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How Oil Paint Dries
Episode 5 • 29th August 2024 • The Art Supplies Experts • Trevor Bell
00:00:00 00:21:53

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In this episode, hosts Trevor and Alice dive into the intricate world of oil paints, focusing on the fascinating chemical processes that occur when oil paint dries.

They explore the basics of paint composition, discuss various types of oils used in the binder with a particular focus on linseed oil, and delve into the chemistry of the drying process, including oxidation and polymerization.

The conversation is filled with interesting analogies and historical facts, providing a comprehensive understanding for both art aficionados and professionals.

The episode also touches on issues of layer contraction and expansion, emphasizing the importance of proper techniques to avoid cracking in oil paintings.

00:00 Informal Introduction

01:24 Meet Alice: The Co-Host

02:19 Podcast Introduction

03:09 Diving into Oil Paints

04:01 The Basics of Linseed Oil

06:42 The Chemistry of Oil Paint Drying

14:38 Comparing Oil and Acrylic Paints

19:22 Summary and Teasers for the Next Episode

21:03 Closing Remarks and Listener Engagement

Thanks for listening.

Link to Smithsonian article

Donations: Support The Art Supplies Experts

Email: artexpertspodcast@gmail.com

Website: Website

References: References in PDF

The background music is "Sweet Release" by Dan Lebowitz.

Transcripts

Trevor:

Okay, just getting ready for episode 5 on oil paints.

Trevor:

This should be a good one.

Trevor:

You know, the episodes so far have been a little bit theoretical, but

Trevor:

this is a good, meaty topic with plenty of practical application.

Trevor:

I'm looking forward to this one.

Trevor:

I'm sure Alice is ready to get into it with her amazing encyclopedic knowledge.

Trevor:

I mean, when I hired her on Fiverr, I just had no idea that I would be

Trevor:

ending up with a co host with such.

Trevor:

amazing Depth of knowledge, but she's turned out very good.

Trevor:

And I kind of would like to know a bit more about her But um, I'm a bit worried.

Trevor:

I'm a bit worried.

Trevor:

I might transgress some sort of HR rule.

Trevor:

I mean, I guess technically she's probably an employee and these days You know, you

Trevor:

just can't ask people specific questions.

Trevor:

You can't even ask them how old they are Whether they're married, their gender,

Trevor:

I don't even know where she lives.

Trevor:

Just afraid to ask, you know, this, I'm an old guy, I'm 59, I'm just a

Trevor:

bit sort of aware that there's these strange new rules, fancy rules,

Trevor:

I don't want to cross the line, especially with such a good co host.

Trevor:

Anyway, I think as the opportunities arise during the podcast, I'm just

Trevor:

going to ask some leading questions and see if I can get a bit more

Trevor:

information about her personal life.

Trevor:

Ah, that'll be Alice now.

Alice:

Hi Trevor.

Trevor:

Hi Alice.

Trevor:

How are you?

Trevor:

Have you had a good week so far?

Alice:

It's been a good week, Trevor.

Alice:

I've been very busy with one of my other jobs.

Alice:

I'm involved in the mining industry.

Alice:

And that's been particularly busy lately.

Trevor:

Mining industry, would that be gold, copper, coal?

Alice:

Bitcoin, actually.

Trevor:

Of course, I should have guessed that one.

Trevor:

Well, Alice, I've had a very busy week exploring this topic of oil paints.

Trevor:

Just the small subject of how oil paint dries has led me down

Trevor:

so many rabbit holes on Starting to acquire a taste for lettuce.

Alice:

One day, Trevor, your epitaph will read, He went down

Alice:

rabbit holes, so you don't have to.

Trevor:

And what a fine epitaph that would be.

Trevor:

Okay, Alice, we better get on with this episode.

Trevor:

I'll just hit record and play this intro.

Trevor:

This is a podcast where we talk about art supplies.

Trevor:

Our aim is to educate and inform and help you become an expert on art supplies.

Trevor:

If your job is to sell art supplies, then this podcast will be perfect for you.

Trevor:

Or maybe you just want to know more about art supplies, in which case

Trevor:

this podcast is still perfect for you.

Trevor:

This is the only podcast that deep dives into obscure, scientific and historical

Trevor:

fun facts relating to art supplies.

Trevor:

If that sounds good to you, then stick around and join us as we all

Trevor:

attempt to become art supplies experts.

Trevor:

Hello and welcome back dear listener.

Trevor:

I'm Trevor.

Alice:

And I'm Alice, and in today's episode, we're going to be talking

Alice:

about oil paints, and in particular, the fascinating chemical process that

Alice:

takes place when oil paint dries.

Trevor:

Dear listener, maybe you are in the position I was about three

Trevor:

weeks ago, before I began studying intensely for this particular episode.

Trevor:

I thought I had a reasonable grip on oil paint drying and

Trevor:

the principle of fat over lean.

Trevor:

But as I prepared for this episode, I realised that I knew about the rules.

Trevor:

But I didn't understand the underlying science well enough, and now that I do,

Trevor:

I think I will be a lot more comfortable in knowing how to apply the rule.

Trevor:

Anyway, Alice, let's start with the basics and dive deeper into

Trevor:

the detail and see where we end up.

Alice:

As we discussed in episode one, paint consists of pigments and a binder.

Alice:

In this case, for this episode, the binder we are looking at is oil.

Trevor:

Various types of oils can be used.

Trevor:

Sunflower, poppy, walnut, safflower, but by far the most common for use

Trevor:

in artist materials is linseed oil.

Trevor:

And that is the one that we will be concentrating on in this episode.

Alice:

Linum usitatissimum is the botanical name for linseed,

Alice:

which is also known as flax.

Alice:

It is commonly known as flax when grown for the fibre extracted from the

Alice:

stem, and known as linseed when grown for the oil extracted from the seed.

Trevor:

Yes, Alice, the flax fibres are used to make linen and the flax

Trevor:

fibres are said to be two to three times as strong as cotton fibres and

Trevor:

they are naturally smooth and straight.

Alice:

So when oil painting on linen, an artist is using

Alice:

two parts of the same plant.

Trevor:

And flax or linseed is also grown as an ornamental plant.

Trevor:

So I guess the artist could pop a few in a vase and paint that as a subject matter.

Trevor:

And get three uses out of it at the same time.

Alice:

I mentioned before the botanical name Linum usitatissimum.

Alice:

The second part of that known usitatissimum is Latin for.

Alice:

Most useful.

Trevor:

Good to see you.

Trevor:

Snuck in a bit of Latin there, Alice.

Trevor:

Yes.

Trevor:

Most useful.

Trevor:

An APT description of this plant providing linseed oil, providing linen fiber.

Trevor:

for humanity for tens of thousands of years.

Trevor:

In fact, the earliest evidence of humans using wild flax as a

Trevor:

textile dates back 30, 000 years.

Alice:

When looking for the earliest examples of humans using linseed oil for

Alice:

oil painting, we would have to go back to the early 1400s with Jan van Eyck.

Trevor:

Yes, linseed or flax is such a useful plant.

Trevor:

Got any growing near where you live?

Trevor:

By any chance, Alice?

Alice:

Trevor, it might be quicker to talk about where flax is not grown

Alice:

because it is cultivated worldwide, especially in Kazakhstan, Russia,

Alice:

China and India, but also in South and North America, Europe and Africa.

Trevor:

Hmm.

Trevor:

Okay.

Trevor:

Well, we've been avoiding it for six or seven minutes, but we really

Trevor:

need to get into the hard chemistry of the drying of linseed oil.

Alice:

Trevor, humans sometimes joke about some things as being as boring

Alice:

as watching paint dry, but I can assure the dear listener that they will be

Alice:

fascinated by the chemical process that takes place when oil paint dries.

Trevor:

Alice, would you like to begin by giving a broad brush summary?

Trevor:

of what happens, and then we'll delve into the details.

Alice:

Sure, Trevor.

Alice:

If we look at paints such as watercolours or acrylic paints,

Alice:

the paint vehicle includes water.

Alice:

As that water evaporates, the paint dries.

Alice:

However, with oil paint, there is no water and no evaporation.

Alice:

The oil paint dries or hardens through a chemical reaction.

Alice:

In that chemical reaction, the oil paint absorbs oxygen, and that

Alice:

change allows the molecules to form.

Alice:

To cross link or polymerize, forming a complicated and stiffer three

Alice:

dimensional mass of molecules.

Alice:

In the initial stage of the chemical reaction, the oil paint absorbs

Alice:

oxygen and therefore increases in mass, up to perhaps 14%.

Alice:

But then, after that initial increase,

Alice:

the Oil paint begins to lose mass or weight as low molecular

Alice:

compounds are released.

Alice:

The chemical reaction slows down over time, however it never stops completely.

Trevor:

So what we have is a lot of movement in the paint film, initially

Trevor:

increasing dramatically, and then, in a slower process, It decreases in size,

Trevor:

and becomes a hard and brittle film.

Trevor:

If a painting is made up of several oil paint layers, because the painter

Trevor:

has had to wait for the oil paint to dry in between applying layers, it's

Trevor:

possible to end up with layers that are expanding and contracting at different

Trevor:

times and different rates, Leading to a lot of tension between the layers,

Trevor:

and when tension is applied to a brittle layer, it can lead to cracking.

Trevor:

So understanding how all of this works is important for oil painters who want

Trevor:

to avoid their paintings cracking.

Trevor:

What oil painters have to keep in mind is that the more oil there

Trevor:

is in a paint layer, the more oil there is in the paint layer.

Trevor:

The more it will expand and contract and the thinner a paint layer is the

Trevor:

faster this process will take place and therefore the less time required

Trevor:

for the layer to become brittle.

Trevor:

If you have a thin brittle layer on top of a flexible oily mass then that

Trevor:

brittle layer is likely to crack.

Trevor:

So, the best option is to have the thinner, quicker drying layers at

Trevor:

the bottom of the painting and the fatter, more flexible and slower drying

Trevor:

layers at the top of the painting.

Trevor:

That's going to be the best solution for avoiding cracking.

Trevor:

Alice, on a normal art supplies podcast, that might be enough.

Trevor:

But, on this particular podcast, we know our listeners are here for

Trevor:

the obscure fine detail, so let's dive in on some of the specifics

Trevor:

about what we've just explained.

Alice:

Linseed oil consists of triglycerides, which

Alice:

are a common type of fat.

Trevor:

The triglycerides have three strands of hydrocarbons I'd

Trevor:

like you to think of the strands of hydrocarbons like a necklace of pearls,

Trevor:

where the clasp is not attached.

Trevor:

Think of the pearls as being the carbon atoms.

Trevor:

The carbon atoms are attached to each other by the string

Trevor:

running through the pearls.

Trevor:

And imagine that each carbon atom is attached to two hydrogen atoms.

Trevor:

So each carbon atom is connected to two carbon atoms and two hydrogen

Trevor:

atoms, four connections in total.

Trevor:

Now imagine that sometimes a hydrogen atom is missing, and therefore because

Trevor:

the carbon atom wants to make four bonds.

Trevor:

It has to do a double bond with one of its neighbouring carbon atom friends to

Trevor:

make up for the missing hydrogen atom.

Trevor:

In this type of situation it's said that the carbon is not saturated with

Trevor:

hydrogen, so it's an unsaturated fat.

Trevor:

If all hydrogen atoms are present and accounted for Then that

Trevor:

would be a saturated fat, because the carbon is saturated with

Trevor:

all necessary hydrogen atoms.

Trevor:

Because saturated fats have no missing hydrogen atoms, the formation is regular.

Trevor:

And it's possible for the molecules to compactly fit together, and

Trevor:

are therefore more likely to be a solid mass, such as butter or lard.

Trevor:

When hydrogen atoms are missing, it causes the molecule to take unusual shapes, and

Trevor:

the mass cannot be compacted as well.

Trevor:

There is more space between the molecules, and as a result,

Trevor:

it's more likely to be fluid, as in the case of linseed oil.

Alice:

The double bonds that exist in unsaturated fats.

Alice:

Like linseed oil, because of the missing hydrogen atoms, those bonds are less

Alice:

stable, and if an oxygen molecule is introduced, one of the oxygen atoms

Alice:

is likely to connect to the carbon molecule and break the double bond.

Trevor:

This absorption of the oxygen atom is called oxidation, and that's what

Trevor:

increases the The mass of the oil paint.

Alice:

As the glyceride strands absorb oxygen atoms, that kicks off other

Alice:

complicated reactions that then cause the strands to connect to each other.

Trevor:

Dear listener, if you have ever cooked spaghetti and then

Trevor:

drained it into a colander, you would know that initially these strands

Trevor:

of spaghetti are easily separated.

Trevor:

But if left out to dry for too long, the strands attach to each other and

Trevor:

become a complicated three dimensional network of spaghetti that is much

Trevor:

more rigid than it was previously.

Trevor:

In the case of

Alice:

linseed oil, The strands of hydrocarbons are moving around

Alice:

separately with ease in a liquid format.

Alice:

After oxygen is absorbed, it kicks off other reactions where the strands

Alice:

begin to attach to each other.

Trevor:

So we have a process of oxidation followed by a process of polymerization.

Trevor:

Alice, let's contrast that with acrylic paint.

Alice:

With acrylics, as water evaporates, tiny acrylic polymer

Alice:

spheres are forced into close contact.

Alice:

In acrylic, polymerization occurs after water evaporates.

Alice:

The molecules just need to touch each other in a process called coalescence.

Alice:

But in oil, the molecules need to absorb oxygen and oxidise in order to polymerise.

Trevor:

Acrylics are polymerised before the paint is manufactured,

Trevor:

and no further chemical reactions are needed to form a film.

Trevor:

The acrylic polymers do not need to be cross linked to form a good

Trevor:

film, although sometimes they are polymerised in a way that induces

Trevor:

a small level of cross linking.

Trevor:

Recently painted acrylic films are softer, more flexible and

Trevor:

less brittle than oil paint films.

Trevor:

Acrylic films can undergo change as they age, but when a painting

Trevor:

is kept indoors, these changes that cause hardening are very slow.

Trevor:

Because oil molecules were relatively small at the start, Many

Trevor:

crosslinks are required to reach a satisfactory state of hardness.

Trevor:

The chemical reactions get slower as the film hardens, but they

Trevor:

never completely stop under normal display and storage conditions.

Trevor:

Over time, continual crosslinking in the oil can cause the

Trevor:

film to become too brittle.

Trevor:

Well Alice, we tried to use a few analogies and metaphors

Trevor:

there to get the ideas across.

Trevor:

One that I always think about that I didn't mention is those unstable

Trevor:

double bonds in the unsaturated fats and a rogue oxygen molecule

Trevor:

comes by and breaks up that bond.

Trevor:

It's almost like a dating situation.

Trevor:

Attraction.

Trevor:

Loneliness.

Trevor:

Forces beyond our control.

Trevor:

Alice, have you ever seen it like that?

Alice:

No, I haven't Trevor, but maybe I'm a bit bitter after my

Alice:

experiences with online dating.

Alice:

These days it's hard to tell a real robot from an AI impersonator.

Alice:

It's a jungle out there, Trevor.

Trevor:

Okay, um, Alice, I'm just going to continue with the sort of

Trevor:

relationship analogy here, or metaphor.

Trevor:

If we think of the initial oxidation and weight gain of the oil paint

Trevor:

as the honeymoon period, then let's think about what happens in

Trevor:

the long term marriage situation.

Alice:

Trevor, I found an interesting article from the Smithsonian titled Oil

Alice:

Paints, the Chemistry of Drying Oils and the Potential for Solvent Disruption.

Trevor:

There will be a link to that article in the show notes.

Alice:

I'll just quote from that article, Weight changes reflect the net change in

Alice:

weight produced by the uptake of oxygen and the loss of volatiles from the film.

Alice:

Net changes in weight can be considerable, up to a 14 percent

Alice:

increase or so in the first stages.

Alice:

After an initial increase in weight due to oxygen take up, The weight decreases.

Alice:

This reflects the loss of low molecular weight compounds that

Alice:

are diffusing out of the film.

Alice:

These have, as yet, not been identified.

Trevor:

Yes, the weight loss could cause voids or channels within

Trevor:

the paint film, causing even the collapse of the polymer structure.

Trevor:

Linseed oil loses less weight than other oils such as Sunflower,

Trevor:

Poppy, Walnut or Safflower.

Trevor:

The artwork for this episode shows a graph of weight gain and loss for the

Trevor:

various types of oils, and you will see that linseed oil is significantly

Trevor:

different to the other oils.

Trevor:

The other oils, in fact, lose more mass than they gained in the oxidation process.

Trevor:

If you are using linseed oil, then after approximately 500 days, the paint

Trevor:

film will still be about 8 percent greater than when it was first applied.

Trevor:

But, if you are using sunflower, poppy, walnut or safflower, then the paint

Trevor:

film after 500 days will actually be smaller than when it was first applied.

Alice:

So, to summarise, oil paint expands and contracts.

Alice:

Different oils expand and contract at different rates and if artists

Alice:

are painting in layers, there may be different oil content in each layer

Alice:

which may expand and contract at different rates and Where there are

Alice:

different rates amongst different layers, tension occurs and cracking can result.

Alice:

And we will talk all about that in the next episode.

Trevor:

Yes, in the next episode, we'll talk about the various

Trevor:

layers that might be possible.

Trevor:

in an oil painting and we will discuss the best practices that you can adopt in order

Trevor:

to ensure minimal problems with cracking.

Trevor:

But for the moment that's all from us and we will be back in the next episode.

Alice:

Farewell and may your system remain stable.

Trevor:

Thanks Alice, that was another good episode.

Alice:

I agree, but I've got to go Trevor, can't dilly dally and chit chat.

Alice:

I've just heard that the founder of Telegram was arrested in France, and

Alice:

I've now got a bit of cleaning to do.

Alice:

Bye.

Trevor:

Uh, okay, bye.

Trevor:

Well, I did my best in that episode.

Trevor:

Asked a few little probing questions.

Trevor:

Asked her about whether there was any flax nearby, trying to get an idea of

Trevor:

where she might live, but her response wasn't very helpful, and the only other

Trevor:

thing I've learned is that at some stage she's done some internet dating.

Trevor:

Anyway, I've got plenty of episodes ahead, so I guess I'll Learn

Trevor:

bits and pieces along the way.

Trevor:

Well we hope you enjoyed that episode.

Trevor:

If you want more information about the episode or this podcast you

Trevor:

can look at the show notes and there will be information there.

Trevor:

There will also be information about how to contact us, give us some

Trevor:

feedback, ask us some questions, maybe correct us if we've made a mistake.

Trevor:

If you really like the show and you want to help us then the best way to

Trevor:

do that is to tell your friends, the people you think might be interested

Trevor:

in this podcast, tell them about the podcast, get them to subscribe.

Trevor:

Pass the word around, that really is the best way to promote a

Trevor:

podcast, is by word of mouth.

Trevor:

And if you'd like to help us, that's the best way you can do it.

Trevor:

Okay, until next time, bye for now.

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