Thursday, June 12, 2014

Scottish 'Dark Ages' - Iron Smelt

For those of you who have following the fine details, I have been invited to lead a special project at the Scottish Sculpture Workshop this August.
SSW is about half way between Aberdeen and Inverness, in the small village of Lumsden (Lumsden, Huntly, Aberdeenshire AB54 4JN, UK)

Turf to Tools is a two week long collaboration project, running from August 9 - 25. The underlaying intent is to use locally available materials to smelt iron using historic methods, then take the iron created to produce a replica of a specific object - also based on a specific local historic source.

The target object is an axe, the one seen on the 'Rhynie Man' picture stone:
The stone is from nearby Rhynie, and is dated to roughly 700 AD (what was a major political centre at that time). The date makes it clearly 'Pictish' (post Roman and pre Norse).

What is likely the best reference for the iron smelting process is from the archaeological site of Culduthel, which is located at the SW corner of Inverness (about 125 km roughly NW). This site is a longer term occupation, but the main iron smelting activities appear to bracket 200 - 400 AD. There were multiple bloomery furnaces uncovered there.
I have not been able to get a lot of details on the furnace archaeology, but some hints from the partial report and summary I do have suggest:

1) Stone chamber at base - a 'slag room' system
2) Clay upper furnace (no specifics on composition)
3) Build over a wicker framework
4) About 30 - 35 cm interior base diameter
5) Most probable bottom extraction

As the size and general layout (and honestly, historic period) does match my main experimental practice, most of the working system prototype is like that earlier work:

For this prototype smelt :

1) Use of 'standard' ceramic tuyere
2) Electric blower for air

I was able to scrounge a wicker laundry basket to use for the interior form on the build. This is tapered, which is an advantage. This form is 60 cm tall, 25 at the top diameter, 38 at the bottom.
Since there is no specific reference made to cobb or organics added to the upper clay construction at Culduthel, the mix will be roughly 50 / 50 clay and sand.

The charcoal I have on hand is Oak. Currently I don't know exactly what species will be available at SSW, but (more or less) this is not considered a major concern at the test phase.

One of the larger unknowns is ore.
I had made an earlier general commentary on the local 'Macaulyite' ore :

Some considerations on Iron Ore for 'Turf to Tools'

This material is specific to the Lumsden / Rhynie area. (Unfortunately, the Culduthel report does not go into any specifics about any ore that might have been recovered at the site.) 

One of the DARC team (our in house geologist Marcus), has suggested the following for the active chemistry of Macaulyite:

Fe2O3 = 78 % 

SiO2 = 10.5 %

Al2O3 = 4 %

H2O = 7 %

With a weight of Fe of 60%, this should be a nicely yielding ore. The combined silica and alumina are a bit high however. I was sent a small sample of the available Macaulyite, and the particle size is about the same as rice grains - which is a bit on the small size. 

I have made up a version of our dependable 'DARC Dirt' analog, with potter's oxide as the primary component and additions to match at least roughly the chemistry above. 

'Spanish Red' oxide = 25 kg

Alumina = .5 kg

Silica = 1.25 kg

Wheat Flour = 2.5 kg

This is dry weight, there is always considerable water weight remaining (which we will calculate via a small sample later). 


Expect several more postings detailing furnace build, iron smelt and the bloom to bar phase!

Note : The smelt itself will take place at Wareham on Saturday June 14, starting about 9 AM, with extraction expected some time between 4 - 6 PM. As with past experimental smelts, this is a 'semi open' event : The truly interested can attend, but please (!) get in contact with me via e-mail if you want to come up. Do expect my major focus to be on this prototype smelt, a training and rehearsal for the SSW project.

1 comment:

David P said...

Maculyite - this may not be as common a mineral in the area as you expect - Several years ago NASA took samples as its the closest on earth to martian soil and its only found in a narrow area some distance from Rhynie.


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

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