Wind & Weathering :

air delivery & long term erosion

Darrell Markewitz, with contributions by Neil Peterson
Peterson generated the various graphs seen in this section

Part Two : 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. ‘

    The primary purpose for mounting the October 2021 experimental smelt was to study erosion of a clay wall smelting furnace over an extended period of natural weathering. At the same time, this smelt also presented an excellent opportunity to both measure air volumes over the entire sequence of effective iron production, but also to allow for the inclusion of shorter measurements of volumes produced from a human powered bellows.
    The major modern intrusion into a truly historical process has been the use of various electric blowers for air supply for the bulk of experimental work to this point. Of a total of 90 smelts, 16 have used various Viking Age inspired bellows types (4 with ‘blacksmith’ sized ; 11 with two different ‘smelting’ sized ; 1 using blacksmith sized linked by a bladder) (2-1) The primary reason there has been a dependence on electric blowers is the considerable labour requirement for human power. Past experience has demonstrated that ideally a separate team of at least three physically fit individuals is required just to operate the bellows, working in roughly 5 minute long shifts. This rotation is required to ensure consistent pumping action, at one stroke per second, with no interruptions, over the 4 - 6 hours required for a full smelt sequence. (2-2)
    Although there were earlier measurements made of air flow rates while using various types of equipment, the accuracy was hampered by the limitations of the instruments used (primarily a digital vane type wind speed gauge) (2-3) Actual volumes were derived mathematically from air speed through pipe size (here 35 mm).


Figure 2-1 : Typical set up used to measure air flow.
Wind speed gauge inserted downstream of the blast gate, nipple for analog pressure gauge.

    The numbers thus computed were transferred as very rough 100 litre per minute (LpM) lines on to the simple siding plate blast gate used to control air delivery from the standard compressor blower (used consistently after Spring 2008) (2-4)). In working practice, during a smelt more attention is paid to the variations in charcoal burn rate, with air amounts changed to establish an ‘ideal’ consumption of a fixed amount of charcoal over time. (Largely based on accumulated experience : see ‘If you don’t get any IRON’ )

Equipment and Set Up :

    Primary research team member Neil Peterson had acquired a considerably (!) more accurate air flow meter in 2020. The Omega HHF1000 flow meter will input it’s measurements directly into a computer lap top, at increments far more precise than needed for these tests. (2-5)  The gauge was configured to make four measurements every second (over this experiment, a total of about 100,000 data points).
    This instrument was first used briefly in experiment # 89 - Phase 3B. (2-6) It was long understood that ‘available’ volume (from the blower) would not match actual ‘flow through’ volume (into the furnace), both because of the constriction of the contained charcoal and because conditions within an operating furnace certainly change over time. Despite the few measurements made during experiment # 89, it was clear that the blast gate increments were indicating air amounts roughly 15 % higher than actual flow volumes. 

set up

Figure 2-2 : Proposed equipment layout

    The extraction phase of experiment # 89 had been undertaken by relatively inexperienced hands, which in turn had resulted in damage to the long used tuyere attachment fittings (particularly the plexiglass viewing port). A complete set of new fittings was created leading up to the October smelt. On the air supply end, this included a Y branch that could be installed in the piping. This had a one way valve attached to the secondary branch (actually a ‘back up protector’ used with sump pumps). The existing ‘smelter bellows’ could thus be installed and left fixed in place. With the air blast coming from the blower, there was only minimal air loss as the one way valve was forced closed by the pressure. By simply sliding the blast gate fully closed, only air produced by the action of the bellows entered the system. Overall, there was found to be negligible air loss through the various fittings, and because the actual flow was being measured, these did not matter beyond simple efficiency.


Figure 2-3 : Air supply fittings.

    The bellows used here needs to be considered speculative, in so much as no direct physical evidence for an actual Viking Age bellows, specifically intended for supplying air into an iron smelting furnace, is known.
This unit is based on the two known historic illustrations of blacksmith’s equipment (there are no surviving artifacts), with dimensions extended to allow for at least a *theoretical* delivery volume in the range of 700 LpM (with 60 strokes per second). (2-7)

bellows plan
Figure 2-4 : Design Drawing Figure 2-5 : Shown in use, Ken Cook during Vinland 3, 2009

Norse Double Chamber Smelting Bellows

    In actual practice, this set produced significantly less air, down to the lower limits of what was considered ideal for effective bloomery furnace function. (For a 28 cm diameter furnace, the ideal would be at least 740 LpM). A series of earlier static tests undertaken indicated an average air delivery with this bellows is in the range of 650 LpM @ 60 stokes per minute, with the output into open air. If the same variation in measuring accuracy is applied, actual delivered volumes into the furnace would be expected to be roughly 540 LpM @ 60 strokes per minute. In past experiments the most obvious effect of the use of this bellows was a dramatic reduction in furnace yields.

    Taken together, the plan for # 90 was to :


Figure 2-6 : Air systems and instrumentation in place, early during the initial ‘ignition’ phase.

Looking at the Data :

    During an actual smelt, the usual is to undertake an activity first, then record the time and details. This means for most even paced actions (like ore and charcoal charges) the accuracy of recorded time points is typically rounded up or down to the nearest minute. For more chaotic actions (clearing the tuyere or tapping slag) the recorded time represents the completion of the task, which itself may have taken several minutes to complete. Obviously the flow meter, sending data to the computer file, potentially allows for considerable more accuracy. The problem is sorting through the huge number of individual data points available.


Graph 1 - Overall air flow over entire smelt sequence

    A) The first overall look places actual flow volumes against the major recorded events, primarily indicating addition of charcoal at an average of about every 14 minutes. (see AIR-OVERALL) (2-9)
    It would be expected that there would be resistance to air flow caused by forcing the blast through the stack, initially caused by the available spaces between individual pieces of charcoal. During the main smelting sequence charcoal is ‘graded’ by breaking through a 2.5 cm grid, and screening out any particles smaller than 6 mm. This size range is likely to change within the burning process as individual particles fall downwards to be consumed.
    At first glance, the air volumes over time are fairly stable, reflecting primarily major changes made in the delivery at the blast gate :

Gate Marking Flow Volume
800 LpM
655 LpM 82 %
900 LpM
795 LpM 88 %
1000 LpM 835 LpM 83 %

    This indicates an average error from the plate markings to the actual flow amounts of 84 %, which repeats the few measurements undertaken during experiment # 89 (average difference at 85 %.).
    At the scale used above, the three periods where bellows provided air supply are easy to see (and are marked on graph 1). There is a noticeable drop in air flow from 19:20 through 19:31 that does not match any observations in the written record.

    B) The second question was how the amount of ore present in the stack (above tuyere) might effect air volume, with larger and larger amounts of ore are being added over the progression of the smelt.
    The 28 cm diameter by 60 cm tall stack has a volume roughly equal to three standard charges of charcoal. This means that at any given point. there should be roughly the last three ore additions still dropping through the stack. This all results in an ever increasing amount of ore present in the stack, increasing during this experiment to as much as 6 kg present. (These amounts indicated as ’Ore - in Stack’.)

ore added

Figure 2-7 : New addition of analog added to the top of a similar furnace
(smelt # 56, June 2014)

    Ore is intentionally added evenly through individual charcoal charges, this because the reduction is a gas process, acting on the surface of individual ore particles. At least as initially added, the ore ranges from roughly 6 to 20 mm sized pieces, but with any resulting dust remaining included. One important variable will be the type of ore, the analog being used for these tests is composed of very fine oxide powder, loosely held together in lumps. This is expected to quickly fragment back into fine particles, and thus may not present much of a block to passage of air through the charcoal.

air 2 ore

Graph 2 : Average air volumes and total ore in the stack, over time

    What the overview data shows is that the amount of (at least this type) ore inside the stack does not greatly impact on the air flow. At the start of the first ore addition, the flow is recorded at 791 LpM, (blast gate at 900). There was use of bellows from 1705 to 1713, and with the return to the blower the gate setting was increased to 1000. There is a slight reduction seen in the average flow of air as ore amounts increase, but even with a lengthy duration of 6 kg ore amounts in the stack, this flow has only dropped to 830 LpM (less than a 5 % reduction).

    C) Was there any noticeable effect on air flow due to slag, both in terms of its gradual accumulation and sudden change in volume due to tapping?
    This would take a much closer look at the individual time point measurements, if for no other reason that events like tapping were normally only noted as approximate times in the overall sequence set.  Slag management efforts are indicated primarily via changes in sound, either distinctive noises or more subtle reduction in loudness. The value of the viewing port fitting becomes clear, as a visual check down the interior of the tuyere confirms the nature of the blockage, with experience determining the best course of action to remedy the problem. Normally any visible sign that the slag has risen to 25 % of tuyere opening is considered a point for intervention.
    One clear indication of a potential problem is a fluttering or thumping sound, caused when the level of the slag rises enough that the air blast is essentially ‘blowing bubbles’ against the liquid slag formed at the tuyere tip. The solution is always some method of lowering the interior slag ‘lake’. Judgment always required, as the important function of the liquid inside the slag bowl is to cover the developing bloom, protecting the iron from destructive oxidation caused by exposure to the direct air blast. Some amount of slag will need to be drained away, but just how this is accomplished can vary depending on intended impact on the working system inside the furnace. A typical slag tapping process only requires a minute or two. Self taps often have a longer duration, with a lower flow rate over the event.

    In this experiment there where four major slag tapping events. Ending at approximate time points 17:37 and 18:02, at the hands of the workers, with self taps caused by the dynamics of the furnace itself ending at about 18:49 and 19:20.
The expected indication of a slag tapping event in the data would be a fairly gradual reduction to the flow volume (as air is increasingly needing to blow back the rising pool of slag), followed by a fairly quick return to the original base rate.
    The following graphs show recorded volumes inside a five minute period, primarily before the time point indicated in the sequence records (shown as the red bar) :


 Graph 3 to 6 : Event time against air volumes.
Note enlargement of air scale for graph 4.

    The graphs do not uniformly indicate the pattern suggested above. For tap one and two, the operators first perceived sounds indicating rising slag levels, which were confirmed by observation down the tuyere.
    Although there is an overall drop in air flow recorded leading up to the tapping action, the difference in air flow at the lowest point is seen as only a reduction of between 5 LpM (tap 1) to 22 LpM (tap 2). As this is a very slight amount overall (only 1 - 3 %), this suggests a well experienced worker can certainly perceive small changed air flow from sound alone. For tap three, the changes are perhaps more typical of what can be expected with a self tap, which usually involves a number of separate, smaller leakages of liquid slag over a period of several minutes.
    Tap four, again a self tap, is the puzzle. There is the expected pattern of slight reduction in flow over time, then a sudden increase in flow, as internal slag volume is reduced. What is unexplained is the quick drop to a lower air flow level from 825 down to eventually 760, an 8 %  change that persists for 11 minutes.

    During this experiment, clearing accumulations of solidified slag from the tuyere was not noted on the sequence records. In any smelt, this process can normally be required to be undertaken a number of times. The method involves removing the viewing port / end cap, and quickly sealing the open pipe with a gloved hand. A metal rod (‘radner’) is inserted through the fingers, again attempting to keep any air from escaping. The radner is forced down on to the congealed slag, braking it loose and pushing it back into the furnace, a motion not requiring more than 2 - 5 seconds. The rod is pulled clear, then the end cap replaced. Checking down the now sealed view port, sometimes this process will be repeated, if not all the encrusted slag is found to have been cleared.
    The pipe fitting leading into the tuyere has an ID of 35 mm. At the furnace end, the tuyere is 20 mm ID. The radner used is a round profile rod at 10 mm (standard 3/8 inch). Very roughly, this means that for the (short) time the rod is being pushed through the tuyere tip, it will be blocking a further 25 % of the cross section.
    Against the quarter second per measurement of the gauge, ideally this combined action should be visible, but without a specific time point reference, is just too difficult to find inside the mass of data.

    D) A major test for experiment # 90 was placing the bellows as the working air supply. The same sequence of operators was employed each time, Schweitzer / Peterson / Markewitz, with differences in both individual duration and consistency expected (largely due to differing body size, age, and overall physical condition of each). No specific method was used to regulate pumping rhythm, but the objective was for 60 strokes per minute. One consideration is that the gauge was recording at 4 points per minute, so would be unable at this setting to catch all the variations during individual strokes.


15:00 Schweitzer 6 541 50%
15:06 Peterson 5 491 42%
15:11 Markewitz 1 477 8%



17:06 Schweitzer 4 549 57%
12:10 Peterson 2 559 29%
17:12 Markewitz 1 542 14%



19:44 Schweitzer 2 532 50%
19:46 Peterson 1 487 25%
19:47 Markewitz 1 501 25%



Overall  average


Table 2-1 : Bellows Air Delivery

    ‘Weighted’ gives an overall average, as contributed by the individual operators (volume against time), calculated to 526 LpM. This compares closely to the much earlier ‘no load’ test and estimate (corrected at 540 LpM). Once again, actual delivered amount is close to 30% below the ‘ideal’ for this furnace (740 + LpM). (2-10)
    Looking at the flow volumes placed against time :

bellows 1
bellows 2
bellows 3

Graph 7 to 9 : Measured air delivery during bellows use, individual operators indicated

    In graph 7 and 8, the break in stroke pattern signifying the change over in operators is quite clear, as is the significant change in delivered volumes from that produced by the electric blower (seen at either end). The lack of a sharp drop in volume on graph 8 suggests a much more coordinated 'hand off' between the operators.

    One simple application of all these results would be to undertake test smelts reducing the available electric blower air flow (as accurately measured) down into the range possible via the use of the twin bellows.

    Of course, a comparison of the blooms created to the few known artifact blooms needs to be made. It may be that the current bellows design itself is the flaw. One alternative, suggested by the Evenstad description (1782), is a twin bellows design where the operator stands on top and ‘walks’ to drive the air against the return action of spring poles.


figure 2-8 : Ole Evenstad’s illustration of a continuous process ‘traditional’ bloomery furnace from Norway.
Of note is the pair of long, thin, bellows chambers, operated from above.
Taken from "Iron Production in Norway During Two Millenia" by Arne Espelund, 1995 (ISBN 82-992430-3-3)

    Given the problem of recruiting a labour pool to conduct a complete smelt employing human operated bellows, there has been consideration of just how to construct a mechanically powered alternative. An attempt was made in 2009 (Vinland 2) to build an electrically powered box bellows, but the result proved over complex, cumbersome, and in the end not especially effective. (So much so that the device was named the ‘Fraken-Bellows’!). Current thought is to replace the blast gate with a sliding plate, driven by a small motor that can be regulated to provide variations in both volume and ‘pulse’.
    Ideally a test smelt would have provision for more complete temperature measurements, sampling points not just up and down the stack, but also across the furnace diameter.

    One clear conclusion to be drawn is that far more detailed event / sequence notes need to be taken, with careful indication of exact times. Ideally there should be a dedicated observer / record keeper who has the task of noting every action undertaken. Over so many experiments, accumulated experience has become a substitute for recording all observations possible, those notes that are made are typically at best only accurate to ‘within a minute’. These two factors combine to make it quite difficult to spot specific changes in air flow, when that instrument is recording at 240 data points per minute.
    There is the possibility of a more detailed academic study of the changes of air flow over time within a complete bloomery iron smelt. This may suggest the content for the next experiment, tentatively scheduled for later June in 2022.

On to Part Three : WEATHERING

Notes :

2-1) For images and descriptions of the various types, see ‘Iron Smelting with Human Power’ (blog post)

2-2) See ‘An Iron Smelt in Vinland
It should be pointed out here that teams employing various ‘Great Bellows’ designs (two stacked and interconnected chambers) are in fact using a type that was not introduced until the 1300’s.

2-3) Earlier comparison of air delivery from various equipment can be found : 'Air Flow Rates

2-4) The standard blower used since 2008 is the AMETEK #116246-04, a high capacity, five stage (US Navy surplus) unit rated at 50 cubic feet per minute / 1400 LpM

2-5) One of the problems effecting all Independent Researchers is limited access to high quality (and priced!) instrumentation. For details of this instrument, see :

2-6) See the section ‘Air Rates’ as part of ‘Archaeology, Experience & Experiment

2-7) This unit was created for the ‘Vinland’ series 2009 - 2010. To date, it has been used for a total of five complete smelt experiments.

2-8) August 2008. A total of 11 individuals operated these bellows, set without any loading. The average pump rate was 63 individual strokes per second, an output of 675 LpM (measured via the wind speed gauge).  One important variable to this test was that the bellows was not placed against a working furnace load.
See ‘Air Delivery Test’ (blog post)

2-9) Key to the columns presented :

2-10) Past experiments have resulted in a significant reduction in produced yields in identical furnaces and ore type and amounts, when these bellows have been used. There is a question about the different quality of air delivery between the constant flow from a blower and the ‘pulsing’ delivery from a twin chamber bellows - and how this might effect the physical dynamics inside the working furnace.

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