This is, by the way, also the method used to produce the black finish on the pots made by the famous potter Maria from San Ildefonso. It is not part of a natural weathering process at all.
Now, if you take violent exception to my statements above and wish to discuss this in a rational manner, I will be happy to correspond about it. Those of you named Anonymous who wish to only insult and threaten me, please keep your frustration to yourselves, but to all others, conversation is an art. Let us practice it together.