Imma Perfetto
Cosmos science journalist
When precious relics are unearthed by archaeologists, it’s the job of conservators to ensure they’re passed on to future generations by protecting them from further degradation.
But a new study has cast into doubt a common technique used in the conservation of ancient metal artefacts. The “significant and alarming” findings indicate that some clear resin coatings commonly used by conservators, react with iron-containing metals and can cause damage.
A team from China’s Beijing University of Chemical Technology developed a non-invasive fluorescence imaging strategy that reveals the early signs of the damaging chemical reactions.
They say it could be used to determine the “conservation state and potential risks for other artifact preservation, minimising the damage to the valuable artifacts.”
Polymer coatings – including epoxy, polyurethane, and acrylics – are commonly used to protect metal artifacts from long-term exposure to light, heat, oxygen and humidity. They are lightweight, transparent, watertight, and can adhere strongly to the materials they preserve.
However, there has been limited research on what happens to polymer coatings as they age and how this affects iron-containing metals. This is because it is difficult to monitor what is happening at the boundary where the materials contact each other.
The new 3-dimensional fluorescence imaging technique, indicates early signs of corrosion and rust on iron-containing metal.
The researchers coated cast iron with the acrylic resin B72 – the most widely used polymer in metal artifacts – and sped up its ageing process by applying heat and UV light for 30 hours.
While they observed no fluorescence on freshly coated cast iron, the fluorescence intensity at the resin-metal interface increased steadily after 3 hours.
“It is surprising to acknowledge that the aged polymers can generate hazardous carboxyl groups and reactive hydroxyl radicals, inducing the oxidation and corrosion of the metal artifacts,” ,” the researchers write in their study, which is published in American Chemical Society (ACS) Central Science.
“In turn, the generated metallic ions could further aggravate the aging of polymer coatings.”
They carried out the same experiment on a rusty iron coin about
1000 years old, from the Chinese Northern Song Dynasty, and found that the aged polymer coating made the artifact rustier.
“These findings raised a timely alarm for the conservation ability and potential threat of polymer coatings on metal artifacts,” the authors write.
They suggest more research must be done to determine how coatings can be optimised for artifact conservation.
“We should strengthen the research on the modification of polymer coatings, including the structural design and preparation optimisation, to exclude the existence of defects or pores in the polymers,” they write.
“Employment of a specific stabiliser, or anti-aging additives, into polymer coatings is also needed.
“These actions would promote the protective ability of polymer coatings and reduce the secondary damage to cultural artifacts.”
