Sunday, June 27, 2021

Icelandic Clay Mix Test - the full smelt

The following is a fast overview of the bloomery iron smelt at Wareham, June 19.

This experiment has the DARC team return to our experimental work based on elements of the 'industrial' Viking Age site at Hals, Iceland, originally excavated by Kevin P. Smith (1). The earlier work can be loosely divided into two phases (2) :

- Phase One / four experiments / 2007 and 2008 / testing individual design elements

- Phase Two / four experiments / 2012 to 2016 / testing use of turf builds, combining elements from phase 1

The current work, as Phase Three, centres on:

a) testing clay mixtures based on those from Iceland

b) testing durability of a turf (sod) supported furnace structure over time

An earlier blog post ("Sticking to It - a clay mix for Icelandic?") detailed the logic behind the clay mixture itself.

Part way through the build process

Based on experience from the earlier furnaces of this series, the wall thickness was set at 4.5 cm, measured against 'two fingers' as shown. An internal form was used, metal in this case, keeping the interior diameter consistent to 28 cm. (This replacing what could have been a wooden straight sided 'barrel' form historically). The clay was constructed to the rough top of the form (about 30 + cm), then the supporting cut sod strips were placed in a circle on the outside. The form was pulled upwards, with the exposed interior filled with a mix of half sand / wood ash. This method has been used many times, the ash mix both supports the thin clay and also helps to dry the walls. 

Built at 35 cm

After the first full course was established, a set of small stones, used like bricks, were placed to frame an extraction arch for the front of the furnace. One large piece of basalt on hand was used as a lintel to eventually support the tuyere and upper sod pieces.

With air system, at first charcoal addition

The build continued to raise the total stack height to 75 cm. The supporting sods were laid in five layers, to a height of roughly 70 cm (at the start of the smelt). The cone created was a bit irregular, extending out 72 cm to the left side as seen, to 56 cm to the right side. Although materials (timber and earth) had been labouriously gathered, it was decided not to box the sod cone and back-fill to a flat upper surface.

The extraction arch was 23 cm wide at the furnace wall, opening to 33 cm at the edges of the framing stones. The lintel slab sat on a slight diagonal, at roughly 20 cm high. There was additionally a small tapping arch cut, 6 cm at the base and 8 cm above the hard base. 

There was no specific attempt to match the air system to evidence from Hals, we chose to use our proven heavy copper tube, inset 5 cm beyond the interior wall. As supported on the lintel slab, and with our proven 22 degree down angle, this set the tuyere tip a bit high in the furnace. In turn this reduced the effective stack height to only 40 cm (normally considered a minimum distance). As this position would also leave a lot of space under the tuyere, a 'soft base' was created with wood ash from the drying fire and a layer of charcoal fines on top, raising the set base to 15 cm below the tuyere. The simplest method to support the normal viewing port and air input connection was to suspend it from a metal rod set well clear of the working opening in the sod cone. 

The primary smelting team :

Darrell Markewitz - smelt master

Neil Peterson - ore and records

Rey Cogswell - charcoal / ore

Kay Burnham - charcoal / ore

Richard Schwitzer - compaction

Kelly Probyn-Smith - safety

Travis Sweet - photography

Slag tap. Earth falling free of the upper sods can be seen as light reddish pieces.

Over the 7 1/2 hours of the main smelt sequence, it proved necessary to make at least four major slag taps to lower internal slag levels threatening to 'drown' the air blast. 

Later into the smelt, it was clear that furnace heat was seriously effecting the sod structure. Earth was baking dry, and pulling free from the root systems and falling downwards between any cracks. This would cause the whole sod cone to slump into itself by the end. There was no venting of furnace gases observed however.

Air flow had been purposefully reduced for this smelt, down to a range considered possible for a human operated bellows of Norse type. Although the long serving electric blower was used, air flow was initially set to roughly 700 LpM, increased about the mid way point to 800 Lpm. (3) This would create an average burn rate at 8 minutes per kg over the main sequence. 

Initially 5 kg of previously gathered iron rich tap slag was added to establish a working slag bowl system. This may have also contributed to the need for later tapping. A total of 25 kg of the DD1 analog ore (51% Fe) was used with combined average addition rate of 11.8 minutes per kg.

Pulling the bloom mass free. Image by Travis Sweet

The bloom mas proved much larger than expected! In pulling it clear of the furnace, the supporting lintel stone was pulled away, resulting in the upper sod layers at the front of the furnace also collapsing, exposing the front furnace liner.

Classic compaction shot - Darrell holding, Neil on sledge : Image by Travis Sweet
With loose gromps hammered clear, before cutting on the press : Image by Travis Sweet
To everyone's great surprise, after some first stage compaction and rough cutting, the resulting bloom, although a bit loose in texture, weighed in at 8.9 kg. This represents a 35% return from ore (4).


Furnace, just after extraction / compaction, with remaining slag bowl seen.

On a quick examination of the furnace (after our ritual post smelt Guinness) :

- The furnace still contained about 2/3 of the slag bowl, completely filling the interior save for a large piece broken away at the extraction arch to expose the bloom, creating a clear C shape. The depression towards the air blast side clearly frames where the bloom was pulled free. The slag bowl remaining is of a size and shape quite similar to those exposed at Hals.

- The front portion of the furnace has clearly broken away, when the supporting stone lintel pulled off, as the bloom mass was finally pulled free. The image above shows it's natural fall position. The sod that had been above this has pretty much totally crumbled away into it's containing dirt.

- There is considerable slumping of the encasing sod cone. The clay liner had actually lifted slightly several times while working the bloom free, but had remained as a solid structure overall. 

Largest of the roughly compressed and cut bloom pieces : 3 kg

to come : looking closely at how the clay walls survived the smelt


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 :

2) 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 :

3) For a furnace at 28 cm, the normal air flow would be set to closer to 900 - 1000 Lpm. see : Air Flow Rates

4) This impressive result may be the effect of the 5 kg of iron slag added as a first step. Our past results for smelts in the 35 - 30 kg ore range have more typically been roughly 5 kg (or less).  

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February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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