Julia Child says in Mastering The Art of French Cooking that to get the full benefit of copper it "must" be 1 eighth of an inch thick. That is 3.175mm. Most quality copper cookware I have seen online seems to be 2.5mm at most.
Was Julia Child wrong? If not who sells 3mm+ copper cookware and how thick is too thick? Also do different pots and pans have different optimum thicknesses? For example stock pots?
E. Dehillerin has a line of 2.5mm-3.5mm in their downloadable catalogue but does not specify the thickness of individual items.