Don't try melting copper in iron

I'm trying to cast some copper and have been having a great time figuring out what won't work ;)  I'm using a coke forge.

The first crucible was a lugnut wrench socket that had broken from the handle. It worked but only poured a fingernail sized nugget.

The next crucible was a 1 1/2" wide pipe forge welded closed at one end. The bottom melted out. I thought this was due to being deeper in the fire than the rest.

Then I made a clay/sand crucible. It cracked before I could add the copper.

The latest one: a tiny cast iron skillet. I figured the base wouldn't get hotter than the sides because it was about the same depth. Besides, there's a ~400°ish degree difference between melting copper and melting high carbon iron, right? Well here's the results.
Moral of the story: DO NOT USE IRON AS A CRUCIBLE FOR COPPER. It looks really cool though!

You're probably wondering why I didn't just buy a graphite crucible. I'm cheap. And ambitious. And dumb at times. I 'll post a picture of the crucible when I get it.:)

That must be what you see when looking into a volcano!

Looks like the heat source may have serious hot spots.

How did you add the melt metal? Just put it in right at the start? That would seem best.

Is this an oven type of thing or does the heat just come up from below?

Keep the container away from the heat slightly, maybe help the heat even out.  

I'm not a smith, nor a metallurgical type, so a Q: What's the usual heat transfer process in casting? Radiation? Conduction? Convection?

I believe they used  electricity directly for aluminum processes in the big "mills".


Regards,
Rufus

On the internet there are some great forge systems that can be made easily. Good luck with the project.

Rufus the heat comes mostly from below. Its just set in a pile of coals. I'm not sure as I haven't tried a gas furnace, I think it's mainly conductive and then radiant heat. In this case it was all conductive.

> coke forge... pile of coals...

I don't know much about smithing, so just speculating along with your pics. Hot in the center, for sure. Guess if you were oxygenating it with forced air, the oxygen was affecting mostly the center. It seems possible that you actually caused the cast iron to burn when the temperature got high enough in the presence of excess oxygen from your forced air. My understanding is that is how a cutting torch works - it actually burns the metal away.

I'm not sure, a smith needs to chime in here, but I suspect most of the heat transfer to an object "in" those coals may be radiant.

Almighty GOOGLE puts the melting point for cast iron at 2300F. and that of copper at 1981F. That's only 319F. difference (15%) and in the absence of really good burn control(s) seems pretty easy to cross over and melt them both. Also, things start to happen  to the structure of cast iron at less than 1000F. Here is a link describing some of the characteristics of cast iron. It seems to be one of the more complex materials we make.

https://www.industrialheating.com/articles/94644-heat-treatment-of-cast-irons

And here is the table (from the above link) describing the "low temperature" heat treatments which cause changes:

https://www.industrialheating.com/ext/resources/Issues/Issues2/2018/Dec/ih1218-htdr-table4-900.jpg


Cheers,
Rufus

Thanks for the heads up I was gonna try melting copper in a similar pan glad I saw what happened to yours. Best of luck to you.