What are the advantages (if any) for unlined copper jam pans?
Apologies for this long question, but I've spent a lot of time searching for an answer to this and so far have not found any satisfactory explanation. But I'll at least share what I've learned in the process.
And if you look into the claimed advantages of such copper pans, they're almost always about the superior heat response of copper. Jam (or preserves or whatever) must be cooked quickly, and overcooking the fruit lessens the flavor. So copper is supposedly ideal for heating and cooling quickly.
As someone who owns a number of copper pans, I know the advantages of copper can be overstated sometimes. But the difference in responsiveness is noticeable in sensitive applications (like egg dishes). So I'm not questioning the potential value of a copper vessel here, even though it might be quite pricey.
But a search for copper jam pans will quickly demonstrate that they are almost exclusively made of unlined copper. Why?
Copper is a heavy metal that is poisonous in large doses. Many people are aware that cooking or holding foods in unlined copper can be hazardous over time. As far as I know, there are two places in the kitchen where unlined copper is commonly justified: (1) copper bowls for beating egg whites, where the copper ions help stabilize egg white foams, and (2) traditional French pots for caramelizing sugar, where the high temperatures needed for caramelization would come close to the melting temperature of traditional tin linings. Such applications are attested in reputable sources like Harold McGee's On Food and Cooking and Shirley Corriher's CookWise. (One further application of cooking green vegetables in unlined copper because copper ions help retain a strong green color is also mentioned by both McGee and Corriher, but not advocated by either: Corriher even warns explicitly against the practice because of the poisonous elements of copper.)
And searches on jam pans and the use of unlined copper seem to turn up loads of references of people worrying about them, and lots of people arguing that since jam is cooked quickly and most people don't eat a lot of jam, the concern for unlined copper is minimal. (See, for example, Serious Eats, Fine Cooking, and this oft-cited blog, with a huge number of comments worrying about just how acidic and low-sugar it's okay for a jam mixture to be before you should use a different cooking vessel.)
All of these sources make the same sort of claims to excuse the use of unlined copper. And most sources severely caution users to be sure not to cook fruit alone in these pans (since fruit can be too acidic and might react with the copper), but rather to always wait and only cook the final mixture diluted by sugar in the pans. (This sort of warning is even sometimes listed by sellers: Williams Sonoma for example notes: "Unlined interior is safe for use with foods with a high sugar content.")
I should also mention that I've tried to find a reputable scientific reference claiming that added sugar makes unlined copper safe to use, but I have so far been unable to find one. Acidic fruit will still be acidic even with added sugar; it may be diluted and buffered, but I'm not sure immediately of the chemistry that significantly inhibit copper absorption just by adding sugar. And jams and preserves are often acidified further with the addition of lemon juice or other acid, which would seem to increase the potential for reactivity with the copper. Moreover, copper salts (which can build up on a corroding copper surface over time; the familiar greenish stuff being the most familiar) are often highly soluble and often highly poisonous, so keeping the interior of your pan sparkling becomes important.
In any case, assuming you keep your pan clean and uncorroded, it may not be a huge concern because jam is cooked quickly and people don't eat a lot of jam, so copper poisoning is unlikely.
But that seems to be an odd argument, since lined copper is now standard in kitchens. Tin, stainless steel, and even silver linings are possible and would make any concerns about reactivity or corrosion moot. And such pans with thin coatings are basically as responsive as unlined copper. So why are jam pans still almost exclusively sold unlined? Neither McGee nor Corriher makes reference to unlined copper for jam-making (which is suspicious). Nor does any other food science source I've consulted give a positive reason for requiring the copper to be unlined for jam.
And even if there were some relatively minor advantage to unlined copper (e.g., Bon Appetit mentions "smoothness" compared to stainless steel, but tin and silver linings are both generally very smooth too), why continue to make these incredibly expensive vessels that become essentially unitaskers in your kitchen? I could imagine other applications for a large wide copper pan in the kitchen, but having it unlined restricts those applications quite a bit to avoid acids, any long-cooking dishes, etc.
Does anyone have any idea for the reason behind this unusual and almost exclusive use of unlined copper in jam making?