Copper thick enough to get a fine, granular distribution of energy across the interior surface can also be so thick that it stores some of the heat energy; when, for example, you’re working with delicate sauces, this can be a real problem. An efficient metal sheds energy as quickly as it takes it on, so when you want your food off the heat, it’s off immediately.
This is why people have strong opinions about thickness and why most high-quality copper cookware is between 1.75 and 3mm. It’s thick enough to distribute energy uniformly, but not so thick that it stores too much of it, and what it takes in lightning fast it can get rid of just as fast. At about 3.25mm in thickness one begins to lose the fast heating advantages with no significant gains in distribution – the pot is just that much heavier and more expensive. The copper thickness of Brooklyn Copper Cookware is between 2.25 – 3.2mm depending upon the specific piece.
“Copper pots are the most satisfactory of all to cook in, as they hold and spread the heat well and their tin lining does not discolor food….. To get the full benefit of cooking in copper, the metal must be 1/8 inch thick, and the handle should be of heavy iron.”
-Julia Child, Mastering the Art of French Cooking, 1961
Note: 1/8 inch is equivalent to .125 (see above), or 3.2mm.