Oil paint thinning and thickening
When we mix up oil paint, we sometimes make it thinner using turpentine or solvent so the paint flows more easily. Sometimes we thicken the paint using wax or impasto medium so that the texture of our brushstrokes is retained. Have you ever mixed up a batch of very thin paint for a wash, and then left it overnight in a sealed container? The next day the paint may still be fluid and usable, but often the pigment will have settled to the bottom or the liquids will have separated. Mixing up a more fluid paint for glazing reduced the paint's viscosity. This allowed the pigment particles to fall to the bottom.
What is Viscosity?
So what is viscosity? In layman's terms viscosity is the thickness of a fluid or paste. The "amount" of viscosity a fluid has tells you how hard you would have to push on it to keep it moving. A more viscous fluid needs more of a push than a less viscous fluid. The "push" moving the fluid doesn't necessarily involve actually pushing (or squeezing a tube) with your hands and fingers. Gravity can pull on a fluid too.
To visualize viscosity think about two identical glasses on a table. One is half full of turpentine, a low viscosity fluid. The other is half full of stand linseed oil, a higher viscosity fluid. A cat comes into your studio and bats both glasses onto their sides at the same time. The liquids start pouring out onto the table. Which glass empties more quickly? Unless you're reading this from some alternate universe with different properties, the turpentine empties more quickly. Gravity moves the turpentine out of the glass more quickly and effectively because it has a lower viscosity.
Viscosity and Paint behavior
If you work with oil or acrylic paint you have probably tried to tweak the properties of the paint by "thinning it out" or by "thickening it up". Both of these actions affect the viscosity of the paint, along with a number of other properties. The "thicker" more viscous paint resists gravity better, and doesn't slide down the canvas - it stays put. The individual small areas within a splotch of the thicker more viscous paint are also better at staying put. If you paint using a thick viscous impasto medium, the viscosity helps keep the brush texture in the paint, because a viscous fluid will take longer to smooth itself out than a non-viscous fluid. Gravity and spreading can't pull the paint completely flat in the time it takes to dry or harden the paint. This is actually a bit of an oversimplification - there are other properties related to viscosity that also need to present to keep paint from becoming smooth before it dries or hardens.
The takeaway so far? Higher viscosity makes a splotch of paint harder to move around. It also makes it harder for the inside of the splotch of paint to rearrange itself. High viscosity paint will smooth out more slowly than low viscosity paint, it will flow more slowly, and drip more slowly, and it will retain the details of a brush impression for longer.
Viscosity and paint as a mixture
But there's more! Paint is not made up of one ingredient. Every minute bit of paint is not all made up of the same molecule like (for example) pure water or pure sugar. Paint is a complex mixture of chemical ingredients. How do you suppose viscosity affects the ingredients in paint?
Well, in a high viscosity paint the ingredients - the pigment particles, additives, and binder molecules - can't move around as quickly. At very tiny scales Brownian motion keeps the molecules and tiny particulates in a fluid moving around. As they move around they follow a pattern that's called a "random walk". Even though all of the little "Steps" in their movement are random, they eventually manage to cover some distance and move away from their starting position - they diffuse within the fluid. In a low viscosity liquid a lot of this diffusion takes place in a particular span of time, whereas less takes place as the viscosity increases. For oil painters the important "span of time" is the time it takes for the paint to harden. Viscosity is one of the factors that determines how much you pigment moves around (diffuses) inside the paint before it can harden.
Viscosity changes can affect paint stability
When we look at historical accounts of oil paint formulations and compare them to modern oil paints one of the big differences is the viscosity of the paint. Historic formulations were quite fluid, whereas modern formulations are pastes. Think about the effects of high viscosity, or of changing the typical oil paint from a fluid oily liquid to a much thicker more viscous paste. In the thicker paste, particles like pigment particles can't move very far before the paint hardens. In the very fluid oil, the particles of pigment can move about quite a bit. In the very fluid historic formulations pigments that "like" to aggregate would have enough mobility to do so, in the modern thick paint they generally stay put and stay mixed in. A change in viscosity is thus a strong candidate for one of the factors affecting historic versus modern paint stability.
Next Up: Oil is a polymer