I decided to test the egg whites in 4 different bowls: copper, stainless steel, glass, and plastic. I kept the egg whites at a consistent temperature and weighed the whites to ensure that each bowl was as equal as possible. I also timed it and ensured that each bowl was whipped at the same speeds for the same amount of time, up until the point that stiff peaks formed. I used a kitchenaid handmixer to mix each batch.
RESULTSTime results. How long did it take for stiff peaks to form?
Copper: 3 minutes 30 seconds
Stainless Steel: 3 minutes 50 seconds
Glass: 3 minutes 55 seconds
Plastic: 4 minutes
Here were the results for volume:
You can see that the copper bowl (lower right) clearly produced more volume. And no I didn't smash it down. I followed the same procedure to transfer the egg whites on each bowl.
Overall, I was kind of disappointed. I thought the copper bowl would make a much bigger difference than it actually did. Clearly it produced superior results, but it just wasn't mind blowing. A copper bowl obviously isn't necessary to whip egg whites because even the plastic worked. But if you are into luxury kitchen items, or you make a lot of finicky egg white-based stuff (like macarons), then you may want to consider getting yourself a copper bowl. I do highly recommend the Mauviel brand. They are posh.