Saturday, June 17, 2023

Bloom to Bar to Blade

 As might be expected, I have a large collection of iron blooms, after over 20 years of experimental smelting. With the assistance of Neil Peterson, I have been trying to get a number of at least the smaller pieces compacted down into working bars. So far this has primarily focussed on sections in  500 to 1.5 kg range, basically half or quarter sections of our typical 3 - 5 kg results. (Noting here that there are a number of much larger, still complete blooms in the 8 - 11 kg range, plus the 16 kg monster made at ’65 for 65’.)

table of blooms (2016, so only to smelt #72 of the current 92)
Importantly, given the various source ores used, and the many variations on furnace design and sometimes method, the actual carbon content of the individual blooms can vary widely. There is everything from soft carbon free through to ultra high carbon (even to unforgeable cast iron) metals.
Any individual bloom itself is likely to have differences in carbon concentration between the bottom and top surfaces as iron is deposited and held at high temperature, usually for hours.

As any individual bloom is built up, the amount of slag also deposited has been found to vary considerably, from very spongy textures through to quite solid masses. The amount and nature of the initial consolidation hammering is obviously a factor here too.
The process of converting a raw bloom into a working bar most typically follows a process of compressing, folding and re-welding. The normal sequence is :
- flatten to a plate (or ‘book’)
- fold and weld into a ‘brick’
- fold and weld into a ‘billet’
- draw out into a ‘bar’
The amount of force required changes as the iron solidifies. This may seem obvious, but in truth just how light the strokes may be need to be at the start of this sequence is counter intuitive without experience. Even without much force, heavier hammers are best employed, to ensure penetration through the entire mass (for me this means switching from my primary 800 gm to a 1000 gm). As the pieces are forged up, it is almost ensured that the there will be additional welds both along the edges (90 degrees to the folds) and often into diagonal flaws.

All this taken together, results in a bar that will often have a distinctive linear, sometimes distorted, physical texture. Along with potential fine lines of slag inclusions, there are blended lines of changing carbon content throughout. These variations from a uniform structure can be made visible on a final object, especially if the surface is later polished and then acid etched.

Creating a bloomery iron bar requires considerable skill, experience, much labour and expended materials. Taken together, these bars are one of the most valuable materials available to an artisan blacksmith.

So what do you make from these small bars, most in the 200 - 500 gm range?
Generally, the highest value small objects for most blacksmiths are knives.

Now I have mostly stayed away from making blades from my bloomery iron. There are several reasons for this, one being that my work here has always been geared towards understanding early Northern European historic examples, not best possible production yields or aiming for high carbon alloys. Bloomery iron in North America has become completely dominated by the knife makers, where the bloomery process itself is seen as only a first starting step, not an objective of itself (something I remain very unhappy about).

forged blades, rough grind at this stage

To be completely fair, the blade forms here were largely a secondary consideration, with the simple tube handles almost an afterthought. This collection represents objects forged up over the last two (Covid) years, many languishing as rough forged blanks until the last month. All the low polished blades (only to 100 grit) have been lightly etched in ferric chloride to bring up variation in carbon contents within the parent blooms. This shows as either lines or mottled patches, the lightest areas having the lowest carbon content (least effected by the etch). Infrequent slag inclusions that remained show as thin dark lines, especially visible in #6 (which was also the starting bar with the highest variation in carbon content throughout).

detail, #5 Tool

In keeping with my general interest in Norse objects, all these knives are V grinds (not the more modern, if stronger, sabre grind). (1) Both #1 and #2 are commissions, so are based on specific artifact sources. Several are intended for small scale domestic use (textiles or food preparation) and generally conform within Period 4B, type C series from Coppergate, York. (2) The two thick and wide blades, straight backed with sweeping curved edges, are a shape suitable for wood carving (form fitting function).

knives as finished

1) Kitchen / Boning :   20 cm blade x 3 mm thick
                    natural antler (caribou) handle
                    core from DARC 11/08 (sparks roughly 1030+)
                    side slabs from Vinland 4 / 2010 (sparks roughly 1020)
                    ‘replica’ (3) of grave find at Ihre, Hellvi parish, Gotland, Sweden (4)

2) Small Norse Domestic : 10 cm blade x 4 mm thick (distal taper)
                    (owner will apply antler handle)
                    from Vinland 4 / 2010 (sparks roughly 1020)
                    a bit longer, but closest to #2829 from Coppergate

Boning #1 (top) and Small Domestic #2

3) Small Carving :      8 cm blade x 5 mm thick (distal taper)
                    natural walnut branch handle
                    from Slag Pit 2 / 2011 (from higher carbon end of the bar)
                    no specific prototype

4) Medium Kitchen :   13 cm blade x 2 mm thick (distal taper)
                    natural walnut branch handle
                    a bit longer than the samples, again roughly P 4B / T C from Coppergate
                    from Aristotle Furnace demo at CanIRON 9 /2013 (sparks roughly 1075)

Small Carving #3 (top) and Medium Kitchen #4

5) Tool :             10 cm blade x 5 mm thick
                    natural antler (caribou) handle
                    from Slag Pit 2 / 2011(variable carbon)
                    no specific prototype

6) Small Kitchen :      9 cm blade x 3 mm thick
                    natural antler (caribou) handle
                    again roughly P 4B / T C from Coppergate
                    from Aristotle Furnace demo at CanIRON 9 / 2013 (sparks roughly 1075)

Tool #5 (top) and Small Kitchen #6

The most probable destination for these knives will be as working tools into the hands of other members of DARC, many of whom have contributed their labour during the iron smelts that created the starting metal.
I will be offering knives # 3 - 6 for sale at the upcoming SCA ’Trillium War’ event over June 30 - July 3. The prices can be expected to be steep however, in consideration of the genesis of the material.

1) I don’t want to get into (yet another) argument here about whether ‘seax’ only refers to the ‘broken back’ shape, or any Norse knife. See an earlier commentaries:
2007, ‘Knives from the Viking Age
2010, ‘Knife? THAT'S not a knife…

2) Ottaway, P., 1992, ‘Anglo-Scandinavian Ironwork from Coppergate’, 1 872414 29

3) I don’t want to get into (yet another) argument here about ‘reproduction vs replica vs interpretation’. If interested, see a detailed commentary about these differences :
2020, ‘Reproduction, Replica or Interpretation’, in ‘The Iron Trillium’, Fall issue, Ontario Artist Blacksmith Assn.

4) Carlson, D., 2003, ‘Viking Knives from Gotland Sweden’, plate ‘Iron knife6’ (top), 91 973304 5 0


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

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