Here's a response I posted to a similar question on the Hotline four years ago. Based on a more recent thread on this topic, I believe it's still helpful.
I sent a note on Friday to the Brooklyn Copper Cookware site recommended by Merrill and received a response yesterday. My question was: "I have a French copper gratin pan, about thirty years old. I'm not sure whether it needs to be re-tinned. It's looking a bit beaten up, with some very slight, superficial blistering. How do I know when it's time to re-tin it?" Here is some useful information they sent: "What you describe is actually pretty normal for tin linings. The slight blistering you note is a sign that the pan was likely heated empty at some point, but unless the blister has broken through to reveal copper, and then a pretty good-sized patch of exposed copper (about the area of a dime), the pan is perfectly safe to use. The tin itself is safe to use if it's still molecularly stable, meaning it's not cracked or flaking, and even then a few fine flakes of tin are not toxic - one takes in more tin ions daily in the form of stannous fluoride in supermarket toothpaste.
Tin can also wear uniformly over many years such that a very thin veneer is left over a wide area. If you can see the pink/reddish copper color showing through there may be fine fissures exposing copper that are not necessarily visible. It's a good idea to retin in this case as well.
Small amounts of copper can leach into food only under acid conditions.
Acid foods cause exposed copper to oxidize, revealing the characteristic greenish color ("verdigris") associated with weathered copper. In the case of iron the same process yields rust - many metals in pure molecular form bond readily with oxygen to cause corrosion, as do elements and structures elsewhere, such as the cells in our bodies. Foods with so-called "anti-oxidant" properties prevent this.
Getting a bit of copper in one's food is not necessarily dangerous as ions of copper are an important trace nutrient. Too much of it, however, and it becomes a burden to cells of the liver and the oxidized form of copper is toxic in the liver. That's why copper has been lined with tin for cooking for about 7500 years. Pure tin is molecularly stable and non-reactive." ;o)
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I sent a note on Friday to the Brooklyn Copper Cookware site recommended by Merrill and received a response yesterday. My question was: "I have a French copper gratin pan, about thirty years old. I'm not sure whether it needs to be re-tinned. It's looking a bit beaten up, with some very slight, superficial blistering. How do I know when it's time to re-tin it?" Here is some useful information they sent: "What you describe is actually pretty normal for tin linings. The slight blistering you note is a sign that the pan was likely heated empty at some point, but unless the blister has broken through to reveal copper, and then a pretty good-sized patch of exposed copper (about the area of a dime), the pan is perfectly safe to use. The tin itself is safe to use if it's still molecularly stable, meaning it's not cracked or flaking, and even then a few fine flakes of tin are not toxic - one takes in more tin ions daily in the form of stannous fluoride in supermarket toothpaste.
Tin can also wear uniformly over many years such that a very thin veneer is left over a wide area. If you can see the pink/reddish copper color showing through there may be fine fissures exposing copper that are not necessarily visible. It's a good idea to retin in this case as well.
Small amounts of copper can leach into food only under acid conditions.
Acid foods cause exposed copper to oxidize, revealing the characteristic greenish color ("verdigris") associated with weathered copper. In the case of iron the same process yields rust - many metals in pure molecular form bond readily with oxygen to cause corrosion, as do elements and structures elsewhere, such as the cells in our bodies. Foods with so-called "anti-oxidant" properties prevent this.
Getting a bit of copper in one's food is not necessarily dangerous as ions of copper are an important trace nutrient. Too much of it, however, and it becomes a burden to cells of the liver and the oxidized form of copper is toxic in the liver. That's why copper has been lined with tin for cooking for about 7500 years. Pure tin is molecularly stable and non-reactive." ;o)
Cheers. ;o)