|Down the interior of the shaft - with the metal form removed.|
|Furnace - front view. Stone framed tap arch below.|
|Working area - The build was framed 2 x 2 m by rail ties.|
|Start of main sequence. Ceramic tuyere installed with air supply. Tap arch blocked with sods.|
|Into main sequence, ore analog added.|
|End of smlet : View down the opened tap arch, showing the slag bowl in place.|
One overall problem was a lack of full consideration of the diameter of the furnace - and what that implied in terms of both charcoal requirements and air volumes.
Our normal furnaces vary from about 25 to 30 cm internal diameter. This build started using a 33 cm diameter metal form (1). After the short (30 minute) pre-heat, the walls had shrunk back somewhat, resulting in an ID at the start of the smelt of 36 cm.
A) Effect on air delivery volumes:
ID = 30 cm : area at tuyere = 700 cm2
air required (at 1.2 to 1.5 l/m/cm2) = 840 to 1050 l/m
ID = 36 cm : area at tuyere = 1020 cm2
air required (at 1.2 to 1.5 l/m/cm2) = 1225 to 1530 l/m
With the start of the main sequence (charcoal addition) the available electric blower was set wide open. (That volume is thought to be 1400 l/m, but this has not been tested 'in line'.)
Another variable is delivery pressure, which in this case could not be modified. (Pressure effects how far into the charcoal mass of the furnace the air will penetrate.) The concern was that even if sufficient air volume was available, the air might not penetrate right across the furnace diameter.
B) Charcoal requirements :
Although the height of various furnaces varies, internal volume is a cube function.
Our typical furnaces require 4 - 5 standard buckets (about 1.8 kg) of charcoal to fill.
For this build the volume of charcoal needed was almost double - a total of 10 standard buckets.
This would certainly effect the 'hang time' of any individual ore particle. (The time the material was inside the reduction chemistry of the furnace.)
The consumption of a standard bucket of charcoal was inside the normal range considered effective on past bloomery iron smelts. The furnace ran a bit hot at first, at a rate of 6 - 8 minutes per bucket. This shifted (as expected) with the addition of the first ore charges. The average was about 10 minutes per bucket, increasing slightly with the later additions of larger ore charges. Ore charges started at 1 kg per bucket, and increased smoothly to 2.5 kg per 1.8 kg charcoal at the final stages.
C) Tuyere :
One effect that was not anticipated was the shifting of the tuyere angle over the course of the smelt. At the start of the smelt, the tuyere was set to the standard 22 degree down angle. However as the smelt progressed, this angle flattened out considerably. By the 2 1/2 hour point into the main sequence, the angle was reduced to 17 degrees down. Measured at the end of the smelt, the tuyere angle was at only 12 degrees down.
This appears to have been caused by the sods collapsing as they dried / organic materials burned away. The tuyere had been placed so that the interior end was resting on the stone slab lintel of the tap arch, but the outside end was only supported by grass sods.
This shifting could have been prevented by placing a small stone support on the outside end of the ceramic tube used.
Next : 'Excavating the furnace & extracting the slag / bloom mass
(1) The reason for using the larger form was an initial intention to line the sod shaft with a thin clay layer. This was suggested by clay fragments found at Hals, ranging up to as much as 2 cm thick. With that clay liner, this furnace would have in fact had an ID closer to 30 cm. Time (and weather) limitations resulted in this clay liner not being added to this furnace.
A successful test of a thin clay liner in an earth cut cylinder was made with 'Icelandic 5', May 2012