Into Phase Three - Continuing Experiments with Iron Smelting,
based on Hals, Iceland

Note : For clarity, a number of terms are supplied with definitions as are used in this commentary. Readers with knowledge of iron smelting are most likely to be familiar with these terms, and are asked to have patience for background explanations.


    The team from the Dark Ages Re-Creation Company, under the leadership of Darrell Markewitz, has been involved in an exploration of Viking Age, Icelandic, bloomery iron smelting methods, now for over a decade. This has been specifically centred on the archaeology of a long duration, ‘industrial’ level production site, located at Hals (west central Iceland). The original excavations there were undertaken by Kevin P. Smith, documented in his paper : ‘Ore, Fire, Hammer, Sickle : Iron Production in Viking Age and Early Medieval Iceland.’ (1) As well Kevin has become a good friend and advisor to our team over the last twenty plus years, and has provided many additional details in private e-mails and discussions. One important consideration that must be made is that the excavations at Hals by Smith were only partial, for example only a small portion of the area most likely to contain the remains of additional furnaces was possible to expose. The 2005 report only describes excavation down to the top part of the individual furnaces, work was halted with the (unfulfilled) expectation of returning later to resume and complete.

This experimental series started in 2007, with an initial concept overview prepared : ‘Working towards an Icelandic Viking Age Smelt - Based on the remains at Hals’ (2)
    Our earlier iron smelting tests in this series can loosely be broken down into two phases :
- Phase One / four experiments / 2007 and 2008 / testing individual design elements
- Phase Two / four experiments / 2012 to 2016 / testing use of turf builds, then combining elements from phase 1
In phase one and two, we were attempting to take the system that Smith had proposed in his early paper - and physically test how various elements would work individually and in concert into a functional whole – of that proposed furnace layout. (3)

    Looking backwards, it is obvious that much had been learned about iron smelting in general in the gap between phase one and two. I did not feel (even at the time) that I really did not have a solid understanding on smelting process until into late 2008 (so not until after the first two of the Icelandic series experiments specifically). The team shifted to our 'Vinland' series over 2009 - 2010, and I did a lot of international project work over 2014 and 2016 - all before the end of phase two in this series. When the initial planning of experimental testing for phase 3 was started in winter of 2021, we had intended to return to the earlier 'boxed turf cone' design. Against this, in spring of 2021, considerable effort had been expended in gathering both timber for framing and earth for fill as would be needed in the construction.
    Long term experimental partner Neil Peterson and myself spent several hours on Friday June 11 (plus many additional e-mails) plotting out exactly how to proceed into a next group of experiments. We were assisted by Rey Cogswell, who brings experience in archaeological survey methods and recently has joined the iron smelting team. Towards designing the current build, we considered again what the evidence from Hals can possibly suggest, especially in light of what we know now (after an additional decade of experience, with over 60 more smelt experiments, since we started this series in 2007). (4)
    It is important to stress that the commentary below is based on our own observations and interpretations over period leading up to a full build the week of June 12 through 17, in preparation to the scheduled smelt experiment at Wareham on June 19. It is well understood that working from a published report and images is most certainly NOT the same as an individual’s direct personal observations on the actual ground itself. (5) To this end full apologies are made (in advance) to Kevin Smith, for any misconceptions or outright errors in understanding that may be expressed below!

Considering Hals (again)

    At this date recovering additional detailed information about Hals is considered 'difficult to impossible'. This puts us back to just looking at considering what we actually know from the original published description - and considering fresh what those elements might really mean.


A revised version of the elevation and profile of Hals group of furnaces.

Modified from original by Kevin Smith.

    Smith had recently provided both a coloured photograph of the central part of the excavated area, plus several new versions of what had been originally much smaller sized black and white diagrams, now with a colour coded key.  The version above has been adjusted and printed on to graph paper, with a grid showing of 10 cm.
Although this may seem obvious, to be ‘fire effected / slag attached’ these stones must have been directly against the interior of the furnace (so any clay liner built around the stones). A functional build would be to frame an extraction arch with stones, laid like a course of bricks, with one larger slab laid over the top as a lintel to support any upper structure. This kind of framing becomes even more important with the kind of turf enclosure seen from the profile at Hals. The cut turf pieces have very little structural strength of themselves, so any kind of gap around the furnace inner surface requires a fire proof material for this framing. In a working furnace the open part of this extraction arch could have been filled by more stones, a clay plate, cut pieces of turf, or some combination of these. How this might be done must be considered an unknown, as this fill material would have to be pulled clear while hot during extraction, so is most likely to have been tossed off the working area for simple safety.

    Another important element to effective furnace operation is the dynamics of air supply. At Hals, there was nothing recovered indicating directly the use of ceramic, copper alloy or iron tuyere tubes. The only remains found suggestive of the air supply system was a single clay fragment with “... a circular, vitrified opening ...” (1d) which was later described as 'suggesting a 5 – 6 cm diameter'.
    This arrangement would not create any damage to metal or ceramic tuyeres, and might even allow the use of bone, possibly even wood tubes. With use of a blow hole, there has been found to be a serious impact on air penetration into the furnace, in turn changing the dynamics of how heat positions. This results in higher, shallower slag bowls, typically only extending part way across the furnace diameter, requiring more careful slag management (so frequent draining / tapping). With a reduced size slag bowl comes smaller blooms. More heat washes back on to the wall surface around and above the insertion point, creating more damage to those surfaces. (6). It needs to be remembered that any variation of the blow hole system requires a fully exposed and self supporting front furnace wall section.
    There is an advantage to positioning any solid tuyere tube so it rests on top of a lintel stone. It would be possible to position a tuyere tube through a stack of turf. However any metal tuyeres rely on freely exposed surfaces to radiate off excessive heat, thus keeping their interior portions from melting. (7) At the very least, the depth of surrounding (and insulating) turf would have to be limited as much as possible.

Slag and Slag Control:

Unknowns :
    Both of these are important to comparing our starting ore analog (8) to the ore actually used, eventually leading to some consideration of both the size of individual smelts and the potential yields. It is understood that certainly the analog’s exact iron %, oxide form, and likely the silica content will not duplicate that of the ore from Hals. Understanding the amount of silica contained in the Hals ore also is critical to estimating how much actual slag would have been produced. The slag bowls remaining at Hals are described as ' 25 - 30 cm diameter and 5 - 10 cm thick'. It is a given that these only represent the *last* smelts - and only a *very few* examples from a long use of the site. Slag bowls of this size could be the result of three variables :

    Controlling slag is essential to furnace operation, so in turn will determine overall furnace design. One huge unknown here is the slag control method employed at Hals. This has been long proven to be critical to the overall design and function of the furnace.
    Excavation recovered a massive amount of broken pieces of slag in a separate pile, but very little of this considered to be classic 'flow pattern' or ‘pillow’ tap slag (d) (1e).
    The thin height of the remaining slag bowls does not suggest the use of allowing slag to drain downwards into a 'slag room' chamber (again, based on past experience in both earlier phases and other experimental smelts). 
    It could be possible that the remaining slag bowls are the result of very small volume smelts. We consider this unlikely however, as there is a very significant efficiency advantage to undertaking large ore additions. The working furnace takes a certain time / fuel expenditure to create the 'functioning system'. In our experience, ore addition amounts sharply increase over time in the later stages of a smelt, and doubling total ore amount is likely to quadruple effective yields. The limit on bloom sizes created is most determined by the physical diameter of the furnace - if large quantities of fuel and ore are available. Working against this are the practical realities of physically manipulating extremely large bloom masses, much over 10 – 15 kg and it simply becomes extremely difficult to apply enough effective force with hand thrown hammers.
    Considering the practical against the archaeology, it may also be :
    Slag bowls are especially broken into pieces during bottom extractions (to expose and release the internal bloom). These hot pieces need to be quickly cleared out of the working area for safety. Additionally it would be expected that when preparing a durable furnace for repeat use, the previous slag bowl would be broken out and removed as waste.
    Tap slag can be expected to be broken clear and moved aside while still hot, again for simple safety reasons. We most certainly save a certain amount of previous iron rich tap slag separately for addition at the first part of a second smelt. This ‘priming’ method has been demonstrated to improve yields, as ore additions go straight to bloom formation, not to creating the functioning slag bowl system. (We had learned this, in combination with suggestions and tests by Michael Nissen and Lee Sauder, circa about 2008. Although you do have to be careful about applying knowledge backwards into the past - it seems likely a group of long duration, 'industrial' iron workers like those at Hals would have figured this out as well?)

Structural Build:


The original proposed furnace layout from ‘Ore, Fire,...’ (1f)
    Smith had originally proposed an overall construction consisting of basically a cone of stacked turf strips, with a cylindrical shaft at least partially clay lined, all surrounded by a timber framed box, the space between the cone and the box filled with earth.

    From our first considerations of creating a working furnace based on this design, the small tunnel proposed into the front wall of the furnace was considered a major problem. There simply was no space available for any kind of slag control method, with the exception of use of a slag pit type (not indicated by the remains). Even for a slag tapping type, this would have to be physically attempted working down at the end of that restrictive tunnel, both difficult to accomplish, and likely physcally hazardous. The required placement of the air blast would have been functionally too low to the base of the shaft, and also seriously limiting any attempt to angle the tuyere not alowing for much space for a developing iron bloom (both of which had proven critical in numourous smelts).
    The archaeology for at least furnaces 4 to 7 indicates a bottom extraction, and again our working experience suggests attempting this down such a tunnel would prove extremely difficult (if not impossible). The alternative would be a top extraction, working down into the shaft while standing on the earthen platform around the furnace. This however leaves a complete circular slag bowl, more of an offset ‘bagel’ shape, rather than the C shapes exposed at Hals.
    Providing for a functional working space at the front side of the furnace thus became a major consideration. This kind of gap necessitated and indicated by the requirements of slag control and extraction method as suggested above. Of course it is certainly quite possible that the first series (remains 1 – 3) were both constructed and functioned differently than those in the percieved second series (4 - 7) at Hals.

    Our previous test of a full scale, timber framed and earth packed build suggested there was no advantage, other than a possible top extraction, to the complete timber box and earth fill around the basic turf cone. The tools used for adding both ore and charcoal almost through necessity require long handles to protect workers from the extreme heat at the top of the furnace. It proved no problem to simply stand outside the raised box when adding materials, in fact not having to climb up on to that top surface every two or three minutes proved a huge advantage. (9)
    Although top of the furnace extractions have been undertaken many times in our past experiments, this method is more physically punishing to the workers. Even when standing beside a typical 60 – 70 cm tall furnace shaft, the upper body, and significantly the face, is exposed to the full heat of the roughly 1100 + C interior. A worker attempting to stand on top of the proposed earth platform would have the entire body exposed to this extreme. Additionally, any of the tools required for extraction (chisels, hooks, tongs) would need to be excessively long to reach down that extra distance to the bottom of the furnace shaft.
    Based on our own working experience, we can see no advantage to the originally proposed ‘timber frame with earth fill’ design, and very clear disadvantages.
    In addition to these practical considerations, one critical factor is the raw volume of earth that would be required to create such a level working platform around a turf cone built to working shaft height for a functional furnace. With a typical 65 – 70 cm shaft, and a turf cone at the suggested 200 cm base diameter, a (very) rough estimate is that some 1.4 cubic metres of earth fill would be required. (10) This would be thickest deposit at the outside edges of the framing, certainly not the situation recored in the excavation diagram.

Clay Body:

    Unfortunately, no detailed analysis of the clay fragments recovered at Hals was ever made. This also includes any specifics about additions of secondary materials to modify characteristics to be more suitable for use in furnace construction (possible sand or organics).
    A good sized clay bank was located within a ‘reasonable’ travel distance (about 15 km, considered an ’overnight walking trip’) by Michelle Hayeur-Smith . (11)  It is unknown if this material does match clay fragments recovered at Hals.
    Working from an analysis and recommendations of team member Marcus Burnham, taken from his work on the Icelandic clay sample provided by Hayeur-Smith, we purchased enough components to match the Icelandic clay, and make up about 45 kg as dry powdered material. When extended with other additives, this is enough to construct our proposed furnace and leave a good amount left over to undertake a second build if required. Based on a suggestion from Peterson, small batches were prepared and tested to at least approximate furnace firing temperatures. This overall experiment is detailed in a separate report - ’Sticking To It - A clay mix for Icelandic Furnaces’. (12)

    It is our feeling that if the original workers at Hals were able to utilize this clay, shortage of supply would not actually be a problem. Depending on details of the build, our typical free standing ‘Norse Short Shaft’ (f) furnaces require anything from 30 - 40 litres of prepared clay mixture. Unless clay was  immediately close to hand, the Viking Age gathering method would certainly have been by ‘the wagon load’.

Phase Three

Our intentions for this, phase three of the overall Hals Icelandic experimental series:
  1. Make a new consideration of the remains at Hals, considering what this suggests (to us) about both a detailed furnace build and smelting method, both individually and as a campaign over time.
  2. Undertake a full scale, free standing, construction, base on current understanding. Specifically, will a turf cone alone provide enough support for the furnace (ie: without the timber box and earth fill)
  3. Use the same kind of components suggested at Hals - but as are available locally
  4. Proceed in a measured pace, with full documentation of the process.
  5. Primary Test 1 : The function of simulated clay body, based on the best possible duplication of the material recovered close to Hals.
  6. Primary Test 2 : How does this furnace perform with repairs and a second firing? In this case most likely the early October regular smelt date, which will introduce some ‘aging’ observations.
  7. Primary Test 3 : How does a well designed furnace, on the Hals pattern, endure over time? In this case an overwintering, with a repeat use 12 months after the original construction.
  8. Long Term Experiment : How will the furnace construction age over a series of winter freeze and summer rains, ideally observed over a decade (or more?).

Furnace Build :
    In anticipation of much later examination of the intended smelt experiments through a process of annual aging, a clean sand base at 100 by 175 cm had been prepared, cut down below natural ground level after removing the grass sod (g), to a depth of about 10 cm below current ground level.  One important aspect here is that no additional framing was required to establish these clearly straight line edges.
Initial sterile sand pad completed.

sand base

Prepared clean sand base area - length of timber laid at nominal ‘front’ if boxed construction.

    It is certainly important to note in the interpretation seen below, that the overall construction was primarily taken from furnace 3, while details of the front tuyere and extraction portions mainly from furnace 7.


Structures illustrated in original profile, as a possible working furnace.
Background grid roughly 10 cm squares.

    To get some concept of the possible original size and details for furnace 3, the scaled drawing above was prepared. The slumping into the shaft was straightened to vertical, and the silt layer corrected to horizontal. The shaft diameter is set to 28 cm, consistent to both the rough dimensions of existing slag bowls 5 / 6 / 7, and to match a metal form used for many furnace builds in the past. The lengths of individual turf layers were then mapped on to this framework, attempting to match  both lengths and tapering thickness. As seen in the profile, the inner three layers sat on a higher earthen layer than the outer two did. The turf lines were extended upwards, so the outside layer would remain roughly 10 cm thick at its upper margin. This suggested a total shaft height (incorrectly labeled ‘stack’ (h) above) estimated at 65 cm. These measurements would guide the actual construction.
    The new furnace construction was placed roughly centred side to side and slightly to one end on the sand base, to mimic the overall size suggested by Smith originally, and echoing the placement of furnace 1. The additional space, located to the ‘front’ of the furnace, would allow for some comparison to the ‘charcoal tongue’ feature recorded in the excavation (seen as light grey cross hatching to the east / bottom).


Showing the placement of the furnace shaft in the working area (in cm)

    As indicated from the plan drawing, an additional raised earth pad was created, this of screened ‘dirt’ (to remove stones via a 1.2 cm grid) laid to a rough depth of 7 cm. This pad was square during the first part of the build, framed with standard 2 x 4 lumber to ease construction. At this point the dirt pad was 94 cm north to south ( tuyere to rear) and 96 cm east west. This frame was removed once the lower part of the clay liner had been built up, and the placing of the first two layers of diagonally stacked sods. With the frame removed, the earth pad was cut and re-shaped from the original rectangle to a circular shape.

Based on the clay fire test results, we have decided to construct the furnace liner of the combination clay / sand / horse manure mix we have come to depend on.
Noted that there is no specific information available from Hals to support this specific mixture.
    As the clay liner had been built thinner and thinner during earlier experiments, it was found there was a sharp functional line between 4 and 3 cm thickness, in terms of keeping clay sticking together to create a wall, and not sticking to hands and pulling free during the build. This was especially a problem in experiment 8, where the sod layers were placed first, and the clay was plastered against the exposed dirt and grass surfaces. For simplicity during this clay build, the walls were constructed using a ‘two fingers’ guide to allow for consistency. (14) Measured later, this thickness was 4.5 cm.


During the clay build : ‘two fingers’ thickness.
    Much past experience has certainly proved it is most effective to push slabs of clay against an internal form. The individual turf layers are then placed against the outside surface as the liner is built up. The supporting form (depending on type used) (14) is removed at some point to allow for the expected shrinking of the clay as it drys. In the past the interior is given support at this point by the addition of a fill mixture of dry sand and wood ash. We consider this the most likely overall build sequence for the furnace itself. If the full timber frame and earth fill was used, the frame would be constructed after the full sod cone was established, the earth fill added last.


Positioning of the individual sod layers.

    In keeping with the structure indicated by the profile at Hals, a total of five full layers of sod were laid. There needed to be an additional small block of sod placed between the inner most, which was basically vertical to the liner, to create the start of the desired diagonal lines. The first batch of mixed clay proved enough to build the liner to 35 cm height.
    Once the clay was constructed to this level, provision for the front extraction arch (and possible slag tapping) had to be made. Working from the excavation diagram and the one photograph available, a set of individual stones were selected that most closely resembled the sizes and shapes of those indicated. These stones were gniess (basalt is not available in Central Ontario), with fairly flat top and bottom surfaces which made arranging them much easier. One larger slab was positioned to frame the open arch created, serving as a needed support for the sod layers which would be placed above to cover the front wall of the furnace. This stone would also serve as a the inner support for the tuyere, to be added later. It should be noted that all these stones were placed outside the clay liner, and so were not expected to exposed to much (if any) heat damage effects. This arrangement would also not allow for any slag from the developing slag bowl to adhere to the surfaces.

first build

First build level completed
35 cm liner / stones framing arch / 2 layers sod

    Once these stones were placed, the first two layers of supporting sod were positioned. The metal form used to control the interior diameter is only 40 cm tall, so it was pulled up and clear. Next the exposed clay interior was filled with the described sand/ash mixture, primarily for support of the structure. Once filled, the metal form was re-positioned for the next build layer, and a second batch of clay mix was prepared. The overall sequence of steps was repeated, eventually leading to the clay liner built up to a total of 65 cm shaft height. A third layer of sod was positioned, then the wooden frame was removed. The earth pad was cut to conform to the slightly offset circular shape of the sods. Next the last two layers of sod were laid. The initial build work was completed by again removing the metal form, and completely filling the shaft interior with more sand/ash mix. This structure was left for two days to allow for a first drying phase.


Overall measurements – top view

    In this construction, the sods used were loosely of two different vegetation types, as cut from locations around the property at Wareham, all to a rough thickness of 10 cm. First was material composed primarily of what had been originally (some 30 years ago!) commercially available ‘grass sod’. This had been little maintained, so also included clover and random Ontario weed plants. This material would form the interior three layers. The outer two layers were composed of primarily ‘quack’ or ‘twitch’ grass (16), which has a much different root size and density. The total amount of cut sods used was roughly 11 square metres.

full build

Build as completed (before air system installed).

    After the time allowed for some drying and hardening of the clay, the ash sand mix was carefully scooped from the inside. Some shrinking of the total wall height was obvious, as well as settling of the sod layers themselves. Using a dry wall saw, the clay was cut into along the line available by the interior of the framing stones. This created an available extraction arch opening 20 cm tall by 23 wide. Although a cut line was created, the section of clay wall was left completely in place. A smaller opening was cut into the bottom centre, creating a port for the slag tapping that was expected to be required. (17) This was a distinctive upside down U shape, 8 cm tall by 6 cm wide. The clay here was pulled free, and slightly re-shaped to allow it to be removed or returned as might be needed.
    The total amount of clay mixture used in the construction of the liner is mathematically calculated as 35 litres volume. The clay component alone, measured as dry weight, was 19.5 kg. As mentioned earlier, the clay was mixed dry wth equal volumes (judged by eye) of sand and shredded horse manure, but no specific measurements of those materials were made separately.
    A quantity of timber had been gathered and prepared, as ‘log’ pieces cut to roughly 2 meters and averaging 10 cm diameter. As well a suitable volume of raw earth had been dug and bagged, roughly 240 litres total. As discussed, none of this material was actually used for this specific build.

    Some attempt to keep time / labour records was made. As all this work was undertaken by a single individual, although well experienced, the time indicated is ‘person hours’. It should be noted that digging was all undertaken using modern steel shovels an other tools.
•    There was no record made of the cutting and hauling sod strips.
•    Hand shredding the dry horse manure used took one hour.
•    Hand mixing and preparing the clay mixture took two hours
•    The total time for setting the earth pad, building the clay liner, setting the stone supports and applying the sod layers, was about 9 hours.
This puts the total build time for this furnace at 13 ‘person hours’ (importantly, not including gathering materials).
•    Although time for digging the earth fill was recorded, this remains highly dependant on the difficult conditions at Wareham (soil typically at least 1/3 rock and stone), and this material was not used.

    The next step in preparation was installing a suitable tuyere and cutting the extraction port and small tapping arch. As the intent of this experiment was to test the simulated Icelandic clay mixture, it was decided to use the proven forged copper tuyere. The possible inclusion of some variation of the blow hole method, although possibly indicted in the archaeology, was considered an additional complication at this point in our testing.


Front elevation, showing finished measurements and tuyere placement

    To reduce as much as possible strain on the clay liner, the heavy copper tuyere was placed so its bottom edge would rest on the top of the lintel slab. This places the tuyere above the extraction arch, where our standard is mounting the air input at right angles to the arch. In keeping with our past experience, the tuyere was set with an overall downwards angle of 22 degrees and with the iinner tip placed extending 5 cm proud of the interior wall. On measuring the distance this placed the tuyere above the existing hard base of the furnace, this distance was found to be an acceptable 23 cm. Unfortunately, with the slightly reduced total shaft height now 63 cm, this allowed for only 40 cm of functional stack distance. This has come to be considered a bare minimum for effective ore reduction in furnaces of this size and type. So to ensure working furnace stack, and to provide a bit of extra against expected complications, an additional 10 cm of clay wall was added to the top of the liner. This material would stand well above the existing line of the diagonal sods however.


Air system in place, at first addition of charcoal.

The final connection to the air system was made using our standard set (obviously modern!) of steel pipe fittings and hose to the electric blower supplying air. (18) To allow for a clear working space to the tapping port and extraction arch, these fittings were hung from a set of steel rods, with uprights located to either side of the front ‘slot’ in the stacked sods. Although certainly differing in detail from the Norse use of a human powered bellows system, it is our feeling that tubes made of heavy leather could easily allow for the same type of offset to air supply.

    Two final elements should be mentioned under this report on preparation for the smelt itself. Both are considered deviations from those used at Hals.
    Our decision was to proceed with our standard, proven, DD1 analog mix for ore (Fe2O3 red oxide + 10% flour as binder).
    Further, we will continue undertake test smelts with roughly 25 - 30 kg ore amounts, plus additional iron rich slag (3 - 5 kg) at the start (to set a working slag bowl system). This sequence has proven it's effectiveness, and also if you can make a 3 to 5 kg bloom, you certainly can make an 8 - 10 kg one.
    The second variation is the type of charcoal being utilized. The most likely species used at Hals would be birch, although it does need to be pointed out that identification of exact type was never undertaken. The experimental work here has used commercially available hardwood charcoal, either maple or oak, as available. For this specific smelt, oak was used. There is a clear relationship between wood species and charcoal density, the same volume of oak containing more contributing carbon than the same volume of (lighter) birch. This suggests that to provide the same reactive chemistry, a slightly stack height would be required for a furnace using all birch charcoal. (It must be noted that no specific experimental tests have been undertaken of this to our knowledge.)

Hals Wareham

boxed cone ? cone

clay liner ? clay liner

shaft ID 25 - 35 cm 28 cm

shaft height plus 40 ? 70

liner thickness plus 3 cm 4.5 cm
build materials clay ?

local approximation

additives  ?  sand / manure

sand type basalt granite

stone basalt gniess

grass strips Icelandic 'turf' Ontario 'sod'
? copper
air supply
bellows +? electric
slag control
? tapping
bottom? bottom
ore type source primary bog analog

oxide FeO(OH) Fe2O3

Fe content ? 55%

silica ? 14%

volume ? 25 kg
birch oak

Comparing Hals to Experiment 9 (Phase 3-A)


a) SLAG BOWL - a dish shaped structure composed primarily of once melted iron rich slag. This is likely to have a depression where the iron bloom itself was pulled free, this depression is formed towards the insertion point of air into the furnace. The slag bowl may have this side broken away, which is most typical of a bottom side extraction of the bloom. (This is seen in the remaining slag bowls excavated at Hals) Some estimate of bloom size can be made by measuring this depression. The slag bowl will also contain pieces of charcoal towards the edges and most certainly on the bottom surface. There are likely to be fragments of reduced iron still remaining, increasing in concentration towards the bloom side depression.
b) SHAFT - the internal height of a furnace, measured from cleared bottom level upwards. The shape of the shaft is most typically a cylinder or a slightly tapering conical section which would narrow towards the top.
c) TURF - used here specifically to refer to the type of ground cover existing at Hals. This will be the upper growth surface with the roots, lifted or cut free of the soil beneath.
d) TAP SLAG - a distinctive, dark black, solid, iron rich slag, very fluid at temperature. Depending on furnace design and wall materials, and especially silica content of the iron ore used, large quantities of this material can be produced. Especially as produced in the later stages over the course of a working smelt, it is often necessary to drain off excess levels of liquid slag, to keep from ‘drowning’ the air blast. Commonly, this means poking a hole into the side of the slag bowl and let the slag run out of the furnace. This creates either long ‘fingers’, or larger pieces composted of rounded layers as the slag congeals. These larger layered masses are referred to as PILLOW SLAG.
f) NORSE SHORT SHAFT – Based on various Viking Age types, a fairly standard furnace build, typically free standing. Clay or clay mix material, walls in the range of 7 – 8 cm thick at the base, tapering to about 5 cm at the top. Normally cylindrical, between 25 – 30 cm ID, total height between 60 – 70 cm.
See :
g) SOD – Used intentionally here to refer to the form of ground cover existing at the work location at Central Ontario. Even at Wareham, two different root structures are created by the natural  ground cover plants, which in turn have differing  physical characteristics. The grass sod material used for experiments here is not expected to be a good match for the ‘turf’ structure existing at Hals in Iceland (the product of an extremely different geography and environment).
h) STACK - the internal height of a furnace, measured from the tuyere (air input) to the top. There is typically some distance below this to contact possible packing, the slag bowl system, and sometimes a gap between the top of the bloom and the actual tuyere.

Notes :

1) Smith, K.P., 2005, 'Ore, Fire, Hammer, Sickle: Iron Production in Viking Age and Early Medieval Iceland', AVISTA Studies in the History of Medieval Technology, Science, and Art, Volume 4, USA
Also available as PDF on line :
Individual page references :
1a) page 192
1b) page 190
1c) page 190
1d) page 191, measurements via a personal e-mail communication
1e) page 193
1f) page 205
1g) page 190

2) Originally published on line, Fall 2007 :

3) A overview of both the Hals site and these experiments is currently under preparation, co authored by Smith, Markewitz and Peterson (‘Now with 70% Less Clay! Experiments with Viking Age Icelandic Turf walled Iron Smelting Furnaces’) A short video overview was presented at the recent EAC 12 virtual conference, available on line :

4) The full documentation for each individual experiment can be found on the web site :
The individual data sets for each of the Icelandic series is colour coded.

5) Repeatedly over this series, and in conversations related to Viking Age iron smelting in Iceland, one significant factor has repeatedly arisen. Markewitz has never been to Iceland. Peterson has made several visits, but has never been to the actual site at Hals.

6) A number of experiments in phase one (and additional, non-related smelts) used this blow hole arrangement, but with a gap between the tuyere tip and the hole. This team has not experimented with placing the air tube directly against the exterior furnace wall.

7) Copper tuyeres, forged from heavy plate (.5 cm range) were introduced to the (North American) iron smelting community by Lee Sauder about 2005, with the DARC team starting to use one in 2012. Pure copper melts at roughly 1085 C (depending on alloy), so within the operating temperature range of an iron smelting furnace. The radiation effect mentioned is crucial, but with the outside surface exposed to open air, there is virtually no erosion effect to the heavy copper, even over dozens of smelt cycles.
The melting point of wrought iron is higher, at 1540 C (although the functional ‘burning’ temperature in an air blast is significantly lower). Sauder once again has pioneered the forging and use of a tuyere made of this material for smelting furnaces. To date, with less tests described, a wrought iron tuyere appears to suffer some damage through a full smelting cycle, but certainly has been found to be robust enough to provide good duration of use.  (Note that the DARC team has not worked with a wrought iron tuyere.)
See :

8) It is well understood that the ‘bog ore analog’ material that is used for this experimental work is only at best ‘approximately’ like the primary bog iron ore (FeO-OH) recovered at Hals :
- Fe2O3 oxide (Spanish Red) with 10% flour binder
- Typically about 55 % iron content overall.
- Silica content of about 14 %
The typical practice, throughout ancient Northern European iron smelting, was to pre-roast natural bog iron ores, a process that would convert the oxide form overall into Fe2O3, and evidence of this process was found at Hals. (1g) Again it needs to be remembered that an actual analysis of the iron ore utilized at Hals was not possible to have been made originally. A consideration of comparing elemental iron content to other ‘reported’ Icelandic primary bog ores can be found :

9) See detailed description of Phase Two, Experiment 8 :

10) Rough estimate based on :
•    200 x 200 cm outside dimension
•    earth fill to 65 cm
•    less 13% for access gap, based on June 2021 build measurements

11) Information provided over a long duration series of e-mail communications between Kevin Smith, Michelle Hayeur-Smith, Neil Peterson and myself.

12) A detailed report available as a blog posting :
Note that the details of analysis and mix proportions are being withheld until a formal publication of this research can be offered.

13) Both theoretical, practical and past measurements have documented that working temperatures within a functioning iron smelting furnace will certainly reach 1150 – 1250 C. Temperatures in excess of 1350 C have been recorded. The use of testing only to 1070 was a restriction created by the top temperature of the propane forge used for the heating here.

14) Detailed in a separate report : ‘Stacking Up – on constructing clay furnace walls’. :

15) When constructing free standing clay furnaces, most typical builds have tapering walls, thicker at the base for additional support of the thinner upper sections. The standard build is 7 – 8 cm thick at the base, tapering to 5 cm at the top. In discussions, Peterson pointed out that there was no particular reason why the clay walls needed to be uniform in thickness in cross section. What he suggested is building so the interior diameter was slightly offset to the exterior, creating a thicker front wall, at the tuyere, than at the rear section. This would provide for the known erosion effect of the hottest part of the furnace, in the past normally seen as an oval shape, extending 10 cm to ether side of the tuyere, 8 – 10 cm below that point, and reaching upwards about 15 cm.
This proposed design has yet to be field tested.

16) This is an extremely aggressive, initially invasive, species found throughout Canada. It has thick roots, but more widely spaced, which quickly choke out other plants once established. One clear difference is that a cut strip of twitch grass will not hold together if picked up from one edge, so the individual sods do not retain the same relative structural strength when applied as a building material as was the case here. (see also definitions c and g above)

17) It is unknown what, if any, silica (as sand) might have been present in the natural bog iron ore available and used at Hals. Since the slag is composed of silica from the ore and melted inner walls, obviously our test may not closely resemble the volume of slag generated by the Norse, and thus the whole dynamics of slag control. From past repeated use of our DD1 analog, it was fully expected that slag tapping would be required.
18) The whole question of how well any electric blower system can simulate probable Viking Age bellows produced air is considered an entirely separate area of investigation. Although a number of potential Norse style bellows have been tested in past experiments, these all require a considerable number of additional workers, beyond the smelt team itself, to operate effectively. See ‘An Iron Smelt in Vinland’ :

Unless otherwise credited
Images and Text © 2021 Darrell Markewitz