Wind & Weathering :

air delivery & long term erosion

Darrell Markewitz, with contributions by Neil Peterson

Introduction :

For the last bloomery iron smelt of the 2021 season, a standard pattern furnace was constructed, then fired using a typical sequence. There were two primary goals to this experiment :
Wind - The recent acquisition of a high quality air flow meter allowed for precise and frequent measurements of the actual 'in line' delivery of air into the working furnace. A by-pass system allowed for the shifting to human powered bellows at a number of points, also with accurate recording of volumes produced.
Weathering - A new furnace was constructed on a clean sand pad, set to one side of the main smelting area. Photographic and video recordings were made of the extraction sequence. The remains of the furnace and the resulting debris field will be exposed to weather, and the aging documented as the features erode. The intent is to continue these observations over the next ten years.

This version, prepared for the web site documentation, has additional images

Part One - The Smelt :

The furnace constructed for these experiments was the long proven ’Short Shaft’ type. (1) In this case the furnace structure was placed on a rectangular plinth formed from fairly uniform thickness limestone blocks. The clay walls were constructed of EPK powdered clay, course sand, and dry shredded horse manure, rough thirds by volume, again a standard mixture. Individual fist sized ‘bricks’ were placed against an internal metal form. To reduce the possibility of cracks generated from clay shrinking against these stones, a ‘Beardsley Break’ was incorporated at the top of the first line of clay. (2)

Figure 1 :
Limestone blocks laid out for the plinth
Figure 2 :
Setting the metal form (note cover of newsprint!)
Figure 3 :
Early in the build, showing the ‘Beardsley Break’.

The initial construction would result in walls built to 65 cm height. On later consideration, It was decided to add a further 10 plus cm (to a total of 77 cm). That initial build would have allowed for roughly 50 cm of working stack, considered effective. It became obvious in the days leading up to the smelt however, that participants were going to be limited (only Markewitz and Peterson had confirmed). Adding the extra height would provide a bit of additional working time, so ensuring no interference with the reduction of ore.

One factor that may have had significant impact on the later durability of the construction was that the builder (Markewitz, working alone) had experienced significant nerve damage to the left hand and was still recovering, and so did not have full strength to apply.


Figure 4 : Furnace interior after the first layer completed, showing minor breaks on inner surface

As was done in past builds, the completed clay structure was wound with rope to support the exterior, then filled with ash / sand mix to both stabilize and assist in initial drying.

Figure 5 :
Completed furnace after initial build,
with rope spiralled over exterior, interior packing
Figure 6 :
 After addition the upper ring

The day before the smelt, the tuyere was installed, the arches cut, and the furnace underwent several hours of a slow drying fire

Figure 7 :
With tuyere installed, arches cut
Figure 8 :
Moisture venting off the clay surface during drying fire

Details :


Figure 9 : Front Elevation of completed furnace

Figure 10 : Overall view of the working area during the pre-heat, with the furnace on the sand pad, the air supply systems to the rear.

Darrell Markewitz was smelt master, with Neil Peterson as lead hand and record keeping, Richard Schweitzer assisting on bellows.

Over the smelting sequence a number of measurements of internal temperatures were made, using an OMEGA HH12B digital meter. Measurements were taken by using a number of different thermocouple types, inserted through the holes cut into the body of the furnace. Each was pushed through to about 2 cm beyond the interior wall. A number of physical problems hampered the use of these probes. There were a good number of fine wire thermocouples available, but these were rated to only 1100 C, and did fail into the higher interior temperatures that were reached. (4) There was also one rigid metal shielded probe, rated to 1330 C. Unfortunately, the holes prepared were just barely larger that the shaft, and so once clay expanded due to heating and slag accumulated, it proved impossible to insert that probe through the furnace walls.

TIME 1352 1402 1430 1439 1521 1530

ARCH 20 1170
30 930
40 780
50 550
60 320
70 250


Table A : Charting the interior temperatures
Distances above furnace base in cm
O/L indicates problems with the meter, no further measurements possible

As was fully expected, there would be a number of slag taps over the course of the full sequence. The first two of these were based on observations (sound and visual) of rising slag levels potentially interfering with air flow. As the first of these were very early into the addition of ore, the first at 6.5 kg (potentially only 2 - 3 kg fallen to bowl level), it was felt the most likely origin of that stag was from interior erosion of the furnace walls around the tuyere. Both of these taps were controlled to small volumes, attempting to drop the interior slag bowl and level of liquid slag, but to retain as much of a ‘working’ system as possible.
In the later stages of the smelt, there would be two much larger volume ‘self taps’. For both of those events, the hot slag was allowed to flow undirected, and initially remain where it solidified

Figure 11 : The results of self tap 1 Figure 12 : The results of self tap 2

Over the course of the smelt sequence there would be three intervals where a human powered ’smelter bellows’ replaced the air blast from an electric blower. (Details on this seen in Part 2 : Wind)

During the course of the smelt, it became clear that the fusing of the individual clay additions during construction was less effective than normal. At least two major cracks, running top to bottom, had developed. This could be attributed to the building process, as the cracks clearly jogged back and forth along joints of the individual ‘bricks’. There was also a clear break between the original shaft and the second additional ring, although this was fully expected. During the actual smelt sequence, these cracks, although clearly visible, were not considered a major structural problem, as there were no actual gaps large enough to vent furnace gasses visible. In retrospect, wrapping the furnace with several loops of iron wire should have been undertaken to strengthen the furnace.

Figure 13 :
Cracking above the front arch
Figure 14 :
At the start of the burn down phase, accumulated tap slag at the front.

To record the actual extraction phase, two different video systems were fixed in place, with two different viewing angles. Peterson undertook the extraction, with Richard Schweitzer utilizing the ‘thumper’ and hammer compacting, Darrell Markewitz provided general assistance and hammer compacting.

(Peterson seen in blue, Schweitzer in silver, Markewitz in grey) (5)

The mass pulled clear is clearly huge, including the majority of the interior slag, still attached to the bloom at it’s core. Initial striking by Schweitzer (employing large wooden ‘troll hammer’) can be seen to quickly knocking off the slag. Once the bloom is exposed, this was worked down with metal sledges. The way the metal compacts into a more solid block is an indication of a relatively ’spongy’ consistency.

top bottom

Figure 15 & 16 : The roughly compacted bloom - top and bottom surfaces (as inside the furnace)

On to Part Two : WIND

Notes :

1) see ‘If you don’t get any IRON…’

2) This feature named after George Beardsley, who had demonstrated it’s use during a cast iron session at the Scottish Sculpture Workshop in 2014.

3) Further details on the reasoning behind, and preparation of this analog mix can be found : Ore Analog Composition (blog post)

4) The various wire probes on hand have been purchased individually over many years, so detailed specifications are uncertain. The method was to insert the thermocouples for just long enough to get a reading, then pull them clear and allow them to cool before moving to the next point. This kind of short duration use will often allow measurements to temperatures beyond the rated melting point of the wires.

5) The first sequence (start to extraction) was shot on a cell phone camera (© Neil Peterson)
The second sequence (duplicates extraction to initial compaction) on a compact video camera (© Darrell Markewitz)

6) These two sequences combined do give a more complete depiction of events. On viewing the second (side view) footage, it is clear that the action of the log ‘thumper’ was in a diagonal direction, pushing against the front edge of the slag bowl at least, and perhaps actually against the interior furnace wall? To be fair, this was the first time Schweitzer had ever undertaken this specific task, and despite the fire suit and face shield worn, was still somewhat restrained working down inside the furnace.

Unless otherwise indicated :
All text and photographs © Darrell Markewitz, the Wareham Forge.