The Physics of Watercolour

Once, following an art course discussion on priming canvas for oil painting, someone said that to be a painter you need to be a chemist. That set me thinking, if an oil painter is a chemist, is a watercolourist a physicist? After all, watercolour paint dries when the water evaporates – a physical process. On the other hand, for oil paints drying is a chemical process of polymerisation. It is interesting to think about watercolour from this perspective.

I’m no traditional watercolourist, prefering a mixed media approach. But in my mind watercolour is the most beautiful of the painting mediums. I love texture and some of the textures and marks that can be produced with watercolour are unique – and much of this is down to physics.


The most obvious physical phenomena used by the watercolourist is probably the use of gravity to control the flow of a wash. This is why watercolourists often work with the paper inclined at a slight angle to the horizontal. But a watercolourist doesn’t need gravity. Astronaut Nicole Stott painted watercolour on the International Space Station, although she did report some novel practical difficulties.

Surface tension

Surface tension makes a small drop of water sit in a round ball on the surface of watercolour paper rather than spreading. But that is on otherwise dry paper – any watercolourist knows that if you want a wash to run, it helps to wet the paper where you want it to go.

A subtle effect characteristic of watercolour is the slight darkening you can get at the edge of a wash. The effect also occurs in coffee stains and academic papers have been written on the “coffee stain effect”, which is quite a complex effect involving surface tension. The illustrations below show this effect in both coffee and watercolour.

The coffee stain effect with coffee …

… and watercolour

Capillary action


Capillary action is what makes brushes, or that other watercolour staple, kitchen towel, soak up and hold moisture. It is also implicated in a watercolour effect that goes by many names – runback, bloom, back-run, or cauliflower. Often dreaded, as they can form accidentally in the wrong place, they can nontheless be beautiful. They occur when a wet wash, or plain water, flows back into a drier wash. One of my aims as a watercolourist is to learn how to force them at will. Timing is crucial, as a wash has to be at the right stage of drying when more moisture is applied. Unfortunately I have a low boredom threshold so watching paint dry is quite a challenge for me.


When you dip a paint-laden brush onto wet paper the colour diffuses. This diffusion is down to another physical phenomenon – Brownian motion. The pigment particles are bombarded by water molecules causing them to move randomly, and the cumulative effect of countless collisions at the tiniest of scales results in your paint diffusing, dispersing and mingling in the familiar wet-in-wet wash.


Two of my favourite watercolour effects are referred to as granulation, I use the terms sedimentation and flocculation to distinguish them.

Sedimentation is the mottled effect that happens when pigment particles settle in the dips on a rough paper surface.It is what most people are referring to when they talk about watercolour granulation. A characteristic typical of many earth pigments and blues, it happens because the relatively heavy pigment particles tend to fall into the dips. So gravity is important and to encourage the effect it is best to work on horizontal paper. It’s best also to know your pigments because some won’t oblige. Sedimentation can also happen in your water jar. Some pigment particles settle to the bottom and others don’t.

Flocculation is where pigment particles coalesce into larger, visible, clumps. If you let them run they can accumulate against barriers (such as a dry patch of paper) and you can get some beautiful flow effects – like the patterns made by a stream in the sand. This is one of those instances where physical watercolour can imitate larger scale natural forms.

Some manufacturers indicate granulating pigments on paint tubes and colour charts. This includes Daniel Smith and some of their pigments are superb for granulation – especially their PrimaTek range based on minerals including blue apatite, sodalite, hematite and amethyst.


The other physical effect that can effect some watercolours is magnetism. Ferromagnetic pigments can be manipulated with a magnet. Daniel Smith’s Hematite Genuine and Lunar Black are the best I have found for this.


Some people think watercolour is “easy”. That probably means they haven’t really tried it. It is easy in the sense that it can be done with fairly minimal resources and mess. However, it is also a very technical medium. To me one of the great challenges is to not so much as control watercolour, but to try to tame it and exploit some of its natural tendencies. Of course watercolourists don’t need a physics background, but much of what one learns about working with it does boil down to developing an intuitive feel for physical properties. Many of the features of watercolour that make it so special are down to physics.

An earlier version of this article was published in the Painters Online electronic magazine in February 2018