks in slowly from both sides and turns the hide brown, right to the middle.
3. Zoom in very close. The hide is a tangle of tiny fibers. The tanning bits grab on to the fibers and link them together. Now the fibers cannot rot, and they cannot glue themselves together.
4. The tanned hide is still soaking wet. It is stretched flat on a frame, and warm air dries it slowly so it does not wrinkle or shrink too much.
5. After drying, the leather feels stiff. A machine with many little pins bends and stretches it again and again. That loosens the fibers, and oils help them slide. Now it is soft.
6. Last comes the finish. The leather is dyed, sprayed with a thin coat to protect it, and polished. Then it is cut and stitched into shoes, bags, belts and balls.

Step through the tannery. Watch what each stage does to the hide.

• Soaking. The hide soaks in clean water. Hair and anything else that is not needed is cleaned away, and the hide swells up, soft and ready.
• Tanning. In the turning drum, tanning liquid soaks into the hide from both sides. A thick hide needs longer, because the liquid has further to go to reach the middle.
• Drying. The wet leather is clipped flat to a frame so it cannot curl up. Warm air carries the water away, a little at a time.
• Softening. Rows of pins push and bend the leather as it moves along. Every bend loosens the fibers a little more, so the leather gets softer and softer.
• Finishing. A spray gun adds color and a thin, clear coat that keeps out dirt and water. A polish makes it shine. The leather is finished.

Change the time in the tanning drum and look at the slice in the magnifier. Then pull the ring to bend the dried strip.

Experiment: What happens when a hide is dried without being tanned all the way through?

1. Set the time to zero and pull the ring to bend the strip.
2. Slide the time up until the slice in the magnifier is brown all the way through.
3. Bend the strip again. What changed?

What we found: An untanned hide dries hard, because its fibers glue themselves together, so it cracks instead of bending. Once the tanning liquid has reached the middle, the fibers stay apart and the dry leather bends easily.

Turn on JavaScript to try it yourself.

Make leather for a school bag from a thick hide: tanned right through, and not too stiff or too floppy. Choose the settings, then tap the machines in order.

Leather is made from hides, the skins of animals such as cows. Most hides are left over from farming animals for food, and making leather puts them to good use. A fresh hide does not last, though. If it stays wet, it rots. If it dries, it turns as hard as a board. People worked out how to fix this many thousands of years ago.

The answer is tanning. A hide is made of countless tiny fibers. Tanning liquid soaks in from both sides and its tanning bits grab on to the fibers. The fibers can no longer rot, and they can no longer glue themselves together when they dry. A thick hide needs much longer, because the liquid has to reach the very middle.

After tanning, the hide is leather, but it is not finished. It is stretched on a frame and dried slowly. Then machines bend and stretch it again and again, and oils help the fibers slide past each other. That is what makes leather soft. Stiff leather for belts gets less of this, and soft leather for gloves gets more.

Last comes the finish: color, a thin coat to protect it, and a polish. Our pickle comparison is a good start, but it is not perfect. A pickle only has to keep. Leather also has to bend thousands of times without cracking, which is why the softening steps matter so much.

Key words

Hide

The skin of a large animal, before it is made into leather.

Tanning

Soaking a hide in a special liquid so it will not rot or go hard.

Tannin

A brown stuff from tree bark and leaves that tans hides. It is in tea, too.

Collagen

The tiny, strong fibers that skin is made of.

Staking

Bending and stretching dry leather by machine to make it soft.

Diffusion

The slow spreading of a substance from where there is a lot of it to where there is little.

Check your thinking

1. What does tanning do to a hide?

   Show the answer

   It changes the fibers so they do not rot or go hard. The tanning bits hold on to the fibers, so they stay strong and stay apart.

2. A thick hide and a thin hide go into the drum together. Which is tanned to the middle first?

   Show the answer

   The thin hide. The liquid soaks in from both sides, and in a thin hide it does not have far to go.

3. Why is leather bent and stretched by machines after it is dried?

   Show the answer

   To loosen the fibers so the leather is soft. Each bend pulls the fibers apart a little, and oils help them slide.

4. What would happen if you dried a hide without tanning it?

   Show the answer

   It would go hard and stiff. Without tanning, the fibers glue themselves together as they dry.

5. Which step comes last in making leather?

   Show the answer

   Finishing: color, a thin coat and a polish. The order is soak, tan, dry, soften and then finish.