Diagnosing The Devil Inside

Using the new non-invasive technology on a plastic statue (left), the researchers were able to show that it is composed of nylon (blue) and cellulose acetate (red). Credit: Anal. Chem.

Similar to a disease, chemical degradation often advances quietly in art and artifacts, without any external warning signals. That is, until a breaking point occurs, and museum staff are suddenly faced with a faded painting or a cracked sculpture.

Like doctors who want to diagnose patients at the early stages of their illness, when treatments are more likely to work, museum conservators also want to assess the health of cultural artifacts at the initial stages of degradation, and they want to do so non-invasively--that is, without taking a blood sample.

Now Matija Strlic at UCL has just published an article about a new diagnostic tool that can visualize the internal degradation of artifacts before the damage is apparent to the naked eye.

Here's how it works: The device shines near infrared light over an entire artifact. The way in which this light is absorbed by the artifact at a particular spot is a fingerprint for its chemical make-up. So if the artifact is starting to deteriorate in a particular spot, the chemical fingerprint will be different there than in places where no deterioration is happening. Using a specialized camera, Strlic's setup can take a picture of the chemical makeup of an object and any damage it's experiencing that is not yet seeable to human eyes.

Strlic's device can identify the chemical make-up of buttons (left), such as wood (right, blue) or polyester (right, bright green). Credit. Anal. Chem.

When I reached Strlic to talk about the device, he was in Cairo, Egypt, lecturing at a workshop about the new technology. Given that Egypt's economy is entirely dependent on their cultural heritage, I'm guessing the device will find use there. But Strlic initially tested the device on materials much more modern than those used by the artists employed by ancient Pharaohs. Instead Strlic focused on plastics and iron gall ink--two materials extremely vulnerable to degradation.

Writers have been using iron gall ink since the 12th century, so a heck of a lot of important documents since the Middle Ages have been written with it. Here's the problem (from Strlic's paper):

Unfortunately, the ink is infamous for its instability and corrosiveness as it induces enhanced degradation of the writing or drawing support, which is a consequence of its acidity and content of transition metals. Acid catalyzed hydrolysis of cellulose in paper is a rapid degradation process and depends on pH; the lower it is, the faster the degradation. In addition to acids, iron gall inks contain a substantial amount of transition metal ions promoting autoxidation of organic compounds present in the ink and in the paper.

A 170-year old document (left) and the visualization of its surface pH (right) to reveal dangerously acidic areas (blue). Credit: Anal. Chem.

Yeah. Not a good prognosis for many archives. There's actually a lot of work going on to help save these documents. But what's cool about the new device is that it can measure the pH from a scan of the document's surface. In the image at left you can see the low pH areas on a document, which are colored in blue and should be worrisome for conservators.

There may not seem to be much going on in this piece of plastic (left), but the new technology shows that some regions (blue) have very little plasticizer and may be in danger of cracking. Credit: Anal. Chem.

As for the tendency that plastics artifacts have to degrade: look no further than a plastic water bottle that's been hanging around too long in the sun. The water's funny smell comes from the breakdown and subsequent leaching of the bottle's polymers and plasticizers--the latter added to keep the bottle maleable. Plastic artifacts tend to leach these components first and then begin to crack. Strlic's technique can warn staff when a plastic artifact or an acrylic painting is heading this unfortunate path.

Author: Sarah Everts

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