ASK THE EXPERT - Tom Ellis, Thompson Work book
As copper comes from the smelter it is 99% pure. The impurities are removed by an electrolytic process where the impurities fall to the bottom of the tank and the 99.9+% copper deposits on a plate known as a cathode. The cathodes are then melted and coast into suitable shapes for processing. During this last operation, some copper oxide forms and becomes distributed as inclusions between the copper crystals. This type copper is sold as electrolytic copper.
At temperatures higher than 751 degrees F hydrogen will diffuse into copper containing copper oxide and reduce the oxide to copper and water vapor . The water vapor which arrives at the interface between the copper and enamel will cause bubbles in the enamel and in extreme cases affect the adherence.
If during this last manufacturing process of melting and casting into suitable shapes a blanket of protective gas is present at all times to prevent the copper from coming into contact with air it will not oxidize. This product is sold as "oxygen free, high conductivity" copper, conforming to ASTM specifications B-170.
The electronics industry uses this type for glass to metal seals, but they demand a certification which includes a scale adherence test and a composition stipulation. The scale adherence test consist of heating a specimen for 20-30 minutes at 1562F and quenching in water. If the entire specimen retains red copper oxide the copper is accepted.
The following incident shows the importance of selecting the proper copper.
A customer who produces a line of giftware using fire scale as a major part of the design found several shipments of copper in which the Scale would not adhere. Several changes in firing procedures were made but the results were the same.
We sent the customer a few pieces of our stock copper and the scale adhered as in the past. We ran an analysis on both types of copper and found the following:
Pieces of each were heated to 1560F for 25 minutes and water quenched. The suitable copper retained a complete layer of red copper oxide with a few small areas of black oxide.
The unsuitable material showed a bright copper surface where all of the scale had popped off. The test was repeated allowing the metal to air cool after being removed from the furnace. The suitable material retained a complete coat of black oxide. Underneath this black oxide was, of course, a layer of red oxide. The unsuitable material retained no scale, red or black. The difference in the two materials was the PHOSPORUS content. Phosphorus was probably added as a deoxidizer. Both of the coppers appeared to enamel satisfactorily. We strongly suspect a further increase in the phosphorus would cause adherence problems.
Webster, Christie, abd Pratt reports scale adheres firmly to OFHC copper when formed at 1470F or higher, and to tough pitch copper when formed at 1560F but has a tendency to come off the phosphorized copper.
The difference between OFHC and electrolytic tough-pitch copper is in oxygen content.
It appears electrolytic copper is the least, an enameler should specify. To be absolutely safe, Oxygen free high conductivity should be specified. Roofing copper purchased from the local hardware store can produce serious problems for the enamelers.