Weathering of Bloomery Iron Furnaces

'Evidence of Absence' : The Erosion of Bloomery Furnaces - Part Two

Smelt 90/D35 - Wind & Weathering, October 2021
Smelts 88/D33, 89/D34, 98/D40 - Icelandic Phase 3, June & September 2021, November 2024

This article is a more detailed look at two specific furnaces, intentionally left in their firing locations and allowed to decompose.
Through the images shown here, the standard placement is with the bottom of the photograph towards the north side of furnace.

Refer back to the wider recording of aging iron smelting furnaces HERE.
(original - May 2025)

" 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. " (1)

Smelt 90/D35 - Wind & Weathering, October 2021

The original intent of this experiment was two fold. First was to get a more accurate measurement of the air volumes produced by our current re-constructon of a potential smelting sized Norse type twin bellows. The second was to follow the erosion of a single use clay built iron smelting furnace on the 'Norse' styled short shaft model, over time.
    A detailed description of the furnace build and smelting process : HERE
    A detailed description of state of the furnace after use : HERE
    The furnace was built on a roughly 2 x 2 m wood framed sand pad, about 8.5 cm deep
    The furnace was constructed of roughly equal dry volume measures of EPK clay (2), course sand, and dry shreeded horse manure
    Furnace walls at firing were about 5 cm thick : see diagram
    This furnace was fired through our standard sequence, a total of 6 hours 40 minutes. Top temperatures measured were above 1350 ° C (3)

w&w

Part way through the smelt, showing cracking.

w&w

About the same view, showing breaking during extraction. Note colour shift at slag bowl level.

Spring 2022

W&W 22

Overall - view towards south, tuyere was on left

W&W 23a

Detail on east side (left)

W&W 23b

Detail on west side (right)

Spring 2023

W&W23

Overall - view towards south, tuyere was on left

Spring 2024

W&W 24

Overall - view towards south, tuyere was on left

Spring 2025

W&W 25

Composite image - view towards south, tuyere was on left
Individual detailed images below (note scales not aligned to north)

Smelts 88/D33, 89/D34, 98/D40 - Icelandic Phase 3, June & September 2021, November 2024

In our current work directed to replicating the Viking Age furnaces excavated at Hals, Iceland, by Kevin Smith :

" Although 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. " (4)

Bold added for my own emphasis

The 'Phase 3' furnace had a core construction used is a conical stack of cut grass sods supporting a cylindrical clay liner (see details). Importantly, the clay used here was an anlog mixture intended to mimic the components from a sample of Icelandic clay material. This was again mixed with beach sand and shredded dry horse manure (1:1:1). The walls of the interior clay liner were approximately 4.5 cm thick when built.
The inital build and first firing of this furnace was in June of 2021 (Smelt 88/D33). It was then allowed to weather over that summer and with considerable repairs was fired again in September 2021 (Smelt 89/D34). It was left exposed until fall of 2024, with a major re-build of the clay liner undertaken in October. It was fired again in November 2024 (Smelt 98/D40).
Assessing the effects of natural weathering to this furnace are more complex, due to the repeated firing and re-builds.

icelandA

June 2021 - Smelt 88/D33 at start (diagram)

icelandA2

At end of smelt, just after extraction

turtle

Turtle and damage next morning

The June 2021 smelt created a larger than expected bloom (at 8.9 kg / 35% yield) , the extraction of which had resulted in removal of the complete stone extraction arch stones (so up to the tuyere level, as seen above). In the early hours of the next morning, a 20 cm long snapping turtle had crawled into the still warm interior, resulting in further damage to the front of the clay liner (the red line seen is approximately the line of the clay before the turtle).

damageA

Weathering at August 2021

icelandB

September 21- Smelt 88/D33 as re-built

icelandB

At end of smelt, after extraction

Spring 2022

	View to West, front at right

Spring 2023

2023 all

2023 east

View to West, front at right

2023 top

Front to lower left corner

Spring 2024

2024 all

2024 close

2024 top

After Smelt 88, this furnace had been left standing to October 2024, through three Canadian winters (as seen above). The combination of large gap at the front of the furnace and slumping of the supporting sods resulted in the largely sintered clay liner titling forward through 23 °. In the attempt to straighten this, the liner broke into several large pieces. These were placed roughly back into their original vertical positions, secured with fresh clay and a new complete stone block front. (see details)

icelandC

Weathering at October 2024

icelandCstart

November 2024 - Smelt 98/D40 as re-built

icelandCend

Cold furnace, after extraction
(image rotated, front/tuyere at bottom)

There had been considerable damage to the furnance after Smelt 99/D40. This was primarily due to the secondary experiment, the use of low volume, plusing, air delivery. With the slag bowl sitting too high, and general failure of the existing clay liner, it was decided to abandon this furnace, but leave it in place for further weathering observations.

Spring 2025

	View to West, tuyere to right.
View with bottom to roughly SE, tuyere to right.	Composite from detailed images below

End Notes:

1) From the more general article : 'Evidence of Absence', The Erosion of Bloomery Furnaces, original 2021, revised 2025

2) EPK is a high temperature firing kaolin clay, the normal recommended firing range (for full sintering) is 1185 - 1240 ° C (cone 6)

3) The combination of digital pyrometer and theromocouples on hand fail at 1350 ° C. The measurements are taken with the probe tip inserted about 5 cm into the interior. Various academic sources suggest the true internal temperature of a working bloomery furnace are between 1400 - 1450 ° C. (R. Pleiner, 2000, 'Iron in Archaeology, the European Bloomery Smelters')

4) K.P. Smith, 2004, 'Ore, Fire, Hammer, Sickle: Iron Production in Viking Age and Early Medieval Iceland ‘, pg 13

Unless otherwise credited
Images and Text © 2021 + Darrell Markewitz