This week we took a closer look at the surfaces of our portraits and tried to identify substances on them by using multispectral imaging. We were visited by Anna Serotta and Dawn Kriss, both conservators at the Brooklyn Museum of Art. Our guest speakers first introduced us to multispectral imaging: the process of illuminating an object in different wavelengths of light, and then capturing an image of the object in different wavelengths of light in order to determine the presence of certain substances based on how they respond to different conditions. They started by explaining seven different types of imaging. We utilize three specific regions of the electromagnetic spectrum—the visible light, ultraviolet light and infrared sections of the spectrum.
We began with visible reflected light imaging (VIS), which records an image in the visible light range when the object is illuminated by visible light—basically a normal photograph. Next came ultraviolet-induced luminescence (UVL), which records in visible light when the object is illuminated by UV light. This method is useful for identifying organic materials on objects, as these substances glow under UV light. Then we discussed ultraviolet reflected (UVR) imaging, which records in UV light while an object is illuminated in UV light. This is useful to identify varnishes or adhesives that may have been added as parts of restoration. We also discussed UVR false color imaging. These are digitally formed by swapping the color channels of a visible light image with the UVR image.
Next came infrared-reflected imaging (IRR) and IRR false color imaging. The IRR image is recorded in infrared light while the subject is illuminated in infrared light (such as a basic table lamp with . This method is especially exciting to use on paintings, as some paints look translucent under infrared light, allowing us to see carbon-based underdrawings. The IRR false color image is formed in a similar manner to the UVR false color image; the visible image is digitally combined with the IR image. Finally, we discussed visible-induced infrared luminescence imaging (VIL), where an image is captured in the infrared light range while the object is illuminated by visible light, in our case LED lights. In this method, the pigment Egyptian blue glows very brightly, making it easily identifiable.
Then came the fun part: imaging our portrait of a man! We captured one of each type of image and found some really interesting things. In the UVL image, the clavi (the stripe on his garment), nose, lips, eyebrows, and eyes all glowed red. This indicates the presence of the pigment madder lake. Additionally, the whites of his eyes and the area around his neck were bright white. We are still considering what sort of substance could this be, by comparing this image to our x-ray image and looking at the portrait under the microscope.
In the UVR image, there dark areas covering the man’s clothing. Since we know that recent restorations were completed in this region, these dark areas may relate to those treatments.
In the VIL image, the portrait was completely black. Nothing glowed, indicating that there is no Egyptian blue on the portrait. This is very surprising since Egyptian blue was a popular pigment at the time! We were very surprised, until, after re-imaging the portrait, we could see a sparkly glow of Egyptian blue pigments along the entire gray-green background do the portrait! This was a good reminder to double-check our results, especially when we expect to see something but don’t.
At the end of class, we looked at previously taken images of the portrait of the young man. Using the UVL image, we determined there was madder lake in the clavi of the young man. But the most interesting discovery we made about the portrait of the young man was the presence of a black marking—possibly writing–in the background above the left shoulder. This showed up in the UVL image but not in the VIS image or in the IRR image. What could these letters be? Is it part of the young man’s name? Or was it marked by archaeologists when the portrait was discovered? We will discuss these questions and hopefully identify more substances next week when we spend more time looking closely at all of the images and evidence together.