Not just once, but TWICE.
Theoretical design of the smelter.
The planned session consisted of background lectures, then a full day with students building a furnace and preparing materials, then the next day actually conducting an iron smelt. The final part of the program would be for them to survey the remains as if it was an archaeological site.
I had intentionally designed the furnace so it stood on a brick plinth. This allows for easy correction of potential slag bowl problems, and also permits either top or bottom extraction methods. Flexibility the key here.
I had also provided a blend of differing ore types. I had just enough of the DARC Dirt 2 analog (9.4 kg) and a bit of granular haematite (1.5 kg) to produce a dependable bloom. Normally the yield on that blend would be expected to be something like 25 - 30 %. In addition I had a good amount of not so great Virginia rock ore, previously roasted (about 20+ kg). This was stuff Vandy and I had picked from one of the mine sites Skip and Lee use. Not the best mine source, and very unskilled hands gathering. In the past this stuff had proved to produce a lot of slag, but not much iron.
My plan was to start off with about 6 - kg of the poor ore, establishing the slag bowl. Then switch over to the high grade analog mix, forming a nice bloom. Then switch back to the low grade material, which should add some extra mass to that bloom. Along the way, there should be greater than normal slag production, especially during the later stages of the smelt. More taping events than normal, so good experience for the students.
Well, things got a bit more *exciting* than that.
Into the second hour of the smelt, there really was a fair amount of slag, perhaps sitting a bit higher than desired. Remember that I was making the students do most of everything - at least as much as possible. During one of the rogering sequences (clearing slag drops out of the front end of the tuyere) something I've never seen before happened. The ceramic tuyere got shoved into the centre of the furnace, actually dropping it inside the shaft. It fell down below its mounting hole in the furnace wall. I tried (frantically) to hook it back up and clear, but without being able to.
Remember this cuts the air blast entirely, so the whole time the furnace is quickly freezing up. As I had told the students repeatedly 'If you cut the air blast any more than about three minutes, the whole furnace temperature crashes, and you can't get it back.' Working like a mad man, I cut a double fist sized hole into the side of the furnace just around the tuyere mount, grabbed out the blocked tuyere tube, then shoved in the steel pipe mount used to attach the ceramic tube. Kevin and one of the students quickly (an working with amazing intuition) slapped on wet clay and back filled the area with sand. Turn the air on full blast.
Amazingly, we got the heat back. Talk about acting in the nick of time!
The left end was broken off through (failed) attempts to grab it with tongs and pull it clear.
It was clear however, the there was most definitely too much slag, and sitting too high into the furnace. So working with Kevin, I pulled clear one of the bottom plinth blocks, then scrapped out the charcoal fines forming the lower base. This exposed the bottom of the slag bowl. Using a technique that Skip and Lee taught me, I tried to lower the bowl, by taping hot slag to flow below the bowl. The principle here is that the hot liquid slag warms the bottom of the bowl, which then sags down lower. Well, than did not work so well. After the first demonstration, I let mainly Kevin punch through the slag bowl and drain excess amounts. We did reach some kind of working equilibrium, at least keeping the tuyere clear for the rest of the smelt.
Even with aggressive tapping, it was getting harder to keep the air blast constant. The damage to the furnace, plus the switch to a steel pipe tuyere (over sized as well), was making it difficult to get the air flow into the centre body of the furnace. The last 'burn down' phase of the primary smelt sequence was rushed. (We were also running out of charcoal).
I decided to blend the two extraction methods, as much for teaching purposes as anything. The last 1/3 of the smelter stack was cleared with the charcoal scoop, working from the top. Then there was a short sequence with the thumper, one of the students attempting to compact the bloom (?) in place. I had hoped this would also drive the slag bowl down into the cleared base area below the bowl. This more or less did happen, but also resulted in blowing open a big piece of the lower area of the furnace above the tap arch.
The end result was a rather lacy bloom, about 2.2 kg in weight. I made an attempt to slice it with a cut off disk on my angle grinder. The sparks generated suggested a bit more carbon content than mild steel, maybe a 1030C equivalent.
On the day, exhausted from frantic activity and guiding a 4 - 8 person student work team, I was a bit disappointed. The ore analog additions should have resulted on something closer to a 3 kg plus bloom (based on past experience). It was clear that there was a considerable amount of reduced and partially sintered iron material that had not yet made it down to bloom level, wasted when we scooped the charcoal.
But like Kevin said at diner later - it was an excellent *teaching* experience. The intent of the whole program was to introduce students to the reality of working experimental archaeology. They had just been given a dramatic example of 'anything can go wrong', 'you have to think fast on your feet' and 'practical experience matters'.
I have to congratulate the participating students (who will be credited in the more formal report). They did an excellent job in all counts. They pitched in full heatedly, followed directions well, were innovative and resourceful as required by the challenges they faced. I would be happy to work with you all any time.
It also should be mentioned that it poured rain pretty much the entire smelt day!
Note that is is from Krysta's original schedule. Some of the Friday build people turned out on Smelt Day (and worked there too). Many of those who came on Smelt Day stayed for the whole sequence.
Instructors : Kevin - Krysta