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Girl with dark hair and dark eyes smiles broadly, wearing a white t-shirt and black sweater with a small golden necklace.
Preservation Science Intern, Sarah Fong.

Tracking Colors: Building a Pigment Reference Database

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(The following is a post by Sarah Fong, Preservation Science Intern, Preservation Research and Testing Division. She is a rising junior at the University of Virginia, pursuing a Bachelor of Science in chemistry with biochemistry specialization.)

Girl with dark hair and dark eyes smiles broadly, wearing a white t-shirt and black sweater with a small golden necklace.
 Preservation Science Intern, Sarah Fong.


This summer, I had the opportunity to work with the Preservation Research and Testing Division (PRTD) on building a database for their reference colorant collection.

The reference colorant collection houses a variety of materials including pigments and allows PRTD to work with materials similar to those of collection items and implement suitable techniques to preserve them. Over the course of my internship, I collected and compiled data from 50 pigment reference samples using three portable FTIR modules.

FTIR, or Fourier-Transform Infrared Spectroscopy, is an analytical technique that uses infrared light to create transmission and reflectance spectra associated with specific compounds. This is a vital technique used in preservation research to determine the identity of a given pigment or material. A lot of information can be gained from an infrared spectrum such as what material components are present, provide a timeframe of a painted object (if there is a known start or end date to the common use of the pigment used), and inform the best preservation treatments.

In the preservation science world, modern technology has eased the process of collecting data through improving the instruments used. What’s neat about PRTD’s array of instruments is its addition of the portable FTIR instrument.

A white box is in the center with a black laptop on the right. There are four other blue and white objects on the left to the white box.
The portable FTIR instrument (middle right) and its four attachments (left). For size reference, a laptop is next to it (far right). Top row of attachments: ATR (left) and ER (right). Bottom row: DRIFTS (left) and transmission (right). Photo credits: Sarah Fong


The ability to capture an IR spectrum using an instrument that’s the size of a laptop is revolutionary, especially in the field of preservation. The portable FTIR instrument allows for an easier and more efficient data collection process. Rather than transporting a fragile and extremely valuable collection item down to the sub-basement lab, which can be risky and potentially cause unnecessary damage, the instrument can travel to the collection item instead and even be mounted above an object. Even using the instrument in lab and stationery on a lab bench is much easier compared to its bulkier alternative.

The portable FTIR comes with four attachments: ATR, DRIFTS, Transmission, and ER. Each attachment has a different way of orientating the infrared light towards the sample, which ends up producing a different spectrum for the same pigment. The goal of my project was to conduct a comparison study among the different attachments, or modules, which will eventually be used to build a reference database for FTIR spectra. With each new module, there came a different set-up as well as sample preparation.

The three modules I used – ATR, DRIFTS, and Transmission – involved a destructive sample collection, meaning a sample of the pigment must be harvested and used up in order to create a sample ready for testing. Luckily, these methods don’t use up much of the sample, which is good when working with items that can’t be replicated. The fourth module, ER or External Reflectance, is non-destructive and more commonly used by the department when collecting data from collection pieces.

The first method, ATR or Attenuated Total Reflectance, uses a diamond crystal to reflect the infrared light from the instrument between the sample and the crystal to create the reflectance spectrum.

A metal arm sits on top of a metal surface with red powder on the metal surface.
A red pigment sample on the ATR module of the portable FTIR instrument. The arm attachment has secured the pigment sample and is ready to collect its spectrum. Photo credits: Sarah Fong


ATR did not require preparation of a sample, so collecting the spectra was quick and simple. The next two methods required the colorant pigments to be diluted to a 1-2% concentration in potassium bromide (KBr) and, therefore, required more involved sample preparation processes.

Metal cup filled with blue powder is on top of a black arm in a box with gold mirrors on the bottom.
The DRIFTS sample is a powdered mixture placed in a metal cup. The gold mirrors at the bottom help reflect infrared beams to the detector. Photo credits: Sarah Fong


DRIFTS, or Diffuse Reflectance Infrared Fourier transform spectroscopy, uses a series of gold mirrors to reflect any infrared light beam scattered from the surface of the sample that sits inside of a metal cup back to be captured by the detector.

To prepare the sample for DRIFTS, I needed to create a mixture of the pigment sample and KBr, that serves as a carrier substance inert to infrared analysis and does not interfere with the measurements taken. This involved grinding the pigment and KBr together into a fine powder, followed by carefully filling up and leveling off the metal cup.

a blue disc sits inside a metal tray inside of a white box.
The transmission sample in the form of a pressed pellet. Photo credits: Sarah Fong


Transmission was the most time-consuming module to collect data on, mostly due to the sample preparation that came in the form of pellets. Transmission creates an IR spectrum by shining light directly through a translucent sample pellet that sits vertical in the apparatus. In order to create the pellet, the beginnings of the sample preparation were similar to DRIFTS in that a pigment and KBr powder must be made. However, transmission required an extra and more physically taxing step of applying a great deal of pressure to the powder in order to form a thin disc, or pellet, specifically designed to fit into the transmission module.

A person in a blue lab coat places a metal cylinder into a metal box.
Placing the pellet press, which contains the pigment sample, into the hydraulic press. 10 metric tons of pressure will be applied to the press, forming the translucent pellet used in the transmission module. Photo credits: Amanda Satorius


Creating 50 of these pellets for data collection was an arm workout! Luckily, PRTD’s hydraulic press did most of the physical labor by applying the 10 metric tons needed to seal these samples. These three modules involved different sample preparations and different experimental samples. Unsurprisingly, each module measured a different IR spectrum for the same pigments.

Four blue and white boxes are on the left of a larger blue and white box. A black laptop is on the right of the large box.
The software used to measure the FTIR spectra displaying the three types of FTIR spectra for the pigment Azurite. From top to bottom, the methods are DRIFTS (blue), transmission (black), and ATR (red). Photo credits: Sarah Fong


The unique thing about having three IR spectra of the same pigment is that it allows for the cross-comparison of each FTIR module with one another to determine which is the overall “best” or most informative FTIR method. It also provides a more holistic approach to analyzing given pigments and serves as a point of reference when all of the information has been compiled into a database.

A document with cursive written in ink is covered in laminate.
Thomas Jefferson’s draft of the Declaration of Independence. Photo credits: Sarah Fong


This summer, I not only had a chance to work with the Preservation Research and Testing Division, but observe the works of other divisions within the Library of Congress as well. I’ve had the opportunity to observe some of the rare collection pieces that are housed in the Library, such as one of Thomas Jefferson’s drafts of the Declaration of Independence and a draft of the Virginia Declaration of Rights.

The work of PRTD is crucial to the continued efforts of preservation science in order to determine the best preservation techniques for the current and future pieces that are part of the Library’s collection. This database is also important for the preservation science community since, as of now, publicly accessible reference databases containing IR spectra of colorant pigments from the DRIFTS or External Reflectance methods are difficult to find.

I’d like to thank Amanda Satorius and the entire Preservation Research and Testing Division for their guidance and efforts in creating this memorable learning experience.

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Comments (2)

  1. awesome!

  2. Great job!

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