Grey Cast (Dark Rings) in Hard Boiled Eggs

Our answer

The dark ring that forms around the yolk of a hard boiled egg is called a sulphur (sulfur) ring. When the egg is boiled the sulphur and hydrogen in the egg white combine to make sulphur dioxide gas, and this gas reacts with the iron in the yolk to form a dark ring. However the egg needs to be boiled for a while to start this reaction and it can be avoided by boiling your eggs for slightly less time and cooling them quickly.

There are many instructions for a perfect boiled egg but for a firm, bright white and a firm but still slightly flaky yolk put an egg into a saucepan of cold water, bring to the boil then reduce the heat to a simmer and let the egg cook for 10 minutes. Immediately remove the egg and cool quickly by either plunging the egg into a bowl of iced water or sitting it in a colander under a cold running tap, until the egg feels cool to the touch.