The neat thing about whipping egg whites in a copper bowl is that as the whisk hits the bowl, tiny bits of copper break off from the surface of the bowl and mix in with your egg whites.
Since copper can bind sulfur groups, those tiny bits of copper bind to the egg white proteins and therefore, those sulfur groups are unable to form the strong disulfide bonds that can lead to gritty, dry whipped egg whites. Therefore, egg whites whipped in a copper bowl stay glossy and firm without as much risk of becoming overwhipped or grainy.