Beating eggs is best accomplished in a copper bowl. I don't think there is a simpler way to put it. A more verbose, scientific explanation is provided by McGee (2004):
It turns out that along with a very few other metals, copper has the tendency to
form extremely tight bonds with reactive sulfur groups: so tight that the sulfur
is essentially prevented from reacting with anything else. So the presence
of copper in in foaming egg whites essentially eliminates the strongest kind of
protein bond that can form...the foam stays glossy and never develops grains.