Saturday, February 17, 2007

Viking Age SWORDS - part 2, ALLOYS

Posted to Norsefolk by Ed Lindey on Thu Feb 15

"Just my opinion but I believe there is sometimes a mistaken impression with
modern people as to what the capabilities of period swords actually were. In
the words of a swordsmith I respect, "A modern alloy sword, properly made
and heat treated, will cut through a period sword without much effort". "

(Once again this, as brief an overview as it is, may be more than many of you wanted to know!)

Ed mentions two aspects, which I'd like to deal with separately. These are technical factors that are most often incorrectly understood - even by many blade makers. To confound this problem, technical language has been increasingly used as mere advertising copy over the last 20 years. This has obscured what was originally precise language - and frankly made it extremely difficult for the layman to figure out just what the heck is being described.

First is 'modern alloy' - correctly identified by Ed (and his swordsmith friend) as being significantly different than historic metals. The metals available in the Viking Age are different from our modern ones in two primary aspects : method of creation and the actual components that make it up.

Iron as a pure element does not exist on the surface of the earth in its pure metallic form (outside of two weird types - which I may get to). Elemental iron reacts relatively with oxygen to form iron oxide - rust. This being the case, to get a bar of metal, you have to take that iron oxide ore and put it into a specially designed furnace and undertake a carefully orchestrated series of steps to reduce the ore and compact it into a mass - a bloom. (Anyone interested in the details should take a look at ).
The process used by the Norse produces metal which has quite distinctive physical properties (a working texture) and chemical content.

Now anyone who has been reading my past posts will have heard me mention 'wrought iron' any number of times. This is a type of metal made using a process which was largely abandoned by about 1900 in North America. True wrought iron has not been made ANY PLACE in the Western World since about 1975 (outside of industrial museums). (At todays date I suspect it is not made commercially any place on earth - period.) Starting in 1855, an new technology was introduced (the Bessemer furnace) which allowed a quite different material - mild steel, to be made easily and in huge quantities. Our modern world is made of cheap steel in all its variations.
To be more correct, the metal used in the Viking Age is 'bloomery iron' - which although it is much more like antique wrought iron than modern steels, it still has some important differences from even 150 + year old metal. Any of the metals created using the bloomery process always has microscopic layers of silica slag embedded within the structure. This gives the material a fiberous texture, and cause the metal to shear as it is stretched and formed - especially if the metal is pulled thin. (As a side - that is why VA 'currency bars' most commonly have one end flattened to a paddle shape, poor quality shows as de-laminated metal. This is always why things like cauldrons are made of narrow strips, which still remain much thicker than modern versions.)
Modern steel has a fine crystal structure, without the imperfections caused by the slag inclusions inside bloomery iron. Although an ancient sword maker would certainly select the best quality metal (least inclusions) available, the different textures of historic versus modern metals would result in quite different physical properties. At its simplest level, the iron would prove more flexible than the steel. It will be more likely to flex under load. It would prove relatively easy to deform it (bend), but it would take a lot of force to actually get the metal to fail. If stressed to the point of failure, the wrought iron would be most likely to start to de-laminate (splinter). The modern steel on the other hand is more rigid and thus less likely to flex, and even if flexed it would have a greater tendency to spring back to its original shape. It would take much more force to push it to the point of failure. Against that quality, the steel would have a much greater tendency to fail by shattering.

The next factor that will greatly modify the base metal, regardless of what method is used to convert it from ore, is the chemical composition of the material - the alloy. The major content of the metal is iron, which typically is going to account for well over 95 % of the material. The first other pure element which can be added to modify the metal is carbon. The second set of modifiers can be 'anything else' - even small trace additions can have significant effects.

In ancient through to relatively recent historic times, there was absolutely no understanding of how to modify the chemical content of the metal, no real 'science of metallurgy'. Accumulated experience through trial and (much) error would lead to ore from certain geographic locations being sought after as the raw material that would be most likely to yield metal bars of certain desireable qualities. This all balanced against the huge variables inherent in the smelting process and the fact at only ores located in very easy to gather locations could be used at all. Many surface deposits widely used during the Viking Age are various types of 'bog iron ore'. These accumulate relatively quickly (within decades) and thus are most likely to also vary in composition over time. Many of the most common additional elements found in natural combination with the iron oxide are in fact undesirable - and serve to reduce the functional characteristics of the smelted metal.
One major exception relates to one of those rare occasions mentioned where relatively pure metallic iron is found on the earth's surface. These are iron meteorites, which contain a relatively high per cent of the element nickel (generally 4- 5%). This alloy was not attainable in ancient times by any other source, so distinctively marks any object with this high nickel content. Generally most iron artifacts in existence from the Viking Age have NOT been tested for alloy content. so it is hard to know how rare the use of metal from this unusual source actually is. Nickel alloys do show up often enough to prove that this material was in fact incorporated into knives at least. The nickel will change the metal by making it both tougher and more rigid. It also vastly reduces the ability of oxygen to attach to the surface, hence slows the effects of rusting. All desirable qualities in the pre-Industrial world.
The situation for our modern metals is quite the opposite. Small amounts of sometimes quite exotic elements are added to the base of iron to finely adjust the resulting characteristics of the metal as it is produced. These effects are so well understood that commercial iron producers will precisely tailor the qualities of the metals they produce for specific industrial applications. ( Because weight is a factor, the front coil spring on a modern imported car is most likely a different alloy combination than those found on the rear of the same vehicle, for example.) Not only nickel (for 'stainless') but chrome, tungsten, cobalt... and many more are added in extremely small percentages to modern alloys, These additions create metals which may have any possible combination of qualities which the modern blade maker can pick and chose from. In this way a bar of metal that is simply ground or milled into a sword shape can have outstanding handling characteristics. These results would absolutely be impossible for the ancient swordsmith to duplicate.

Of all the possible elements that can be added to iron to change the quality of the metal, carbon is the simplest to combine. Modifying carbon was within the technology of the Viking Age swordsmith, and the effects were at least loosely understood. Additions of even small amounts of carbon (up to 2 %) can drastically change the handling characteristics of the resulting metal. For the blacksmith, useful amounts all range below 1% carbon. As carbon content increases, the metal becomes both harder and more rigid, unfortunately the trade off is the harder material also becomes more brittle. Some useful comparisons: modern mild steel has roughly .2 %, a truck spring has .5 % and a metalworking file about 1%. Comparison uses for those same carbon contents would be: standard table knife (at .2%), a sword blade (.5%) and a skinning knife (1%).
Every blade maker working with historic types of metals (not modern alloys) has to deal with what is called the 'bladesmith's dilemma'. A low carbon metal will be flexible, and thus survive impact, but it is soft and may bend and will not stay sharp. A hard high carbon metal will be hard and stay sharp, but is more likely to shatter on impact. This is why heavy chopping knives and long blade weapons historically have lower carbon contents than small detail cutting tools.

One of the simplest (technically, but involving much skilled labour) is by layering together soft and hard metals so as to lend specific qualities to certain areas within the body of a blade. In the Viking Age, a commonly seen method used for knives is welding a small piece of higher carbon metal to a larger block of soft iron. When forged to shape, the sliver of hard material becomes the cutting edge, in turn supported by the wrought iron. A more elaborate version is piling a stack of plates of alternating carbon content and welding them into a block. The most complex technique is forming several of these layered blocks then drawing them into twisted rods and welding several to form the core of a sword blade. This is the method to 'pattern welding' which creates distinctive herring bone patterns seen on the highest quality swords from the Viking Age. The purpose here is not mere decoration, but a union of hard and soft, rigid with flexible, down the spine of the sword. (There are several commentaries on pattern welding on the blog)

Once the ideal carbon content of metal for a specific type of blade has been chosen, there is yet another way of selectively altering the relative hardness. Any given piece of carbon / iron alloy can be modified down its length by carefully controlled use of the heat treating process.

That is the topic for part three...

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

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