persistent black layer inside copper pipes

[gwall]

I know there are a lot of threads on this, but I still can't get the inside the copper pipes on my new still clean. I've run boiling soapy water through them, soaked them overnight in vinegar, and distilled water, vinegar, and cheap wine multiple times. A bunch of crap came out, but there is still a layer of black crud inside all the joints that will not come out.

I think it is flux, or an oxidized layer, or something. I've scrubbed the joints that I can reach with a toothbrush, which helps a little. But they are still black, even though some comes out on the toothbrush. And there are many more joints that I cannot reach.

Has anyone had a problem like this? Any advice would be great.