Also you need to rough sand the mating surface of the flat joining plate. I rough sand in order to give it a "tooth". Not sure if that is really required, but ever since I started doing that, I have had a lot fewer failures. This was a really coarse (60 grit) sandpaper, and you can really see the roughness in the picture above, especially compared to the untouched piece above. The real reason is to take off the oxide coating which covers all metal which has been exposed to the air, and if more than a few days were to go by before I could solder the pieces, I would need to do the sanding again. Don't get any fingerprints on the sanded part from this point forward, fingerprints will prevent the solder from adhering and forming the cupric-argent alloy with makes the joint stronger than just the solder alone.
Above, I have placed the workpieces onto a steel soldering surface. You never heat the workpiece directly, you only heat the soldering surface. Pretty simple, a chunk of steel. I use steel because the solder won't stick to it. Because the torch is under the workpiece, you need to be careful of any drips of solder or flux. I had a tiny drop of molten silver hit the centre of my thumbnail yeasterday. Burnt a divot in the thumbnail, and stung like a bee sting! The lessons I learn the hard way!
I have smeared the sanded surfaces with soldering flux. This is a resin based flux....the resin carries some interesting chemicals which when heated, form a powerful acid. This acid will strip off any oxide which might interfear with the joining of the workpiece sections.
For solder, I am using pure silver, mostly because I will be able to electroplate over the silver later on, and most lead solders are a royal pain to plate over. Silver is more expensive, and heats at a much higher temperature than lead, but it is pretty nice to work with.
The solder actually forms an alloy with the copper in the brass, it doesn't just glue the pieces together. Therefore you need to "tin" it first, then clamp it very tightly so that there is a very thin layer of cupric-argent alloy. Any extra sqeeze out will be pure silver of course, and we use that to fill in gaps and holes as required.
This is the solder paste. It is important you use a solder paste which is compatable with silver....they all look and smell alike of course, the resin is just a binder for the zinc oxides which will actually form the acid. Since zinc oxide is really poisonous when it is in smoke form (but harmless otherwise) a good ventilation system is essential. I always get massive headaches whenever this stuff is smoking off.....and the mask only seems to help a bit. A fume hood would be a good idea I suppose.... Normally, I just ventilate the area really well, and stand back.