When Color is Sensitive to Light

With Still Life with Five Apricots by Adriaen Coorte, a painting with migrating arsenic

My name is Marcie Wiggins and I am participating in the NSF-funded International Research Experience for US Students (IRES) this summer in the Netherlands. I have been interested in conservation and cultural heritage studies since high school as a unique, challenging way to utilize chemistry. Pursuing chemistry and art history as an undergraduate at the University of Maryland, I was able to further explore this interest through internships at the Library of Congress and the Smithsonian’s Museum Conservation Institute. Now, I continue this focus as an analytical graduate student at the University of Delaware. Through my experiences in cultural heritage science, I have seen the various ways cultural heritage studies have benefited through partnerships with the sciences.

Paintings provide a challenging chemical system for many reasons, one of which is the multiple layers of paint an artist can use in compositions. As a result, below the visible surface there can be many distinctive layers interacting and affecting the object. My summer research, mentored by Dr. Katrien Keune, will be addressing a recent phenomenon concerning arsenic paints, such as orpiment (As2S3), where arsenic degradation products have been found throughout other layers in a painting, from wood support to the top varnish layer. These pigments are sensitive to light, which accelerates degradation. As arsenic sulfide pigments photo-degrade, they turn into arsenic trioxide, which is slightly soluble. As a result, it is believed the arsenic is transported throughout the paint layers with water, where the arsenic may react with other compounds as well. This can pose a risk for conservators interested in removing the old varnish layers and can influence how they handle the object’s treatment, as this can causes bright oranges to loose some of their color.

A cross-section of orpiment paint over a lead white ground prior to degradation

To study the transportation of arsenic species as a function of relative humidity, model samples of orpiment on differing grounds will be exposed to a range of relative humidities in open and closed systems. These types of weathering experiments are common in cultural heritage research to reconstruct objects degradations to better prevent the processes. This is similar how car coatings are put through extreme conditions to improve them. Cross-section samples will be taken during the process allowing us to study the multilayer system. This means samples will be embedded in resin, turned on their side, and polished to expose all the layers on the paint sample. Over these past two weeks, I have been setting up humidity chambers using salt solutions and monitoring the systems. I am now focusing on making cross sections of the controls and initial samples, which will next be analyzed with light microscope, mapping-FT infrared and mapping Raman spectroscopy, X-Ray diffraction, and Scanning electron microscopy with elemental analysis.

The iconic “I Amsterdam” sign outside of the Rijksmuseum

Working at the Rijksmuseum in Amsterdam in an effort to tackle these cultural heritage questions is a dream come true! During my art history classes, many fantastic paintings were housed in the Netherlands, so it will be great to see those same paintings in person finally. But more importantly, it is exciting to work alongside conservators and scientists on such a vast collection of cultural heritage objects. Already I have gotten to not only work with historical cross-sections, but I have gotten an up close look at these objects in the conservation labs, instead of just being sent cross-sections to analyze from across the world. A conservator, Nouchka de Keyser, showed me a Jan Davidsz. de Heem painting containing arsenic paint to analyze alongside my reconstructions, which has helped put the whole project into perspective. Having worked in and with museums in the US for several years, I have wanted to broaden my experiences beyond the US. I am looking forward to learning the different approaches and techniques abroad to return to Delaware with. I hope to forge collaborations and partnerships that will last beyond this summer, as I have already found this a great place to work and a lovely country.

Paintings, Not as “Still Life” as you might think

MyPicture1 name is Lindsay Oakley and I am a Northwestern University graduate student working in the Netherlands this summer as part of a NSF sponsored International Research Experience for Students (IRES) focused on investigating questions in cultural heritage science.  I was first introduced to the field as an undergraduate at the College of William and Mary. When I was struggling to settle into a major, deciding between chemistry and history classes that I also enjoyed, I met a chemistry professor setting up a collaboration with the paintings conservator for Colonial Williamsburg.  I volunteered to get involved and was quickly fascinated by the way that the scientific study of objects can reveal insights into people and technologies of the past. But historical knowledge also serves to help interpret scientific results, and this synergy leads to new understanding.  This experience led me to Northwestern and a project in partnership with the Art Institute of Chicago where I have continued studying paint and new tools that can be applied to understand it and preserve it for future generations.

Many perceive paintings as static objects, capturing an image from an artist’s mind in perpetuity.  In reality, from the time an artist touched a brush to canvas until you encounter the work 10s or even 100s of years later, quite a lot has changed. The colors may have faded, a wrinkling or cracking pattern may have developed or new chemical species that are products of aging and degradation reactions may have formed and started to disturb the surface of the painting.  (For an interesting article on this last example, you can read more here.) But how do these things happen?  What physical or chemical processes are responsible?  These can be difficult to observe and quantify.  Sometimes the processes are just too slow (at least for one PhD student!) and sometimes we need to study the mechanisms of interest at the molecular level.  In these types of dilemmas, we can turn to computer models for help.

If you zoomed in on a painting down to the molecular level, this is a representation of what you would find. Long chain molecules that make up the paint binder link together to form a network that small molecules can diffuse through.
If you zoomed in on a painting down to the molecular level, this is a representation of what you would find. Long chain molecules that make up the paint binder link together to form a network that small molecules can diffuse through.

This is why I am so excited to be here in Amsterdam this summer.  Over the course of the next seven weeks I will be building a computer model of an oil paint layer and exploring how small molecules move and diffuse through it, while working with some of the leading experts in modeling and paint dynamics. For aging and degradation reactions to occur, reacting species have to “find” each other in the paint layers. With experiments we can observe where these species end up, and this study will seek to answer complementary fundamental questions about how they are transported.  We will model the oil paint as a field of atoms that exert forces on the molecules of interest, either attractive or repulsive.  Setting up this field will be the main research challenge I will face in the coming weeks.

At the Amsterdamse Poort, an old city gate in Haarlem.
At the Amsterdamse Poort, an old city gate in Haarlem.

While most of my time will be spent on the computer, being in the Netherlands offers wonderful opportunities to work closely with other scientists and conservators interested in these unanswered questions as well as the opportunity to walk across the city and look at the big picture first hand.  Staring at some great Dutch masters seems to put all the screen time in perspective and I am so excited and grateful to get to explore a new part of the world.  I am looking forward to reporting what I learn in the coming weeks, so stay tuned!