Honestly, I don't wash out, cook, boil or microwave my egg shells. I prefer to follow the natural route. When I crack an egg for a meal I let the shell and leftover egg content dry indeterminately under the sun, by a window. When I have accumulated enough egg shells to warrant some crushing (usually a full dish plate), I check them to see if anything is still wet, put the dry ones in a bowl, and use a potato masher to crush them. After that I pass the mess in an instant food chopper for a finer size of egg pieces to simulate advanced decomposition, then put everything in the calcium bowl of my hens. They think it's a treat so they eat it with enthusiasm.
The one thing I -don't- do is give my hens the egg shells of another flock, commercial or otherwise. Coccidiosis transmits by poop, and my paranoid mind prefers to avoid any potential contamination by diverting any outside egg shells to pot fills. Plants that are demanding in fertilizer seem to love having the bottom of their pots filled with egg shells for drainage, I've tried it on one last year and not only has she doubled in size since then, she doesn't seem to need fertilizer anymore. She's even giving me flowers!