I'll pass along what I "think" I know. A magnets flux and Nickel-brazing rod flux are two very different things.
The oxygen in air reacts with just about everything it touches to some degree. Copper oxide is green, aluminum oxide is grey, iron-red, etc. The hotter something is (during welding/brazing) the more of a reaction you'll get, and the oxidized bits are weaker.
The dry coating around a stick-welding rod is a flux powder. During the welding process, it forms a no-oxygen cloud around the melting metal. My continuous wire-feed welder has flux in the core, with the welding rod being a hollow tube shape thats wound up on a spool. When you're copper pipe brazing, the goop you spread on the joint before applying the torch chemically eats away the brownish start of oxidation to reveal shiny copper, and when you apply the torch it bubbles into a no-oxygen gas just enough to make a strong joint.
If I've screwed up any part of this, please jump in.
PS I seem to remember reading Edison used nickel-electroplated steel plates, so you wouldn't need a huge amount of nickel. I know it must be more complex than just Nickel, Iron, KOH, and water, but even adding a few problems it may be doable...
