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Author: Subject: thermite (copper/brass)
JumpedAngel
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[*] posted on 17-2-2005 at 17:14
thermite (copper/brass)


Hey all

i am a newbee to this site with little chem knowledge, i have a engineering desidn background (mech./structural) so i have had reasonable exposure to polymer chem and metalurgy.

i have been involved with restoration of antiques for well over a decade now and often put things aside to fix later when i do not have the right 'fix' available.

one such project i have put aside some time ago involves the thermite process, i need to cast (in situ) a new leg onto an antique bronze ornamantal pot, i can use damp clay moulded around existing legs to form a mould, but i am unsure of what formulation to use and also what initiator.

i know copper/bronze thermite is possible,
i just didnt pay enough attention when i last came across it, so:-

what would be a good way to produce the finely divided copper/bronze,

would i still be using iron oxide as the oxygen source

would a K/permanganate / Glyserine initiation suffice

where can i get K/permangonate in Oz or what can i use as an alternative initiator (magnesium strip? what else?)

cheers and many thanks in advance :cool:
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chemoleo
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[*] posted on 17-2-2005 at 18:31


Well the best way to go about this would be
1) find out the exact composition of bronze, the percentage of copper and tin
2) obtain the respective oxides
3) mix them in the correct ratios, with the correct amount of Aluminium filings (not powder, it's too energetic)
4) ignite with a couple of sparkling candles.

The problem will be a good separation between the aluminium oxide and the resulting metal mix. Further, tin may volatise at these (hot) conditions.

Best is to test this first with 200 g batches or so, and see how the metal casts.

Much better is, however, just to get bronze, a propane burner, to melt this and to fill this into the cast. You'll invest much less time that way, less materials, and less experimentation.
Unless it's the experimentation you are after ....




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JumpedAngel
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[*] posted on 17-2-2005 at 19:59


cheers chemoleo, yeah that propane torch solution was considered, however, i thought that perhaps without the right flux etc the metals would not flow into one another creating a good join

sparking candles, now thats made me think, cheers m8
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uber luminal
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[*] posted on 18-2-2005 at 00:04


how fortunate. I ran a CuO+Al rxn just yesterday.
Except I was trying to get a mixture of Cu and Al2O3, which is very easy to do.

The total load was about 130 grams, to yeild a puck about the size of a small Mcdonalds sausage patty. I never did the mass out on it, because there is allways a signifigant loss with any exothermic redox.

My puck was made with a traditional crucible with a small hole in the bottom which allows the load to fall into a mold. my mold in this case was a graphite puck shaped mold. (Foundry sands will instill the burnt cationic binders to the surface of the cast, which contaminates and makes things smell bad)

This was done with CuO(black). I have plans to attempt the same rxn with about half the load, tommorrow morning. I will use Cu2O (red/gold) this time, to see if there is a different intermatallic spread of the Al2O3.

No, KMnO4 will not ignite this mixture. I was not pleased when I tried to use this on Tuesday and it wouldnt run.
on wed, I used Mg turnings atop the redox mix. then placed the oxidizer ontop of that, then the sugar on that. I got some good thermocouple readings from the igniters, (was able to tell what was going on, without having to look at the rxn. (btw, if you ever get this to work, the light given off is getting close to white, and will blind you, so dont watch it). sadly the thermocouple alloy melts around 1300C, (this is where it peaked before giving err data), so i was not able to see just how high the temp went. (this will depend on how big your load is, and what is being reduced).

however, for what you are trying to do... I do not think it will work. Granted this is a fun way to cast stuff, Copper is a different animal than iron. Not only that, but you are trying to mix tin in with it?

I dont see this happening. Your tin will either melt 1st(if you just put tin right in), or if you use a tin oxide, it will go 1st to super heat and absorb oxygen like mad.
The copper will get hot, and will stay hot, dont get me wrong, but it doesnt stay hot for long. and especialy long enough to pour it into a mold. Oh yea you copper will also be absorbing gasses from the air, and will form gas pockets in your cast. AND be mixed with Al2O3, theres no getting away from that. these things will make your casting somewhat fragile

Ok. so it might work... IF you use a lot. like 2.5 times the ammount of copper you need. What I have in mind is a special pouring ladle that pours from the bottom of the cup, and is walled further up. So if you were to use a Kg of this and let the rxn go, and immediatly poured from the base of cup, you might get a good intermetallic mix, and get it into the shape you want with the 1st half of the load. The rest of it will probably be crap. you will still hit the gas absorbtion snags when pouring but you want it to look rustic, right?. If you try to use flux initialy, you will lose it in the thermite rxn.

Model rocket igniters work really well to start these rxns. Since that is what they are meant to do. be careful. once a thermit rxn goes, it goes to completion. theres not a lot you can do to stop it (in most cases lime, or hydr lime wont do much. however dry ice or liquid nitro will put them out) They get hot, and will send powder and particles everywhere, which sometimes are unreacted, but hot enough to react with things they land on. They also make a huge a mount of smoke, you will want to do this outside, on a plot of grass you dont care about (so when it turns black for your adventures, you wont mind)

edit. er. I just read "i can use damp clay moulded around existing legs to form a mould". thats not a good idea when we are talking about pouring metal above 500C.
1. your mold wont stay together. Water is a very very powerful force... in fact its the most powerful thing on the planet. even if you think your mold is dry, the water that remains within the clay will still be there. steam will crack your mold 10 times over. 2. If you use moist clay when pouring, you will get nasty gas bubbles in the cast, as well as risk spattering molten metal. the 3 worst molten metals to be burned with are Aluminum, Copper and Tungsten. If copper or aluminum spatter on you, your screwed. By the time you are able to brush it off and cool your burn off, you allready have a severe 2nd or 3rd degree burn (due to the TC of the metal, it dumps heat like mad. and tungsten... well if tungsten gets you, I would think it would be the least of your concerns.)

[Edited on 18-2-2005 by uber luminal]
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JumpedAngel
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[*] posted on 18-2-2005 at 14:20


thanx u/l

being such an utter newbee i hadn't even noticed the bang-bang section to this site when i first posten this thread, now i've been going over it with a fine tooth comb.

this is such a great site for practical thermo chem stuff, cheers guys
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