Wednesday, December 11, 2013

Idea for smelting metal in an electric kiln

I digress to a subject not exactly ceramic ... although it involves clay, firing in a kiln, and an understanding of kiln atmosphere and high temperature chemistry....

I removed some ancient, leaky copper pipe from my house and replaced it with a PEX system. Could I now melt this copper into a solid ingot in my electric kiln? Could I make an ingot of high-carbon steel from old nails?

Iron melts at 2800 °F and boils at 5182 °F.

Copper melts at 2000 °F and boils at 4643 °F.

Metals must be smelted in a reduction atmosphere ... or they will form oxides rather than pure metal.

I watched a great NOVA video available on YouTube: Secrets of the Viking Sword (2012). In this video, blacksmith Rick Furrer (of Door County Forgeworks) smelts iron ore into high quality steel in a tightly closed clay crucible containing iron ore, crushed charcoal, sand, and a small amount of glass. The crucible and contents are fired to 3000 °F in a small kiln packed with charcoal and fed continuously with air via bellows. The measured amount of charcoal included within the crucible, provides both a reduction atmosphere and the carbon required to make carbon steel. The glass acts as a flux to melt the sand which forms the "slag" that removes impurities from the iron. In the video, Furrer makes an ingot of very high quality steel from which he forges a spectacular sword.

So, ... I'd say the answer to my question of whether I could make steel in my electric kiln is: "No, you can't get it hot enough". To make steel, the iron needs to be well above its melting point and be runny liquid. My electric kiln will not go much above 2300 °F ... which is below the melting point of iron. I would have to go to 3000 °F to make steel from iron nails. If I wanted to make carbon steel, I could do it in a kiln similar to the one in the video, fired with charcoal, with air blowing in throughout the firing. An easier way would be to fire it with propane. However, the kiln would have to be made of very high temperature brick - not the 26K brick used for most ceramic kilns.

How about copper? Could I chop up my old copper pipe, put it in a clay crucible with some charcoal (to create a contained reduction atmosphere ... like a little saggar) and melt the copper into an ingot that would take the form of the crucible? Yes. I probably could do this in a 2300 °F firing in my electric kiln. Copper melts at 2000 °F and boils at 4643 °F. If I were to seal the crucible the way Rick does it in the video, the reduction atmosphere would be contained within the crucible ... and do no harm to the elements of my electric kiln.

Another thought: Since iron is still solid at 2300 °F (cone 9), vessels used at this temperature can be made of iron. You can melt copper (or other soft metals) in an iron crucible. You can pour liquid copper into an iron mold. You could use an iron saggar for ceramic work. Here is a video that shows a person melting copper in an iron crucible and pouring the liquid copper into a cast iron mold (click). You can find cast iron molds like this on ebay.

Could I melt copper in a wood stove? No way! Down in the hot coals, the wood stove can reach 1780 °F (cone 07) ... which is 220 °F shy of the melting temperature of copper. However, I could melt aluminum or lead in a wood stove. Aluminum melts at 1220 °F and lead melts at 621 °F.

It is an interesting subject. I did chop up the copper pipe using large pruning shears. I don't know if I will actually get around to melting the copper scrap into an ingot. If I do it, I will probably make a one-time-use clay crucible, let the molten metal cool in it, and later break the clay away from the remaining metal ingot.