Tuesday, June 1, 2010

Stack furnace v1.0: high firing

Not to ruin the ending for you, but this is where stack furnace 1.0 started to go south. The previous day I had run two wood fires through to burnout in the forge, with no ill effects. Today I planned to do charcoal and forced air. While I had purchased bellows for this purpose, an acquaintance suggested that I just get a heat gun and some dryer duct from the hardware store, and start with that. Being short on time before my trip, I decided to go the surer route, and bought the gear.

The setup turned out to be quite easy. I loaded the furnace to the brim with charcoal, lit the pile from the bottom, used a wooden wedge to keep the duct in the tuyere, and sat down to watch, periodically removing the vent to reload the charcoal.

At the beginning of the first reload, I began to hear a curious sound, a lot like popcorn popping. Those more knowledgeable readers will be wincing right now, but after checking that there were no cracks or faults developing, I assumed it was some impurity in the charcoal and blithely continued. The next odd symptom was that the ash didn't seem to be burning as completely. There was a large accumulation of unburned crap in the bottom of the forge, and it continued to grow as I fed more charcoal into it.

The third symptom was clear, however: the edges of the tuyere cracked and widened it to about 200% (and later 300%) of its original size, causing the duct to drop out. At this point, with the better view into the forge afforded by the (now quite large) tuyere, I could see that the accumulation of "ash" was actually an accumulation of spalled clay. The popping noises were caused by fire-hardened clay cracking off as less hard (and still wet) clay behind it generated steam and popped it off.

The clay body was still too wet, and I was eroding my forge from the inside out. Below are some pictures of the whole thing, the inside, and a close up of one "edge" of the tuyere

Well, I was ready for my vacation anyway :)


  1. Jeff - take a look at the (not very organized) descriptions of Skip Williams' 'Aristole Furnace'. This converts scrap iron materials into middle carbon steel pucks of bloomery like metal. The size is about the same as your test here. The key is mixing 50 / 50 dry clay with shredded dry horse manure. The plant fibres strengthen the clay and reduce steam cracking as well. There should be something : http://warehamforgeblog.blogspot.com/2008/07/aristotle-furnace-demonstration.html

  2. I had been thinking sand, or maybe straw, but the more I read, the more it looks like actually using manure has been the most successful road. Where am I supposed to get horse manure in suburbia?!? Perhaps it's time for a roadtrip to horse country.