gmeyers
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Copper (I) oxide/Carbon reductions
I'm currently running some Cu2O/carbon reductions in the context of making bronze (Bronze Age style) and I got
two quite strange results.
I'm using a charcoal fired paint can furnace (details here http://developing-your-web-presence.blogspot.com/2007/06/coffee-can-foundry-making-bronze-part-5.html), technical Cu2O and BBQ lump charcoal (also used as the furnace fuel).
The reduction proceeds well and quick and going by weight the conversion is more or less complete. On the second run I added 20 w% charcoal on top of
the stoichiometric amount, to compensate for any carbon that simply burns off in the hot, oxygen rich environment.
The trouble is that although theoretically this reduction should be slagless (CO2 being the by-product), I get
nonetheless some black gunge which doesn't melt and is hard to chip off the copper metal.
I tried dissolving it in 36 w% HCl and although clearly some CuCl2 (from some CuO?) is formed, the solution is a dark black liquid. Filtering didn't
change anything: the filter caught only some ash (from the reducing charcoal, as well as some that came "flying in" from the charcoal fuel during the
reaction), but the filtrate remains very dark coloured (twice filtrated). 
It's possible that the lump charcoal contains small amounts of a light hydrocarbon to make it easier to light but I can't really see how this can
cause the problem. I used the same charcoal for SnO2 reductions in the same furnace and no such gunge was formed.
I'll now be trying another form of carbon as the reducing agent, such as Norit or battery graphite electrodes.
Does anybody have an idea what this slag/gunge might be and how to avoid its formation? Perhaps some kind of fluxing agent? 
Gert from England.
[Edited on 24-6-2007 by gmeyers]
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12AX7
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BBQ charcoal does tend to have some "flavor" to it. I've never bought a sack of it though. I made my own...
Metal is supposed to be melted with flux. Try something low-melting like borax.
Tim
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gmeyers
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I've been recommended a glassy/borax flux for melting bronze. Borax alone might be too volatile at 1084 C (1974 F) or above (MP of Cu), not sure.
Worth rtying in any case... 
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12AX7
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Nah, salt (NaCl, KCl, etc.) is too volatile, borax just melts. Borax is too thin, actually. What you really want is a silica slag, roughly the
composition of glass but other oxides help (boron, magnesium, aluminum, etc.).
Tim
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gmeyers
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Are we talking about the same borax? Sodium tetraborate? MP 75 C, BP 320 C?
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olmpiad
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Gmeyers, have you ever heard of a term called a Borax Bead Test? Essentially, you add a metallic oxide to molten borax, which then will disolve the
oxide into it, and harden into a glassy slag with a color characteristic of the metal from the oxide. Well, essentially, with the borax as a flux, you
will be doing a similar process, except you will not be trying to acheive a glassy bead, but the metal from the oxide. Now, to the point: Borax WILL
work for copper. I know this from experience when doing borax bead tests on copper minerals. It is actually quite hard at times to PREVENT the Cu from
coming out of the bead as the metal. I'll try and take a picture of some of them if I can. If you use a glass/borax flux, you will essentially be
making a cupric borosilicate glass slag, like pyrex glass.
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gmeyers
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olmpiad:
Yes, I've done Borax bead tests (20+ years ago, so I'm a bit rusty on that).
Remember, I need to cast the metal, so the temp will be >1084 C. A glassy slag, thinned with borax would probably be best. The lowest melting glass
is bottle (about 66% SiO2, if I recall well). What would be a good glass/borax ratio, do you think?
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12AX7
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That's water of hydration. Would you actually believe that a glass-former like boric oxide would itself evaporate at 320C?
Tim
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gmeyers
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| Quote: | Originally posted by 12AX7
That's water of hydration. Would you actually believe that a glass-former like boric oxide would itself evaporate at 320C?
Tim |
.10 H2O, right?
Would boric acid do to make such a glassy flux. I've got plenty of that. Driving of the water and you get B2O3, the basis for borax?
Any ratio of glass/B2O3 to recommend?
Thanks... 
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12AX7
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Not offhand, I never weighed anything (I could care less if your inner chemist is barfing at this fact  ). It's easy enough to adjust... toss in some sand, lime, soda, crushed glass, borax, clay, dirt, etc. etc. Remember
the rough amounts you added, then make a batch and test it. If it's not quite right, then add something to the batch, stir it up and try again.
FWIW, I prefer a sticker slag. It doesn't melt and protect much while scrap is melting, but it does spread out when the metal heats up and melts. At
the pouring temperature, it's sticky enough that it remains in the crucible while pouring, or can be skimmed off with a steel rod before. Alumina and
silica are good thickeners. Silica and boric oxide (boria, as such?) are good cleaners.
The most important aspect of a slag is to trap crud (oxides, etc.) and seal the surface so gasses don't get in or out (zinc is present in many bronze
alloys, and is gaseous at pouring temperature), giving nice clean metal.
Tim
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guy
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Check out this site. It's like youtube but for chemistry. There is a video of the Cu2O/C reduction
http://www.science-tube.com/index.php?c=chemie§ion=0...
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gmeyers
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| Quote: | Originally posted by 12AX7
Not offhand, I never weighed anything (I could care less if your inner chemist is barfing at this fact ). It's easy enough to adjust... toss in some sand, lime, soda, crushed glass, borax, clay, dirt, etc. etc. Remember
the rough amounts you added, then make a batch and test it. If it's not quite right, then add something to the batch, stir it up and try again.
Tim |
The black art of fluxing... I'll probably start fooling around with
bottle glass and borax first.
Tim, in the case of in situ formation of the metal through reduction, when would you add the flux? At the end of the reaction,
during reaction or would you sit it at the bottom of the crucible, right from the start? In the latter case the newly formed metal should sink through
the flux and be protected right away. Just guessing here: no experience with fluxing at all (apart from using LoSo on Al melts a few times). The
potential problem here is that the flux could dissolve some of the Cu2O before it gets to react with the carbon... Personally I'm guessing to add it at the end or near the end .
Guy: what that YT vid shows of course is what you get when running the reaction at a low temperature: copper powder, not really all that useful...
Interesting site though! 
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12AX7
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I'd probably put it in with, just so it fuses automatically. I wouldn't put it at the bottom. But if I discovered that tends to make a dispersion of
copper droplets, I'd flux afterwards.
Tim
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gmeyers
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I got my borax today so tomorrow I'll be melting some bottle glass with 20 w% and 40 w% borax (based on anhydrous sodium tetraborate) to see how that
goes.
No-one here seems to have an answer regarding what the slag might be, so I guess I'll have to try and find out more.
[Edited on 29-6-2007 by gmeyers]
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not_important
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I've been thinking on the 'slag' you got, and have a bit of speculation. First off, oxygen dissolves in copper to some degree, as do the copper
oxides. Your slag could be copper oxides formed as the copper cools, in effect formed by oxygen coming out of the copper.
Then, if your charcoal has much mineral content, easy to check by burning a crucible full of just the charcoal, you could be ending up with a small
amount of a high melting slag that's glued to the copper, possibly containing a small amount of carbon.
What were you filtering that dark solution through?
Wouldn't a Bronze Age flux likely consist of ash and sand? That would give a K-Na-Ca-Mg silicate glass, with enough ash it could be fairly fluid.
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gmeyers
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| Quote: | Originally posted by not_important
I've been thinking on the 'slag' you got, and have a bit of speculation. First off, oxygen dissolves in copper to some degree, as do the copper
oxides. Your slag could be copper oxides formed as the copper cools, in effect formed by oxygen coming out of the copper.
Then, if your charcoal has much mineral content, easy to check by burning a crucible full of just the charcoal, you could be ending up with a small
amount of a high melting slag that's glued to the copper, possibly containing a small amount of carbon.
What were you filtering that dark solution through?
Wouldn't a Bronze Age flux likely consist of ash and sand? That would give a K-Na-Ca-Mg silicate glass, with enough ash it could be fairly fluid.
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The liquid was first stood overnight, without any precipitate forming. Then filtered twice on standard labware filterpaper. The substance is clearly
in solution or as a fine colloid.
Yes to the Bronze Age type flux. But I'll try a 21st century flux first...
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squizzy
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i suspect its Copper oxide. Copper gets an oxide coating when molded so exposed to the atmosphere you will get more.
Try heating the slag in an excess of HCl and see if its fully converted (safety precautions). It should give a green liquid that will plate iron.
Less likely is a substance called copper glance which forms in smelters, its a black fusible substance soft and fusible with even a candle flame. Iron
impurities can make it harder. In your case id say its the oxide though.
In casting bronze and copper too id imagine a later of charcoal is often used to cover the surface preventing excessive oxidation during the melt. And
a wooden stick, even green wood used to deoxidise the melt
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gmeyers
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Sure, copper oxide seems the most likely culprit here. Some of the slag did dissolve in HCl, yielding green CuCl2. But it strikes me as odd that most
of it didn't dissolve in 36 w% HCl (although more of it could have done the trick) and that standing overnight and filtrating doesn't capture it...
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