Hallo Zan,
Electrolytic Copper is normally nearly always oxygen free. You must not buy it by the ASTM-Standard. The ASTM B 170 is especially for cable-maker from interest and really more expensive as normal Electrolytic copper. I use since eternal times copper waste which I buy for the scrap-price by a friendly plumber. I never had any problem with these copper.
Edmund
Hi Zan,
I have recently spoken with Tom Ellis from Thompson Enamels and mentioned your question about the ASTM B-170 oxygen free copper - If you have the Thompson Enamel Work book - pg. 11 - you will find a whole lot of information by Woodrow Carpenter on the subject of Copper and the testing that has been done - He also said that the Oxygen free copper is an electrical grade - almost pure copper - where as any other copper you buy will have unknown alloys which might not be compatible with enamels, i.e. roof copper - and the only way you will find out is by experimenting - Call Tom Ellis and he will be able to help you - and if you don't have the WORKBOOK I would call them up and ask for one - the information and testing completed by Mr. Carpenter is invaluable.