I've seen various places refer to the evidence and possibility of a Medieval or Renaissance use of Differential Temper, some more certainly than others, but the problem is I've never really seen 'smoking gun' evidence for this process, mostly just things which are enough to warrant the possibility and a tentative prediction that some might have used it. Nothing really decisive for its existence in the Medieval or Renaissance period. As much as I've searched though, it's hard to find a really in depth discussion of the topic to get myself closer to the truth. So, in my frustration, I'll propose the question; how much evidence for this technique is there, really? How 'valid' of an idea is it considering the limited knowledge we have of the specifics of the subject at hand? I've heard variously that there is little documentation for this, but is the physical evidence enough to maintain the concept as a plausibility for some smiths during this time?

Thank you for your consideration.

Are you referring differential temper or differential hardening? They are not the same thing. I cannot answer your question directly but will comment that with the simple (low-alloy) steel/carburized iron that was used during these periods, differential hardening can and sometimes will happen whether it is intentional or not. These low-alloy steels are referred to as "shallow hardening" because they will only harden to a certain depth, and when quenched in slower mediums such as oil, they may harden only the thinner areas of a blade, leaving thicker areas and parts of the interior un-hardened. I have had this happen with 1075 and 1095 steels when quenching in oil and it caused visible hardening lines similar to a hamon.

If you have not read it yet, this article- http://www.myArmoury.com/feature_bladehardness.html will give you an idea just how difficult it is to answer your question with any certainty, unless someone has a writen reference to such techniques. Varying hardness is typical but to what extent this was intended by the makers is open to speculation.

Early medieval swords, of piled construction, probably did have varying hardness due to premium more hardenable materials being utilized in the edges. That is more of a hardening characteristic that was inherent in the material construction.

Differential tempering (usually a two step process where the cutting edge is insulated from or not in direct contact with the higher temperature second heating source) sounds tricky to execute on a double edged broadsword. I guess the center core section of a broadsword could be set upon a heated bar. I am curious how others think it would be accomplished?

European swords were often made to wrap a soft core around a harder jacket (that was pretty much universal among sword-wielding cultures), but I don't think even pattern-welded Norse weapons ever had a true hamon. I seem to recall Chinese weapons with them, though; both single and double edged. The thing to keep in mind is that the blade geometry of a katana was completely different than, say, a Type XVa, and the armor that these swords opposed was also different. I don't believe that a sword of Western design (particularly one with any kind of agressive profile or distal taper) would have gained anything by tempering of that specific variety. It seems to me that you'd just be asking to snap your nice pointy tip off by making it too brittle. A Japanese sword, by contrast, often has very little distal taper and almost no profile taper until you reach the kissaki. That makes it easier (in my admittedly layman's understanding of the process) to support such radical differences in the hardness of the steel between edge and spine.

I have seen some short custom pieces (Don Fogg, et al) with this sort of thing done, but I have no idea how well the function of the weapon is served by mixing these two worlds, or how well it would hold up in sword sizes.

Axes and chisels are traditionally given a partial quench (the edge and maybe an inch or two back are quenched), then the residual heat is allowed to 'creep' towards the edge until the oxides hit the right color (somewhere between straw/gold and peacock/apple), then it is quenched again to stop the process.

I do not know how far this tradition goes back, but it is possible they have always been tempered this way....

Dr. Alan Williams writes in a paper freely available at the Gladius revista's website:

The methods of heat-treatment employed in the Middle Ages included:

(i) Full quenching (to form an all-martensite structure). If the
overall carbon content was low, a full quench without tempering might
be employed.

(ii) Slack-quenching (not quenching fast enought to produce an all martensite
structure, but a mixture of martensite, and other products;
bainite and/or pearlite). The sword could be plunged into oil, boiling
water or some other quenching medium less drastic than cold water. The
hardness would be less than that of an all-martensite structure, but its
brittleness would also be less. Tempering would not be necessary, unless
the carbon content was high. An interrupted quench (i. e. plunging into
water for a few seconds, withdrawing and then quenching again) might
well produce a similar mixture of transformation products.

(iii) Slack-quenching and tempering. Reheating a slack-quenched
steel would yield a mixture of products, some of which would be difficult
to resolve, and many of which would have become aggregates of carbide
particles whose origins could not be diagnosed. It is possible that
some such process was responsible for forming Piaskowski's < cicular
troosite), and Anteins' <(incompletely tempered structure),. This process
was probably employed to harden some of the swords discussed in this
paper, but it cannot now be determined by optical metallography.

(iv) Time-quenching. A now obsolete process whereby a blade
would be plunged into the cooling liquid and then withdrawn after a fixed
number of seconds. The transformation structure first formed would
be tempered by residual heat. The products would be a mixture of tempered
martensite and bainite in bands. (9)

paper published in

Consejo Superior de Investigaciones Científicas
http://gladius.revistas.csic.es

Hi Joshua

When you look at the evidence provided by Dr. Williams research and combine that with the descriptions of adjusting the hardness of items in literature I think we can say they did use a variety of tempering regimes to accomplish what they wanted in material qualities. While they may have not described it as such, they where adjusting the temper of the piece. The caveat must be applied that they did not have hardness scales or any way to check temperature exactly or even to monitor time the way we do, so these were inexact operations compared to the way we approach a project today but they were able to get results they wanted for specific tools and weapons.

Best
Craig

I imagine that rigorous trial testing a sword, or any other tool, I would imagine, was also part of the process. at least for anyone who wanted to risk potentially expensive product for better business.

I mean, at least until said smith has a good system working for him.

Sirs-Alot of the problems we face in this dicussion are caused by the fact that a smith had no way to keep a" patent" except by keeping the process he discovered to differentally harden or heat- treat a blade was to keep it a secret. So we are reduced to guessing.

I do not think we need to look at each smith developing a system for themselves in the historical frame work. This is a modern concept and one that even today is not quite accurate. In fact almost no one could develop the knowledge of how to make a good sword in a life time of trial and error in complete seclusion. Just to many variables.

I think it would have been easier in period for the information to flow from generation to generation than it is today. The idea of a lone smith producing great blades from raw ore is just not accurate to the records we have of how the weapons where made. I am not suggesting that anyone is advocating this model but it is crucial to realize that all of these arts where traditions handed down from one generation to the next and that no one was out there trying to figure it out on their own. But rather would have been part of a shop or production group that would be concerned with producing swords the right way over time. Yes, surely the human element of talent and commitment to end results would fluctuate over time but this would be the human factor not the art .

In the context of their knowledge and skills they were doing things in a scientific way. It was a science based on experiential repetition. The world around them was governed by what they could see and infer. Not what was observed and reported in object detachment. Thus when one sees in a period text the reference to "Master Alchemy", it is not a reference to a particular individual but rather the acknowledgment of the art of this science or body of knowledge. It was something that we see written down and commented on in text. It was not available to the average joe but would have been something that a metal worker would have knowledge of.

Best
Craig

The chemistry of medieval era steels is relevant. Most of medieval swords with known composition have inappropriate chemistry (too little carbon on average) for a modern spring steel type of heat treat. http://www.myArmoury.com/feature_bladehardness.html Some wootz or damascus specimens may have been exceptions, but, they tended to be legendary for being able to withstand something like a 30 degree bend without taking a set. I'd love to know how many museum specimens actually exhibit spring temper like quality, but, doubt anyone is going to subject them to that kind of test. (Verhoeven did mechanical tests on fragments of bulat, wootz, and a couple of others and concluded that localized mechanical behavior was at best comparable to, but not quite as good as, modern "mild steel.")

Given a simulated reconstruction of similar lower carbon chemistry, I would go for a fast slack quench (recipes similar to 15th century and later ones surviving describing equivalent of low salt brines and oils such as "blood of stag in rut" and "animal fat." Similar simple recipes are still popular with current blacksmiths who use variety of fat and soap based "goops" for quenching instead of expensive oils that are hard to acquire in small quantities.) Reheating of the blade and quenching in an organic medium (carbon rich -case hardening by wrapping in goat skin and reheating, also historical for files and such) could be used to case harden the edge and surface to a shallow degree. Making a blade in such a way that it could deflect up to 30 degrees and spring back would be very difficult as proposed above. I suspect this is why so many archeologists refer to historical blades of pre-rennaisance era as "iron swords."

http://www.myArmoury.com/talk/viewtopic.php?t=16009 According to Sa'ar Nudel, this crusader sword can be flexed for 30 to 40 degrees and return back true.

Another thing that influences the depth of hardening in simeple steels is grain size. Courser grain increases the depth of hardening but also weakens the steel. If the grain gets too small it will prevent hardening altogether even though it will make a very tough blade.

I wonder if anyone has any knowledge of the chemical composition of the crusader sword?

I do believe that good spring temper swords were sometimes made, perhaps even consistently, by those with ideal ores and careful refining processes (not burning the carbon out of it in forging). The ability to assess a random supply of ore chemically and know what to do with it is 21st century technology though. It is hard to carburize steel much above 0.4 to 0.5% carbon without a specially controlled atmosphere. Crucible steel is at least one example where an ideal atmosphere was achieved with period technology and uncommonly good (carbon content in about the right range to start with) raw ore.

The feature article illustrates a lot about this. A simple carbon sword needs to be fairly homogeneous in carbon content, and well above 0.4% carbon to accept a good spring temper. I suspect the plots represent all data from a lot of swords sampled in different areas. Even if you allowed for the possibility of all of the swords being uniform in carbon content, only a minority would be good candidates for spring temper. If you factor in the reality that many have concentrations of carbide from failed tempering, which does show up as high readings in samples, the really good ones (like the crusader example) were probably a pretty small minority.

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