They come when an egg is over Boiled or Cooked. The gray layer or patch is caused by a reaction between the iron in the egg yolks and sulfur in the whites. When the eggs are cooked too long or at too high a temperature, they form a drab compound called “ferrous sulfide.” You might also notice a distinct sulphury odor when you peel the eggs. To avoid this reaction, cook the eggs gently at a low temperature. The American Egg Board recommends covering the eggs by at least 1 inch with cold water, and bringing them barely to a boil. As soon as they boil, remove the pot from its burner and cover it. After 12 minutes, large eggs will be perfectly — and gently — hard boiled. But we recommend soft boiled eggs which are easy to digest.