Awesome question! We’ve all heard the expression that something’s “like watching paint dry”… but what exactly are we watching?
First, we must understand how paints are made. Most paints will consist of three basic things:
Pigments: the compounds that give paint its color.
Pigments for sale at a market stall in Goa, India.
A binding medium: usually some kind of polymer that carries the pigments, sticks to surfaces, and hardens to form a continuous layer once the paint has dried.
Solvent: because the other components tend to be very thick on their own, a solvent is used to dissolve the binding medium and lower the viscosity of the mixture.
Composition of watercolor paint
Sometimes, there are also additional additives that modify the properties of the paint for certain purposes, such as better durability, consistency, or drying speed.
Paints are a type of mixture called a colloid. A colloid is a mixture containing tiny particles suspended (but not dissolved!) in another substance. Other examples of man made colloids include mayonnaise, body lotion, detergent, and whipped cream.
In the case of paints, the pigment is the tiny particle that is suspended in the solvent-binding medium solution. When paint dries, the solvent portion of the mixture evaporates away, leaving the pigment suspended in the binding medium in a hard layer. These evaporating solvents are also largely responsible for the “smell” of the paint you are using.
Diagram of paint drying
So why doesn’t paint dry out in the can? Well, the speed at which the paint dries depends on how much surface area is exposed to the air. When you paint something—for example, a wall—the surface area of the paint increases dramatically relative to the amount of paint present. Accordingly, the rate of solvent evaporation also increases dramatically.
In a tube, can, or other container, the portion of paint that is exposed to air is very small, so the solvent will only be able to evaporate very slowly. This means that it is unlikely that the paint will dry out in the time that you are using it. If you were to leave a can of paint open for a long period of time, you might notice a sticky or hard film forming on the top—this is the result of the solvent evaporating from this top layer. However, if you mix the top layer that is exposed to the air back in with the rest of the paint every once in a while, the overall change in the amount of solvent in the system is small, so you will not notice any difference in the consistency of the paint.
Additionally, since evaporation requires energy, paint that is heated or in a warm environment will usually dry faster than paint in a cooler environment.
Lastly, there are many different kinds of paints, and although most follow this basic procedure of drying via evaporation, each type of paint works a little differently. To read more about the differences between types of paints, please click on the link below.
Answered by Rachel R, Expert Leader.