Re: Christopher Gray and "masonry buildings where the owners made a
conscious decision not to clean".

I often give the example of "controlled failure" in the cleaning of the
Main Building at Ellis Island.  We worked with two of N.Y.'s best
conservators and chemists specializing in stone along with Pro-So-Co's
people on the building scafflod to get the brick, limestone and granite to
clean up.  Every time we got it worked out, we'd move down the line or
around the corner and it the materials would behave dramatically different.
Stop.  Reconsider.  Reconvene.  Retest and retry.  Move on.  Fail.  Stop.
Etc....

After a few too many rounds of that, we ended up steaming some of the
blackest of the black stains, but left most of the dirt and staining alone.
You may ask why did we want to clean it in the first place.  Stains can
tell the story of survival.  That approach may not work for Madam Gotrocks,
but that's what we and the Park Service finally concluded.  Some crusts and
organics can weaken stone, so some remedial work should be considered.  But
I am most amazed at the really dramatic visual difference a good pointing
and flashing job will do to keep a wall dry, letting rain wash the building
naturally.   And like the ivy on Lexington Avenue,  the stains will likely
return before long.

--Jim