Friday, July 04, 2014

Pictish Iron : Bloom to Bar

Continuing the commentary related to the 'Turf to Tools' project coming up from August 9 - 24 at the Scottish Sculpture Workshop.

An initial test run of a Pictish Late Iron Age bloomery iron smelting furnace was made in Wareham on June 14 :

Kelly Probyn-Smith holding the freshly extracted bloom mass

The data from the smelt:
Burn rate - average 10 - 11 minutes per 1.9 kg charcoal bucket measure
Total ore added - 31.1 kg (28.3 kg 'dry' weight)
Ore additions range from 1 - 2.5 kg per bucket
Total charcoal used - 61 kg
Total time - 1:15 for preheat / 5:30 main sequence to extraction
Finished bloom weight - 5.2 kg
Yield - 18% (from dry ore weight)

Resulting Bloom - only partially scored

The important next step in having usable iron is compacting the spongy and slag containing bloom down into a dense (and crack free!) working bar for the blacksmith. This is a step often overlooked by the archaeologist, and poorly understood and researched.

The next day, we were joined by David Robertson and one of his students (Mike). The session was to at least start the process of compacting the bloom towards a working bar. Using my hydraulic press, the rough mass was compacted (at welding heat) down to a flat plate about 5 cm thick. This plate was then cut into four roughly pie shaped pieces. At this point a more accurate estimate of carbon content via spark test could be made of the interior. The estimated result was a surprising .60 to .70 carbon - a tool making material.
Both David and Mike worked at compressing their quarter pieces down towards a possible block, to their credit working all hand tools (in a coal forge).

One of the resulting quarter sections

A couple of days later, I took the section seen above and forged it down to at least a first stage 'block'. The piece I started with weighed 1278 gm.
I had the services of my 50 lb Robertson air hammer, plus my 30 ton hydraulic press. All heats via my large coal forge (bottom blast). Hand hammer was a 1000 gm.

The full working cycle (number of heats) :

(compress surfaces on hydraulic press, at welding X 3)
(cut in half, then to quarters on hydraulic press X 2)

Light welding by hand, surfaces and edges X 6
Attempt to work down edges on air hammer (not effective) at welding X 2
Compress on hydraulic press to brick at welding X 3
Work 4 sides to block on air hammer at welding X 2
Weld back both ends by hand X 2
Pull to billet, starting on side surfaces at welding on air hammer X 2
Flux and weld by hand over centre flaws on top and bottom X 2
Draw to even billet on air hammer X 1
Clean up to final shape by hand X 2

Near the start, after edges were welded down by hand

Total heat cycles - 21 (+5)
Heat cycles at 'welding' - 19 (+3)
Cycles by hand - 12
Cycles using press or air hammer - 9 (+5) *
Total elapsed time - 2 1/2 hours *

* These two numbers are significant in relating the use of modern powered equipment to historic practice, working with human muscles alone. If strikers using sledges were employed, there would still certainly be some increase in number of working cycles / time expended. (Guestimate - increase by 30 % or more.) Starting with a 1.2 kg mass, it certainly would be possible for an individual to have worked these processes alone. (Guestimate - increase by x 4 - x 6 work cycles)

Resulting 'block', top and bottom surfaces

The 'block' at this point is still not completely refined. At this point the weight is 957 gm. Size is 16 x 4.5 x 2 cm. You can see that there is a major crack that runs into the bottom side about the mid line, about 3 mm deep.
The next step will be cutting along the crack, then adjusting the two pieces to the same width and length. At that point they will be welded up again, then drawn out slightly to ensure there are no more major flaws.

At this point:
Bloom to block loss = 25%
Ore to block yield = 13.5%

Hopefully the resulting billet will be compacted concisely enough that it can then go on to the final sequence - forging an actual replica of the axe-hammer. The billet is roughly the right size / volume for making one of these  objects.

One of the important observations here is the huge amount of effort - and overall losses, involved in producing a working bar for the blacksmith's hands in historic times. Besides the obvious use of power tools, an important difference is the use of rock coal instead of the historic charcoal as the fuel.

This should make us think differently about the value of those objects created by ancient smiths!

For more details on the Turf to Tools project (several photo essays and commentaries) :

For information on the Scottish Sculpture Workshop

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

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