How is paint made?

1. One swipe of the roller and the old wall disappears under fresh color. Paint goes on as a liquid and turns into a tough, dry skin. What is in the can?
2. The color comes from pigment: powder ground so fine that each bit is far smaller than a grain of sand. A bright white pigment is added to most paints because it hides what is underneath.
3. Powder alone would rub off like chalk. So paint needs a binder: a liquid glue that dries clear and holds the pigment on the wall. Water is added so the paint spreads easily.
4. In the factory, a fast spinning blade mixes pigment into the liquid and breaks up every clump. Smooth, well-mixed paint covers much better than lumpy paint.
5. Each batch is tested. Paint is spread over a card with black and white squares. If the squares show through, it needs more pigment. The color is checked against a sample too.
6. The paint is filled into cans. On the wall, the water dries away into the air. The binder pulls tight around the pigment and leaves a thin, tough skin of color.

The magnifier shows a drop of wet paint, zoomed in thousands of times. Tap each ingredient.

• Color pigment. These tiny particles are the color. Each one soaks up some colors of light and bounces back the rest, and that is the color you see.
• White pigment. White pigment bounces light back in all directions, so the light never reaches the wall behind. That is what lets paint hide an old color.
• Binder. The binder is a clear glue. In the can it floats as tiny soft balls. As the paint dries they squash together and stick the pigment to the wall.
• Water. Water carries everything else and makes the paint runny enough to brush on. Once the paint is on the wall, the water dries away into the air.
• Helpers. Small amounts of helpers do special jobs. Thickeners stop drips, others keep the pigment from sinking to the bottom, and others stop mold growing in the can.
• Dry paint film. This is paint after it dries: pigment locked inside a thin skin of binder. It is thinner than a sheet of paper, but it can last for years.

Change how much pigment and binder go into the mix. The roller paints a test stripe and a cloth rubs it. What makes paint hide? What makes it stay on?

Experiment: What happens to paint with lots of pigment but hardly any binder?

1. Slide the pigment up until the squares are hidden.
2. Now slide the binder all the way down and watch the cloth.
3. Slide the binder back up. What changes?

What we found: Pigment does the hiding: more pigment covers the squares better. Binder does the holding: with too little, the color rubs off like chalk. Good paint needs enough of both, and not too much water.

Turn on JavaScript to try it yourself.

Mix fence paint that hides in one coat, does not rub off and does not run. Pigment costs the most, so use only what you need.

Paint has to do two very different things. In the can it must be a liquid, so that you can brush or roll it. On the wall it must turn into a hard, dry skin that stays put for years. Three kinds of ingredient make that possible.

The first is pigment, a powder ground extremely fine. Pigment gives the color, and it also hides what is underneath. Most paints contain a brilliant white pigment that bounces light back before it can reach the wall. More pigment means better hiding, which is why cheap, watery paint needs coat after coat.

The second is binder. Think of it as glue. Without binder, pigment would be like chalk dust: it would brush straight off. The binder dries into a clear film that locks the pigment in place and sticks it to the wall. Lots of binder makes paint shiny and easy to wash.

The third is the liquid, usually water, which makes the paint flow. After painting, the water dries away into the air and plays no further part. The chocolate milk picture helps here, because both are tiny bits spread through a liquid. But milk stays liquid, while paint is designed to lose its liquid and leave the solid bits behind, stuck fast.

Key words

Pigment

Very fine powder that gives paint its color and hides what is underneath.

Binder

The glue in paint. It dries into a film that holds the pigment on the wall.

Hiding power

How well a paint covers up what is underneath.

Latex

Tiny soft balls of binder floating in water.

Pigment share

How much of the dry paint is pigment, compared with binder.

Check your thinking

1. Which ingredient hides the old color on a wall?

   Show the answer

   Pigment. Pigment particles block and bounce the light, so you cannot see what is underneath.

2. What would paint be like with no binder?

   Show the answer

   It would dry to a powder and rub off. Binder is the glue. Without it, the pigment is just dust sitting on the wall.

3. A paint is nearly all water, with only a little pigment and binder. What will it do?

   Show the answer

   Run down the wall and hide poorly. Too much water makes paint runny, and there are too few color bits in each brushful to hide anything.

4. Where does the water go when paint dries?

   Show the answer

   It dries away into the air. The water evaporates. Only the pigment and the binder are left on the wall.

5. Why is paint ground and mixed so thoroughly in the factory?

   Show the answer

   Smooth paint with no clumps covers better. Clumps waste pigment. Spread out evenly, the same pigment hides much more.