Thursday, July 06, 2017

a 'Celtic Iron Age' Bloomery Furnace

If you have not been following recent posts, you might want to look back to see the development of this prototype...

As a fast introduction, this is my suggested furnace build for the demonstration upcoming at the Scottish Crannog Centre event, August 5 & 6, 2017

I have decided to keep to a known and proven layout :
- 40 cm stack above tuyere
- 20 - 25 ° down angle on tuyere
- at least 10 cm base (floor to tuyere)
- set the tuyere proud of inner wall

What will mark this system as 'Celtic' is :
- use of semi-drum bellows
- slag pit design

I see the most likely 'design flaw' is the available air volume from the 'semi-drum' bellows.
Because of limited workers, and the very real transport restrictions, I am going with a single tuyere / bellows combination.
To try to balance that, I've reduced the interior size of the furnace :
- at 20 cm the ideal air would be 375 - 470 LpM
- at 22.5 cm the ideal air would be 500 - 620 LpM
My theoretical output on the semi-drum bellows I built is about 500 LpM. So this suggests with the smaller furnace diameter (and good workers!) this system should function as hoped.
Honestly, I feel that creation of a lacy, lower yield bloom is more likely. Not really a problem of itself, as this is more like to represent actual Celtic Iron Age results.
There is a balance between interior furnace volume (heat created) and exterior surface (loss through radiation). I have successfully run furnaces at 20 cm ID before. There is a tipping point some place between 15 cm and 20 cm where there is too much loss off the exterior against limited heat volume in the interior.

I'd like to try the slag pit filled with bundled grasses. The ideal (from an experimental archaeology stand point) would be to be able to cut a sheaf of oats or barley. I have run this same system (slag pit with thin clay cap) a couple of times using small sticks - with good results.

The leather Y tube is something first made up for the Vinland Series, originally intended to replace the modern steel pipe fittings that normally are used. (The purpose of the Y is to allow a straight line down into the tuyere, allowing for visual inspection and clearing of blockages as needed.) A second advantage proved to be providing a flexible coupling between moving bellows and static (and fragile) furnace. Working in a more historic context, a simple wooden plug seals one branch of the T. Sound then becomes the indicator of when the slag builds too high or freezes in a drip, either case partially restricting air flow.

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

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