Today I tried brazing the nicest of the scrap lugs in my little collection. It turned out alright.
The photos above are prior to soaking the flux off of the joint, the photos below are after. Since this was a test, I only submerged the joint itself, leaving a bit of fuzz down the tube.
Flux is a paste that you coat the area you're brazing in to prevent oxidization. Basically, getting steel up over a thousand degrees makes it rust pretty much instantly... which keeps you from getting a good bond with the filler (low fuming bronze, in this case) that you're trying to use as glue to hold the pieces of metal together.
I definitely baked some of the flux on there, indicating that I overheated things, or kept them hot too long for the flux to keep up. Flux that ISN'T burnt comes right off when you soak it in warm water. Flux that IS burnt takes scraping and sanding. Next time it'll be more flux and a bit lower temperature if I can keep things flowing.
I spent a bit playing with ways to take off the burnt flux. It's surprising how tenacious that stuff is. Techshop has a media blaster (like a sand blaster... but with other stuff in there), which should help stripping that stuff away, but it's broken at the moment. It's a good idea for me to get familiar with doing this stuff by hand anyway.
When I cut open the joint I discovered that I had pretty solid penetration of the filler, which is the main goal here. The exceptions where at points where the tubes didn't join together cleanly... which is what happens when you rush through shaping the joint because it's "just a test". Little gaps, the filler will bridge... big old "god, was I really that lazy?" gaps are a different story. Luckily, I've already completed mitering most of the tubes for the actual bike, and they're joining up pretty well, so this won't be an issue on them.
That's it for today. Next week I'm going to try making a head tube badge by coating a sheet of copper in paint, using a laser cutter to burn the paint away based on an image, and then use acid to eat away the copper everywhere without paint on it.