The Chemistry of Oil Painting

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?

The old Symbiartic blog banner started out as oil on slate. Tough to photograph, fun to scan. I have no patience and scanned it wet.

As an oil painter for the past 20+ 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 Eyckbrothers. In reality, craftspeople and artisans were already using oil for some time previously. We also now know that oil paints were in existence even earlier in Asia, thanks to paintings found behind statues blown up by the Taliban at Bamiyan.

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.

Yellowish, flexible Linseed Oil on the left; Clear, prone-to-cracking Walnut Oil on the right.

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. 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:

The Mond Crucifixion by Raphael uses two types of oil on different elements to preserve colour and paint film. And the effect still lasts after 500 years, eh?

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 they absorb 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.

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.

Here you can see the light beams (blue) bouncing multiple times through the oil layers, off oil membranes and off the colourful pigments. This bouncing gives oil paintings their luminous glow.

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.

I’m not a chemist — I could be wrong. Feel free to offer corrections and tips of your own in the comments. When this post was originally published in 2011, some lively discussion ensued in the comments on both Symbiarticand Lines and Colors and I think they’re worth checking out.


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


Originally published on Symbiartic on Scientific American at on August 2, 2011. I have made some light edits.

13 Things I’ve Learned About Being A Science Artist Online


After celebrating 9 years of blogging on The Flying Trilobite, I’m going to get all old guard and pompous and established and drop some wisdom about best practices for science artists online.

  1. Show off. Saying “I am too busy making art to spend time online” means you are too busy making art no one will see. Visual art is a performance art.
  2. Yes I said before coffee. Start making art before you even have coffee in the morning. Get up an hour early and do art before you go to your day job. You’ll spend the rest of the day with your creative muscles buzzing because you gave them a workout and that feels good.
  3. Make pie. Don’t do the hyper-competitive thing and only talk about yourself and your own work all the time. It’s tempting, I know: we’re all trying to make it, and live the dream of full time art-making all day. But doing that will cause you to miss out on the camaraderie of community with other science artists. Promote other people too. Remember: you are not giving away your piece of the pie by retweeting someone else’s art: you are making a bigger pie for the potential audience to feast on.
  4. Sell your very soul. Don’t try to separate art from artist. More than ever, artists are a part of their art. Your online presence will award you contracts not only on your talent, but also your personality.
  5. Get with the times. Used to be that things were centralized around The Blog and comment sections were lively places. That’s not the case anymore. People read the link on Twitter or Facebook when you share it, and then comment on that social media platform instead. It may be a little messier, but skipping the latest-greatest social media platform means missing out on those conversations about your work.
  6. Have an art avatar. Update your blog once a week, but put some art up every day on Twitter. Remind people that you are talented and not just amazingly Twitter-witty.
  7. Get on Twitter. So you have a blog and you have Twitter ( both non-negotiable in my opinion.) You may want to sign up for a lot of social media sites to try them out, but pick two more and focus on building an audience and business with them. Artstation? Instagram? Periscope? Vine?
  8. Spending money to make money is a lie. Go cheap when you start out. Use free blog software to build a portfolio. Skip joining professional illustrator groups and find community in Google Hangouts and industry hashtag discussions. Spending time to make connections is the truth.
  9. Proudly Google yourself. Learn how to use Google Search by Image and Tineye to monitor how your work is being used by other people. You need to be proactive in protecting your work.
  10. You’re scared, I know. Let people share as much of your art as you can. Sharing ≠ stealing.
  11. I’m telling you to work harder. Crowdfunding, selling on Etsy or print-to-order like Redbubble all offer the potential to take some of the burden off of working 2 jobs and making art in the wee hours. So treat it seriously. Have a plan and be professional.
  12. Oh that ol’ thing? Chances are you’re building new connections all the time. Dust off your older artwork and share it again for a fresh batch of eyes.
  13. Winning. Remember that as science artists, our feeds are filled with the very best of the internet: exciting scientific discoveries and delightful, disturbing, intriguing art. We are creating sciart utopia. We win.

+ + +

This post originally appeared on Symbiartic on Scientific American, by Glendon Mellow March 29, 2014. I’ve done a bit of updating to the list of platforms and added an extra tip.

Beefy arm and theropod doodles

A few doodles while away at a friends' cottage for the Victoria Day weekend. 

Originally shared on my Twitter and Instagram.  


 Turning a sketchy doodle into a beefy guy's tattoo.

Turning a sketchy doodle into a beefy guy's tattoo.

 Anatomy and planes all screwed up. Don't care. Cottage doodle. #longweekend #sketch

Anatomy and planes all screwed up. Don't care. Cottage doodle. #longweekend #sketch

  ...and now there's an inked theropod. #sketching

 ...and now there's an inked theropod. #sketching

 Theropod sketch. #doodling #sciart  

Theropod sketch. #doodling #sciart  

Amazing how good carving a little time for some undirected sketching feels. Like aligning bicycle brakes. And I think I could turn this into a more elaborate tattooed-dinosaur piece... 

What If All the Images Went Away

Can you imagine what science communication would be like without images?

Again on Twitter and Facebook, I find myself levelling criticisms at particular sites and railing against improper image use in science communication. Again.

After arguing with (arguably) allies in science communication I was fed up. Fed up with the attitude that unattributed images are just a (small) sacrifice for the net good of science communication to the populace at large. Fed up that photographers, cartoonists & illustrators are considered by many to be lesser professions than scientists & educators. Fed up that rapid image sharing (oh I’m sorry: “curation”) can trample so many creators and yet lead to fame and fortune.

I found myself saying once again, “can you imagine what science communication would be like without images?

And now I’m going to show that to you. Click on the images for maximum effect.

* * *

The 4 blogs I am showing below are all ones I consider to be excellent at science communication in all aspects: compelling reads coupled with effective, often astonishing images. These bloggers, in my opinion make every effort to attribute and use images correctly. They link back. They name sources, just as good science blogging should.

Deep Sea News

See Cocktail Party Physics’ post here

* * *

I wish that once a year, popular browsers like Chrome, Safari and Firefox could somehow block all images online as an awareness campaign.

Despite feeling discouraged last week about what it will take to see a phase-change in how image creators are treated online, I still believe there is hope. And thanks to those of you who encouraged myself and others who were wading into some ugly debates. Here are a few hopeful tweets I shared.


Many thanks to Annalee Newitz (io9), Dr. Craig McClain (Deep Sea News), Jennifer Ouellette (Cocktail Party Physics) and Ed Yong (Not Exactly Rocket Science) for letting me stripmine their blogs in order to make these non-images. Apologies to your bloggers, illustrators, photographers and designers.

Before anyone goes scurrying off to see if they have ever screwed up and forgotten an attribution, let me assure you that these bloggers would welcome the correction. I didn’t approach them because I am certain they are perfect: I approached them because they show respect to creators consistently.


Originally published at This post originally appeared on Symbiartic on Scientific American, by Glendon Mellow March 18, 2014. I’ve done a bit of light editing on tense in the first paragraph. This post has also been re-blogged on Medium

So You Want to Hire a Science Illustrator

You’re proud of the science communication you’ve written for your blog. You want to add visual excitement to an announcement about science outreach. You need to illustrate your findings for the paper that has taken years of your life to study and write-up. You know illustrations, photos and cartoons make your writing much more likely to be read on social media.

You want to hire a science illustrator, but are not sure where to start.

Here’s some things you should know.

Where to find science illustrators

You want someone who knows what they’re doing.

Is accuracy important, or are you trying to convey a mood to accompany your writing? Perhaps something instructive, or funny? What type of image will help explain your message? An artist can help you figure that out.

How to pick one

Most illustrators have areas of expertise. It’s a good idea to look around at some portfolios or ask for recommendations in the community. For example, I tend to do oil and digital paintings about prehistoric life, humans, and microbes. I’m not the best person to ask to do a design-heavy infographic about sanitation hazards on kitchen appliances.

Think about your audience. If you’re describing the evolution of dinosaurs and birds to high school students, you may want something funny to keep their attention, rather than something cute like you would for grade school.

Be open to new styles. A lot of the best science illustration pushes boundaries in favor of artistic value to communicate effectively, rather than act as a stand-in for a photograph.

How to approach a science illustrator

Be excited about your blog post, paper, book draft or study. Be excited to find visuals to match what you poured into it.

Be prepared that their years of study, school, and practice honing their ability is going to cost money and take time — and for it to be worth it.

Don’t ask them to do work “for exposure”. Illustrators laugh about that, bitterly. For example, see David Thorne’s classic post Simon’s Free Pie Chartsand the @forexposure_txt Twitter account which quotes real people asking for free work.

If you can’t pay, or can’t pay much, it’s best to ask about something the illustrator has already created. Many illustrators are okay with re-sharing their back catalogue work that may otherwise be gathering dust. Not all feel this way, as the back catalogue may be their bread and butter for licensing. But if you have a specific piece in mind, it may be worth asking if it’s available.

If you work for a charity or non-profit, you still have to pay a plumber to fix the broken taps or a service provider for your phone. So keep that in mind about illustration.

Money and contracts and stuff

This section might well be your main reason for reading this post. Spoiler alert: I am not posting how much illustrators charge.

Most illustrators have their own contracts or Agreements about money and usage. All professional ones will want to use one, even for minor jobs.

Contracts and Agreements are not created out of the aether. Illustrators have guides such as the Graphic Artists Guild Handbook: Pricing & Ethical Guidelines and modify from there. You might want to get a copy if you are hiring often.

That Handbook I mention above has prices for a lot of scenarios, but not all. Want to hire someone to make a Twitter background reflecting your research into theoretical physics? The artist may decide how to charge based on a living wage and estimate of how long it will take to complete. $300 might sound steep, but if it takes them 20 hours to produce, that’s still only $15/hour.

Copyright always lies with the image creator by international law. Paying someone to make the image does not change this.

If you want to pay someone to relinquish all of their copyright, it can cost a lot, up to 500% of the original price. Ask yourself: do you need to? Is this for a website written only in English? A textbook in 3 languages in Asia? Illustrators prefer to retain copyright so they can re-license the image to other markets. At bare minimum, they will want to use it as a portfolio piece, to enter contests in their field and other forms of self-promotion.

If you are hiring an illustrator to create work for you from scratch, many will ask for a deposit, send sketches, and then reach a “kill-point” in the contract where if it is killed or substantially modified, payment will be expected anyway. Most charge for revisions past a certain point.

Don’t ask the authentic pen&ink illustrator you hired for that classic black and white look to “turn the head on that hominid by 30 degrees”. It’s not a 3D computer render.

It’s also useful to describe to the illustrator who the audience is, in terms of numbers and demographics. Potential audience goes a long way in making decisions about pricing.

Remember it’s worth it

You’re reading this using the greatest image and communication tool ever conceived, the internet. Images are the future of communication.

These tips aren’t pitfalls: most illustrators are excited and hungry for new challenges and projects. Don’t be shy.


Illustrations by Glendon Mellow, top image has photo references by Morgan Jackson.]

Originally published at on July 27th 2013 by the author. Some edits made to the list of where to find people, and the images. You may also reshare this post on Medium

Elusive Dimetrodon-Sphinx

Dimetrodon-Sphinx. I've never been able to digitally colour this piece to my satisfaction. Sketches of Dimetrodon-Sphinxes have been around in my artwork for about 20 years. The idea of pre-Mesozoic animal/human hybrids really appeals to the sense of deep time I want in my mythological art.

 Dimetrodon Sphinx by Glendon Mellow

Dimetrodon Sphinx by Glendon Mellow


Really need to get back into daily sketches. Hopefully they will appear here and on my Instagram: @FlyingTrilobite