Historical Sword Heat Treatment pt2
Control of the hardness of steel (iron with an alloy of carbon) is a crucial process to create blades and weapons. Iron without any carbon can only be slightly work-hardened by hammering creating stresses in its structure (the same techinque used to create a harder edge on bronze weapons). When iron has a bit of carbon as an alloy (usually 1.2 % or less) it is steel. If it has more it becomes cast iron and not suitable for blades due to brittleness and no ductility. Our Medieval predecessors thought of it as hard or soft iron or as steel being a purer form of iron then the iron with no carbon. They would then strive to add characteristics to the material using ingredients that exhibited these attributes from their surroundings that they desired. Dr Fabrice Cognot, Phd described it as the medieval craftspersons intention to change the materials temperament as opposed to a modern concept of temper being influenced by the process and recipes described.
Quenching
As we saw in part 1 the medieval smith had an experiential reference base to work from. Using tradition and simple tools they were able to craft the great metal work we see from this period. Pol Hausbuch MS 3227a illustrates the understanding they had of what types of material and hardness they needed for different objects. We will look at some of these and see the intersection of our understanding of the process with theirs.
MS 3227a
We start with the tools to make weapons. Blacksmithing was known as a base art one on which all others must rest for the tools to do most other things must come from the smith.
"The hammers with which one pounds files or all weapons and with which one wants to scrape metal – the temper is thus. Take one part “rueberetich” (possibly a white-radish?) and one part horseradish and one part earthworms-larva and one part buck’s blood when the buck is in the rut – the temper truly/fully has the four elements. Pound that all together and press out the water; and what ever your wish then to harden in that, polish/file it first* and then harden it therein (in the juice of the four ingredients)."
-note: Here we see practical elements we recognize today as beneficial to a quenching process being advocated. They see the different elements contributing their characteristics to the final product, we recognize factors that we still use today in heat treatment. Today many smiths use brine as a quenchant instead of water, this is due to the problem of steam envelopes being created in a pure water quench. This will insulate the piece and slow the quench to a point it will not harden. The salt in brine or the other elements in the medieval quench add particulates to the water or base liquid and the steam envelopes collapses before they can form. Modern steels with their high carbon content and alloys often will need specialized quenching oils and additives to not create cracks or fractures in the material itself. In the medium to low carbon content of most medieval steel these recipes would work well.
The last bit in this description addresses the polish as important as well. It is usually beneficial to quench a piece when it has all of the scale from forging removed. This allows for even heating and hardening. It also reduces the material to the least amount needed to quench thus less work after it has been hardening. This means less wear on valuable tools and time in production. New scale may form as it is quenched but a successful hardening quench will shock or flake off the scale that forms. It can be used as a sign of success and the color of the steel can indicate the tempering temperature achieved in an interrupted quench.
“If you want to get a real temper (i.e. ‘super-temper’); So/thus take “dragon-root” with stems and leaves and everything and also lots of “ice-herb” (Eyeserkrawtes) and strew that into lukewarm water and when it is thoroughly boiled/steeped, take it from [the heat] and let it cool and even get fully cold and harden what you want in that.”
-note: Here we see a recipe that is plant based and includes the entire plant. It is in essence a room temperature vegetable soup. The inclusion of the plant would again add a good deal of particulate matter into the quench medium increasing the ability to cool the metal quickly enough to achieve a working hardness for the object being heat treated. Some of the plants described in fact have concentrations of silicon dioxides and calcium carbonates, both of which are used today as flukes and refining chemicals in the smelting of iron. This may have helped with reduction of scale in a quench.
In our third installment we will look at some of the ways MS 3227a describes releasing the hardness from a hardened tool or weapon, what we call tempering today. This resulted in a workable tool or blade that would not be so hard as to be brittle which would break or difficult to sharpen.
Translation by Keith Alderson, 2008, unpublished. Additional translations consulted unpublished Matt Galas 2002.