I have acquired a considerable amount of data on "artificial" or
applied chemical patination of copper, bronze and brass, but less on
the natural tendencies of the materials under various atmospheric
conditions (salt air, sulfurous air, clean air, etc.)  We have a job
with roof cheneau exhibiting the familiar pale weathered copper-green
coloration with black spot in the recesses. Question is how far from
copper into a castable alloy, (I assume bronze, therefore some
tin...) must this fabrication be?  (Some bronzes go to a wonderful
Hershey's chocolate color.) And given the degree to which rain
run-off is staining the stone below, is there a practical way to
arrest the staining activity, perhaps by (gulp!) sealing the surface?
Recommendations on required reading?

FYI: Re field application of a copper patinater: We have tried out a
commercial product, Nordic Green, which with expert application (very
easy to screw up) seems to achieve a reasonably stable immediate pale
green coloration on relatively new to moderately brown exposed
copper.  Time will tell if the treatment lasts and continues beyond
it's initial masked state (the product appears to have a masking
effect in addition to initiating the patination process) toward an
ultimately stable natural green state.

--Jim

P.S.  I've enjoyed the lively discussion on paints and oils.  That's
where this BP forum really...shines.