Brushstrokes & Molecules: Unveiling the Chemistry Behind Oil Paint

The Science of Art by Mot Tuman

Uncover the chemistry of oil paint! Get ready to explore the secrets behind those vibrant colors and the magic that makes oil paintings timeless. 

Join Mot as they explain the chemistry behind oil paints in this article with a full video. We will go into the chemistry of one of the most popular mediums around which is oil paint.

Find in this article: explanations, images, video and faq’s below

The Science of Art, Uncover the chemistry of oil paint
The Science of Art by Mot Tuman , Uncover the chemistry of oil paint

In this Science of Art article: What makes oil paint different from other mediums is the oil can have varying properties depending on what type you use and you can’t just use any type of oil within your oil paint. Oil paint is going to eventually dry even if it’s over a very long period of time.

See Full Length Video below!

Drying vs. Non-Drying Oils

We’re going to be talking all about the chemistry behind one of the most popular mediums around which is oil paint. 

What makes oil paint different from other mediums is the oil. Oils can have varying properties depending on what type you use, and you can’t just use any type of oil within your oil paint.

If you were to make your own at home, you can’t grab cooking oil off your shelf, like your olive oil and your avocado oil, and have it behave in a way that will work for oil paint. Oil paint specifically uses a type of oil known as drying oil. 

Screen capture from The Science of Art: If you use cooking oil in your paintings, it will never dry

These are different from the types of oils that you find in your kitchen, which are non-drying oils. They have chemical differences that make it so your cooking oil isn’t going to dry out in the bottle, but the paint that you use to oil paint with is going to eventually dry, even if it’s over a very long period of time.

The main difference between drying and non-drying oils is that non-drying oils contain less of a very specific type of acid that wants to react to oxygen so that the paint can harden.

Oxidation

In order to dry, acrylic paint and watercolor paint have some amount of water that is inside the paint itself. As it dries, that water is taken away through evaporation. All you’re left with is the pigment. For oil paint, while oil can evaporate just a little bit, oil molecules are a lot heavier than water molecules, and aren’t going to want to vaporize nearly as much.

So instead of drying through evaporation, oils are going to want to dry through oxidation. Now, oxidation is a very fancy term that refers to the molecules of oxygen in the air reacting with your oil. When the oil molecules come in contact with that oxygen, it is essentially the secret ingredient that they use where they’re then able to start linking to each other.

Screen Capture from The Science of Art: The oxygen creates the link between molecules

All of these molecules within the oil are linking to each other. When these molecules are apart you have a liquid–you have your liquidy oil paint–but as they start to bond together and form chains, it solidifies and it hardens into what we have as dry oil paint.

That’s the reason why you’ll find that when you have oil paint left out on your palette over a long period of time it will start to form a skin on the outside and you’ll still have wet paint on the inside. As soon as that wet paint is then exposed to oxygen it will also start to harden.

Screen capture from The Science of Art: Cover your palette and put it in the freezer if you want to keep your paint

What Is Fat-Over-Lean?

Another key thing to think about when we’re talking about how oil paint dries is a very well-known oil painting rule called fat-over-lean. This rule is very important in order to keep our paintings archival.

So, this fat-over-lean rule is really only relevant if you’re going to be painting over a long period

of time. If you’re doing an Alla Prima painting where you’re completing your painting in one sitting, this rule is helpful, but it’s not as important as when you’re going to be painting over several days, weeks, or even months. This rule really applies to when you have one layer, you’ve completed that layer, and you let it touch dry before you start the next layer.

The reason that fat-over-lean is very important is because the oil content in paint can affect the drying time and also the flexibility of your paint. Keeping that flexibility in mind is really key to preventing cracking as your paint dries and hardens. 
See our full helpful guide on: Fat Over Lean For Oil Painting

How To Paint Fat-Over-Lean

When you start a painting, you want to begin with very thin lean layers with very little fat. Oil is something that is very high in fat, so generally those first layers are going to be thin with very little oil content and you’re probably going to be thinning down your paint with a bit of solvent, like if you were doing an underpainting.

Screen capture from The Science of Art: Mix with solvent to create “lean” paint

You would then start your next layer by adding a bit more paint or a bit more oil, like a linseed oil, in order to increase the fat content. As you increase the fat content, you should be adding more and more with each layer. This is so that your paint adheres properly to the layer before it.

If you have a lower amount of oil in your top layer than your layer beneath it, then it’s not going to want to adhere properly because it doesn’t have as much of those fatty molecules to cling on to.
For oil paint made with refined linseed oil binder: Tusc & Pine Artists’ Oils

The other thing to keep in mind–the reason that you want to increase fat as you increase layers–is that when your top layer has more fat it’s going to be more flexible. This is important because we’re waiting between each step for your layer to become touch dry before continuing to the next.

When that bottom layer starts to dry, your oil paint is going to start to shrink just a little bit and as that bottom layer starts to shrink it’s going to pull on the layers above it. If those layers aren’t flexible and don’t have a higher content of oil in them, they’re going to cause cracking.

Different Oils Found In Oil Paints

I talked a little bit about the differences between drying and non-drying oils but even within the category of drying oils there are several different types that you can use. One of the most popular types of oils that we use in oil painting is known as linseed oil. It is one of the most common oils and it’s also the most traditionally used. 

Linseed oil is one of the most popular because it creates the strongest film out of any of the types of drying oils that we have available. It also dries the fastest of all the drying oils that we have available. The trade-off is that linseed oil also tends to be the most yellow–though it is a very pale yellow–in comparison to some of the other oils out there.

Screen capture from The Science of Art: Linseed oil is also known as flaxseed oil

Since linseed oil tends to be one of the more yellow versions of our drying oils, there are other oils, like poppy and safflower oil, that are paler but don’t create quite as strong as a film and don’t dry quite as fast. 

Oftentimes, you will see in white paints they will use safflower oil. That is why it’s common in a lot of white oil paints for them not to dry quite as quickly as some other colors, but it also means that it’s really good for your final highlights on your page since it’s going to be slower drying than the layers underneath it.

Screen capture from The Science of Art: Great White oil paint with safflower oil

The other well-known type of non-drying oil that’s used in oil painting is walnut oil. Walnut oil is probably the most similar to linseed oil in terms of its properties. It’s non-yellowing, it’s fairly strong in terms of its film, and it also dries fairly fast, though not quite as fast as linseed oil does. The trade-off is that it also has a somewhat glossy effect in comparison to other popular oils. 

Frequently Asked Questions

How long does it take for oil paint to dry?

A: Oil paint will become touch dry within a few days, depending on what environment it is in. However, an oil painting will not fully cure until around 6 months after it becomes touch dry. This is because oil paint dries through a slow process called oxidation.

What is drying versus curing?

A: Drying occurs through the evaporation of the paint’s water-content. Curing, on the other hand, occurs through oxidation, where the paint comes into contact with oxygen and begins to harden through a chemical reaction known as polymerization. 

What is fat-over-lean?

A: Fat-over-lean is an important oil painting rule that is followed in order to maintain a painting’s structure when painting over several sessions. As each layer becomes touch dry, the next layer must have a higher fat content, a.k.a. more oil, in order to adhere properly and prevent cracking. 

What oil is in oil paint?

A:  Oil paint uses a type of oil known as drying oil. Drying oils, such as Linseed, poppy, or walnut oil, will cure over time through oxidation. Non-drying oils, like what is typically found in your kitchen, will not cure over time and are not ideal for paint. 

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