Copper is traditional not only in brewing, but in cheese-making. Traditional Mountain cheese including Gruyere, Emmentaler (aka Swiss cheese) and Parmesan were all traditionally made in large copper pots, and still are to this day in the Alpine parts of France, Italy, and Switzerland.
Hardcore traditionalists insist that copper creates a unique flavor that just cannot be replicated with stainless steel equipment. It is believed that a chemical reaction which takes place between the copper and the milk helps some of the ripening bacteria and creates certain flavors. I have also heard that copper oxides are a yeast nutrient in beer making.
All copper should be cleaned, however. Excessive copper is definitely poisonous, but in trace quantities is an essential mineral.
I also thought I'd respond to the comment that "if its on sale it must be safe!"
Originally Posted by yashicamat
Does anyone have anything to add to Nerro's comments? I thought copper immersion chillers were sold by homebrew shops . . . . such things would surely need to be safe in order be on sale?
I couldn't disagree more strongly. There are scores of things onsale, for internal ingestion, which are clearly unsafe. To name a few: Aluminum pans for cooking, Beer in tin cans, MSG in food products, methanol in soft-drinks, etc...
There are also somethings on sale which are "not suggested for internal ingestion" by the FDA, but which will probably end up getting used for that purpose anyways and are probably harmless if not outright healthy in small quantities. For example, Sassafras root, Comfrey Leaf, Yarrow leaf, Wormwood, etc...
Did you know that Sage has a higher Thujone concentration than Wormwood? (Thujone is the neurotoxin which has been demonized as the culprit for absinthe's intoxicating ability)