'Evidence of Absence' : The
Erosion of Bloomery Furnaces
Being a loose draft for what might become a formal study?
(latest edit - June 1, 2021)
One of the huge problems in practical attempts to use
archaeological evidence of ancient bloomery iron smelting
furnaces is simply - not much of those furnaces survive.
In most cases, all that can be expected to be recovered,
after hundreds (if not thousands) of years, will be the vary
base level of the furnace, basically the slag bowl below
tuyere level, with perhaps the base level wall section that
surrounded that. Usually there is no sign of the tuyere, or
any indication of how it might have been mounted. If the
walls had been constructed some clay mixture, there may be
some indication by way of that inner part of the walls that
had reached high enough temperatures to sinter into a
ceramic. The action of rain has simply washed away any raw
clay that composed the outer section of the original walls.
Even the sintered portions are sure to have been subjected
to years of alternating freeze and thaw cycles, further
fragmenting even these remains.
Right : A series of images showing the stages of
excavation of one of the furnaces at Skogar, Iceland. This
furnace, showing only about the bottom 30 cm of height
recovered and its clear lower base construction of stacked
stones, served as the conceptual prototype for the
experimental furnace seen below. (1)
Image taken from : St.
Sigur∂arson, G., & Zoëga, G., 2013,
'Skógar í Fnjóskadal - Fornleifarannsókn 2011-2012',
Bygg∂asafn Skagfir∂inga Rannsóknaskýrslur, 2013/140 :
page 29 (available
as a PDF)
Condition of the stone block furnace after
the winter of 2019-20, at preheat of the November 2020
smelt. (View to the west.)
Remains of the same furnace, after the
breaking down for extraction, and left over the winter of
2020-21. (View to the south.)
The images above make a dramatic example. This specific furnace,
part of the ongoing Icelandic series, was constructed originally in
June of 2019.
One important difference between this test furnace and the prototype
from Skogar is that the ancient remains are only of a shallow, stone
lined pit, below ground level, with no superstructure recovered.
This would be a 'slag room' type in operation (slag draining into
this lower chamber). The test furnace was built totally above ground
level, and is a slag tapping type. It had been used for a total of
five complete smelts, the last being the '65 for 65' in
October 2020. The furnace had been protected by wrapping it in
plastic over the winter of 2019 - 2020. For the extraction during
'65 for 65', the upper portion of the furnace was broken down by
pulling off each of the hot stones, which were placed clear of the
working area. This dismantled the furnace down to the top of the
slag bowl (roughly just below tuyere level). The jumble of stones
remaining show heat effects, most with slag attached to their inner
surfaces. The slag bowl itself is mainly intact, with the depression
inside where the bloom had been pulled free.
Our experimental work at Wareham is typically investigating
variations on furnace layout, typically related to specific historic
furnaces or methods. To that end, most typically a given furnace
build is often only used once or twice. (2).
As I have commented on many times over the years, despite millennia
of human iron making, involving what most certainly must be tens of
thousands of furnaces, it is quite rare to find much more than a
slag bowl (relatively in-destructible glass) surrounded by a ring of
stones or vague circle of clay. (3)
But just how does a furnace, after it is
abandoned, disintegrate over time?
Well, I had any number of used furnaces at Wareham, and have set
about undertaking at least some loose observations. So far this has
involved a total of seven individual furnaces, built using a number
of differing materials. (4)
Group of aging furnace shafts (May 24. 2021).
The two oldest furnaces ( 2004 / 2004) are located separately.
Also not visible is June 2018, hidden behind 'DD1 Test'.
TABLE : Furnaces Under Erosion
Comparison images of use, as moved, current condition Links back to original smelt reports
- mix not recorded
(likely chopped straw cobb)
- thickness not recorded
- fired 3 1/3 hours
- complete, moved
- clay / sand / manure
(in metal shell)
- EPK with shredded horse - 5 cm as fired
- 3 uses of furnace
- fired 18 hours total
- base walls and slag pit mass,
- clay / sand / manure
- EPK with shredded horse
- 7 cm as fired
- fired 6 hours,
- base and broken wall segments,
- stone block / clay fill
- gneiss / EPK mix
- variable thickness (+ 10 cm)
- 5 uses of furnace
- fired 32 3/4 hours total
- base walls and slag bowl
mass before moved
larger pieces seen were moved image by Neil Peterson
all remains April 2021
image May 2021
moved - May 2021
upper seen above
lower right wall clay 5 cm + slag total 8 cm
lower wall at tuyere clay 3.8 cm + slag
total 6.5 cm
to far left seen above
upper wall segment 2 cm (top) / 2.7 cm (bottom)
This whole process was undertaken originally without much of a
What furnaces were moved to allow aging was fairly random,
based more on the condition of an individual furnace after end
use, rather than any attempt to select different wall
There are a sequence of images over successive years. This was
not done every year after a furnace was placed. Most images were
taken either April or May, after another winter exposure, but
before grass had obscured surrounding details.
Specific measurements (before the ones indicated here) were
not made. An attempt has been made to include those earlier
images with a scale.
There has never been any attempt to measure temperatures
reached through a cross section of clay wall during a smelting
Attempts have been made to measure
internal temperatures inside furnaces over a course of a
smelt. The results have to be considered spotty at best, largely
due to the quality of the measuring equipment available.
Attempts to measure temperatures are only sometimes made.
In any firing, internal temperatures into the range of 1150 - 1250 C
are certainly produced (to allow effective reduction of ore to
creation of a metallic bloom).
How this temperature is applied through the thickness of the clay
mixture walls remains unknown. Outside surface temperatures of the
furnaces are generally not measured.
Clays generally start to vitrify into ceramics starting in the 900 C
+ range, with the recommendation
to 'bisque' fire into about 1000 C. (5) Depending on the specific
clay (and additives) the optimum 'full firing' temperature is can be
+ 200 C (depending on the clay type).
In the chart above, the early use of naturally dug clays ('Blue
Mountain Red' and 'Albany Brown') proved a problem, as both of these
are a low fire temperature clays. There was thus excessive melting
of interior walls, resulting in large amounts of clogging high
silica slag. Although mainly done for reasons of reducing raw labour
(placed against low cost!) the switch to the higher firing 'Bell
Dark' commercial clay solved much of those problems. Durability and
price being considered, our standard build has become using the
'high fire' clay EPK, which normally would be fired to the 1200 -
1350 C range, again about the same internal temperatures we have
In conversation with my experimental partner, Neil Peterson, some
important considerations were raised :
- In any furnace firing, there is certainly a marked difference
between the internal wall surface (to, if not above the sintering
temperature) and the outside surface. The outer side of the wall is
not likely to be raised high enough (plus 870 C) to generate actual
sintering. The transition of temperatures through the wall thickness
will also graduate upwards in a tall furnace.
- There is likely a limit to the possible penetration through the
walls, even with repeated firings. It actually unlikely that those
furnaces detailed above that undertook multiple firings will have a
greater sintering effect than those with single uses. Most single
firings last an average of 4 - 5 hours at temperature.
The one visible exception to this was seen with the furnace used for
the 'hot swap' smelt. Here, one full cycle was undertaken to bloom
extraction, then a second working team immediately re-filled and
undertook a second full smelt sequence. This furnace ran over a 9
hour operation total, and one result was that a large hole was
burned through the wall around the highest temperature area at the
One of the things that becomes apparent in terms of erosion over
time is the effects of a freeze / thaw cycle against relative
porosity of specific mixtures (where water absorbed by the ceramic
freezes, greatly expands, then effectively shatters the material).
There are three main types :
- Straight Clay : seen in June 2004 / October 2007
The best effect of the use of sold clay is seen with the October
2007 build, when solid 'bricks' were cut from fully compacted clay
blocks. The solid composition is appearing to let very little
moisture penetrate the fully sintered ceramic, resulting in very
little frost damage, even with over a decade of exposure to Ontario
- Clay / Sand : (The standard used by Lee
Sauder, which takes more care in construction, but does result
in a very durable construction.)
- Clay / Organic Cobb (5) : A) Copped Straw : November 2005 /
June 2007 (as grass) / June 2008
Horse Manure : October 2018 / October 2018
The use of chopped straw leaves easily visible, fairly large
(several mm wide) 'tunnels' through the sintered wall material.
These most easily allow water to penetrate into the remaining walls,
which leads to shattering of the material with the effects of frost.
This penetration to expansion effect of freezing is less dramatic
with the use of shredded horse manure, where the individual organic
pieces are an order of magnitude smaller in length and diameter.
So, what is the point?
Section of the furnace wall after the November 2005 smelt.
Slag adhering to the inner surface, with a fragment of the ceramic
tube tuyere in place
The graduation in colour, indicating the progression from an
interior sintered ceramic (dark brown)
through to dried clay on the outside surfaces, is clearly seen.
neither standing furnace walls nor clay linings were preserved in situ, small
frost-shattered fragments of fire-hardened and vitrified silty
clay have been found scattered throughout the slag heap.At least one of these
preserved the edge of what appears to be a circular, vitrified
opening for a bellows nozzle. These, the first ceramic furnace
linings reported from Iceland, were recovered only by
water-screening bulk soil samples from the slag heaps.Although small, they
are clearly remnants of once more abundant, expediently
produced, fragile ceramic linings that have not withstood the
intense freeze-thaw cycles characteristic of the Icelandic
setting. The remnants of these ceramic linings from Háls are no
more than 1-3 centimeters thick and would certainly not
have allowed the furnaces to be self-standing.Nor do sections
through the furnace remnants suggest that thicker, unfired
clay walls were once present.However, slag lumps
found adhering to samples of this clay-like matrix as well as to
fire-reddened slabs of basalt suggest that both materials may
have been used to line the furnace shafts. " (6)
Bold added for my own emphasis
Taking reference from
the various eroding furnaces above :
Current Thickness (cm)
3 - 5.7
straw cobb ?
2 - 3.8
It is clear, that even over a 'short' time frame, that :
Fairly quickly, the effects of water on any exposed furnace
wall wash away any raw clay on outer wall surfaces.
There is a continuing fragmentation of exposed sintered walls,
especially those that are porous from the inclusion of organic
The effect of heat penetration causing sintering is not
dependent on starting wall thickness.
The depth of sintering may be relatively consistent.
These sintered remnants appear to be in roughly the
same thickness range as the fragments recovered at Hals.
One question most certainly is raised by the last (highlighted) part
of the quote above :
Nor do sections through the furnace remnants suggest that
thicker, unfired clay walls were once present."
I personally most
certainly lack training in, or experience with, archaeological
excavation technique. Saying that, is is unclear to me how easy it
would be for a field examination to tell the difference between what
originally had been 'plain dirt' and 'raw clay', most especially
after 1000 years of weather effects.
In our own test series attempting to replicate the 'framed turf
cone' system represented at Hals, one of our testing variables has
been to gradually reduce the thickness of the clay liner. It was
found at thickness less than 5 cm, there were significant problems
with both manipulating the clay itself, and in durability of that
liner, even over a single smelt cycle. At 3 cm, it became extremely
difficult to get the clay to actually stick in place against the
fragile dirt surface of the stacked turf pieces. (7) Although a
change in the build process may help to solve the construction
aspect, the serious problem of these thin clay walls failing (to
destruction) still remains. (8)
One clear indication from the examples presented may be that in fact
the original walls at Hals were built to a more functional 'usual'
thickness. It is the combined effects of a limited sintering
process, coupled with the combined effects of water and freezing
cycles, that are then most likely to have resulted in the small
diameter wall fragments recovered archaeologically.
1) Obviously, the Viking Age furnace (#23) from Skogar only remains
as the below ground construction. This furnace was operated as a
'slag room' type (a chamber excavated, well below tuyere level, to
contain any drained slag, rather than slag tapping). There is no
clear indications of the upper structure. So what originally had
been constructed was not necessarily built of stone blocks. The test
furnace was completely above ground level, and operated as a slag
tapping type. On complete excavation of the furnace after the last
use, our experimental furnace also had a ring of stones attached
around the final slag bowl (similar to that indicated in the
2) To date of writing, there have been a total of 49 smelts
undertaken here at Wareham. For these, a total of 37 individual
furnaces have been constructed. (If I have my count correct!) This
difference in approach is significant to that seen with most others
in the Early Iron movement here in North America, where most
typically the stress is on production, with furnaces built to
withstand as many uses as possible.
3) I would refer interested readers to the 'gold standard' - Radomir
Pleiner, 'Iron in Archaeology, the European Bloomery
Smelters'. Once almost impossible to find, this cornerstone
reference is now available as a free
PDF download (!!)
4) It could be considered 8 in total, if the stone block build seen
above could be included. That furnace has been recorded via scaled
and grid photographs in May 2021, but only after a single winter's
exposure. The plan is to clear off the remains, to free one of the
main working spaces in the smelting area for future experiments.
5) Getting easy access to hard data on the firing dynamics for a
specific clay body has turned out to be much more difficult (even in
the world of the internet) than expected. I found the simple and
direct commentaries provided by Leslie Milne at The Pottery Wheel
6) Adding organic materials provides three effects to the wall
structure. Depending on the size of the individual pieces,
especially during the initial drying and shrinking phase, the
organic material acts like re-bar in concrete, holding the clay
material together and reducing potential cracking. Next, as the
walls are first heated, remaining moisture flashes (explosively!) to
expanding steam. The holes inside chopped straw especially, gives
this steam some place to vent. As the wall material starts to heat
beyond the roughly 250 + C range, the organic material burns away,
leaving a somewhat porous texture. These air spaces create an
insulation effect. (Although it should be noted that the dynamics of
this effect has never been measured specifically.)
6) ' Ore, Fire, Hammer, Sickle: Iron Production in Viking Age
and Early Medieval Iceland ‘ Kevin P. Smith - 2004,
7) Details of the 8 experimental tests in this series are found in 'Now
with 70% Less Clay! Experiments with Viking Age, Icelandic, turf
walled iron smelting furnaces', Smith, Markewitz &
Peterson' (pending). This paper was prepared as part of the 2021
EAC12 Conference, with a video overview available : https://youtu.be/7Ltz5NG2BP0
The individual smelt reports are also published (marked in tan) on
the full documentation web site : www.warehamforge.ca/ironsmelting
8) A separate report dealing with potential build methods for the
clay walls, specifically as this relates to constructing the Hals
type furnace, is under preparation.