Sunday, August 13, 2017

Celtic Iron Age SMELT

at the Scottish Crannog Centre - Saturday August 6, 2017

(ok - not quite as planned...)

'Celtic Iron Age' Slag Pit Furnace
Electric Blower 
DD1 type Bog Ore Analog

Furnace at 'touch off'
Furnace - prepared clay with sand and shredded horse manure
Slag Pit - roughly 25 cm square / 45 cm deep / filled with wheat sheaves
Dimensions - 62 cm tall / 22 ID at top / 30 cm ID at base / about 25 ID at tuyere
Ceramic Tuyere - Output ID = 2.5 cm / Length 40 cm (made by Graham Sheffield)
Tuyere Setting - 23 down / 4 cm proud / 16 cm above base

Extraction - Assisted by Dirk Spoedleter
 Average Burn Rate = 9 min for 1.75 kg (roughly 10 minutes for 2 kg)
Average Ore Charge = 1.5 kg per charcoal measure

Total Charcoal Used = about 45 kg
Total Elapsed Time = about 4 hours (main sequence)
Total Ore = 20 kg
Hammering Extracted Mass - Shona Johnson & Pete Hill
Results ? Sintered Iron 'Gromps'

Iron Production = 2.94 kg of sintered iron gromps (collected from hammered 'mother')
Yield = 15 % (but elevated because mass not a compacted bloom)

This was an attempt at running a 'Pre Roman' style furnace (tentatively 500 BC).
As illustrated by  Thijs van de Manakker the pit below the furnace was capped off with a clay disk. The initial disk, supported by a grid of light branches, proved not heavy enough to prevent the drying fire from effecting the original wheat sheaf fill of the lower pit.
On smelt day, the pit was refilled with a mix of straw and reeds, with a thicker (2.5 cm) disk cap. However it happened that this thick (very!) wet clay cracked (explosively!) as the furnace heated.

This caused the heated charcoal to drop too early in the heating cycle, before a truly effective slag bowl could develop. In turn the reduced and sintered, but still fragmented, iron dropped out of the heat zone. Without the usual high position of a full slag bowl, this iron could not collect while hot enough to 'condense' into a compact bloom mass.

Given the frantic pace of preparing for the smelting demonstration - the first ever at the Scottish Crannog Centre - this still proved a fairly good result. Especially an excellent example of the processes (and often difficulties!) involved in Experimental Archaeology.

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

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