Saturday, May 01, 2010

More on Pattern Welding - what and why?

(See yesterday's article - same source.)

Why would you want to layer metals for a blade in the first place?

The reason is what is known as the 'Bladesmith's dilemma' . If you make something out of soft, low carbon, iron - it would be flexible and survive impact. But the edge would not last very long. If you make something out of hard, high carbon, 'steel' - it would stay sharp, but at the risk of it shattering on impacts. A problem for any long, high stress, cutting edge - read sword. Remember that the direct bloomery furnaces used to make metal in the first place DO NOT produce metals like the ones we use today. Quality was more random.

One way to solve this dilemma is combine the soft (for flexibility) with the hard (for edge holding). This basic method is understood by working blade makers in cultures the world over and through history. The simplest example is a rugged tool like an axe. Fold over a long piece of soft iron. Where the two ends meet, insert a small sliver of very hard steel. Forge weld together. When you grind to sharpen, you cut back to expose the hard metal for the cutting edge, leaving this supported by the larger mass of soft metal. (Most VA axes are actually punched and lap welded, but leave that, ok?)

A more elaborate technique (for those who have not bothered looking at the extensive articles on the blog I referenced yesterday dealing with just this topic) is to make a pile of alternating hard and soft plates, then weld those solid.
Depending just when and where you are referring too, there are different traditions of what to do next. (Independent invention in N. Europe, Middle East, SE Asia, Japan).

In North Europe the tradition is to take the layered block, stretch it out to a long thin rod. Twist the rod. Make at least two, twisted in opposite directions. Weld together. Most typically take a *medium* hard piece and weld it around the outside for the cutting edge.
The combination of hard and soft, making that set of reversed diagonal lines, effectively results in two coil springs running up the centre of the blade. This absorbs shock on impact.


You are going to rough forge to shape, then polish. Remember that you have combined a hard and a soft metal. When you polish, the soft metal cuts faster than the hard metal. So in fact there is going to be a subtle difference seen between the two layers. If you are only working with a grinding stone, especially a rotatory one (either electric or hand powered) the subtle effect of hard and soft may not be seen. This gets obscured by the variations caused by irregular grinds.
Once you start to use any polishing system (hand or powered) that employs a hard and flat surface, rather than a curved surface, you will certainly see the effect of differential cutting. I personally go from hand grinder to sander using about 60 grit. Even at this first rough shaping stage, the lines in the pattern can be seen. (And trust me, I just did this exact thing on Wednesday - blog article with step by step photos pending.)

If you look carefully at the full size image (click) you can just make out the lines of the differing layers along the cutting edge. This is a direct scan from the actual blade at life size, made after the first polish step at 60 grit.

There is certainly a big question about just how finished a polish would be applied to historic surfaces. Any blade maker would absolutely require the forged surface to be ground / polished far and flat enough to remove any pits resulting from the forging process. These pits cause stress concentration points - and are most likely to be where a blade will fail (bend or break). All other things (alloy, heat treating, basic method) being equal.
Although it is certainly possible to achieve a mirror like finish with simple sand and leather polishing, I personally consider this unlikely for VA blade surfaces. Remember the bulk of polishing would be done with long bars of sandstone (the whetstones found). If you have never seen any of these, they are huge. Imagine using a bar as long as your arm, almost as big as your wrist. These also provide a long flat cutting surface (see above).

A good number of existing VA blades show quite complex arrangements of twisted bars. Alternating directions, mirror images of straight and twisted section, multiple core rods. There is not any technical reason for this complexity, in terms of function. This has to be the result of a desire for artistic use of the method. This does suggest that some method of making these patterns more visible was also employed historically. The Norse are well known for visible display of wealth / status.

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

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