Friday, November 09, 2018

'Roman' / MoAF-I : Results

(continues a rather lengthy post describing the furnace build)

One thing to remember here is that the intent of the Nov 3 experiment was to record internal temperatures = to see if high enough temperatures would be produced, and maintained, to allow for potential iron smelting.

In this, the effort can be seen as a success. Thanks to instrumentation provided by Neil Peterson, there was much better temperature data records than normally generated.

As detailed in the previous description, the addition of ore did not exactly follow our well proven 'best practice' :
  •  There was no initial addition of iron rich slag, which has proven to quickly establish a working slag bowl system, increasing both yield and bloom density. (1)
  •  The amount of ore added was on the small end. In the past it has been found that the first ore is primarily creating that same working slag bowl system, with the later amounts mainly contributing to iron mass on to the bloom. (2)
  •  The quality of the bulk of ore used was questionable, with past uses of the same lot of material seen to result in low yields at best. (3)
A last important point was the overall layout of the base section of the furnace. The depth below tuyere level was set at 20 cm - intentionally quite deep. In the past it has been found that the most effective working range is 10 - 15 cm. If there is more available space to hard ground level, usually any extra is filled with charcoal fines. This was not done in this experiment. Primarily to negate the effect of accumulated ash and small charcoal pieces seen to be blocking air intake during the first firing of this furnace.

The following morning (so plus 16 hours), it was found that the charcoal had all burned away inside the furnace, but the exterior base was still warm to the touch. In actual fact, it would be another three weeks until the furnace was actually opened...
Nov. 3 - after cutting
The exterior of the MoAF-I furnace still remained in extremely good shape. There had been one major crack during the initial drying, which ran up to the lower of the two potential working ports cut for the bead making experiment. (This port can be seen, as sealed, roughly to the centre, left.) The intent for the November 3 experiment was to section the existing shaft, then lift off the top portion for conversion to a new 'short shaft' furnace. A starting cut was made 70 cm down from the top, using a zip disk on an angle grinder. The wall thickness proved to be about 6 cm - a bit deeper than the disk would cut. The last depth was cut with a small dry wall knife. A metal plate was slid through the cut, and the top section of the shaft carefully slid off and positioned on a prepared base plinth. (4)
View of interior, showing tuyeres with mass below
When the lower interior of the furnace was exposed, the heat effect on the clay was clear. The furnace walls were remarkably clean, with little attached slag or fragments of sintered iron attached. The individual tuyeres were still roughly the same length as when they were installed. There was considerable material fused and piled on top of each, what appeared as partially sintered (but reduced) iron, mixed with slag. There was considerable erosion effect to the tip of each of the tuyeres, now appearing to have the original mild steel pipe replaced by fused slag. They had however retained roughly their original insertion length of + 7 - 8 cm.
Judging from the heat effects, it appeared that the placement of the tuyeres was in fact quite effective in concentrating the heat into the centre of the furnace. (Rather than washing back on to the walls and erroding them, which likely would have been the case with a shorter insertion length.) (5)

Neil Peterson undertook the excavation of the furnace base. Recording the process was done through an extensive series of scaled photographs he made (only a few reproduced here).

Walls removed to about tuyere level (image by Neil Peterson)
Detail of lower left, before 'cleaning' (image by Neil Peterson)
The upper walls were cleared away, working down to tuyere level.
There was no clear bloom mass in evidence. The top surface was roughly bowl shaped, higher under each of the tuyeres and depressed to the centre. The material had a 'crumbly' surface texture, looking like reduced but only partially sintered iron with a lot of slag included. Unfortunately no exact measurement of the depth of this surface below tuyere level was made, but looking at the images, it appears to be about 6 - 8 cm lower to the centre.
One of the tuyeres removed (image by Neil Peterson)
In the image above, one of the tuyeres, with its attached debris, is positioned so the brick line is roughly the same as the furnace wall. The point of attachment through the wall is clearly visible. The congealed slag from the inner tip, will be in a vertical line (under gravity). Between the two indications, it is possible to find the original angle that the tuyere had been installed.

Another tuyere, image rotated to 'square' (image by Neil Peterson)
At least some indication of the minimum depth of the slag mass at the tuyere tip can be determined. The slag still attached to each tuyere forms a rounded 'drip' at its lowest end, so each had to have formed above that lower mass. The measured distances show at least 4 and 6 cm. (*)

After the tuyeres were removed, the remaining wall material was cleared down to expose the slag mass.
The slag mass exposed, surface cleaned
This surface was cleaned, first by brushing, then by using a tube to blow off ash from the fire and dust from breaking the walls.
In the image above, one further course of clay additions was been removed from the left side. You can clearly see the difference in heat effect. The upper level (closer to tuyere blast and thus the full furnace heat) shows slag attached to its inner surface. Clay shows sintering (white) from the inside outwards, carbonization of the horse manure organics, then dry baked clay to the exterior. In comparison, the next course down (just above the brick plinth) is only baked dry clay. The height difference between these layers is no more than 10 cm.
There was a clear 'necking' observed, with a change in appearance in the material above and below.
Mass exposed, side view - rotated to level (image by Neil Peterson)
The upper material formed a bowl shape, seen here to lift a good 6 - 7 cm towards the outer edges. The upper material had a thickness of about 7 - 8 cm at the centre down to the top of this reduction in diameter and change in material.
Slag mass removed, the right edge broke clear - image altered to place 'neck' at horizontal     (image by Neil Peterson)
With the lower mass completely pulled free of the furnace, the difference between the lower and upper sections was easily visible. One edge of the upper, lacy, material remained adhered to the clay walls, and broke free of the central mass (seen to right above).

Later in the day, the large mass was broken open (we still had hopes there might be an iron bloom inside...). The mass broke apart into several large chunks, and along the central 'neck' line.

Part of the lower, slag block (top uppermost)
As was expected, the lower section, below the 'neck' was one solid mass of iron rich slag. This was found to be almost totally free of charcoal or bubbles, a solid black colour of iron rich slag.

Upper mass - bottom side (against lower slag)
Upper mass - side towards tuyeres
The upper part (above the neck) had an entirely different construction. This proved to be lacy 'foils' of iron, formed around pieces of charcoal and containing a lot of slag. At best it appears only about 25% of this mass is actually iron. The 'foils' of iron can be seen as the white lines and spots in the images (especially in the bottom view).

next posting - Conclusions...

Prepared with some observation checking help from Neil Peterson.

1) What I call the 'Nissen Method'. This was demonstrated, to great result, by Danish experimenter Michael Nissen at the 2016 Pruszkowski Festival of Archaeology. In brief,3 - 5 kg of crushed iron rich tap slag is added as the first series of charges. This material will immediately form the effective slag bowl, so as ore is added after, it goes directly to iron bloom formation. Typically this method has been found to increase effective yields by about 10% (overall).

2) In relation to above. In the past it has been found (depending on ore type) that about the first 8 kg of ore goes into forming the working slag bowl system. Material added after that is generating bloom mass. This is especially true of those ore charges added to the end of the sequence. Our normal process here is to use roughly 30 kg of ore, with the expectation (with our DD analogs) of a bloom return of 20 - 25 % / 3 - 5 kg. Both raw size and yield % will sharply rise with larger ore additions. Remember that the overall intent of the various experimental series is primarily testing various furnace types and detailed builds - not production per se.

3) This specific ore lot was found to be generally 'poor' - with considerable slag, small blooms, and low yields, from earlier uses : Nov 5, 2006 / June 9, 2007 / Oct 27, 2007

4) Nov. 3 - 'Gromps' smelt. Report still pending.
One factor is that Neil proceeded to 'excavate' the MoAF-I furnace, while I was guiding a second team in preparing for a second experimental smelt. Both furnaces under the smelting area at Wareham, about 1 metre apart!

5) Through past experience, using higher volume air (bellows or electric blower), it has been found the 'sweet spot' for the tuyere tip is about 5 cm proud of the interior wall. When using mild steel pipe, these almost always will burn back to maintain that '+5 from wall' distance.

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February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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