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.