What is the difference between a pigment and a dye?

Both pigments and dyes are used to color different materials, but the way in which they do it is very, very different. It’s all got to do with solubility – the tendency to dissolve in a liquid, especially water. Think for a moment of two glasses of water standing side by side – the one is mixed with salt, the other with mud.

Solubility
The muddy water is brown, with all sorts of dirt and mud particles suspended in the water. Give it enough time and the particles will settle on the bottom of the glass. This mixture is called a suspension.

Now take the salt water. Mixing the salt with the water results in it completely dissolving in the water. Put a lid on the glass to avoid evaporation, and you won’t get a layer of salt particles settling on the bottom of the glass. So this mixture is called a solution.

Pigments, like the mud in the above example, are finely ground particles of color that get suspended in a liquid (usually not water, and called the dispersing agent or vehicle). Whereas dyes are chemicals that, like the salt above, get dissolved in a medium (such as water) to create a colorant.

Application
The differences between pigments and dyes don’t stop there. How the colorant actually attaches itself to the material also differs greatly between pigments and dyes. While dyes chemically bind with the material, pigments are painted onto the surface, so they physically bind.

The pigment, mixed with the dispersing agent, is applied to the base and forms a coating on top. The dispersing agent basically makes it stick to the surface of the material.

Dyes, on the other hand, chemically bind to a material – they’re not painted on as layer but actually become part of the material. Whether the dye will attach itself or not depends on the chemical nature of the material being dyed and the dye itself. Sometimes, a dye needs another chemical to bind and that chemical is called a mordant.

More differences
When it comes to lightfastness, pigments are much more lightfast than dyes. Light, especially sunlight, destroys the color in a dyed object by breaking open electronic bonding within the molecules. Dyed material is therefore often not stable and can fade, sometimes rather quickly, when exposed to light. (Though this is true for some pigments too, and why art museums tend to keep their lights on dim to protect their paintings).

Uses
Typically, dyes are uses in the textile and paper industries. Leather and wood are also usually dyed. As are waxes, lubricating oils, polishes, and gasoline. Food is often colored with natural dyes – or synthetic dyes that have been approved as safe for human consumption. Pigments, on the other hand, usually color rubber, plastic and resin products.