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PRTD Intern, Jessica McKenzie

The Art of Color Analysis: Using Analytical Chemistry in Pigment Research

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The following is a post by Jessica McKenzie, preservation science intern, Preservation Research and Testing Division. She is a fourth-year student at Georgia Institute of Technology, pursuing a bachelor’s degree in chemistry with plans to continue her education in analytical chemistry.

A girl with dark brown hair and glasses smiles.
PRTD Intern, Jessica McKenzie

 

A lot of people think of chemistry and think of organic chemistry. It’s an infamous subject in which you generally take individual components and combine them in specific conditions to create something else. Analytical chemistry, in my eyes, is the opposite of organic chemistry. For one, the class usually doesn’t make people change their majors – if you stick with chemistry long enough to take this course, you’re in too deep to back out now. In all seriousness, analytical chemistry typically uses methods to look at a sample and determine its composition. From the compositional data, a wide range of questions can be answered. In the preservation world, these questions include those of authenticity or how a sample can be better preserved.

My project at the Library of Congress expands upon research initiated by a previous intern,  . Sarah worked with Fourier Transform Infrared Spectroscopy (FTIR) to create a spectral database from 50 raw pigments using three different methods. FTIR measures the absorption of infrared light into a sample, which gives information about the bonds in the sample and helps determine the composition. Creating a spectral database will assist members of the Preservation Research and Testing Division (PRTD) in recognizing trends about these pigments and will help them to identify unknown pigments more quickly. My research works to expand Sarah’s database with two more FTIR methods. It also works to investigate how these pigment spectra change when they are applied to paper.

A rectangular piece of paper with written notations at the top and 7 small painted squares in a grid pattern correlating to a single pigment painted in 7 different binder media.
Paint-out reference card of a blue pigment, verdigris, from the PRTD’s colorant reference collection. This reference card has seven different binders.

 

Reference cards such as the one above were analyzed using FTIR because each swatch on the page uses a different paint binder. Although reflectance FTIR is considered a surface technique, it does slightly penetrate the substrate. This results in the binder and paper also appearing in the spectra when it is measured. Comparing these spectra to the pure pigment spectra allows us to see what peaks are persisting from the pigment.

A microscope image showing a blue, crystalline pigment applied to paper.
Image of the verdigris pigment taken with a MiScope, a digital microscope, under 40X magnification

 

Verdigris is one pigment I studied in my research, and it is a good example of why expanding the spectral database to include pigments on paper is an important next step in this research. The next image shows my two methods of FTIR using the pure pigment in comparison to the pigment on paper.

Graph showing three curving lines that depict the patterns of light absorption at different wavelengths. The x-axis is measured in wavenumbers. There are four boxes around these lines where the absorptions are similar across the three graphs. The boxes are labeled A, B, C, and D.
Two FTIR spectra of pure pigment samples (pink and blue, lower two spectra) and one spectra of the pigment applied on paper (black, higher spectra). Note that the y-axis values are not correct, as the spectra have been shifted to better show the similarities.

 

In the image, the peaks that are showing on all three spectra have been boxed off. I have ignored the range from 3800 to 3200 wavenumbers because this is the area of the spectrum where OH bonds show up, and this peak is often influenced by the water in the air. It can be seen, however, that sections A and B are showing as significantly smaller peaks in the paper spectrum than in the spectrums of raw pigment. Meanwhile, some peaks have been completely obscured by larger peaks in the paper spectrum. The fact that so many peaks can be obscured by the paper makes it very difficult to distinguish peaks when looking at samples. The development of this database is very important to be able to determine what peaks are likely to show up regardless of what type of paper or parchment the pigment is applied to.

A long map is positioned on a rolling cart. Next to it, a rectangular instrument with a sensor in the front is positioned on a tripod. This instrument is tilted downwards so its sensor is directed towards the map.
The portable FTIR setup for analysis of the Portolan chart.

 

During my time in PRTD, I was able to assist in taking FTIR measurements of a Portolan chart from the 16th century. This beautiful map was brought into the lab for a pigment analysis, so it was incredible to experience the practical applications of my research. The map had small doodles of ships in the water, and an intricate compass rose. It had four different colored pigments, and multiple spots with each pigment were analyzed to determine if the colors were the same pigment across the map and what the pigments were.

A small drawing of a sailing ship.
A picture of one of the ships decorating the map.

 

Because PRTD is developing techniques for portable instrumentation, there is much more flexibility with how rare media from the collections can be analyzed. Items that are difficult to move do not have to make the trip to the lab. Instead, the lab can be brought to the item. In this case, the FTIR was secured onto a tripod, which allowed the instrument to be tilted so it could focus and scan the Portolan. If the instrument were a bulky benchtop version, simply scanning the item in this manner would not be possible. The preservation scientists would need to maneuver the item in a way that just isn’t good for it, or sampling would have to be involved. Portable instrumentation allows PRTD to explore further applications of analytical chemistry and utilize them to learn more about the many collection items of the Library of Congress.

I would like to thank everyone in the Preservation Research and Testing Division for giving me this opportunity, with special thanks to Amanda Satorius for being an amazing mentor throughout this experience. I have really enjoyed this internship and working with these brilliant individuals at the Library of Congress.

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