When you're preheating the copper and graphite the little bit of moisture you see at first is not from the copper or graphite, it's from your torch. When a hydrocarbon like propane burns the carbon atoms make CO2 but the hydrogen atoms that make up the rest of it form H2O. Your torch is putting off steam. You can see the same thing out of any car muffler. At idle sitting at a stop light some of that steam tends to condense into water and it'll spill out when they take off once the light changes. The reason why it shows up at first but not once you've got it hot is because at first the material is cold enough to cause it to condense. Once you've got it nice and hot and it's above the boiling point of water it's not going to condense anymore. It's not that much different than hot moist air condensing on a cold windshield. Give it some time to heat up the windshield and now it's back to removing the fog. It's not because the heater is somehow "driving water out of the glass", it's just hot enough that it's not condensing on it like the outside of a cold drink. As for the rest of the technique, You probably don't want to go at the mold so heavy with the wire brush. It'll last longer if you're more careful, I forget which manufacturers in particular but common wisdom is to use a softer brush like a stiff natural hair brush. It doesn't really matter much up at the top and if anything a wire brush is better for cleaning up the weld and the wire. That graphite is super soft though so try to avoid scraping at it with anything harder than necessary. Using a torch is much better than fiddling around with a little flint striker in thick gloves, if you've already got the torch, which everyone should be using to preheat and degrease the wire and mold, it just makes sense to use it to ignite the thermite. Even if the slag on top of the weld comes out not looking so terrible you should still chip it off so you can get a look at the weld underneath. If you have a bad fitment of the mold or some oil was left on it you can get some defects and that little nugget of slag you left on the top could hide it to where you wouldn't be able to tell there was anything wrong with the weld. Chip it off so you can get a good look at your work and leave it clean going into the ground. Also when lighting it off, don't put your body directly in front of the vent in the side of the mold. If there's a bit of moisture that causes it to burp out some of the burning thermite, you definitely don't want it spitting it in your direction. Your welding gloves won't help you when that happens. What's really unintuitive is that the primary components of the thermite powder itself is just finely powdered aluminum and corroded copper dust. Aluminum actually burns with quite a vengeance if it's finely powdered and it'll strip the oxygen off the copper oxide to burn the aluminum.
When you're preheating the copper and graphite the little bit of moisture you see at first is not from the copper or graphite, it's from your torch. When a hydrocarbon like propane burns the carbon atoms make CO2 but the hydrogen atoms that make up the rest of it form H2O. Your torch is putting off steam. You can see the same thing out of any car muffler. At idle sitting at a stop light some of that steam tends to condense into water and it'll spill out when they take off once the light changes. The reason why it shows up at first but not once you've got it hot is because at first the material is cold enough to cause it to condense. Once you've got it nice and hot and it's above the boiling point of water it's not going to condense anymore. It's not that much different than hot moist air condensing on a cold windshield. Give it some time to heat up the windshield and now it's back to removing the fog. It's not because the heater is somehow "driving water out of the glass", it's just hot enough that it's not condensing on it like the outside of a cold drink.
As for the rest of the technique, You probably don't want to go at the mold so heavy with the wire brush. It'll last longer if you're more careful, I forget which manufacturers in particular but common wisdom is to use a softer brush like a stiff natural hair brush. It doesn't really matter much up at the top and if anything a wire brush is better for cleaning up the weld and the wire. That graphite is super soft though so try to avoid scraping at it with anything harder than necessary. Using a torch is much better than fiddling around with a little flint striker in thick gloves, if you've already got the torch, which everyone should be using to preheat and degrease the wire and mold, it just makes sense to use it to ignite the thermite. Even if the slag on top of the weld comes out not looking so terrible you should still chip it off so you can get a look at the weld underneath. If you have a bad fitment of the mold or some oil was left on it you can get some defects and that little nugget of slag you left on the top could hide it to where you wouldn't be able to tell there was anything wrong with the weld. Chip it off so you can get a good look at your work and leave it clean going into the ground. Also when lighting it off, don't put your body directly in front of the vent in the side of the mold. If there's a bit of moisture that causes it to burp out some of the burning thermite, you definitely don't want it spitting it in your direction. Your welding gloves won't help you when that happens. What's really unintuitive is that the primary components of the thermite powder itself is just finely powdered aluminum and corroded copper dust. Aluminum actually burns with quite a vengeance if it's finely powdered and it'll strip the oxygen off the copper oxide to burn the aluminum.
Very interesting. Have done cathodic protection welds but not nearly as complex.
ty for the video
Hope it helped you!