Carbon Content
As we discuss the heat treatment techniques ( part1, part 2) used by Medieval smiths it is important to remember their raw material consisted of wrought iron and iron alloyed with carbon to create a steely-iron mix. This material would have a range of carbon content from none up to approximately 1.3 percent of the whole. This variability occurred not only in each smelt but in each object made. The quality and cohesiveness of this mix would have been quite variable by maker, smelting batch, and ore source. This means that the individual item, whether a hammer or sword, would most likely have a carbon content that was not consistent throughout its whole.
The amount of carbon in the material at any point regulates the hardness one could achieve with any of these methods outlined in the descriptions from MS 3227a. This indicates that it would be essential that the smith be able to grade the materials available for different projects. If the item is pure wrought iron it will not harden in the heat-treating process. Once they choose their material, the process most likely to attain the desired results would be used to help them overcome the shortcomings that their raw material presented to them.
Metal is cool
Metals are composed of a crystalline lattice structure; the only molecules present being those that are impurities in the material. The use of heat treatment alters this crystalline structure, placing the lattice into varying degrees of strain. This occurs as the carbon and iron atoms are trying to occupy the same space in the lattice.
Prior to heat treat the carbon is in a body centered cubic formation.
When brought to forging temperature, approximately 1250 degrees Fahrenheit, or critical temperature, the carbon is now in a face centered cubic position.
So each grey dot is a carbon atom on the face of the cube, again the cubes are interlaced.
When cooled very quickly or “quenched”, the result is a tetragonal body centered formation where the carbon and iron are frozen in a position of strain; this makes the item harder than its original state.
The resulting structure is captured in the faced formation by the quickness of the quench. If the object were allowed to cool slowly the carbon would recenter.
Science geeks can see deeper discussion of structures above here
Quench
The quench or cooling to arrest the formation of the lattice in the new position must occur in a few seconds at most. The strain that results can be so hard as to make the item brittle and easily broken or, if carbon content is high enough, even crack.
The techniques of heat treatment allow the smith to graduate this hardness and so allow the tool to have a useable surface that will not suffer failure when used in a given application. The reheating of a quenched item at a lower temperature is today called the temper. It results in the reduction of the strain applied by a full quench. Thus, any heat treatment process is a balance between the hardness desired and the durability or ductile nature needed for a given object to be functional. In a blade this is the factor contributing to edge durability.
Etching of alchemist at work, this would have been the study of chemistry, metallurgy and other areas we identify as science today.
Medieval Temper
This complex relationship was understood by the medieval smith as an achievable result but not on the complex level described above. The Pol Hausbuch MS 3227a describes the need to release the hardness induced by some types of quenching. At the end of the recipe section we come to a series of treatments to reverse the hardening process. In the first case of these the item is being treated to achieve a hardness that is less but not fully soft, what we would call today being tempered. This is what allows a sword edge to hold its edge well but still be soft enough to allow resharpening.
“If you wish to release the hardness from the iron; take human blood, and ‘leave it out’ till the water comes to the top, then strain the water into a glass and keep that. And when you then want to release the hardness, so take the hardened weapon and hold it to the fire until it is so hot that it ’slurps - swallows-up’ (slinde) water. Then brush the water with a feather on the edge, thus the hardness is let out and it will become more flexible (linder).”
-note: Here we see the use of blood plasma as a temperature indicator. The medium will be relatively consistent and mirrors closely some of the brine solutions used in modern heat treatment. The dominant components of this plasma would be sodium chloride and water.
The particular sound “slinde” is the “tell” that would allow an experienced craftsperson to narrow in on a particular temperature range. Taking a hardened piece and tempering with heat so the edge achieves a working hardness. This allows for good edge holding ability but also a durable edge as described earlier. Without this activity the object, a sword especially, would be prone to breakage and chipping. So we see the use of available components being used to "release" the hardness as they experienced the procedure. Today we would understand it as controlling the temperature to reduce the strain in the carbon iron lattice.
If you have not seen them as yet check out parts 1 and 2 of this series.
Translation by Keith Alderson, 2008, unpublished. Additional translations consulted unpublished Matt Galas 2002.
Read Part 4