What Are the Colloids in Paint?

The paint you see everyday on simple things like walls or fences is more complex than you may give it credit for. Paint is one of the many colloidal substances you use everyday. Knowing what a colloidal substance is will give you more of an understanding about how paint works, and how master painters used to make their own paints from ingredients as simple as egg yolks and plants.

A colloidal substance is a liquid, solid or gas suspended in a substance made up of a different state of matter. The states of matter are liquids like water, solids like ice and gases like steam. All three of these examples are H2O molecules, but they appear in different forms. Sometimes, when you combine molecules in different states, they change, like the way that sugar dissolves in water. The solid sugar grains become liquid. A colloid is a particle that does not change its state, but rather is suspended in the other substance. For example, smoke is made of solid particles of ash suspended in air, which is gaseous. The ash is a colloidal particle. Bread is made of air in baked dough. The dough is a solid, and the gaseous air is a colloid spread throughout it. Paint is made of particles of pigment that float in a liquid, often water, or linseed or other oils. The tiny bits of pigment are colloids that give the liquid color.

Egg tempera paint was the most common artist's paint used through the Middle Ages. The artist made it himself, mixing together egg yolks, water and a ground-up plant or mineral for pigment. The water gave the mix liquidity, so it could be worked. The plant or mineral gave it color. The egg yolks kept the particles from settling in the water, keeping them spread throughout the mixture. This made paint, which scientists later defined as a colloid because of how the pigment stays suspended in the mix.

Paint retains its color when drying because of the colloidal property. Spreading the paint with a brush or roller thins the mixture so it dries faster. The paint particles are still suspended through the liquid, but as the liquid dries, those particles are left behind. There is no more liquid, so what you have left is the solid particles of pigment -- dry paint.

The colloidal particles of solid material show different colors, depending on from what they were made. Before the advent of synthetic pigments, these were the ground plants and minerals artists used to mix their egg tempera and even oil paints. Examples of these materials abound. Titanium oxide was used to create a white paint, which gives us the color titanium white. Indigo paint was made from the indigo plant, which also gave its name to the color it created. Verdigris is a green-blue derived from copper acetate. Until the 1900s it was the most vibrant green available, but becomes brown or black with age. Copper acetate is rarely sold today because it is toxic. Vermilion was made from another toxic substance, a mercury sulfide called cinnabar. It produced a vibrant red with a hint of orange. Carmine is another red pigment. It is less orange than vermilion, and was created by using ground shells of cochineal beetles.