Friday, June 23, 2017

Iron Smelting in the Celtic Age (three)


In casting around for prototypes for the upcoming demonstration project at the Scottish Crannog Centre, I keep coming back to the work of Thijs van de Manakker.

Thijs works at / with the Eindhoven Museum in the Netherlands. This is a living history museum, centred from pre-history to the Medieval period.
One of the activities there, which Thijs has lead over the years, is experimental iron smelting based on the Celtic Iron Age. You may note that the video record below is from 1999 - two years before I even started inv

(Below ported over from YouTube - you may have to click on the title to get the intended content. The full set of videos are on Thijs' web site.)



Looking over the process illustrated above - these are the things I notice (*):

1) Build : The mix is a fairly rough blend of chopped straw, what appears to be locally dug clay, and sand. The consistency is softer than I normally use, with what appears to be a sequence of thick rings applied, then left to sun dry to firm up before the next layer is added.
- This method results in very thick wall, with clear breaks between the layers. The outside is clearly blended, but the interior has been left very rough. I did wonder at this when I initially watched this first video (but the logic becomes clear later).

2) Layout : The completed furnace appears to stand roughly 70 cm tall. It is clearly flask shaped, looking about the same outside diameter at the base as the height.
- Given the thick walls, this suggests an interior diameter at tuyere level of about 50 cm, perhaps 30 cm at the top opening. (All WAG).
- The furnace is a slag pit type (seen briefly at the start of the construction phase, later in the smelt when slag is drained). It looks like this is a smaller pit, lined with sticks, is placed to the front of the furnace below the tap arch. (Rather than a full pit under the entire furnace?).
- There are two tuyeres, set at base level, opposite each other and so also 90° to the tap arch. These are both basically set dead flat. (I would be concerned about slag levels.)
- The actual tuyeres appear to be lengths of modern steel pipe. These are quite long, the purpose appearing to keep the bellows operators well back from the furnace itself. (Likely done for both safety and to keep the working area around the furnace clear. This becomes especially helpful during the extraction phase.)
- There is no specific way to tell if these were set proud in the interior - or how far they may have burned back during the smelt.

3) Air : There are two good sized leather drum bellows supplying air. Obviously there is a flap input valve on the top. It is not clear if there is an exhaust valve.
- From the video you can see the pump rate is about one stroke per second each.
Also that the two bellows are being blown identically, with one operator setting the pace, the second (less experienced?) following.
- A (very WAG) guess on the size of each is about 30 cm wide, with about 30 - 35 cm height of air being expelled each stroke. This (extremely WAG) suggests about 3 litres per stroke / 180 litres per minute / 360 LpM combined. Taken against that estimated 50 cm ID, I would consider this marginal air volume, and likely to effect the bloom yield and density.

4) Slag Management : The tuyeres are set up fixed and in a straight line. (Not the Y tube with removable plug / view port that we use.) This means all the slag control is in the hands, and ears (!) of the smelt master.
- As the tuyeres are set flat to the ground, this is certain to result in the slag bowls forming high, requiring careful monitoring to ensure the tuyeres are not 'drowned'.
- Given the lower volumes of air blast, I would expect either two separate slag bowls, or jointed to a lobed shape (like a kidney bean), shallower in the centre.
- In the later stages of the smelt, you can see several slag taps. The bowl is punctured at the front edge - I would suspect on a diagonal back to the individual slag bowls. The excess slag runs to drip into the front pit.

5) Extraction : The logic behind the construction style of the furnace, with the corregated interior, becomes clear here. The lines between the individual rings of clay are easily broken free, allowing the furnace to be dismantled in easy steps as the charcoal burns down.
- The use of the wooden chisel tool is a nice touch. A bit lost when the clearly modern metal tongs are used to pick up the clay pieces. (But certainly much easier than using a more historic wood shovel or wooden folded tongs.)
- Once the charcoal is burned down close to tuyere level, you can clearly see the 'bright spot' outlining the two separate bloom masses. As expected, these are located just to the front of each of the two tuyeres.
- On the extraction and first quick cleaning / compaction of each, it is clear that the iron produced is mainly a number of smaller pieces most significantly quite lacy in consistency. (Note how much slaggy 'mother' breaks away on hammering, and how quickly the hot iron core collapses into a very small mass.) The last piece extracted appears to be as large as all the others combined. I'm not surprised it was formed on the tuyere side of the 'lead' bellows operator.

6) Compaction : The use of the hand held stones initially is certainly a 'primitive' touch. I'm not sure exactly how accurate that might be historically? (Use of hand held stones was recorded in African traditional method, but wood hafted stone hammers are certainly part of the archaeological record by the Celtic Iron Age.)
- The use of a second, quite different, 'slot' forge is clear for the compaction phase. - Quickly the hammer stones are abandoned in favour of metal head sledges.
The piece being worked is appears to be the largest iron mass extracted. Over the sequence you can see it compacting nicely to a dense iron billet. It certainly looks to be working up like good soft iron, judging from the effect of the hammering!

The end result (2 kg?) is worked to resemble the 'double pyramid' trade bars known from the Celtic Iron Age.



(*) Although I know Thijs loosely through past e-mail conversations, I freely admit at the point of writing this I have not specifically approached him on more accurate details. Specifically for exact measurements and things like consumption rates and yields. I will be doing so!

WAG = Wild Ass Guess

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?

Friday, June 09, 2017

Iron Smelting in the 'Celtic' Iron Age... (one)


One of the problems with researching the actual historical prototypes for the upcoming demonstration at the Scottish Crannog Centre - is finding some actual prototypes.

(Very) Loosely, this will be framed up as :
500 BC
Scotland (ideally the area around Perth)
Iron Age technology ('Middle' ?)
Is that 'Britons' / 'Celts' / 'Picts' ?

I've chosen to refer to the current SCC project as 'Celtic Iron Age'.
I freely admit - no matter what language I chose - the terms are a bit loaded.
What to the Scots themselves use as the 'ideal' term for their ancestors? The people living at the edge of the Highlands, 'before the Roman Invasion'.
- 'Picts' generally is used to refer to the peoples of especially North and East Scotland, 'post Roman to pre Viking'. One of the dominant differences (on many influencial levels) is the the Picts are primarily a Christian culture.
- 'Britons' generally is a much wider grouping, referring generally to 'pre Roman' - but over the geography of England primarily. I feel it reasonable to distinguish between Scotland and England - although at period of interest, neither nation existed.
- 'Celts' does imply connections to the wider group across Europe, with shared material culture. So it is fair to say a term not so specific in terms of geography or even time. It is the term most often used at the SCC itself.

As I understand it, there is some question, at least based on existing archaeology, if the people building and living in the Crannogs on Loch Tay had actually been smelting their own iron. Or if instead, the iron was imported from outside, as either the intermediate stage 'working bars', or even as the finished objects. The excavations by the team of the Scottish Trust for Underwater Archaeology have only recovered rare objects made of iron. This is pretty much what should be expected, given the nature of ancient object preservation, the severe limits of underwater archaeology - and the raw value of iron objects within their original historic context in the first place. One object recovered, from under the platform of the original Crannog, was a small iron knife blade. Certainly a personal disaster for the original owner!

Unsurprisingly, there is not much solid archaeology to go on.
The excavated remains of iron smelting sites for Scotland is very limited. Not especially surprising, as 'occasional' working areas don't leave much of an observable trace to begin with. Furnaces are constructed of clay, perhaps with stone supports, but in any case either wash away or are shattered by weather. The slag always remains, but again in itself presents little in way of evidence. The valuable iron is of course absent! So even in the best situation, it is more likely traces of an iron smelting event will be most likely discovered within the excavation of a much larger occupation complex. (see earlier work related to the Culduthel site)


(next - looking at some other experiments)

Monday, June 05, 2017

#2 - Celtic Iron Age Bellows


Continuing from the last post ...
What I've built so far:

'Semi Drum' Bellows - top view, extended
So at best I have to consider this a hypothetical design.
Once again (as with my long discussions here on Viking Age twin chamber bellows), there is no archaeology to guide in type or design. In this case there are not even illustrations, much less surviving artifacts (or fragments).

As I suggested in the previous post, I remain unconvinced that the simple open top 'bag bellows' will supply enough air for an * effective * bloomery furnace operation (1).( When I look at other experiments, I most usually see very poor penetration of air into the furnace interior, exhibited by the temperature gradients visible. )

Admittedly, the raw size of the bellows above was almost solely dictated by the leather materials I had on hand. I cut the largest pieces possible from what was about a half full hide leather skin. (This also has used up the last available piece of leather I had of a suitable thickness for bellows sides.)

What you see above is made from one single rectangular piece, folded in half and stitched up the two sides - the same basic construction method as the bag bellows type.  This gave a measurement on the open (top) end of 2 x 25 1/2 inches (51" / 130 cm total as circumference).

The oval top plate was then cut from available 12 x 1 inch rough pine, again utilizing the best width possible from that material. This created an oval 19" / 49 cm x 11 1/4 / 29 cm in dimensions

Top Plate - inlet holes to hand size
Given the oval overall shape thus generated, it was clear that two hand operation would be available. I made a decision not to include interior input valves. Instead, the operator's hands would create the seal - as they pushed down on the plate to collapse the bellows. As with the 'one goat skin' interpretation, a loose leather strap would serve to lift / expand the bellows. I cut the input holes to fit my (smallish) hands - to 3 " / 7.5 cm diameter.

Now I did get a bit more elaborate on the internal structure:

Interior view - showing securing the base straps - and the (modern) output valve.
I had considered how to hold the unit down securely. I decided the easiest way to accomplish this would be using heavy leather lacing across the four lower corners of the bag. These could be tied to wooden pegs hammered into the ground. To ensure there would not be damage to the leather where the crossed thongs exited the bag, I cut a smaller wooden plate for the interior. The thongs are knotted in the centre, then secured in place with a large leather square, itself tacked down to the wood based. (This of course uses unlikely metal fittings. I can imagine the laces running through simple holes in the wooden base as a more historical method.)

Past experience has shown that unskilled operators will often end up sucking air back into a bellows from the output end. As this unit is intended to be attached to a smelting furnace, this unwanted reversal would be sucking back gasses at a temperature range of 1100 - 1250 C. Certain to burn up the leather!
So to prevent this, I did add a very modern 'cheat' to the design. A standard plastic sump pump one way valve was inserted and tied into the output leather tube. This does reduce the available output diameter slightly - to 1 1/8" / 3 cm ID.

The short leather output tube was sized to easily allow the insert of a standard plastic sump pump hose - at 1 1/2" / 4 cm OD. Of course in use this tube could be mated directly to the tuyere. (2)

View of the expanded bellows from the side.
At full expansion, the height of the unit is 10 " / 26 cm. When fully collapsed, the effective height is 4" / 10 cm. This gives a rough 'loft' of about 6" / 16 cm.
Computing the potential volume produced using the formula for a regular oval tank (Pi x major axis x minor axis x length / 4) suggests 17 litres per stroke.
Given expansion of the leather sides, and that the bottom is not another flat oval, this calculated volume is likely high. Experience with other bellows units built in the past certainly suggests the practical working volumes are often closer to 50 % of the calculated theoretical.

Lifting the bellows on the fill stroke (only one hand - the other was on the camera!)

Even so, this all does suggest that this version of a semi drum bellows might easily produced as much as 500 litres per minute, based on 60 strokes per minute. Over the length of a smelt, this is more likely to drop considerably, but hopefully a volume of 350 LpM should prove achievable.
If the smelting furnace intended for the Crannog Centre demonstration is built to the smaller 22 - 25 cm ID, this amount of air should prove workable, if on the lower end of the effective range.

One of the other considerations here is equipment transport (!). This construction lays fairly flat, and is not so large as not to fit into a suitcase. It also has almost no metal parts - often a major concern in these days of airport paranoia.


1) Weasel Words Here:
Certainly you can get * some * iron from a furnace using low volume air. The resulting yield will be extremely low. The density and quality of the iron created will be extremely low. So if your objective is balancing over effort and expenditures of materials over the entire ore to working bar production cycle, low air is just not effective. Other researchers using low volumes have reported 'ore to bar' ratios of 10% (or even less). The loss at 'bloom to bar' is especially high.

2) Our standard practice is to use a Y tube between air source and tuyere. Fitted with a simple wooden plug (in historic context) to the third branch. This allows both easy observation down the tuyere, and clearing obstructions with a long thin 'Radner' tool.
 

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

COPYRIGHT NOTICE - All posted text and images @ Darrell Markewitz.
No duplication, in whole or in part, is permitted without the author's expressed written permission.
For a detailed copyright statement : go HERE