(Note that all these pieces are going to be worked up into a formal paper at some point, readers are advised to refer back to the earlier related postings and excuse a certain lack of integration between the segments!)
The furnace wall fragments recovered in the archaeology were described as 'kaloin'. There is a natural clay source in the general region of LAM, which has been used by the re-enactors at Norstead for their own ceramic production. I have no idea if that source matches the materials recovered at LAM. The modern potter's clay available to us has the following components:
|EPK Kaloin||46||37||0.8||2450 F|
|Bell Dark||59||27||1.1||2400 F|
|Old Mine 4||58||27||1.3||?|
The furnace used was built of primarily Old Mine 4 on the lower half, though this included some previously mixed clays of unknown origin. The upper half of the structure was built of freshly mixed Bell Dark. I had contacted Pottery Supply House (our supplier) and had been advised that all this clays had a 'slumping temperature' in excess of 3000 F.
The bloom had been originally weighed on a spring type fish scale (always inaccurate!) Now putting it on the accurate scale, the total bloom weight is in fact 4.89 kg!
I also took some of the LAM analog and heated it on the top of the hot
gas forge to remove the water. We normally compute our yields as bloom against dry (roasted) ore.
sample start (as used in smelt) = 87 gm
sample dry (remove H20) = 80 gm / 8 % water
Applied to the smelt, that means the 18 kg of ore used actually
contained only 92 % 'solids' ingredients = 16.6 KG
TOTAL YIELD (4.9 / 16.6) = 33 %
(Our normal expected yields for a smelt of this size range 25 - 30%)
I had always felt the originally suggested yield estimate for LAM of 20% was far too low, especially in light of the high iron content of the ore also recovered from the site.
When the analog was mixed, it would have been 85 % oxide / 5 % silica /
10 % flour. In the smelter, that organic 10 % just burns off and does
not add to the chemistry. (For Arne, the 'oxide' listed is potter's 'black iron oxide', which chemically is 93 % Fe3O4 / 3.5 % SiO2 / 3.5 Al2O3) Applied to the smelt, that means the 16.6 kg dry weight used actually contained only 90% 'active' ingredients = 14.9 KG
When we collected up all the fragments of slag, and furnace wall, I got a
very rough weight of 54.1 kg recovered. (Note that Neil Peterson made a complete photographic record of the entire working area to pin point the debris pattern. ) I am not easily able to directly weigh the slag separately as it is almost impossible to separate the attached slag from the fused furnace wall. We did work on a fresh sand layer, and were able to hand sort slag fragments down to about 'peanut' size and gather all these.
So this makes our OUTPUT weights at 54.1 + 4.9 = 60 KG
Input is a bit less accurate. We do have the 14.9 'active' from the ore.
I used 'very roughly' 45.5 KG (dry weight) of clay in the furnace
construction. This was one full bag (at 50 lbs) dry, and 'about' a half
bag of dry, plus about a further half bag worth of clay previously mixed to wet. (Admittedly that 45.5 kg is pretty shaky!) Still - that gives us a rough INPUT of 45.5 + 14.9 = 60.4 KG
Total INPUT at 60.4 against OUTPUT at 60 KG
Those are real rough, especially since the amount of clay used is a loose estimate! But it is close enough to input = output to at least show a quantitative result.
Also of note: Slag amount (debris at 54.1 - clay at 45.5 ) = 8.6 kg
So working ore input against slag + bloom output:
Smelt INPUT at 14.9 against OUTPUT at 13.5 KG
Given the estimate for the slag weight, again the numbers are close enough to balance. It should be noted that the actual amount of slag gathered at LAM was 10 kg. Perhaps that number is closer to reality than was thought?
We had specifically designed this experiment so that :
- the ore analog matched the available Fe content in the LAM sample
- the smelter size matched that suggested by the archaeology
- the amount of ore used was equal to the estimated slag plus calculated bloom (15 + 3 kg)
Our test proves:
- the function of the 20 cm furnace diameter
- the suitability of the ore analog
- agrees with slag amounts from LAM
- improves on production estimate from LAM
The primary differences in this test from the Viking Age at LAM:
- use of an Fe3O4 oxide, rather than Fe2O3 (although the raw Fe amounts
- use of electric blower, rather than human powered bellows (although
delivery volumes match).
I am am EXTREMELY pleased with these results!
I am in the process of slicing off roughly a 1/3 segment of the bloom. The metal inside the cuts also appears quite dense. A close approximation of carbon content can be made by an experienced eye looking at the nature of the sparks generated. The lower side of the bloom looks to be a nice soft low carbon iron, with the upper surface containing significantly more carbon, perhaps as much as .3 % (Ideal for heavy tools like axes!).