(Dr Smith)
First, interesting that after 4-5 repeated smelts the base of the furnace stack itself (below the tuyere) was still essentially unconsolidated, poorly fired or unfired clay cobb, plastic enough to slump 10 degrees forward. Unlikely that anything this "soft" this would preserve through 800 annual winter/summer freeze/thaw cycles and yet we're always looking for the bottoms of the furnaces, expecting them to be preserved. Makes perfect sense that the best fused/fired parts of the furnace wall are going to be above and around the tuyere but it's important to realize also that the base may well be ephemeral from an archaeological perspective.
Showing the remains of a clay smelter left to weather since June 2004
(D)
I would put that line actually bellow the tuyere. The heat effect is consistent with the location of the entire slag bowl mass, forming a gradient from highest temperature effect just above the tuyere to a level marking the bottom of the slag bowl. The way we set our bases, this marks a line roughly at the top of the tap arch. With the angle of the tuyere and size of furnace being pretty constant, the heat is not penetrating much beyond about 20 cm below tuyere level.
The clay cobb bellow that line is just mud. So depending on the construction of the furnace, I would expect to see unaltered earth, then maybe disturbed earth, then the ring of raw cobb. Inside this would be whatever remains from the extraction process.
If you pulled from the top, the slag bowl might just be left in place. That would leave you with the classic shaped bowl. Below that would likely be some combination of ash, unburned or partially burned charcoal and maybe charcoal fines. The looser this lower packing is, the more likely there would be 'icicles' of slag from the bottom of the slag bowl. There might also be the same shapes of bright white cast iron. We have also seen plates of this metal forming between the slag on top of the fines layer we use. (Although I suspect our high air rates and temperatures have something to do with that material).
One question for us is what happens next. Our process has been to knock loose the cold slag bowl to prepare the smelter for its next use.
If you pulled from the bottom, you first dig out all the lower packing material through the tap arch to leave a space. Then normally you would try to puncture the slag bowl to let some of the liquid run into the space created. This serves to help heat and loosen the structure of the slag bowl. Next you either pound on the top or work to free the edges of the slag bowl from the top. With some levering from the bottom, you work to loosen the entire slag bowl and get it to drop down into the space. Then the whole slag mass, with bloom still in place, is pulled out in one large piece. In the past this is then dragged forward a couple of feet, Then most of the loose slag is removed by striking with chisel ended heavy iron rods. (This is how Skip and Lee extract)
So this method pulls the entire core out of the smelter. Nothing would remain of the slag bowl save some fragments still attached around the edges. There would be some mix of slag / ash / burned charcoal, very low in the smelter base. This at the level of the raw cobb. I would expect a much more disturbed artifact - likely one that pretty much results in loose pile of mixed elements as it collapses.
(Dr Smith)
Second, interesting how the patches in the walls finally burned/melted through with subsequent influx of external packing material and consequent production of excess glassy slag. And interesting, as well, to find that 4-5 reuses was the limit for the utility of the patched stack. How long, though, had that stack been standing (i.e. over how many winter/summer cycles?). Would be interesting to know how many times in one summer a single cobb-built stack could be used before it became necessary to tear it down and rebuild it, as the "typical" structure of the Norse smelting sites is based on a series of overlapping furnace bases representing the reconstruction of a furnace on more-or-less the same site over and over again...yet it's not that clear how much time (or how many smelts) each base in the "stack" represents.
Clay cobb smelter - after 5 smelt experiments
(D)
A number of things here:
Limits on re-use - we are finding typically that our smelters loose something in the range of 2 - 4 cm of wall thickness each use. This effect most drastic just above the tuyere. (We have found that tuyere angle effects this - remember that this smelt had that angle at only 10 degrees due to winter slumping. The ideal has been found to be 22.5 degrees. The lower angle suggested that excessive melting would in fact take place.)
Also significant for this smelt was the double smelt. In the past we have had a gap between smelts that allowed us to repair and patch up the lost wall thickness.
The repair before this experiment was done using a clay that is known to have a lower firing / melting point (local 'Blue Mountain Red'). This likely was an important factor.
The heat profile in the smelter is a torus. This washes back over the walls above the tuyere (relates to angle again). The distribution should be roughly symmetrical. What happened was that the excessive erosion was primarily on the right side of the tuyere - the area which was inset into the bank and had a layer of ash and sand packing against it. Again I would say this is significant. Free standing clay smelters can radiate heat off their surface and thus avoid excessive heat build up. There is some balance between clay properties and ideal wall thickness.
This particular smelter has gone through one entire calender year, plus two sets of firings (June and November). In our area it is pretty dry from mid June through to mid August there is always a lot of rain In September and October. The smelter had a piece of sheet steel over the top through all this time, but the exposed surface (roughly half) was open to winter snows. When the snow melted the interior base actually had several inches of standing water in it (this at the level of the raw clay cobb.)
So it may be that a single cobb wall smelter, especially if free standing, would withstand even more uses - providing that it was fully patched between smelts. This smelter had started to develop some fairly serious top to bottom cracks - several of which were leaking jets of hot gases. The cobb mix helps to tie together even cracked furnaces. My best guess is that after 5 - 6 uses of the furnace that it might just be too damaged to continue using the same structure. The damage done during the extraction process is actually more significant to the ability to re-use the structure. Most of the patching is done at the base around the tap arch - most especially if a bottom extraction is used. Skip and Lee have never gotten any more than two uses of a Flue Tyle furnace - and there is almost no wall erosion with that system. The base at the tap arch is the problem.
Depending on your access to clay (!!) building a new smelter is not that big deal. At best two days for two people (thats including digging and preparing the clay). Given the physical difficulties of top extraction (heat related), and the vast reduction in charcoal required for a second 'hot swap' smelt, it may just have been more efficient overall just to re-build entirely.
(Dr Smith)
Third, I was wondering whether you plan to dismantle this furnace prior to rebuilding it? If so, it would be interesting to know how oxidized/burnt/modified the soil appears beneath the furnace itself and in the matrix that surrounds the bottom 20-25 cm of the furnace base. Again, Icelandic archaeologists tend to look for burned/oxidized/intensely fused soil beneath and around the furnace base to convince themselves that they have found a furnace base. Yet, if the lower 15-20 cm of the furnace stack itself, below the tuyere, is still unfired clay and the charcoal charge beneath the tuyere remains unburned (and I well remember finding unburned paper in the base of the furnace we ran in 2002), it seems questionable that the sediment/matrix/soil beneath the furnace will always be fused/heat-reddened/burned/fired and perhaps questionable, as well, that the matrix outside the cobb core will always be intensely thermally altered unless there has been a disaster, like having the wall melt through. Does this make sense to you?
(D)
Our plans right now are to uncover this furnace and leave it exposed to the elements. With the hole blown in one side and the cracking I don't think we can actually mover the thing without breaking it into pieces. We backfilled the trench we dug at the front to allow the second bottom extraction effort with sand. This mainly to give us a flat surface in the work area. We will build our next smelter to the left side, so we can leave the current smelter in place for some time.
What you mention certainly suggests a useful experiment would be the ability to probe for temperature at various levels of the furnace structure. I have a tool coming that may allow for some attempt at that (I think - Its actually an electric meter - but it comes with a thermocouple attachment. This may take some number crunching to covert electrical measurements to temperature.) As you mention, we have pulled unburned paper out from under the slag bowl, (Mind you that WAS that first attempt here - where we did almost EVERYTHING wrong!)
I would suggest that there is minimal to zero effect on the condition of the underlaying soil. Remember however that when we dug in this smelter, we cut a larger hole then back filled the gap between the cobb cylinder and the undisturbed ground with that ash and sand insulation layer (also provided drainage !) We also raise our smelters up and create a base layer with charcoal fines. We did this initially to provide insulation from the damp soil underneath - which proved not a problem. It also allows the slag bowl to settle slightly below our estimate of its correct level. If you simply used bare earth for your base, then remember the slag bowl would form on top and transfer heat directly. I would think that you might end up with more of a flat bottom to the slag bowl if you worked directly on the soil. The distance of the tuyere from the base earth layer is likely the greatest effect.
All this suggests (yet again) things that we may be doing that are different that what was done in the Viking Age. I will take some more images of the weathering smelter from 2004 (seen above). What is happening there is that the sintered clay is becoming a standing ring surrounded by a wide smear of clay mud. As the ceramic is exposed, it cracks from freezing and largely falls into the hollow where the slag bowl was extracted. Any visible heat effect is limited to the walls of the smelter itself.
We have always dragged off any slag that leaks out of the smelter away from our work area as soon as it cools enough that we can pick it up. This mainly to keep our work area clean and safe to move around it. This last smelt we had a huge amount of just waste glass slag. It mostly had enough iron contained to turn it black - but not much more of than that (not magnetic or we would have re-cycled it). Originally I wanted to also record our slag volumes and weights - but with the extra produced because of the hole in the smelter this information is not helpful to an understanding of ore to bloom conversions. Anyway, I'd guess we have at least enough of this material to fill two milk crates. Time to start our first slag pit?
As always - more questions than answers!
Darrell
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