Paint

Last updated: August 18, 2009.

If you don't like it, paint over it. It's certainly true that paint has the power to change things. You can brighten up a room with a colorful picture in much the same way that you can make your house look more attractive with a fresh coat of paint. This is probably why we think of painting as a kind of "alchemy"—a way of using chemicals to change something we don't like into something we do. But have you ever stopped to think exactly what paint is or how it works? Let's take a closer look!

Photo: An artist's oil painting palette.

What is paint?

Paint is protection—much more than just color in a tin or a tube. You don't necessarily paint things just to make them look better. You paint the outside of your home to keep the rain out. We paint cars and bicycles partly to make them look good, but also to stop the metal inside them from going rusty. On airplanes and the Space Shuttle, paint is one of the things that protects the structure when air, rushing past at high speed, creates friction and heat.

Photo: Spray-painting cylinders in a chemical plant. There's no real need to paint things in a factory for the sake of appearance, but a coating of paint helps to protect things from water and chemicals in the air. Note how painters wear protective clothing and breathing masks to save them inhaling the potentially harmful solvents in the paint. Photo by courtesy of US Department of Energy.

What sort of chemicals are inside paint?

You might think paint is just a color chemical dissolved in a liquid to make it spread, but it's a bit more than that. Most paints actually have three main components called the pigment, the binder, and the solvent. (The binder and solvent are sometimes collectively called the vehicle.) There are also typically a number of additives to improve the paint's properties in various ways, depending on where and how it's going to be used.

Pigments

Photo: Left: Paint pigments on sale in Turkey. Photo by courtesy of Wayne Noffsinger, published on Flickr under a Creative Commons Licence.

Photo: Right: Titanium white paint is so-called because it's made with titanium dioxide pigment.

The pigment is the color chemical in a paint. It looks a certain color because it reflects some wavelengths of light and absorbs others (see our article on light for an explanation of how colors work). Traditionally, metal compounds (salts) are used to create different colors so, for example, titanium dioxide (a bright white chemical often found in sand) is used to make white paint, iron oxide makes yellow, red, brown, or orange paint (think of how iron turns rusty red), and chromium oxide makes paint that's green. Black (arguably not a color) comes from particles of carbon (think what your burned toast looks like and you're getting close to a color chemical known as "carbon black"). Different pigments are mixed together to make paint of any color you can imagine.

Binders

Pigments are typically solids, so you couldn't use them to paint by themselves. They'd be difficult to apply, they wouldn't spread evenly, they wouldn't stick to paper or a wall, and they'd wash straight off if they got wet. That's why paints also contain substances called binders. Their job is to glue the pigment particles to one another, but also to make them stick to the surface you're painting. Some binders are made from natural oils such as linseed oil, but most are now made from synthetic plastics. Visualize the binder as an invisible skin of plastic with a colorful pigment dispersed through it and you can see just how a paint gives a layer of protection.

Solvents

Mix a pigment and a binder and you get a thick gloopy substance that's difficult to spread. Ever tried painting a wall with treacle? That's what using a pigment and a binder is like. It's the reason why paints have a third major chemical component called the solvent. As its name suggests, a solvent is something that dissolves something else. The solvent's job is to make the pigment and binder into a thinner and less viscous (more easily flowing) liquid that will spread evenly (that's why paint solvents are sometimes called thinners). Once the paint has spread out, the solvent evaporates into the air, leaving the paint evenly applied and dry beneath it. When you apply a really nasty paint and there's a smell lingering for days while it dries, that's the solvent evaporating into the air.

Water is the best-known and most versatile solvent we have and it's widely used in water-based paints, including emulsions (for walls) and watercolor paints (for paintings). When you paint a picture with watercolors, you're using water as a solvent to dissolve some pigment on your brush that you can easily spread on the paper.

Other paints (including oil and gloss paints) use solvents made from strong organic (carbon-based) chemicals extracted from petroleum. If you leave paints sitting in tins and jars, gravity gradually separates them into their different chemical components. Typically you find the solvent sitting on top as a reasonably clear, thin fluid with the binder and pigment making up a thick, opaque sludge underneath. That's why it's always important to stir tins of paint before you use them.

Photo: Gloss paint uses oil-based solvents so it spreads evenly. It's usually much thicker and more opaque than water-based emulsion and the oily solvents have a powerful smell that can linger for days afterward. Photo by Brian M. Brooks courtesy of US Navy and Defense Imagery.

Additives

Apart from the pigment, binder, and solvent, most paints also have chemical additives of various kinds. For example, ceramic substances can be added to paints to improve their strength and durability. Fluorescent pigments added to paints make them glow in the dark. Additives in paint designed for outdoor use can help to make things waterproof and rustproof, protect against frost or sunlight, and keep them free of mold and mildew.

Photo: Painting safety stripes. Photo by James R. Evans courtesy of US Navy and Defense Imagery.

How paints are made

Although there are many different types of paint, they are broadly all made the same way. First, the pigment is prepared. If it's made from a metal salt such as titanium dioxide, it'll be dug from the ground as a mineral ore, so it will need to be refined in various ways to remove impurities. (Having pure pigment chemical is essential to ensure the final paint has a uniform color.) The pigment chemical might start off as a lump of rock, so it needs to be ground into a very fine powder. It may also need to be physically or chemically treated to change its color in subtle (or not so subtle ways). It might be roasted, for example, to make it darker. Once it's been ground to a powder, the pigment is mixed with the binder by a huge, industrial machine that works a bit like a giant food mixer, and solvent and additives are added as necessary. That's not the end of the process, however. Because it's vital that each sample of a particular paint looks exactly the same color as every other sample, the mixed paint has to be sampled and compared with previous batches. If the color isn't exactly right, the factory workers add extra pigments. Extra solvents are added if the paint is too thick. Once the paint is the right color and consistency, it can put into cans, bottles, tubes, or other containers and shipped to the stores.