I have been working with copper and making good progress in achieving a strong blue/green patina. However the finish does not have the black highlights running through it like appears on natural very old patinas as observed on old church copper work etc. I'm trying to obtain this bluish/green with a dark or black mix through it so as to closely approximate truly old patinas.
What is this black overtone and how could I reproduce it artificially?
Thank You,Dan Scarbo
- No. Adams, Massachusetts
The Black Finish on the copper that occurs with the blue green salt ammonia vapor bath is hard to get on regular copper sheet. This is because most modern industrial sheet metal is produced in an electro-plating method which introduces an oxygen bond to the metal, making it also mildly more brittle. The sheet if taken cold rolled does not respond well to the patination blackening because of this bond. Most old church work was done by individuals who cast their own alloys of copper sheet and produced their own sheet metal at the forge. the gradual heating and cooling in a carburizing flame bond the free oxygen to carbon making Carbon Monoxide when heated to a high enough temperature for the molecules to free themselves (Red, to mild orange). if attempting the blue green finish on modern sheet copper give it a few hot passes with a cutting torch to anneal it and then polish back to unoxidized metal to patina.Todd Hansen
- Kansas City, Missouri