this version : This was the original full length draft,
before an initial edit and correction of the footnotes
into a more standardized method. In conversion between the
various methods (RTF / PDF / HTML) there has been shifting
of the formatting. This version first published on line,
November 27, 2020
The development of effective bloomery iron smelting has progressed in the past several decades from the first repeated experiments into documented, effective, methods. This progression has primarily been the result of work of often isolated individuals, many with great practical experience as artisans, but most often with little formal academic training. The overall result is a patchwork of recording methods and descriptions, with greatly differing degrees of detail. Experience contrasts Experiment, with many different approaches, and often objectives, as the goals. The author, drawing on almost 20 years of experience with bloomery iron smelting, and afoot in both the artisan and academic worlds, suggests some standard measurements and record keeping methods, presented in a hope to make sharing of information easier.
As this article is also intended for first time experimenters, and both artisan, and academic, experimenters, indulgence from well experienced iron makers is requested over any detailed explanations.
Background : The State of an Art?
Over the last three decades, bloomery iron smelting has moved from the largely theoretical to the practical. Although there were certainly earlier attempts via experimental process to build workable furnaces, most of these attempts were basically unsuccessful, at least in terms of actual iron production. Early researchers too often undertook limited test series (one or two attempts) and concentrated far too much on slag, not on the production of metallic iron itself. This could be understandable on two counts. First, the nature of academic research does not normally lend itself to long drawn out series (especially with repeated ‘failures’). Second, actual artifact blooms are at best uncommon, verging on rare. In this way the desired production outcome in terms of yields, consistency, even clues from shapes, was not clearly represented via archaeology ( 1 ) In contrast, what does survive, in massive quantities, is the waste product of glassy slags, presenting an obvious target for academic study.
The latter 1980’s through the early 1990’s saw some shift, primarily from outside the established academic circles. Increasingly, individuals with more practical backgrounds and a more general training in traditional / historic techniques, primarily working at European ‘open air’ museums, started attempting to duplicate historic processes. Generally, the intent was to investigate functional methods leading to iron production,often within a framework of public demonstrations. The acquiring of considerable experience was often more the result, rather than the conducting of formal experiments. The long sequence of tests at Eindhoven, the Netherlands and Lejre, Denmark especially stand out. ( 2 ) On the more academic side, the pioneering series by Peter Crew from England must certainly be noted. ( 3 ) Gradually, some of this work would filter back into the academic literature, primarily through journal articles or conference proceedings. The fact that so much of the practical work of iron production was in the hands of individuals who were not primarily academics, so not trained or tied into those formal reporting systems, most certainly influenced the ability to share results.
From the late 1990’s into the early 2000’s in North America, interest into bloomery iron appeared within the artisan blacksmith community. This was largely due to the work (and inspiration) of Lee Sauder and Skip Williams, based in Sauder’s studio workshop located in Lexington, Virginia. ( 4 ) A significant event was Michael McCathy’s Early Iron Symposium in 2004, the first full scale gathering of those interested in small scale bloomeries on this side of the Atlantic, which would continue as an annual event in 2005 and 2006. Sauder afterwards hosted a series of workshops for fellow enthusiasts, all skilled artisan blacksmiths, annually from 2005 - through 2011 ( 5 ) . The focus of these 10 -14 day long workshop events was to determine, through repeated physical testing, just what individual factors would combine to create the best functioning small scale bloomeries. It is important to remember that the objective here was not the duplication of historic methods, although regional and historic prototypes were the starting inspiration for the test furnaces. The intent instead was to develop dependable methods of bloomery iron production, both in terms of high ore to bloom yields and overall quality of the resulting metal. This last aspect was an especially important consideration, as a reflection of possible historic processes. Certainly the desired end product of ancient iron makers was also a quality working bar, refined from the smelted bloom. The necessary second step of bloom to bar had rarely been considered by early academic experimenters.
Early Iron 2 – 2005 (L-R) ‘Skip’ Williams, Jake Keen, Mike McCarthy, Lee Sauder
Although some description of these practical working methods would be delivered into the academic realm, the primary impact would be through physical demonstrations and workshops lead by these experimenters for other artisan blacksmiths. ( 6 ) From about 2010 onwards, bloomery iron as a raw material attracted the interest of especially the blades mithing community in the USA. Here the focus would remain on production and metal quality, not necessarily on historic method, or formal experimentation. The clearest example of this is the dominance of electrically powered blowers as the air supply.Because knife making is most commonly the final intention here, there has been considerable emphasis on the carbon content in the resulting working bars, especially over the last decade.
In Europe, the pattern had been largely different. Here most of those interested and working with bloomery iron have remained within the open-air museum / living history population. An increasing number of sites have attempted iron making, with prototypes drawn from their own individual historic / regional archaeology. ( 7 ) Although success is still often measured in terms of yield and quality, the focus remains on historic elements.These efforts most commonly are framed as special events, almost always as demonstrations before the visiting public. There has also been the development of a number of regional to international scope symposiums in Europe, especially over the last decade. ( 8 ) As interest in bloomery iron as a material has filtered back into the blacksmithing community across Europe, increasingly the overall result is a productive blending of approaches, experiences, and skills seen at these events.
‘Iron at Thy’ Symposium, Heltborg Denmark, 2008
As Experimental Archaeology as a discipline becomes more formalized, bloomery iron smelting is increasingly being used as an educational situation for the purposes of teaching ‘how to run an experiment’. In this case, the results, in terms of effective iron production, become less important.
Experiment or Experience?
Obviously, the current field of bloomery iron making is composed of individuals with a wide range of approaches, training, available resources, and objectives. As the field of Experimental Archaeology has gained increasing acceptance, and new generation of researchers also increasingly values those with practical working experience, the initial gap between academic and artisan is also diminishing. Organizations such as EXARC (Europe) and ReARC (North America) have specifically laboured to bridge the divide between the ‘doer and the thinker’. ( 9 )
Those attempting bloomery iron smelting as a personal exploration almost always have huge limitations created by the absence of any institutional support:
- Although there are a number of clear (and accurate) guides to effective methods, written by experienced practitioners, these have been largely published on the internet, and academic bias often obscures the real value of these tutorials. ( 10 )
- The cost of the raw materials required for building even a small furnace, to provide the needed fuel and ore for a single sequence, is considerable. Added to this is the physical labour required to prepare materials, build the furnace, then actually run a complete smelting cycle. ( 11 )
- Self supporting individuals are simply quite unlikely to have access to (or money to acquire) any but the simplest of recording instrumentation. Specialized tools may be imitated by less effective choices, simply because of availability or cost. ( 12 )
- With no specific shared training, approach, or even intentions, there certainly was no shared concept of what observations might be important, or how to document an individual experience. At best, there might be some notes taken, most commonly only intended for that individual to reproduce their own results.
- Many (if not most) individuals attempting to make bloomery iron, are not intending on sharing the fine details of their own experience, so may undertake very limited record keeping. There is a noticeable tendency to hold ‘secret knowledge’ of hard earned methods to individuals, or at best small groups. A working blacksmith is most often focused on making an unusual material, just as the first step in the larger creation process of a unique object. At best they may chose to report rough ore to bloom yields,perhaps approximate carbon content (particularly if, as many in the USA are currently,intending blades as the finished objects).
One important divergence in data recording is that American workers are most commonly using USA / Imperial units (inches / pounds / gallons), with the rest of the world using the Metric system. Although conversion of units is certainly possible, it is the opinion of the author that Metric units should be used as the standard language of science.There is also no standardized set of descriptive technical language. This often is aggravated when translations are made from the differing languages used by individual European experimenters. It is also the option of the author that English language terms should be adopted as the universal (at least for reports translated over into English). At the least, local ‘jargon’ terms should be clearly defined. ( 13 )
Bloomery iron smelting is an extremely complex operation. There is a clear relationship between ore type, furnace material, slag control method, air supply, and fuel used. All contribute to the ‘best’ overall furnace design, physical process, and thus the results of the smelt. Just what might constitute a set of useful measurements and descriptions varies considerably between individual approaches. Regardless, there are dozens of effecting variables, many with fairly tight tolerances between success and failure. It is suggested here that there needs to be a minimum set of information recorded, if for no other reason than to allow the repetition of successful operations (or avoiding errors!), even for those more concerned with iron production. ( 14 ) Those intending a more experimental approach most certainly need to record far more detailed descriptions of materials,equipment, and process to allow for useful comparisons to the work of other researchers.
What has become clear, on two decades of observation, is
that at the current time, there is no standardized set of
expected measurements within a very wide spread
community.The list of individual elements provided below
is most certainly extensive, perhaps in the view of some,
over complex. It is also understood that some measurements
may be beyond the capabilities of individuals (ore iron
content as the best example). Although it is not the
intent of this paper to provide a primer on effective
furnace design and operation,those individual factors that
have proven most critical (in the experience of the author
at least) are underlined. Those elements which may not
apply to all in italics.
One : Describing Intentions
There is most obviously going to be a direct relationship between the person who is undertaking the smelting process, and what their intended end results might be. Someone mounting a public demonstration may be most concern with information flow and even visual drama (and not actual iron production results). Those without significant institutional or economic support are unlikely to have access to instrumentation (ability to measure temperatures is a prime example). There are a number of aspects when describing both the process and the resulting iron that are very subjective, often based on previous smelting or associated metalworking experience (A good example would be the‘feel’ of a bloom as it is compacted, and how this helps define density or even carbon content). Those most focused on the end result of converting ore through bloom into working bars is likely to be most be most concerned with overall bloom yields and metal‘quality’ - and may have little interest in slag remains at all.
Demonstration at CanIRON 8, Fergus Canada, 2011 (blacksmith’s conference) / Experiment at L’Anse aux Meadows NHSC, Canada, 2010 (living history presentation) The author with members of his smelt team
1) Event Type - may be
one or more of :
i) experiment (give objectives?)
ii) production (desired outcome?)
iii) demonstration (what is the audience?)
iv) teaching (what level?)
2) Prototype Reference -
may be some combination of a specific historic period or
regional / ‘ethnic’ type (an example : A ‘Medieval’
dated furnace from Iceland or one from Japan, would
certainly be quite different!). With furnaces built for
production purposes, again give some design reference as
3) Team Composition -
number of people, roles undertaken
4) Previous Experience -
some suggestion of the background of the team leader and
the participants. A group of working blacksmiths, a team
of trained archaeologists, or a group of high school
students - all would bring quite different skills and
observational abilities to a smelt undertaking.
5) Geographical Location
6) Physical Conditions - typically out of doors, so temperature, wind, precipitation.
Two : Describing Materials
a) Material Type
Iron Content - as per cent (if known)
What form of iron oxides are present, and in what amounts?
Additional Components - as per cent (if known)
Silicon and Aluminum oxides generate slag. Phosphorus present in natural bog ores can have a significant effect on the iron created. Water content can seriously change consideration of ore weight, so calculation of yields.
Roasting - if undertaken
f) Particle Size - as added to furnace
a) Material Type - species (charcoal), or some alternate?
b) Particle Size - as added to furnace
3) Furnace Build
a) Material Type - primary material used
b) Additional Components - other materials added to mix / proportions
c) Construction Method - as description of process
Three : Describing the Furnace
A special element to be
described : What kind of furnace is being used?
A ‘problem’ here is that early classification systems to describe furnace types almost always centred on slag control methods, not the upper (working!) construction of the furnaces. ( see 1 ) This is somewhat understandable, as archaeology most commonly only uncovers the very base level of ancient furnaces. Position of the tuyere, as important as this is to effective function, or total height of the shaft, were often completely unknowable. This is complicated by the many ‘traditional’, often regional or language based, terms in use.As experiment enriches archaeology, hopefully some common terminology will emerge?
The following elements are indicated based around what the author defines as a simple‘short shaft’ furnace, intended for slag tapping, and finally bottom extraction - as illustrated. The drawing includes variables that also relate to alternate builds (plinth, slag pit, ‘blow hole’).
1) Furnace Design (letters in capitals
refer to illustration above)
A & B) Interior Diameter - ideally at tuyere level, include at top if different (circular or square? Two directions if oval / rectilinear)
C) Total Height
D) Stack Height - from centre of tuyere to top of furnace
E) Base Depth - from centre tuyere to functional bottom level of the hollow furnace
F) Packing / Soft Base Depth - being loose material added above ground level,changing the effective Base Depth (if used)
G) Plinth (base blocks) Height - here a set of base blocks / stones raising height of upper shaft off the ground (if used)
H) Plinth Wall Thickness - (if used)
I) Slag Pit -here a hole below the furnace for draining slag into (if used)
i) depth below ground level (bottom of shaft), plus cross section dimensions
ii) fill material used
iii) capping method at top of pit (if used)
J + K) Tap Arch (here an opening specifically for draining liquid slag) - (if used)
L + M ) Extraction Arch - meaning a larger opening at the base of the furnace allowing for pulling the bloom out, if larger than above (if used)
N) Wall Thickness - (at top and bottom if varies)
O) Tuyere Interior Diameter - (taper measurements as applies)
P) Insert Angle - measured from horizontal (plus or minus)
Q) Distance Proud - from centre of tuyere to inner wall
R) Tuyere Length
S) Tuyere Outside Diameter
(S - O = tuyere wall thickness)
T) Offset - tuyere to extraction / tap arch - relative position of these elements
U) Blow Hole - here a method were the tuyere is offset in front of a larger diameter hole cut into the furnace body (if used)
i) hole diameter,
ii) measurements from furnace base
iii) position related to extraction arch (if different)
2) Air System
a) Tuyere Material
b) Air Supply Type -
If electric blower :
i) equipment type
ii) delivery volume control method
If human powered air : (* A * see detail below)
i) equipment type
iii) stroke rates
c) Air Delivery Volume (* B * see detail below)
d) Air Delivery
Pressure (if measured)
e) record of Changes / Stoppages
Four : Describing Method
1) Reuse of furnace - if applies
a) details of
interior condition and previous erosion
b) repairs if any
2) Temperatures - (if measured - * C * see detail below)
a) equipment type
b) Probe Placement
d) Time and Measurement
i) material type and sizes
iii) introduction of air (when and how much)
b) Ignition Phase
i) time from first charcoal to full column ignition
ii) time to first ore addition (if different than i)
iii) particle size (if different than for standard charges)
c) Charcoal Charges
i) as kg weight against time added (burn rate) (see detail * B * below)
ii) method of addition (as a single large amount or as smaller portions?)
4) Ore Charge
a) as kg weights
b) method of addition (evenly through charcoal or as single pile?)
c) placement pattern on surface (e.g.: ‘c shaped’ vs ‘centered’ ?)
5) Modifications as occur
a) clearing tuyere ?
b) downwards probing into furnace ?
6) Slag Tapping (if used)
a) Time Point
c) Slag description / type
d) Volume (as estimate)
6) Burn down
a) charcoal charges above last ore
b) elapsed time to extractionc) remaining height before extraction
7) Extraction Method
/ bottom (side) / furnace destruction
b) Compaction in Place (if used)
c) Actions described
a) equipment / method
b) secondary heating to repeat?
c) number of compaction sets
d) hot cutting (if undertaken)
Five : Describing Results
1) Total Charcoal
2) Total Ore
3) Bloom (* D * see detail below)
c) Condition / Density
d) Carbon Content
e) Calculated Yield (ore in to bloom out)
5) Total Elapsed Time
6) Waste Produced (if collected)
a) Gromps ( 15 )
b) total Tap Slag
c) total Furnace Slag (bowl and fragments)
7) Furnace Erosion (as dimensions)
7) Furnace Erosion (as dimensions)
Sample of experiment record : Field notes into data frame
Details Above :
Determining relative quality of any bloom, in the
absence of fairly elaborate (expensive) equipment is
very much subjective and based on related experience.
During the consolidation series, a well practised
blacksmith can estimate, through sound and hammer feel,
both the general solidity and also rough carbon content
(hardness) of the metal. The carbon content can be at
least roughly estimated by ‘spark testing’ over a cut
interior surface. ( 21 )
Mounting even a single iron smelt is a large investment in time, labour and materials.Looking back over many experimental smelts, it is clear that there is a complex set of individual variables which combine to determine the best possible results. Changing more than two or three of those ideal aspects may result in poor returns, or even no significant iron production at all. What works for one local ore may not create the best yield or bloom quality with another.
Bloom made at the ARTEfakty Event, Poland 2016
In discussions between experienced and first time iron makers, the two most common questions are : ‘Will this work? and ‘What went wrong?’. Core to understanding both of these questions is always ‘What did you do?’Obviously for the scientific experimenter, recording as much available data as possible is critical. Often a full iron smelting sequence results in damage to (if not outright destruction of) the furnace when extracting the bloom. Subsequent reporting may have reduced value if full documentation was never made. For the production iron maker, even small changes in method often result in large variations in yield or quality. In these situations, careful records are most likely to allow for better predictability over the intended bloom. It has been certainly demonstrated that recording details tends to help control technique - sloppy work tends to generate random results For both groups, recording the basic set up of the furnace, and the operating sequence undertaken, will best allow repeatability. The author would like to stress the immense value of photography, both the extensive use of still images, and where possible, short video segments. A huge amount of data can be easily recovered ‘after the event’ through the examination of images!
The many references to both the continuing work, and private discussions with, Lee Sauder, should certainly indicate the massive debt owed to him personally over 20 years of guidance, joint work, and friendship. I would also like to thank my smelting partner Neil Peterson for his hard work taken together on experimental iron smelting over the last 20 years, and for his preview and edit of the draft of this paper. Various members of the Dark Ages Re-Creation Company have contributed labour and taken part in many of the experiments and public demonstrations of bloomery iron smelting I have undertaken since 2002.
Footnotes / Endnotes
1) Iron in Archaeology, The European Bloomery Smelters, Radomir Pleiner, Archeologicky Ustav AV CR, 2000 pgs 235 - 243
2) Early Iron Production - Archaeology, Technology and Experiments, Lars Norwich (editor), Historical-Archaeological Experiment Centre, 1997
3) 25 Years of Blooming Experiments - Perspectives & Prospects, Peter and Susan Crew, text of presentation at HMS Conference, 2010 Peter Crew has certainly published many papers concerning his extensive work, this article provides the best overview.
4) A Practical Treatise on the Smelting and Smithing of Bloomery Iron, Lee Sauder & Skip Williams, in Historical Metallurgy 36(2), 2002
5) The author, along with Mike McCarthy, Lee Sauder and Skip Williams, formed the core of this initial ‘Early Iron Group’. Descriptions of the individual events were published as part of the larger documentation on the author’s web site : www.warehamforge.ca/ironsmelting/index.html , ongoing since (at least) 2007
6) Darrell Markewitz : Quad State Roundup, Troy Ohio / 2008 Dark Ages ReCreation Company : CanIRON Conference, Annapolis Royal Nova Scotia / 2010 Lee Sauder : ABANA Conference, Rapid City South Dakota 2012
7) Some significant examples :
Weldon Iron Research Group, England (Tim Young)
Eindhoven Museum, the
Netherlands (Thijs van de Manakker)
June 2014 Museum Thy,
Denmark (Jens Olessen)
Viking Centre, Denmark (Michael
Iron Smelting Days, various
European countries (since 2004) ARTEfakty’,
Pruszkowski Festival of
Archaeology, Poland, 2016 the Woodford
Furnace Festival, Ireland
(since 2018) Dymarki
(Festival of Archaeology),
Poland (web site in
International Iron Smelting Days, various European countries (since 2004)
Pruszkowski Festival of
Archaeology, Poland, 2016
the Woodford Furnace Festival, Ireland (since 2018)
Dymarki (Festival of Archaeology), Poland
(web site in Polish only)
“But if you don’t get any IRON…” Towards an Effective Method for Small Iron SmeltingFurnaces
Darrell Markewitz, 2012 (originally published in the ExARC Journal)
Making Steel (web site) Jesvs Herandez - (now only available via the Internet WayBack Machine) :
Quoting from the ongoing series by the
author, in Ontario Canada.
One complete build and use cycle for a ‘short shaft’ furnace, 30 cm ID and 60 cm tall, 30 kg ore smelt. (in CDN $ / ‘work’ hours/ 2019) :
Furnace materials - clay, sand = $40
Charcoal - 100 kg = $235
Ore - as red oxide ‘analog’ + $70
Furnace build = 6 hours Fuel / Ore prep = 3 hours
Smelt = 27 hours (3 bodies x 9 + hours)
Total : $350+ / 35 - 40 work hours (plus specialized tools)
12) A clear example of this was the initial scorn placed on what academics called ‘Vacuum Cleaner Smelting’. Independent workers commonly used scrounged household vacuum cleaners, reversed, as air supply. Largely this was done because of the high cost (and difficulty in even finding) suitable powerful and reliable blowers.This had an important impact on record keeping on the early researchers in Denmark especially, where almost everyone was using virtually identical, small, aluminum housing,vacuum cleaners. So widespread was the use of this specific unit for air supply, few ever bothered to ever consider measuring or recording air volumes (‘Just use The blower…’was a common refrain from smelters in the area) .
13) Two good examples:
- European authors often use the term ‘coal’ to describe the fuel they are using. At least to North American readers, there is a big difference between wood ‘charcoal’ and rock mineral ‘coal’. The author has seen this kind of language perception problem come up many times inside discussions with fellow experimenters from across the world, especially if translations are involved.
- Over the years, the author has seen the term ‘bog iron ore’ be applied to many quite different natural materials, with separate deposit mechanics and physical characteristics.At the extreme, ‘bog ore’ can be (and often is) used to describe ‘any iron bearing material deposited by water’. (At one occasion, a limonite rock ore, eroded and washed downstream hundreds of miles from its source, then recovered along an old river bank, was described as ‘bog ore’.) It is suggested here that a clear description of any ‘bog iron ore’ material and its deposit method be provided.
14) Most especially important to first time workers in the field. To often requests for advice on ‘what went wrong’ reveal that even the simplest measurements were never made. Many do not fully appreciate how what might appear small modifications or simple substitutions can drastically effect outcomes. In North America, one of the guiding concepts among the original ‘Early Iron Group’ was freely sharing of all information collected from personal experience and research.
15) ‘Gromps’ are defined here as pieces of combined slag and iron, with enough metal content to be magnetically sorted, but which have not remained solidly attached to the core bloom itself. These are often struck off the bloom during the initial compaction phase. This material may also be removed later from the slag mass within the furnace. [honestly you might as well do a glossary as a part of this article]
16) The clearest example of this is seen with those teams reporting use of ‘Viking Age’ bellows, which can be seen to range from small twin chamber units to (later medieval) huge sized ‘great bellows’ types. All this when in truth there is not a single physical artifact of any bellows from that period, most certainly nothing ever recovered related to the iron smelting process.
17) A long series of personal conversations with other iron smelt experimenters over the years, most significantly with Lee Sauder.
18) A working iron furnace will have individual consumption rates that undergo significant changes over the progress of a working smelt. The type of the furnace, and the type of the ore used may also effect these rates. It has been found in the author’s experience and observation that one of the best indications of the developing cycle inside the furnace is the elapsed time between individual standard charges.
19) Several ‘historic’ types of time measurement were tested during the ‘Vinland’ experimental series, undertaken by the author’s team at L’Anse aux Meadows NHSC (Parks Canada) in 2012. These included a ceramic pot filling with water, and chanting songs (which also had the added benefit of maintaining bellows pumping rhythm).
20) The evidence and the secrets of ancient bloomery ironmaking in Norway, Arne Espelund, Arketype Forlag, 2013. Espelund has gone one step further, suggesting that there was an intentional two stage process, with a first smelting cycle used to create a ’semi refined product’, which then was later subjected to another full smelting effort to produce higher quality blooms (both in terms of size and density).
21) Care must be taken to
assess the interior - and multiple areas
across the surface. Past assessment of many
blooms produced by the author has shown that
the both the outer and inner portions, as
well as the top and bottom surfaces, of a
single bloom can vary considerably in carbon
content. Estimates by ’Spark Testing’ is a
method well known to working blacksmiths,
but at best only provides a rough
approximation of carbon content.
Statements by some groups of producing "excellent quality iron" are virtually meaningless as indications of characteristics.
About the Author:
Darrell Markewitz from Ontario, Canada, has been a professional artisan blacksmith for over 30 years. Known as a specialist in the Viking Age, he has contributed to a number of major museum exhibits, including extensive work for Parks Canada at L’Anse aux Meadows NHSC. His interest in bloomery iron smelting started there in 2001, and his extensive practical and experimental research has included participation in a number of international projects and academic conferences over the last two decades.Past work is well documented on an extensive web site : www.warehamforge.ca/ironsmelting , plus on his blog : warehamforgeblog.blogspot.com