The issue is real and relates to the bronze medals, which are an alloy of copper, tin and zinc, with an inset hexagonal piece of iron reclaimed from refurbishment work on the Eiffel Tower. It was known from the outset that this would make them vulnerable to moisture, including sweat, and it was planned to use a chromium trioxide based ‘varnish’ for protection. This had been successfully used in the past but there was a last-minute regulatory change arising from an EU Directive which prohibited the use of chromium trioxide in relation to concerns about toxicity.
The Paris Mint could well have predicted the forthcoming ban but was caught with its pantalons down and hurriedly switched to a new type of varnish which had been inadequately tested. The new varnish allows ingress of moisture and creates a corroded ‘crocodile skin’ pattern on the surface of the medal itself which then causes the varnish to begin flaking off and worsening the problem.
It’s not the first time that the Paris Mint has produced defective workmanship relating to its varnishing processes. In 2023 the Chinese multinational Huawei returned a batch of 12,000 medals to the mint. They had been intended for distribution to Huawei’s top employees but the first recipients soon began to notice cracks appearing below the varnish.