Saturday, June 17, 2017

Iron Smelting in the Celtic Iron Age (two)

From the last post, you can see that the upcoming bloomery iron smelting demonstration (Scottish Crannog Centre - August 5 - 6) is going to be framed:
500 BC
Scotland (ideally the area around Perth)
Iron Age technology ('Middle' ?)
'Celtic' / pre Roman

What might that look like?

- As with other early history bloomery iron smelting, the furnaces are likely to be small.
- Air delivery is going to be produced by smaller, human powered equipments (see the earlier discussion on possible bellows types).
- The ore type most commonly exploited is a primary bog iron ore.
- The furnaces may be some version of a 'slag pit', rather than the later 'slag tapping' type

My normal 'go to' is Radomir Pleiner's Iron in Archaeology, the European Bloomery Smelters.
This is almost the only overview survey of Early Iron for Europe. This reference is however not organized in a fashion that makes sorting to a specific geography / cultural / date sample the easiest. There is in fact not very much indicated as 'Celtic' in the index. (The archaeological examples are sorted by furnace construction type, and only into major forms, largely based on the slag management method employed.)
Typical of the results of attempting an internet based research into Early Iron in Scotland

Now, the raw dynamics of a bloomery furnace remain the same for anyone (1) :
- Furnace needs to be constructed of some material which can withstand temperatures in the 1200 C range.
- Internal diameter needs to be plus 20 cm. (Experience has shown that below that size, the working heat volume to surface area loss ratio becomes so high that the furnace just will not get to the needed temperature range.)
- The effective working height of the upper stack needs to remain at plus 40 cm. 
'Ideal' Short Shaft (Viking Age) furnace
 - The air systems available are not able to effectively penetrate very far into a working furnace. In turn this typically results in a smaller bloom (usually with very lacy consistency). One way seen both historically and in modern experiments to combat this is to use multiple tuyere points, which although individually are limited in effect, combine to both ignite a larger furnace volume - and create a number of smaller individual blooms in a single firing.
The second effect of the lower air volume systems (bag or drum bellows types) is to limit the maximum effective diameter of the furnace. With single tuyere / bellows combination, this is most likely in the range of 25 cm. A furnace certainly could be built larger, but practically only a small part of the interior volume will be effectively involved in the reduction process.
- It is most likely the original smelting efforts would be 'occasional' - rather than a more intensive ('industrial') scale. This certainly limits the number of furnaces constructed originally on any given location, and by extension reduces the chances anything but the most fragmentary archaeological evidence may be discovered.

" SMELT 2010 was an experimental archaeology weekend held in the National Heritage Park, Ferrycarrig, Co. Wexford with the primary aim of smelting Irish bog ore in a reconstructed bloomery furnace. We had some success, producing iron, but no usable bloom. "

What you can tell from the video:
- slag pit furnace (core using river reeds)
- construction of clay / sand / manure mix (2)
- furnace is very wide and squat
- the stack height above the tuyere is limited
- single tuyere (ceramic tube) (2)
- use of paired bag bellows
- ore is gathered bog iron
- charging amount of 1 kg charcoal to 1 kg ore is indicated

I have sent a request for more information to this team, as the video does not give much by way of technical details :
- dimensions of the furnace
- total ore added
- total charcoal consumed
- burn rate
- air volumes

When I watch the video - a couple of things do stand out:

A) The air system is only heating at best the front half of the furnace volume. (This most obvious with the sections shot at night, during burn down).
With the single tuyere, and the bellows system used, this is about what I would expect.
B) The duration of both the build and the smelt itself is quite long. Although not specified, from the lighting it appears they started the furnance pre-heat quite early in the morning, and certainly worked well into darkness. Plus 12 hours?
C) At a point in the sequence where they have already started adding ore, a comment is made of the volume of steam coming off the outside furnace walls. This normally indicates water being driven off from the clay structure. Water seriously impacts the overall 'energy budget' of the furnace, robbing heat that should be going to increase the interior temperature. Ideally the main sequence is best not started until no more steam is visible on the exterior walls (ie the furnace is fully dried). The construction shows very thick clay walls, plus there is a comment about the clay mix being initially too wet.
D) They certainly have created a volume of iron rich slag. Iron?
The night time image of the mass that was extracted certainly appears to my eye to be primarily slag. A slag mass with an interior bloom shows a very distinctive colour gradient. The slag cools quickly to dark, while the dense bloom will stay much hotter for a considerably longer time. There should be a distinctive, bright 'nugget' of iron visible inside the larger slag mass. (Normally the camera easily captures this - as the camera records further down into the infra-red light than the human eye does.)
E) At the end of the video can be seen a very small and lacy fragment described as containing iron. This appears what we would call a 'gromp' - metallic iron to be certain, but too light and lacy to be effectively condensed into any workable iron.

Next - 'Bloomeries of the Scottish Highlands'...

1) ORE - This has proven to be the single largest modifier for what will prove to be the most effective individual furnace design.
The second important modifier is 'material culture' - European, African, Japanese cultural concepts of work organization, even 'ritual' practice, have a serious impact on how separate groups have chosen historically to construct and operate furnaces.

2) IRELAND - I had been in e-mail conversations with a group attempting a reconstruction of an Early Iron Age smelt - at about that same point in time. (Of course I've lost / can't find the contact names!) Given the similarity of the clay mix and the use of kiln support tube for the tuyere seen - I do wonder if this is the same team?

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

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