It seems like a simple task, but boiling an egg for slicing or Salade Nicoise calls for technique. A perfect egg should be rich and luscious with tender white and bright yellow center. It should never be dry and crumbly in the yolk, or worse, marred with that green patina you sometimes see.
I hesitate to use the phrase "hard cooked" because what you really want is an egg that's pliant in the yolk with slight darkening toward the center. After all, cooking eggs is mainly about getting the yolk right.
Where Does the Green Ring Come From?
The good news is that the ugly green gray ring that sometimes appears around the edge of the yolk is harmless. The bad news is that it's fairly disgusting to look at. The other bit of good news is that it can be totally avoided.
The ring forms when sulfur in the egg white reacts with iron in the yolk. The most likely cause of is overcooking, but apparently (according to a University of Nebraska posting) it can also be caused by an abundance of iron in the cooking water.
How to Avoid the Dreaded Green Ring & Get Perfect Firm Egg Yolks
I've been using this method for several months now with perfectly consistent results. I hope you find the same success.
Take cold or room temperature eggs and put them in a pot of cold water. Put it on the stove and bring the pot to a boil. Once the water has reached the boil, remove the pot from the heat and set your timer for 10 minutes. After the timer goes off, plunge the eggs into cold water and relax. When you crack open your eggs, it all has gone to plan, you'll find a bright yellow, slightly moist and dense, delicious yolk.