The other day I was melting metals in a small homemade furnace which burns charcoal, using a little stainless steel cup as a crucible.
I melted and cast aluminium no problem, then moved onto trying to melt bronze. I added a bit of copper wire and tin to the crucible, the tin melted
immediately, and as it started glowing yellow hot, I stirred it to try to melt the copper.
There was a hissing popping sound and a very bright flame, and a lot of sparks shot out of the crucible. It looked almost like burning magnesium or
even thermite. after about 3 seconds it stopped, and the copper wire was gone, and a hole was melted in the crucible.
Any ideas what happened here? I have no idea what happened. My only theory is that i somehow accidentally made thermite in the crucible. There would
have been a bit of aluminium left in the crucible, but I figured it would be fine, I'd just get aluminium bronze.
I may include photos later.bnull - 12-3-2024 at 07:09
Unless the copper wire was very, very oxidised, and there was aluminum enough, I would rule out thermite. The copper wire was clean and shiny and
stuff, right?
I was thinking, maybe the molten tin burst on fire. I don't know if burning tin would be hot enough to melt steel. The MSDS says "May be combustible
at high temperature". What if the copper wire acted as catalyst? You just had to break the oxide coating to expose the melt to oxygen in the presence
of copper so the tin would burn at a lower temperature.
Note: Removed an idea that was too farfetched to be true--a highly exothermic phase formation, like in
the synthesis of Raney nickel.
[Edited on 12-3-2024 by bnull]phlogiston - 13-3-2024 at 06:42
Are you -absolutely- sure it was really tin, and the copper wire wasn't any sort of copper-coated light metal? It sounds like a zinc or magnesium
fire. Was there any residue that can tell you something? Burning metals usually give off copious amount of oxide smoke that deposit nearby. Zinc and
magnesium oxides are white when cold. Zinc oxide is thermochromic (yellow when hot).
Copper oxide thermite gives a deposit of brown/black copper/copper oxide metal. But I think a significant thermite reaction is highly unlikely with
any solid mass of copper. There is simply not enough oxide to allow for a prolonged reaction with enough heat output to melt the crucible.
Maybe the enthalpy of alloying tin and copper is very high? I tried googling it but can't find anything about it. On the other hand, even if it is I
expect it would just heat up significantly and maybe melt your crucible, but not actually burn.
[Edited on 13-3-2024 by phlogiston]Sulaiman - 13-3-2024 at 07:23
There will also be lots of stuff in the furnace due to burning charcoal,
especially if not fully charred/calcined,
eg CO may be involved, or carbon soot,......
PS I once poured liquid iodine into a (probably similar) stainless steel pot -
with a second or two of fury the iodine corroded/perforated the pot.
I know that iodine is a halogen, but it's far down the group and quite tame as a solid... D'oh! clearly_not_atara - 13-3-2024 at 07:33
Zinc seems like a likely culprit. Often alloyed with copper and tin, low bp, potentially flammable vapor. Morgan - 13-3-2024 at 08:06
Maybe this?
"The researchers found that steel/iron-based structural materials corrode outwardly and inwardly while forming intermetallic compounds when exposed to
high-temperature liquid metal tin. This is because iron, the main component of steel, reacts with high-temperature liquid tin."
The corrosion rate is too low (some millimeters per year) and doesn't explain the bright flame. And the carbon (whether in the form of soot, monoxide,
or whatever carbonaceous substances were present) wouldn't do that.
Bronze formation--as far as I remember--is not that exothermic; that's why I edited out the idea of a Raney-like alloying of copper/aluminum/tin.
I would stick with the metal fire, either tin or zinc, both autoigniting above 1000 °C[1], the temperature where things glow yellow. The
sparks, the high temperature[2], the bright flame, it sounds like a metal fire.
@knowledgevschaos: Would you mind to repeat the operation, preferably using the metals from the same source?
Repeat the experiment, try a few variables?
Sulaiman - 13-3-2024 at 09:50
Morgan - 14-3-2024 at 06:55
With air, probably excluded in this example if they were making hydrogen, the reaction might go even faster. Can't really tell the details without
reading the write-up.
"We learned that molten tin can attack stainless steel, but we could not find any data, so we prepared this experiment. The molten tin absolutely
destroyed the stainless steel. The two samples that are still relatively intact were coated with flame-sprayed tungsten and ceramic respectively. The
coatings, where they survived, protected the stainless samples." https://www.reddit.com/r/metalworking/comments/y7ok0b/tin_ca...
[Edited on 14-3-2024 by Morgan]knowledgevschaos - 14-3-2024 at 17:22
I got my tin from a pewter cup, which had a stamp on the bottom saying something along the lines of 97% selangor pewter, so I have little doubt it was
almost pure tin.
I did not experience metal fume fever, which I would have if it were burning zinc. I also didn't notice a layer of zinc oxides, but I'll take another
look.
Can tin burn below it's boiling point?
Edit: all of the copper wire I used was destroyed, so I can't test if it was pure. I guess I could test the remnants in the crucible.
[Edited on 15-3-2024 by knowledgevschaos]bnull - 18-3-2024 at 03:20
