|MadSci Network: Biochemistry|
Berenice and Chealsea,
Thank you for your question. When shell eggs are hard cooked, the protective coating is washed away, leaving bare the pores in the shell. This might make the shell slightly weaker but not much. The shell itself is mostly calcium carbonate which does not change much during boiling. It does, however, react with acids so if your cooking water is acidic or contains any vinegar, the shell will weaken and might even dissolve completely. Inside the egg you might see a green ring on a hard-cooked yolk which is the result of overcooking, and is caused by sulfur and iron compounds in the egg reacting on the yolk's surface. The green color can also be caused by a high amount of iron in the cooking water. So, in short, boiling does not significantly weaken an egg (at least I don't think it does, try it to find out!!).
Try the links in the MadSci Library for more information on Biochemistry.