I was at a cooking demonstration the other week, learning how to make jam. The demonstrator used an all-copper pot, a kind which is common in jam and candy making. One of the attendees asked why the copper pot wasn't lined in tin, which is common these days, because copper is supposed to be bad for us. The demonstrator wasn't sure, but he really appreciated the ability for the copper to quickly and easily transfer heat. So I figured I'd do some research, and see if there might be something special about jam making that makes it safer to use with copper.
Unfortunately, copper toxicity is in that annoying intersection of rarity and preventability that makes it difficult to find really good answers on the subject. Confounding the issue a bit is that copper is an essential nutrient as well, though being harmed from a lack of copper is equally rare. I do know that some countries and states have outlawed the sale of all-copper cookware, but it's a lot easier to outlaw copper than it is to determine if copper needs to be outlawed, so I'm not entirely convinced on that front.
Here's what we know: most adults need 1 to 2mg of copper per day. If you take copper in doses of 1000 or so times that amount, it'll probably kill you. For children, divide those numbers by 2. For infants, there's no good data, but try to avoid giving them extra copper. If you have a condition called Menke's disease, you'll have trouble absorbing copper. If you have a condition called Wilson's disease, you'll have trouble getting rid of it. And, of course, you can get nutrients from your cooking vessel, including copper.
In general, this looks pretty cut and dried. If you were to cook a food that absorbed 1g of copper from your pot in one go, chances are good that you would notice. That's a ridiculously large amount of copper to pull in at once. An 11" copper pot, which is pretty large, ships at 11 pounds. Drop 1 pound of that and we're looking at 4500g. If you cooked in that pot every day for a year, and you lost 1g per day, you'd lose 10% of the weight of the pot in a little over a year. And while I expect very few of you use any given pot every day, it's still crazy to think that you'd pull out that much copper in one go.
On the other hand, Harold McGee writes that we can only excrete copper in limited doses per day. So if we did happen to absorb too much copper on a regular basis, eventually it could build up and cause problems. We clearly don't want that. Therefore, it makes a good deal of sense not to have all of our cookware be pure copper.
There are factors that modify the copper absorption in food. Very acidic foods will pull off more copper. Tarnished copper will be absorbed into food more quickly than well maintained copper. Storing food in copper rather than just cooking with it and transferring it out will allow the food to absorb more copper.
Given all of this, let's consider jam. Given there are probably six jars of copper made in one batch, the batch cooks for a relatively short amount of time, and in one sitting, you're unlikely to eat very much jam (unless it's really, really good jam, and even so), you're probably safe eating jam made in a copper pot.
If you eat a jar of jam every day, and that jam is made in a copper pot, you may want to switch to another type of jam that is not made in copper pots. If you have Wilson's disease, you want to avoid eating the copper-enriched jam. If you are very young, you will probably want to hold off on eating copper-enriched foods regularly. Otherwise, and understand I am writing this as someone with no medical training whatsoever, jam cooked in a copper pot should be just fine. The gap between "not enough" and "too much" is a wide one.