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 20 litres per stroke / 600 litres per minute / 1200 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.
(corrected against bad original bad math!)

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