Beneath the Surface: The Return of the Green Lantern

Hello and welcome to the first post of “Beneath the Surface”, the blog that keeps you up to date on the latest activities at Colonial Williamsburg’s Materials Analysis Laboratory, where the 18^th century meets modern-day science. Here, we use sophisticated analytical instruments to better understand the art and artifacts that make up our varied collection.

I’m Kirsten Travers Moffitt, the Foundation’s first Materials Analyst (as well as Associate Conservator of Painted Objects), and I’ll be your guide.

I would like to start by sharing a current project that highlights the capabilities of the lab and the impact that analytical work can have in the historic area.

As some of you know, one of CW’s major projects right now is the reconstruction of the Market House. Many branches of CW have been involved with the research, planning, and construction of the site, and the tinsmiths have been working hard to craft period-appropriate lanterns to hang there.

Historically, many tin objects were painted for protection and decoration. In keeping with this treatment, the lanterns will be painted with hand-ground paints, but the question is: what color should they be painted? What pigments and binding media (for instance, a linseed oil or varnish) should be used?

This was a perfect project for the Materials Analysis Laboratory.

To help answer this question, I was asked to analyze the paints on several lanterns we have in our collection, dating from the 18^th to the early 19^th centuries. I examined each object carefully and collected small paint flakes from each with the help of a surgical scalpel. I prepared cross-sections of each paint sample and examined them under the microscope to see how many layers were present, and which was the original. This technique, called cross-section microscopy, allowed me to see other important characteristics such as the general color of each layer, the consistency and thickness of the paints, and the presence of dirt and wear at the surface- a dead giveaway for coatings that have been exposed for long periods of time.

And the winner is…Green! Most of the lanterns had green as their first paint color.

But the color was only one part of the mystery. What were these green paints made of? Other techniques in the lab were applied to answer this question.

I analyzed each lantern with our portable XRF (X-ray Fluorescence Spectrometer), an instrument that uses x-rays to analyze the elemental composition of a sample. Any element above sodium (Na) can be detected with this method, which is invaluable when characterizing pigments. For instance, the presence of mercury (Hg) and sulfur (S) in a red paint would strongly suggest that the red pigment vermilion (HgS) was present.

I also carried out polarized light microscopy (PLM) for pigment ID, which is a geological technique that allows one to observe the individual pigment particles and make identifications based on their optical properties such as color, morphology, and refractive index. Online databases and reference collections at CW were also used for comparison.

So far, the data indicates that all of the green paints were made with a combination of blue and yellow pigments, rather than a single green pigment alone.

All of the greens contained Prussian blue (Fe₄[Fe(CN)₃]₃), a blue pigment with incredible tinting strength, introduced in 1704 and found in many 18^th c. paints.

All of the paints also contained yellow ochre (Fe₂O₃ • nH₂O), an iron oxide pigment collected from the earth. Colors range from browns and reds to oranges and yellows, and are what gives certain soils their distinctive colors. Ochres have been in use since antiquity because they are cheap, available, and very durable.

But here’s where it gets complicated- some of the green paints also contained chrome yellow (PbCrO₄), a bright yellow pigment that was not introduced until c.1810. This means that these paints post-date the 18^th century.

Chrome yellow pigment will, of course, not be used in the paint replication, since it would not have been available during the colonial period.

So far, the findings suggest that the market house lanterns should be painted with a green paint made with Prussian blue and yellow ochre.

The next step is to analyze the binding media with FTIR (Fourier-transform infrared spectroscopy), a technique that uses infrared energy to identify organic materials such as resins, oils, and waxes. This should tell us if the pigments were ground in oil, or a varnish, or perhaps both.

I’ll continue to work with the lantern samples to learn all I can, and convey the findings to the tinsmiths so they can move forward with painting, and I’ll keep you posted on their progress.

Thanks for reading, and until next time…keep looking!