I believe these are impurities. This is talking about copper mines, producing the copper for the bronze casting.

So they are looking for the signature lead impurities in that copper. They use isotope analysis to find the lead impurities in the copper.

Isotope ratios are very easy and reliable signals compared to impurities. Impurities can be all over the place depending on where the materials were mined. Impurities could have also been added inadvertently during the casting as well.

Perhaps based on time-mapping known regional mining activity