John Palmer November 2007
"Copper
Copper has the highest heat conductivity, is easy to form and was traditionally used for making the brewing kettles or “coppers.” Copper can be readily soldered, brazed and welded with the proper equipment. Soldering and brazing should be more than adequate for most brewery uses.
Copper is relatively inert to both wort and beer. With regular use, it will build up a stable oxide layer (dull copper color) that will protect it from any further interaction with the wort. Only minimal cleaning to remove surface grime, hop bits and wort protein is necessary. There is no need to clean copper shiny-bright after every use or before contact with your wort. It is better if the copper is allowed to form a dull copper finish with use.
However, you need to be aware that copper can develop a toxic blue-green oxide called verdigris. Verdigris includes several chemical compounds — cupric acetate, copper sulfate, cupric chloride, etc. — and these blue-green compounds should not be allowed to contact your beer or any other food item because they are readily soluble in weakly acidic solutions (like beer), and can lead to copper poisoning (i.e., nausea, vomiting). To clean heavy oxidation (black) and verdigris, use vinegar or oxalic acid-based cleansers like Revereware Copper and Stainless Steel cleanser.
For regular cleaning of copper and brass, unscented dish detergent or sodium percarbonate-based cleaners are preferred. Cleaning and sanitizing copper wort chillers with bleach solutions is not recommended. Oxidizers like bleach and hydrogen peroxide quickly cause copper and brass to blacken; these oxides do not protect the surface from further corrosion, and are quickly dissolved by the acidic wort. Copper and other trace metals are beneficial nutrients for yeast, but the amounts that are dissolved from non-passive oxides can be detrimental to
the batch.
Copper counterflow wort chillers should not be stored full of sanitizer or water. Any biological deposits can lead to corrosion in both water or sanitizer. Copper should be rinsed thoroughly with clean water and allowed to drain
before storage."