One thing I don't think that AB mentioned...
NATURAL LUMP HARDWOOD CHARCOAL ONLY!
Never use briquettes for this cooking method. Briquettes have all sorts of binders and additives, like clays and borax.
You can also do the same thing on coals from a hardwood fire. I've done it before with oak (not that great, as you get a tannic, sour/bitter still quite edible), and maple (very good).
"just put a charcoal briquet on their plate."
I did that to Mtnbkr's wife some years ago. She was a "Joan d'Arc" steak afficionado. She was not really amused at the joke.