Whether to Retin or Not
Generally, if the lining issues you’re seeing do not penetrate through to the copper pan body, then it’s perfectly safe to use and may yet last many years in regular use. If the damage penetrates to the copper body then we recommend a more watchful posture. Although it takes quite a lot of exposed copper for there to be even slight risks, the copper has to first corrode under fairly concentrated acid preparations to form any of its potentially toxic byproducts, such as copper sulfate and sulfide (“verdigris“, or the green-gray cast characteristic of weathered copper; for example, the Statue of Liberty).
Verdigris is a protective layer for the copper, which chokes off its own advance, but it can transmit to food, especially (again) acidic preparations. Verdigris also imparts a strong metallic flavor, but before it’s concentrated enough to do so it will give you plenty of warning to its presence by being very easy to see – it appears as a bright greenish streak where it’s formed in any wound to a tin lining. The onset of verdigris in a copper pot is a signal that it’s time to get that pot serviced.
If you have any doubt about damage to a tin lining, you can test the lining’s integrity by wiping a bit of undiluted white vinegar on the wound and letting it evaporate at room temperature. If you see any verdigris flecks in the wound you can start thinking about getting a new lining, less so because any potential exposure the verdigris is ipso facto dangerous to you, and more because penetration through to the copper layer indicates the compromise of the tin envelope and the likelihood that tin crystals will start to collapse outward from the wound. Tin itself has no known toxicity (it is, in fact, a micronutrient), so even if a bit of it flakes into food that’s not cause for concern. However, the lining’s disintegration process will begin to accelerate from this point.