Sunday, June 04, 2006

Bloom to Pattern Weld Question

> Can you take me full circle in understanding? How does this explaination of the nature of the > blooms relate to pattern welding?
> Posted by Karen to Hammered out bits at 6/02/2006 09:05:38 AM

May be that my original posting was a bit vague .

Point is that any bloom is likely to have a range of carbon content through its mass.
You have to fold and weld the bloom to consolidate it to a solid bar. As I have only worked up three or four bloom to bar pieces, I don't have the widest experience or samples yet. At this point the individual pieces have ranged from as little as two to as many as four folds and welds to solidify into a solid workable bar.
Depending on the starting bloom fragment, this could automatically lead to an effect a lot like an intended layering of different carbon plates - in a flat stack at least. The effect with just a small bloom is not going to be as dramatic as what I normally do with a pattern welded blade. This also is more likely to be noticed with small whole blooms - as opposed to pieces cut off a larger bloom mass (which are more likely to be more consistent from one side to the other). There are not going to be as many individual layers - for a decorative pattern weld I typically start with nine alternating layers - often with three or four differing metals (carbon and alloy contents)

Of course true pattern welding is far more complex. The pile of alternating plates are first welded to a block. This roughly compares with compacting the bloom to a bar. In the pattern weld, there will be several, normally identically composed blocks made. These are then drawn to long rods. The rods are then twisted - in the simplest form, one is twisted right and one left. These two rods then form the centre core of the sword. In some period blades (Sutton Hoo and Gilling West) there may be a number of these core rods, with quite complex arrangements of twisted and straight sections. (Sutton Hoo has 8 core rods, Gilling West has six.)
To create the billet that the sword is forged from, two more bars are welded to the core to produce the two cutting edges. These may also be layered in a flat stack, the Sutton Hoo sword has about 150 layers along the edge. The edge block should not be twisted, as this will create a cutting surface that alternates soft then hard through the twist. Not the ideal design as it will mean irregular wear and a poor performance overall. Some blades, like the Gilling West, have a single material (usually a mid carbon) for the edges.

Back to the original thought. The concept of adding a small piece of higher carbon metal to create a cutting edge by welding it on to a larger block of softer iron was well understood by ancient smiths. If for no other reason than to extend the supply of the carbon material. A good example of this is seen with axes, which many historic samples have either lap welded (carbon metal on one side) or inset (carbon surrounded by two soft iron) edges.
Extending this last - the inset edge - would lead to the flat stack type of layered steel. This might have been done originally to extend the available carbon metal, or even to take a number of smaller fragments and weld them together to a more useful larger chunk. If you made a kitchen knife, the effect of acidic foods (say onions) would selectively discolour / etch the blade to make the visual aspect obvious.

I see this as the progression. Either a small inconsistent carbon content bloom or a deliberate layering of carbon and soft iron into a smaller functional tool, likely a kitchen knife. The everyday use of the knife eventually displays an interesting discolouration or wear pattern. Someone has an inspiration to intentionally control this for artistic effect.

2 comments:

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