What chemical properties give oil paintings their luminous glow and deep darkness?

Why do they crack?

What kind of oil is used?

Is it safe to use the oil painting medium on a fresh dandelion salad?

As an oil painter for the past 17 years who used to manage at a fine art supply store and notably not a chemist, I'll do my best to explain. Don't slip on the floor, and remember to soak your cleaning rags in water before disposing of them in the metal bin. They can spontaneously combust, you see.

Introduction to what paint is and isn't

All fine art paints share a few properties that make them different from say, dyes. Paints are essentially pigment particles bound in a sticky, transparent medium, whereas dyes or soluble in liquid. So oil paints are pigment bound in oil, acrylic paints are pigments bound in acrylic polymer medium, and watercolours are pigments bound in a water-soluble medium called gum arabic. Fabric dye and fabric paint are therefore not the same thing.

There can be other agents inside a tube of paint these days, that slow down or speed up drying, that lend texture, or help stubborn pigments bind to the medium. (Inexpensive paints often have too much binder in them and can cause discolouration over time - check out this post by artist Jonathan Linton on his blog Theory and Practice for some empirical tests.) But at their root, all paints are pigment+medium.

Quick History Lesson

Within Western Art History, oils overtook fresco painting and egg tempera painting in popularity relatively quickly. The Master of Flemaille is sometimes credited with beginning the practice of using oil paint for fine art purposes, though more often the credit is erroneously given to the Van Eyck brothers. In truth, craftspeople and artisans were already using oil for some time previously. ¹

The superior qualities of oil make it easy to understand why it took over from other mediums. Fresco, such as Michaelangelo's Sistine Chapel Ceiling, are essentially pigment bound in plaster. You had to mix just enough of the correct colours for one "go" or "pass" at a section, and estimate how much detail you could achieve with it before it dried before your eyes. So something like the subtle blending of God's robes or tones and shadows of Adam's skin in The Creation of Adam had to be estimated in multiple swatches, each with its paint mixed just before application. A difficult task. Oil paint on the other hand, may not fully dry for weeks: you can play with it, correct its tones and even erase missteps from the canvas and start again on a section. Blending becomes open for experimentation.

Types of Oil

Even in the Renaissance when oils first inspired artists to delve wholeheartedly as a medium, a number of oils were tried as vehicles for pigments. And their properties differ.

  • Linseed Oil - made from flax, linseed is the most popular due to its flexibility and resistance to cracking. It does have a strong tendency to yellow with age, however.
  • Walnut Oil, Poppy Oil and Safflower Oil - much less likely to yellow, these thin, clear watery oils are much more prone to cracking.

With these different properties, how do they come into play when actually painting? Well one of the Ninja Turtle Old Masters had it right: analysis of Raphael's The Mond Crucifixion (1502-3) shows that the ground, figures and green robes were painted using linseed, and the blue sky painted with nut oil.² This way, the yellowing of the figures and ground were an acceptable trade-off due to their subject, but the blue sky was considered better off being cracked and bright blue than yellowed and smooth. Painting below:

Watching Paint "Dry"

Watercolour and acrylic paints have water as part of their medium - they dry by evaporation. But oil paints don't. They dry by what's called a siccative quality. That is theyabsorb oxygen from the air. This has the undescriptive definition of

(Chemistry / Elements & Compounds) a substance added to a liquid to promote drying: used in paints and some medicines

[from Late Latin siccātīvus, from Latin siccāre to dry up, from siccus dry]

Essentially, oils have a rate of autooxidation from the air, they absorb oxygen and harden. I've often described this as putting Jell-o into an enclosed container and adding tons of pineapple chunks to it: the oil is the Jell-o and the air is the pineapple - you can only add so much to the enclosed bowl and it will stop jiggling. Perhaps I haven't got this analogy quite right. But now I want Jell-o.

As oils harden, there's an interesting problem: oxygen is absorbed through the paint surface, meaning if the paint is very thick, you can see a different "drying" rate on the paint's film than on the first layer's applied on the canvas. The surface could be hard and the oils underneath still squishy like yummy lemon Jell-o. (Warning: oils processed as art supplies are not cleared for human consumption.)

Fat Over Lean

One of the main appealing properties of oil painting are the glazes. By adding a small amount of pigment to the relatively clear oil medium, you can very subtly tint an image. This is called glazing. Most Renaissance Old Masters (think the Ninja Turtles and their peeps - Artemisia Gentileschi not April O'Neil) used a toned underpainting and then built up several of these thin glazes of colour on top to create astonishingly realistic figures and scenes. The translucence of the paint film allows for sophisticated ranges of flesh tones. But then we hit the problem of the upper layers of oil glazes drying before the lower (first) ones do - and this is where cracking comes from.

Okay, another analogy: imagine the top (newest) layer of oil is stretching as it dries out hardens, and it stretches to the max. Its surface is expanding because it is absorbing oxygen (not evaporating water). Now, they oxygen eventually begins to hit the layer below. And it stretches and expands to the max. But they layer above is already dry, how can it expand any more with the one below pulling it!? >crack<

Like a big cookie on a pan. Slide an uncooked cookie under a cooked cookie, a bigger one and stretch and heat up that dough: as the bottom cookie dries and expands its surface, it will crack the smaller cookie it is now stretching on its surface. >crack< Nomnomnom.

To get around this, painters developed the Fat Over Lean rule. With each layer of glaze, add an increased amount of oil paint to the layer. (Less pigment, more oil.) This way, the rate of oxygen begin absorbed by an oily (fat) top layer will be slower than the hidden lower, less oily (lean) layers, and hopefully they will saturate with oxygen and harden at approximately the same time.

This leads to other tricks and techniques too. If you use too little oil in an early glaze, it can obliterate the drawing or painting underneath that you want to show through all the thin transparent glazes. It also can make the paint too pasty and thick, which is unworkable for fine detail. So, in the early, lower glazes, sometimes solvents such as turpentines are added. The loosen the paint, disprese the pigment particles, and then kindly evaporate in a big hurry leaving the old that's left to be covered by another turp+pigment+oil layer that has a little less turp and a little more oil. And so on.

To answer the question above about the dandelion salad, oils themselves are not harmful (though not processed to be safe for food). An open container of say, safflower oil on the table will do no more harm to breathe in than some extra-virgin olive oil with Balsamic vinegar and a few chili flakes on your table for bread. It's the solvents you have to be especially wary of. Even some of the odorless ones have harmful vapors, although it's possible nowadays to buy non-toxic alternatives. I'd be happy to recommend some I've tried if anyone has email requests (this is not an infomercial).

Patron Saint of Pigments

In Renaissance Italy, the patron saint of painters was St Luke - who was also the patron saint of doctors. Painters didn't have a Guild of their own, they belonged to the same as doctors. Why? Besides the mythology of the saint himself, it was for the practical reason of painters and doctors both frequenting apothecaries for medicinal and artistic ingredients.

The pigments in oil glazes add another property and challenge to the artist who up until about 150 years ago, had to mix each batch of paint by hand. The pigment particles are not all the same size, and do not all disperse at the same rate within the oil medium. What this means is some colours will have more oil, and others less. Yeah you see it coming: the glazes following the fat over lean rule are best applied in certain orders to reduce cracking upon hardening.

As an example, let's say you're painting a red rose, with all it's subtle shadows and highlights. To get ideal results in your glazes, you may want to apply the glazes in this order: manganese blue, cadmium red, quinacradone red, alizarin crimson. Mostly this will not matter to modern oil painters, but it can still have an effect even today. Most true alizarin crimsons will have up to twice as much oil content as a lead-based white.³

Bouncing light

What's the point in all these complicated glazes? Just to mix colour? Not just - they add luminosity to the painting. You see, when light enters the hardened oil paint film, it passes through several distinct layers of mostly transparent paint. And sometimes, before being reflected back out to the surface, it bounces off of one of the colourful pigments, and back down to the layers below, and then out. Sometimes it will bounce on the boundaries of the separate glazes before bouncing out to meet your eye. And this is what gives oil paintings their glow and their deep deep blacks. The dancing behaviour of the light in the complicated multiple layers and their colour pigments.


The New Oil

Consider this little afterword the start of another conversation for another day.

Oil painting gave artists the tools necessary to create images that can be corrected easily due to their long drying times and that seem to glow due to their layers. As an oil painter myself, these are highly prized qualities. And the last several years, we're seeing another technology that prizes these same qualities of easy correction and luminosity. Digital painting has exploded in popularity with programs like ArtRage (used to create the simple image above), Photoshop, Corel Painter, and the shareware Gimp. Ctrl-z is the new solvent, and pixels the new luminous colours. And I don't think it's an accident. What would pioneers like The Master of Flemaille or the Jan Van Eyck have done with current technology?

If they're like me, they'd want to experiment with the ease of the new tools but still stick their fingers in the sticky paint, smell the soft odor of the oil, and play with their pigments.


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I hope you've enjoyed my contribution to Chemistry Day here on the Scientific American Blog Network!

I'm not a chemist - I could be wrong. Feel free to offer corrections and tips of your own in the comments.


1. History of Art, Fourth Edition. H.W. Jansen, revised and expanded by Anthony F. Jansen, 1991 Harry Abrams Inc. p.425-426. (Link leads to newer edition)

2. The Artists Handbook. Ray Smith, 2000 Alfred A Knopf. p.180

3. The Artists Handbook. Ray Smith, 2000 Alfred A Knopf. p.182