This is sort of a plumbing question, but I dont belong to a plumbing forum so I thought I would ask it here since I have been on this forum in the past and us refrigeration guys are very good solderers of various types of metals, more so than the average plumber I feel. I am almost embarassed to ask this question, however twice I have run into a problem soldering this old type of copper tubing. I call it old because both houses I have ever tried to solder this tubing in were built in the fifties. I have been working in refrigeration since 1990 and just recently started working for myself. And I now do all types of home repair and maintenance as well as refrigeration. But recently I had a job where a lady wanted some new cabinets installed and all the plumbing in them had to be replaced to install a new configuration of faucet and dishwasher, well im thinking excellent install some new copper (she specified copper), well when it came to soldering a coupling on the 1/2 inch pipe that was the "old stuff" the solder, just would not stick, flow or anything, whether I used my mapp gas or oxy-acetylene with regular plumbing solder. So I did what I did when I ran into this problem once in the past, I used my 45% silver solder and wahlah, it sticks. Anyone ever run into this, this tubing im talking about is black in color due to age, but when you rough it, its definitely copper. The only other time I've ran into to this is in a house my folks owned. I can only explain it as a weird make up of elements or the alloy in the tubing or something, it makes me feel like an idiot though as I can solder really well. All my new fittings after the old connection are a piece of cake, its like I know how to solder again. I knew as soon as I saw this tubing it was gonna be an issue because I saw it once before.