Some protective resin coatings may damage metal artifacts

Conservators and museum technicians protect precious archaeological metal objects, such as tools and weapons, with clear coatings, leaving preserved and unobstructed views of these detailed treasures.

However, researchers have reported in ACS Central Science that some of the resins used for these coatings react with iron-containing metals and can cause damage. The team developed a noninvasive fluorescence imaging strategy that reveals early signs of these damaging chemical reactions and confirmed its utility on ancient artifacts.

Polymer coatings, including acrylic resins, are commonly used to protect metal artifacts from long-term exposure to light, heat, oxygen and humidity. The coatings are in many ways ideal for this application because polymers are lightweight, transparent and watertight, and they can adhere strongly to the materials they are preserving, including waterlogged wood.

However, there is limited research on what happens to polymer coatings as they age and how that might affect iron-containing metals, such as steel or cast iron, because it’s difficult to monitor where the materials contact one another. Current options include peeling away or dissolving the polymer, risking damage to the artifact, or imaging techniques that are nondestructive and fast but don’t give a clear, high-resolution picture of the chemical interactions within this thin space.

So, researchers Rui Tian, Chao Lu and colleagues developed a 3D fluorescence imaging strategy to light up the carboxyl groups that indicate early signs of corrosion and rust on iron-containing metal.

Initially, the researchers observed no fluorescence when they used the imaging technique to look at metal freshly coated with a common acrylic resin used for preserving metal artifacts (a copolymer of ethyl methacrylate and methyl acrylate). They then sped up the aging process of the resin by applying heat and UV light for 30 hours. In observations, the intensity of fluorescence at the resin-metal interface steadily increased after three hours.

In a proof-of-concept demonstration, the researchers tested their fluorescence imaging technique on a rusty iron coin of the Northern Song Dynasty from an archaeological excavation. They coated the artifact with the same resin and expedited the polymer’s aging process with heat and light. The aged polymer coating magnified the production of damaging carboxyl groups, making the already rusty artifact rustier. However, on a new piece of iron, the aged polymer coating did not speed up surface rusting.

The team says this information could help develop new polymers with stabilizers and antiaging additives to better protect metal artifacts.