'Sticking to It' - a clay mix for Icelandic?

 Part of our ongoing experimental series investigating a possible bloomery iron smelting furnace, as suggested by the archaeology of the site at Hals, Iceland. ( 1 )

The clay mix being tested was based on an 'in team' analysis of materials gathered by Michelle Hayeur-Smith from a natural deposit 'fairly close' to Hals. (Noting that it remains unknown if this was the clay material actually utilized for the furnace builds there). ( 2 ). Team member Marcus Burnham suggested a set of components available at Pottery Supply House, which would in combination approximate at least the chemistry of the Icelandic sample. ( 3 )

These individual components were dry mixed by hand, giving roughly 50 kg of the clay powder (hopefully enough for two builds with reduced wall thickness). ( 4 )

I made up a total of four small batches for next step testing :
- clay mix with water
- wet clay mix plus powdered slag (66 / 33 slag) - this 'copper shot' sand blast slag ( 5 )
- wet clay mix plus fine sand (50 / 50 by weight)
- dry mix / fine sand / shredded horse manure by volume (ie our current proportions using the EPK clay base)

In each case, the various mixtures of materials holds together well, without being too 'sticky', and all are nicely plastic.
It is not expected to have any physical problems building a furnace wall structure with any of these mixes.

The individual mixtures were pressed into small cylinders, filling a standard toilet paper cardboard tube. Each 4 cm diameter by 10 cm long.

- These were placed in a toaster oven set (very roughly) for 90C, over a duration of about 4 hours total.
I'm not sure that temperature was actually reached, because when I pulled them out, I could still handle each with bare (blacksmith's!) hands.
The centre part of the paper cylinders could be depressed - suggesting there was still water remaining. This thought to be the effect of the paper not easily passing moisture (?) .
- The cylinders were then placed in the rear section of my gas forge on a metal tray. (At top operating adjustment, this forge has been found to produce temperatures into the 1100 C range) The forge was lit, with a flow at a greatly reduced level.

Showing results of first (shortened) heat cycle.
Number 1 to left, through to number 4 to right

The material was heated enough to burn off the paper on the upper surface, but this remained solid and attached on the lower side. This suggests the the top had to reach at least 230 C, but the bottom surface (furthest from flame) did not get that hot.
You can see clearly that cylinder 1 - the straight Icelandic mix, has already crumbled.
Cylinder 2 - wet mix plus slag, has also become brittle.
Cylinder 3 (wet / sand) is showing cracking - but remains whole
Cylinder 4 (dry/sand/manure) remains unaffected (that lower crack seen was from
the initial packing of the material)
As it turned out, the forge propane ran out after a short time (20 minutes?). So in this it is hard to estimate just how hot the forge interior would have gotten.
The cylinders were allowed to sit in place inside the forge (so slow cool), overnight.

The following day I refilled all my propane cylinders, then repeated the test.
Again the tray was placed at the very rear of the forge as it was lit. A low propane rate to start. The material was left in place for about 30 minutes (until the forge interior was at full heat) then the tray moved forward to the centre, under the gas jets (where I normally would place metal to heat). ( 6 )

Temperature probe was inserted into the gap between # 3 and #4 (to right).

After 55 minutes total, a temperature reading was taken, the probe wire between the cylinders, tucked slightly underneath. Reading was 980 C.
At that point, you can see that cylinder 1 is composed of crumbled pieces.

The forge was then adjusted for higher temperature (increased gas flow and air combination).
The materials were subjected to another 85 minutes at this temperature, which was measured again to 1070C.

At end of second heating period (open door for image has cooled interior slightly).

You can see that the surface of cylinder 1 has slumped and started to fuse.
Cylinder 2 has developed some serious breaks.

The forge was turned off - and left overnight with the door closed to slowly cool. 

Cylinder 1 (dry mix with water) - has melted
Cylinder 2 (wet / 33% slag) has badly cracked
Cylinder 3 (wet / 50% sand) has developed a number of cracks, but mainly towards the surface. A bit of surface fusing (point directly under the gas jet)
Cylinder 4 (dry mix / sand / manure) shows no significant effects (again, bottom piece was separate at the start)

I took each of # 2 / # 3 / # 4, and applied pressure by flexing between my hands. I attempted to keep the force as close as possible - and applied to what I judged a 'reasonable expectation' level of pressure.
 

( Cylinder 1 - only the small disk seen earlier could be handled, the result was it easily crumbled )
Cylinder 2 - not much force was required to break along the large cracks - you can see from the dark coloured interior, this crack had broken completely through during the firing.
Cylinder 3 - held together effectively, despite surface cracks
Cylinder 4 - no effect to reasonable force (earlier separated piece shows internal texture)

The conclusion I draw :

Sample #1, our potential Icelandic clay mix alone, is not able to survive temperatures even at the lower end of expected iron smelting range. Combined with it's relative fragility even during the early heating cycle of a furnace, this material on its own is not deemed suitable for furnace construction (without modifications).

Sample #2, with 2/3 wet clay / 1/3 slag (by weight), was considered highly speculative at best. (Included mainly because I had the material!) This offering one possible version of 'Where did the silica come from?'. Iron rich slag could have been on hand (at least after a first smelt attempt). In the test, this mix proved relatively fragile. It might be possible functionally to use this mix, with greater care in mixing, construction and drying. Generally, this is considered quite unlikely to have been used.

Sample #3, with 50% wet clay / 50% fine sand, certainly would be an effective build material. First note is that the sand used here is of unknown composition, but likely Ontario granite based at about 70% silica (higher than Icelandic basalt based). It has been well demonstrated (Sauder) that high sand mixes can better withstand iron furnace temperatures, but at the same time do require more care during the construction phase to eliminate just the kind of surface cracking seen in the test sample. This is considered a possible effective mixture for the Hals build.

Sample #4, using our developed standard of equal dry volumes of clay mix / fine sand / shredded horse manure, was clearly provided the best properties of the four mixes tested. As proven in the past, the sand provides heat resistance and stability at temperature, the horse manure re-enforcement during the build and drying phases. Even with what is demonstrated as a significantly lower point clay material, this is considered the 'best possible' mixture to use for the upcoming full build test.

1) The original description of the Viking Age, 'industrial' level iron smelting site at Hals was provided by Kevin P. Smith in his 'Ore, Fire, Hammer, Sickle : Iron Production in Viking Age and Early Medieval Iceland' The earliest evaluation towards evolving a working system can be found in 'Working Towards a Viking Age Icelandic Smelt'.
Currently Smith, Peterson and myself are preparing a detailed description of the 2007 - 2016 experimental series, to flesh out our recent presentation 'Now with 70% Less Clay' which was delivered at the 2021 EAC12 Conference.

2) Unfortunately, for a number of reasons, an actual analysis of the any of the clay furnace walls recovered during excavations is available. Such tests were not undertaken at the time, not being within the scope of the original project there. At this point (after some 20 years!) is is quite unlikely to be able to arrange for this currently

3) With the kind financial assistance of Neil Peterson, who donated funds to cover the purchase of the clay materials indicated, plus charcoal to supply the next three full smelts in the current experimental series. 

4) Sorry. Due to problems experienced over the last several years of hard won information derived from the DARC team experimental work, I am intentionally holding back on listing the exact details here. A further paper is under construction, describing our newest interpretation of the excavations at Hals, and how this applies to our current experimental series over 2021 - 2022.

5) This material is composed of 'medium' sized particles, primary components are 53-60% (Iron Oxide), 32-37% (Silicon Dioxide). 

6) Readers need to remember that the camera records light and colour of heated surfaces differently from the human eye. See an earlier blog post.