Oil paint doesn’t dry by evaporation. It oxidizes. Now, on the surface, it’s a really easy concept to grasp, in general... Oxidation is oxygen chemically combining with things.... rapid oxidation: explosion; fast oxidation: fire, slow oxidation: rust! Apples turning brown is somewhere on that spectrum, too, and humans oxidize, as well... we, our bodies, like a nice alkaline Ayurvedic environment, and sugars like soda and candy make us acidic, then we oxidize and age more quickly. Simple, right? We can, on rare occasions, spontaneously combust, but I think it takes a high blood alcohol content.
Rose Bud 5x5 (2021) oil on canvas.
The speed by which oxidation occurs has a lot to do with the potential available electrons (oxidation state, valence, maybe)... and available oxygen, of which there are plenty of both. From my college chemistry, I recall that acids have a H+ ion and bases have OH- ion and that opposites attract. So a little H+ and OH- makes H2O (water) sort of thing. It gives off or absorbs heat, too, and I cannot remember which.
I’m teaching a painting materials class, and this weeks topic was oil painting, so being the conscientious instructor that I am, I thought I would dig myself a wee bit deeper into the chemistry of how oil paint dries... which I’m going to share. Toe testing, mind you, not a full on assault. My girlfriend refers to my blog posts as ‘going deep’ when I talk about artistic things that make her eyes glaze over. Hopefully, this will be more a Dance of the Wu Li Masters and less of a Zen of confusion. Did I mention that, in addition to science, writing is not one of my strong suits?
First, let me say that pigments are minerals and that minerals are crystals that get their color from their crystalline structure. Ice is a mineral, but most rocks and things have metals in their molecules. My sister studied math, chemistry, and physics and once told me that at a certain level it’s all the same subject... I have not seen quantum physics applied to art, yet, but it’s probably coming.
Organic chemistry is the study of carbon based things like living things. We a are apparently a carbon based life form, although we are, also supposedly, mostly water... like 80%. I would think that it would make us a water based life form, but what do I know, as I’m just an artist. Organic Chem had stinky lab classes, and regretfully, I didn’t make it that far; after, Chem 101, 102, and 103 kicked my academic behind several times over and over (another blog post, another time, perhaps).
As an artist, but not as a chemistry geek, I’m interested in the not completely understood process by which oil paint dries. As I’m teaching about it, I did the only logical thing and googled “oil paint.” Can I just say, I love Wikipedia.
So, oil paint is, basically, just pigment plus vegetable oil. Not just any vegetable oil, though. It has to be a drying oil. H
ere is where we go down the rabbit hole, but relax, I’m only going to throw a bunch of words around. After reading about all sorts of drying (causative) oil chemistry, I can attest to why drying oil paint is not completely understood. Not all oils dry (semi-drying, etc.), and some oils that do dry are not vegetable oils, like fish oil is a drying oil. Who knew? One of my students asked me if that would make the paint smell like fish. And while I am on the topic of drying, I just want to add that they measure something called the Iodine Value (IV) of oils in order to determine how much Iodine an oil can absorb. The more Iodine the merrier. So 600 years ago, painters figured out linseed oil was a fast drying oil... without the Iodine test! Just saying.
We talked about the mineral pigment being a crystal... the oil component of oil paint is fat, vegetable fat, but a fat nonetheless. So, I Google “fat.” I did say how much I love Wikipedia, right? Fats, as it turns out, are made of a molecule hub or a carboxyl group (carbon, oxygen!) bonded with an alcohol group to form a glycerol, with a long chain or chains of fatty acids attached made from aliphatic (not aromatic) hydrocarbons (hydrogen and carbon atoms, try and keep up!). Vegetable and animal fats with 3 long chains of fatty acids attached to the glycerol are called tri-glycerides. See, easy peasy!
Stay with me here. The fun is just getting started. When these carboxyl groups attached to an alcohol group like an ethyl- or propyl- along with the aliphatic hydrocarbon chains of acids have an OH group (alkyl, like alkaline, remember, above) are replaced with an oxygen, in turn, it forms what’s called an ester. Glycerides are fatty acid esters of a glycerol (see, painless). So much for semantics! Well, not quite done... hang in there... metal soaps are related to these fatty acids and minerals (think drying oil paint). I did not Google “metal soap,” thankfully, but suffice to say, sodium (metal) hydroxide (OH-, a base, alkaline) makes a type of soap with which we wash our hands. Apparently, proteins are very shape intensive, viruses are proteins and soaps flatten their asses, making them ineffective, and, and, soaps just act like magnets with poles to lift bacteria off our skin so they can be removed by rinsing. So, wash your hands.
As an aside, these chains of fatty acids can have various shapes, various bonds (single, double, triple), but no rings (hexenes) and can have from 2-48 carbon atoms. If there are no double bonds it’s called saturated and if there are one, two, or three double bonds somewhere in the chain... they are poly-unsaturated (counterintuitive if you ask me, but whatever). There you have it saturated, unsaturated, and polyunsaturated fat! Learned ya’ something. These acids can make things like amino acids and deoxyribonucleic acid (DNA). To think in 7th grade, I could actually chart the 8 amino acids in DNA.
On to paint drying. Here is where it gets a little sketchy. So, you have these three polyunsaturated ester chains of fatty acid (triglyceride) and then oxygen atoms from the air start mixing with it to create this cross-linking of (and I’m guessing here) the ester chains that is called polymerization or polyesters, plastic, basically, and the metal ions like iron, cobalt, and manganese from the mineral pigments act as a catalyst (speeds up or facilitates the chemical reaction). The hydrocarbon fatty acid chains dry (oxidize) in order of the number of double bonds in the chain (linleic, then linoleum, and last oleic). Unlike rocket science, they do not know the exact chemical process of drying paint.
Wasn’t that fun? I discovered what saturated, unsaturated, and polyunsaturated fatty acids are, polyesterization of triglycerides (although not what makes it happen), carboxyls, alkyls, alcohols, polemerization (plastic), cross-linking, and metal soap. Soap, stockings, plastic, and oil paint... all in general, and no chemical equations or diagrams... just dry oil paint.
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