Copper-phosphorus filler metal

This is a brazing alloy that only works for copper and copper alloys. The phosphorus in it acts as a flux; it binds oxygen and keeps the copper surface clean during brazing. For iron or steel, though, this stuff is absolutely off-limits—it forms brittle phosphides and the joint breaks right away. You need to clean the parts well so the filler metal flows nicely, and keep the gap tight. If you get the flame too hot, the phosphorus burns off and you get porosity in the joint. Let it cool slowly after brazing, and the connection will stay tight.