Saturday, June 03, 2017

Celtic Iron Age - BELLOWS ??

Given that the rough dynamics of an effective iron bloomery iron smelting furnace are set in fixed science...

What is a specifically 'Celtic' working furnace going to look like?
Here I am using the Scottish Crannog Centre rough date target of about 500 BC.

One of the biggest specifics in my mind is the air system - most specifically the bellows type used.

Pair of simple bag bellows used for a bronze casting furnace (from Ancient Tools & Crafts) (1)
There are often references made to 'bag bellows'.
This is a very early historic type, basically a rectangle of leather, stitched up two sides, with a pair of sticks framing the open, upper edge. Most typically, the type is illustrated in use for metal casting furnaces, one hand opening and closing the bag. This obviously this small size and method will greatly limit the possible delivery volumes. Not at all a problem for bronze casting furnaces - which most certainly has proven quite effective. Same for the requirements of blacksmithing forges.

 It may be larger, worked with one hand on either side of the open edge. This will certainly greatly increase the active volume. At the cost of increased effort of course! Remember that overall size and the required air volumes for an effective iron smelting furnace are easily an order of magnitude greater than that required for a simple bronze / jewelry casting.

Bag Bellows are often described as the type seen in various African iron smelting traditions. Early observer reports describe the size as 'one goat skin'.
What I have observed of film of this system in use shows very rapid stokes (reported as high as 120 per minute!). The individual strokes at those rates show as extremely short, and without little force applied. In combination, this will combine to produce only low total volume - and certainly very little penetration into the body of the furnace itself.

Jens Olesen, likely at Eindhoven, the Netherlands (date unknown) (2)
A fully developed type is the 'drum' bellows.
In its simplest form, the open top of the bag has been replaced with a wooden plate, either oval or circular. Leather can be conserved by using two matching plates, one top and one on the bottom. This also will have the effect of increasing the 'open' interior volume. Adding some hoops of stiffened material (simplest being bent twigs) can greatly improve delivery volume. The stiffeners keep the sides of the leather from collapsing inwards during the fill stage of use.
The true drum bellows will also have valves, at the very least a simple circular flap valve at the top (input) side

Unidentified re-enactor, at Military Through the Ages, Jamestown Virginia (my image - 1998?)
 A transitional type is what I am going to call a 'semi-drum'
The inspiration for my current design is based on what I had seen while a participant at Military Through the Ages in the late 1990's.  This event is a juried competition for historic re-enactors - of all all time periods. (The second year I was there, the span ranged from Celtic Iron Age - through to Viet Nam War!)
You can see what the fellow above has is basically a 'one goat' skin. One 'leg' extends to attach to the ceramic pipe tuyere that feeds this blacksmith's forge. (Itself a shallow clay lined bowl dug into the ground, filled with charcoal.) The 'neck' of the skin is sealed with a wooden plate. Interestingly, there are no fixed valves in the system. The top plate has a hole in it. The leather strap spans this hole. On the fill stoke, the hand moves against the strap as you lift up. On the output stroke, your hand covers the hole - and becomes the valve. Obviously this is somewhat less efficient than the full drum design.

Next Up - what I've built (so far)...

1) Ancient Tools and Crafts has a great deal of helpful information. Particularly detailed observations / build instructions for bag bellows and simple metal casing furnaces. 

2) Eindhoven Museum - under the direction of Thijs vander Manakker
Thijs is most certainly one of the pioneers of experimental archaeology applied to bloomery iron smelting. (Expect further references to his work as this series proceeds.)

Note : for this article, I did pinch most of the reference images from various web sites - I have tried to cite these as possible. 

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

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