There are disadvantages to the traditional copper bowl: it’s expensive, and a nuisance to keep clean. (Copper contamination is negligible; a cup of foam contains a tenth of our normal daily intake.) Fortunately there’s a nonmetallic alternative for controlling reactive sulfur groups. The sulfur bonds form when the sulfur-hydrogen (S-H) groups on two different protein molecules shed their hydrogens and form a sulfur-sulfur (S-S) connection with each other. The addition of an acid boosts the number of free-floating hydrogen (H) ions in the egg white, which makes it much harder for the S-H groups to shed their own H, and so slows the sulfur bonding down to a crawl. A good dose is ⅛ teaspoon/0.5g cream of tartar or ½ teaspoon/2ml lemon juice per egg white, added at the beginning of the beating.