Tuesday, 11 October 2016

The metallurgy of Oriental weapons

This blog post looks at the science behind kris manufacture. It's even more technical than the last post about kris, so I'm making this an extra mid-week posting for people with refined taste. Don't worry: normal service will be resumed with my next effort.

Iron and steel

Swords are essentially made out of iron. Pure iron is quite soft, which isn’t ideal in a sword. But if you heat iron in a charcoal furnace, some of the carbon from the charcoal combines with the iron to give you steel. Steel is much harder than iron, which is a good thing, but as you add carbon it becomes much more brittle, which is not. A brittle blade shatters too easily, leaving you holding a few inches of broken sword for the last few seconds of your life.

The trick is to have just the right amount of carbon, giving you a sword that is flexible enough not to shatter but hard enough to do the whole cutting into your enemy and killing him bit. Getting this balance, especially with primitive equipment, is as much art as science. Not for nothing were the first Malay smiths regarded as sorcerers.

Iron and steel were first made in ‘bloomery’ furnaces. Bloomery furnaces are not particularly hot, and the iron never melts. Iron produced in this way is never pure, always being mixed in with slag from the ore. 

Although iron has been made in this way for thousands of years, the limited quantities produced and its poor quality means that early kris were probably made using meteoric iron. Meteoric iron is usually found in small quantities and contains many impurities, including carbon. Depending on the amount of carbon, meteoric iron might be soft and flexible or hard and brittle. The early smiths discovered that if they beat together needles of iron from different sources they could produce a weapon that combined flexibility and hardness to give an ideal blade. A kris would therefore contain at least two kinds of iron and a good kris would have seven. The kris of the legendary hero Hang Tuah contained twenty kinds of iron.

By the 11th century smiths had developed ways of making crucible steel. Here iron and carboniferous material (e.g. plant matter) were heated in a crucible for several days. While bloomery iron was produced using air drawn in naturally, crucible steel was produced using bellows. The temperature in the crucible is much higher than that in a bloomery and the result is that the iron melts. The slag can be separated out, leaving a purer metal with a higher carbon content.

Making the kris

As crucible steel became available, it was possible to produce a flattened bar of steel that could be shaped to form the core of the sword. This steel would be hard but brittle. Softer iron was therefore welded both sides of the steel core, providing a protective layer of flexible iron. Another layer of soft steel was then welded onto that to form a harder (but not hard to the point of brittle) outer coat.

As the blade is worked, the spine of the blade is left thick, with the alternating layers of steel and iron providing a strong, yet flexible  weapon. Towards the cutting edges, though, the blade is ground away, so that at the very edge we are left with just the hardest steel from the core. This gives a vicious cutting edge, but, though it may chip, it will not shatter because of its protective iron coat.

The iron is not hammered on as a simple sheet, but is folded  into twisted strips before being welded onto the core. It is these twists that introduce the striations that will generate the characteristic pamor. Shaping and grinding the rough blade into finished shape reveals differing levels of the respective layers. Treating the blades with arsenic further emphasises the pattern, as does cleaning the blade with lemon juice or other fruit acids. (Some people still clean kris blades by cutting a lemon with them. The acid, though, is essentially eating away the blade and modern conservators would discourage this approach, however dramatic its short term effectiveness.)

The hammering is a vital part of creating the sword. Twisting, heating, and hammering further hardens the steel by changing its crystalline structure. It also strengthens the bond between the different layers of iron and steel. The hammering, though also adds to the waviness of the pamor, which are such an important part of the spiritual element of the blades and their aesthetic appeal to the modern collector.

With the different layers of iron and steel beaten and welded into a single blade, all that  remains is the final shaping of the weapon. The blade is heated once again and then, having been hammered into its final shape, it is plunged into cold water, not only completing the shaping process but also hardening the outer steel (which changes its crystalline structure) while allowing the iron, protected by the outer steel coat, to retain its vital flexibility.

This, then, is the way that kris were traditionally produced. The patterning was both a product of the manufacturing method and appreciated for its aesthetic and spiritual properties.

Crucible steel and the development of wootz

Meanwhile, the way in which steel was produced was becoming more technologically advanced. Smiths discovered that if crucible steel was left to cool very slowly it would form  large dendritic crystals of a ferrous carbon compound, cementite (Fe3C). If such steel was carefully forged at low temperatures, it would retain these large crystals which could (after polishing and micro etching) form a visible pattern on the blade surface. This is known as watering and steel produced in this way is often referred to as ‘Damascene’ steel or, in India, wootz. Fig 1 shows a comparatively crudely patterned wootz blade.

Fig 1

Usually the patterning is much more subtle. By careful hammering and working of the steel, smiths could work the watering into regular patterns. On this blade (badly displayed and almost impossible to see properly – yes, I’m looking at you, Wallace Collection) you can (just) make out a pattern of horizontal stripes across the blade (known as the ladder of the prophet) interspersed with rosettes. (Click on the pic for a larger, clearer view.)

Fig 2

Here’s a much less beautiful – but more easily seen – example of watering on a knife from Syria.

Fig 3

As you can see, there are similarities between the pattern produced in a damascened blade and that produced by pattern welding. Because Damascene steel was viewed as exceptionally strong (and is said to never need sharpening) swords made of wootz were particularly valued. (Game of Thrones fans can think of Valerian steel at this point.) Some blades were therefore pattern welded so that they could be passed off as damascened. At the same time, some blades were so polished – especially by Western collectors – as to almost entirely eliminate the patterning. (That's one of the reasons that the pattern is so difficult to see in figure 2.) The result is that, although the blades have very different qualities in use, it is often impossible to tell how the blade was made by visual inspection. (Note that I describe the blade in figure 3 as Damascene judged on appearance and provenance. It may well not be.)

Analysing blades by neutron diffraction

You can take a sample of the metal for analysis, but many museums are understandably uncomfortable with the idea of chipping bits off some of their finest exhibits. Fortunately, it is now possible to establish the nature of the metal by a process of neutron diffraction.

When neutrons are passed through metal some of the neutrons are distracted as they interact with the atoms in the metal. When they are passed through steel, the pattern of diffraction allows you to estimate the proportion of carbon in the steel. This will show whether the steel was produced by the bloomery process or if it is crucible steel.

If the steel is homogeneous, then the results will be the same whatever the angle at which the beam enters the metal. However, if these steel contains cementite dendrites, then the results will vary according to the angle at which the beam enters the metal because the carbon within the steel is not evenly spread. Neutron diffraction can therefore be used not only to state whether or not the metal is crucible steel, but also whether or not there is watering. Because the watering is a function of the crystalline structure, it extends throughout the sword even if it's surface appearance has been removed by over polishing. This approach, which reveals some sorts to be finer than they appear, also enables us to say with confidence that some swords which seem to have damascened blades were actually produced by pattern welding, as there is no evidence of dendritic cementite. (Pattern welded swords may be made from crucible steel. For example, much iron contains a small amount of phosphorus and if irons with different phosphorus content are welded together in the manufacture of a blade, it is possible to obtain a watered blade through pattern welding.)

The manufacture of wootz developed in India. It is unlikely that wootz was produced in Malaysia, but there was an extensive trade in wootz ingots. It is therefore quite possible that some later kris were produced with damascened blades.

The development of neutron diffraction analysis of metal weapons has led to a reassessment of some museum pieces. For example, many 19th-century Indian axe heads, richly overlaid with silver and/or gold turn out not to have been made of steel at all, but just plain iron.

Neutron diffraction does allow us a better understanding of the way these swords were made but, despite our sophisticated technology, there is nobody nowadays who can produce  the exquisite watered  patterns  of the finest blades of the past.

Acknowledgements and further reading

I was inspired to write this after a two-day conference at  the Wallace collection (All Depends Upon the Brave:Recent Research into Museum Collections of Ottoman, Middle Eastern and Asian Arms and Armour). I'm very grateful for all that I learned there. I have drawn particularly on the presentations by Prof. Alan Williams and David Edge. 

For an excellent discussion of the making of  pattern welded blades with details of how a modern smith achieves this effect, see http://englishhistoryauthors.blogspot.co.uk/2014/07/a-sinuous-and-deadly-beauty-pattern.html

Fig 1 is reproduced using a Creative Commons licence: awrose/Foter/CreativeCommons attribution-NonCommercial-NoDerivs 2.0 Generic (CC BY-NC-ND 2.0)

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