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This is Asami and it's Asami only today. This is the first of what I hope to be a series of
semi-regular episodes where I summarize and share some interesting papers that I come across in the
conservation science field. And this is sort of my literature review record, I guess. It's not
really formal but I'm trying to learn more broadly about the conservation science field as a whole
because it's relatively new to me as well. So I'm gonna feature some older papers,
some newer papers, so not always super up-to-date, but I'll pick out a few,
um, maybe more than a few interesting papers that encompasses mostly chemistry just because
that's my background, but I'll be more interested also in sort of optics and machine learning and
other stuff that's more closely related to my current project. Anyway, I think it might be fun
to, uh, one, practice my skills to share scientific knowledge to general audience and also
bring you guys along the journey of sort of behind the scene of what happens in the museum labs.
So without further ado, let's go to the first episode. I'm actually going to split this
paper episode into two. First, where I sort of share the background information, historical
context, a little bit of basic chemistry to, uh, so that you don't have to, you know, freak out
when we actually do go deeper into the paper that I want to talk about. So that will be the second
episode. Uh, so the next episode, this is going to sort of be, uh, to put everyone on the same
page, um, and get ready for the second episode, um, where I talk more nitty gritty about the
techniques that they use, look at the data and sort of think and share what I thought about it.
So this, uh, paper, uh, is titled time-dependent ATR-FTIR spectroscopic studies on fatty acid
diffusion and the formation of metal soap in oil paint model systems. That was mouthful,
to say the least. Don't worry, we'll break it down into easier chunks. And in this episode,
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I'm only gonna focus on the last half of that title, the fatty acid diffusion part and the
oil paint model system. So to give you a little bit of historical background, uh, this was around
the end of 1990s. A research group studied Rembrandt's the anatomy lesson of Dr. Nicholas
Tulp. It's a famous, famous painting by Rembrandt. If you Google it, in fact, I might try to put an
artwork on this episode if I could. If you see it, you'll go, oh yeah, I've seen this in some
kind of textbook. Anyway, so they were studying this old Dutch painting, uh, and they found out
on the surface of this painting were a bunch of little crater-like holes,
uh, about 100 to 200 microns in diameter. And this was not reported in the previous sort of,
you know, condition report. Uh, this is a documentation that conservators keep a record of,
um, you know, when they do these checks of artworks. And they found out that this was
never reported previously, but they do see now this, you know, a bunch of crater-like items.
And what happens when you have these, like, microscopic scale craters, but a lot of them
on the surface of a painting, is that it creates uneven surfaces, which catches the light
in a different way. Um, most notably, it's going to scatter the light a lot more than it would if
it were just a smooth surface. It would scatter the light in a lot of different direction.
As a result, the overall effect to the visual experience as a viewer is that the painting is
going to look a little bit foggy. It's going to look a little bit like you're looking at it through
a smoky glass. Now, the effect most likely was not that bad. Uh, you know, not exactly like
behind the smoky glass, but I think it was evident that the painting surface was damaged.
And it made the outline a little bit blurry and foggy. So, clearly, this is a huge problem.
And the museum and the research group didn't really know where they came from. So,
they reached out to the larger conservation science community, asking, has anyone seen this
type of weird stuff on their paintings? Does anybody know anything about it? Um, you know,
what sort of situation, under what circumstances did you find these weird crater-like items?
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And they found out, to their surprise, that this actually were a pretty common problem,
but not just for, you know, 16th, 17th century Dutch painting, but it's also found in a huge
geographic and historical distribution. And they found these crater-like holes on Picasso, Van Gogh,
William de Kooning, and Georgia O'Keeffe. So, everyone from Europe to America,
from 16th century to 20th century, you know, huge range. But the only common thing they had
is that this was found in oil painting. A further research showed that these crater-like
items were made of lead and zinc soaps. So, that was great to characterize that,
and they found out that this indeed is the culprit of, you know, resulting in this bubbly
crystalline surfaces that creates uneven surfaces, and also some delamination flaking off of the
paints problem. So, good to know what these are made of, but they still didn't know how they come
about or how they started appearing. So, let's take a step back and think about what is inside
an oil paint system. First of all, oil paint system has the pigment, which gives it a color,
and pigments, in conservation science, technically is referring to inorganic compounds
that has some sort of color that sits in the visual range of 380 nanometers to 700 nanometers.
So, what are inorganic compounds? It sounds almost like an opposite of organic compounds.
Organic compounds are made of carbon, hydrogen, nitrogen, oxygen, and inorganic
is technically any other compounds that has something other than these elements,
but in this context, in the pigments context, these other elements are usually metals,
and specifically transition metals. So, things like cobalt blue is a good example. Cobalt
metal center is sitting in a ring of porphyrin-like rings of organic molecules, so a bunch of
carbons, basically, and the existence of cobalt in the center of this ring of carbon molecules
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gives it an absorption spectrum that absorbs the red color region,
which is why, to the human eyes, we see blue from cobalt blue.
So, these type of pigments and transition metals, it doesn't have to be cobalt, it can be copper,
iron, or zinc and lead, like we see in these paper. And so, just think of, for now,
what are pigments? Pigments are molecules that gives color and that usually has metals in the
middle of some kind of organic network. That's all you need to know. And oil paint, obviously,
has to have oil, and drying oil is the typical sort of common name for this type of oil that
people use for oil paint medium. And drying oil is made of triglycerides. It's a type of fat
with a high degree of unsaturation. So, what does unsaturation mean
if you have done organic chemistry in high school and if you still remember it? First of all,
very good job. But second of all, don't worry. I think you can remember that carbon bonds to
four things, typically. And unsaturation means they are missing hydrogens. So, of the four
things that they can bond to, if they have missing hydrogens, that means it's bonded to
some other neighboring carbon in a way that has less hydrogens around it, which means
it is doubly bonded to the neighboring carbon or triply bonded, or maybe it's sitting in a ring
network so that the bond order from one carbon to another carbon is about 1.5 bond orders.
So, anytime you basically see any double or triple bonds or if the carbon is in a ring format,
you can call it unsaturation. And this unsaturated part, when they are exposed to oxygen in the air
and as they dry, they begin to polymerize and make this network of molecules. So, oil,
as all is liquidy when they apply the paint, and as they dry, they form this thick coating,
hard layers, which is why once you dried your oil painting, it's hard to touch. When you touch it,
it's a solid. It's hard. It's not liquidy once it's fully dried. So, now we have established
that an oil paint system has pigments, which gives it a color, and oil, which is typically
unsaturated oil. So, oil with a lot of double bonds in this context.
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And so, when these two are mixed and as they dry, they start forming this metallic center
pigment and complexing it with the fatty acids in the oil to form a polymer.
And this type of reaction is a variation of reaction called saponification, and this results
in a formation of metal soap. And metal soap is a complex of metal ions from the pigment
and a long chain saturated fatty acids. What are saturated fatty acids? This is different
from the initial oil that we put because those are unsaturated fatty acids. These ones,
they're saturated, which means they are only singly bonded. The neighboring carbons are only
single bonded to each other. And so, this metal ion, when they find a long chain of saturated
fatty acid, they like to form a metal soap. And this metal soap, they can aggregate, they can
crystallize, and they cause these protrusions or unevenness on a surface of the artwork,
which is what we see as a crater-like characteristics on the surface of the artwork.
So, this is why people wanted to know how exactly these are formed, and not just how they are
formed, but how long does it take? What sort of time span are we talking about? Is it going to
happen in a matter of, you know, a few hours, a few minutes, or is it more like a few years,
maybe even a few centuries? We didn't know that, and to understand that, the group of researchers
for the next episode looked into the mechanism, the kinetic studies of how these metal soap
formation happens, and also tried to identify what are the factors that contribute to the
formation of metal soaps. And they used different conditions, different sort of recipes of model
system in order to understand this. So, for this next episode, I'm going to go straight into the
paper and talk, discuss about the technique that they're using, ATR-FTIR, and also go have a look
at their data and see what they come up with. So, tune in! Bye!
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That's it for the show today. Thanks for listening, and find us on X at Eigo de Science,
that is E-I-G-O-D-E-S-C-I-E-N-C-E. See you next time!
