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]

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...

That's water of hydration. Would you actually believe that a glass-former like boric oxide would itself evaporate at 320C?

Tim

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...

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

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!

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]

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...

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...