Part 97 - Metallurgy & Maille, Pt 1
by Lord Thomas the Black
Metallurgy & Maille, Pt 1
Welcome back to another edition of Blackmaille!
This month, we begin a three-part
series on the science of metallurgy, and how it pertains to maille. This month,
we’ll take a look at the history of metallurgy itself, as well as some basic
concepts of metallurgy (tempering, annealing, alloys, etc.). Next month, we’ll
look at how this information applies to maille armor, and the month after, we’ll
examine how advances in metallurgy helped shape the rise and fall of the armor
A Brief History of Metallurgy
Metallurgy is the scientific understanding of metals. It includes classification systems, investigation into molecular and crystalline behavior, and the effects and uses of metals[i]. No substance has been as important as metal in the story of man’s control of his environment. Advances in agriculture, warfare, transportation, even cookery are impossible without metal. So is the entire Industrial Revolution, from steam to electricity[ii].
Gold was likely the first metal discovered and worked by man. Bright, incorruptible, and easily malleable, gold could be found in pure form in streams and riverbeds[iii]
From about 7000 BCE, a few Neolithic communities began hammering copper into crude knives and sickles, which work as well as their Stone Age equivalents and last far longer. This intermediate period between the Stone Age and the first confident metal technology (the Bronze Age) has been given a name deriving from the somewhat awkward combination of materials: the Chalcolithic Period, from the Greek chalcos (copper) and lithos (stone)[iv].
The use of fire made possible two significant steps in the development of metallurgy: The casting of metal, by pouring it into prepared molds; and the smelting of mineral ores to extract metal. Objects made from smelted copper, from as early as 3800 BCE are known in Iran[v].
Sometimes the ores of copper and tin are found together, and the casting of metal from such natural alloys may have accidentally provided the next step forward in metallurgy. It was discovered that the two metals, when cast as one substance, were harder than either metal on its own[vi].
The cast alloy of tin and copper is bronze, a substance so useful to mankind that an entire period of early civilization became known as the Bronze Age. A bronze blade will take and hold a sharper edge than copper, and bronze ornaments and vessels can be cast for a wide variety of purposes[vii].
The technology of bronze was first developed in the Middle East. Bronze was in use in Sumer, at Ur, around 2800 BCE, and in Anatolia shortly thereafter. It appeared in the Indus Valley around 2500 BCE, and progresses through Europe from about 2000 BCE. At much the same time, it is found in China. From about 1500 BCE the Shang Dynasty produces bronze objects of exceptional brilliance[viii].
In all these regions, it is the rulers who use bronze, as a luxury for themselves, or as a weapon for their armies. For the ordinary people, the Stone Age survives well into the Bronze Age[ix].
The next great development in metallurgy was the discovery of iron, around 1500 BCE. Much more difficult to work than copper or bronze, iron has a melting point too high for primitive furnaces to extract in pure form from its ore. The best that could be achieved was a cluster of iron globules mixed with sludgy impurities. These impurities were then forced out through repeated heating and hammering[x].
By 1200 BCE, it had been discovered that iron could be improved if it is reheated in a furnace containing charcoal. The carbon from the charcoal transfers to the iron and the result is steel. Steel can be worked just like iron, and it will keep a finer edge, capable of being honed to sharpness. From the 11th c BCE on, steel gradually replaces bronze in the Middle East. It became essential, then, to have a good steel blade, rather than a bronze one[xi].
The Riddle of Steel
As said before, the first metal to be smelted from ore was copper, and the first copper-smelting furnaces were probably modified pottery kilns, where copper ore was heated with charcoal, with the following results[xii]:
Charcoal burns to form carbon dioxide (C + O2 = CO2). At higher temperatures (@ 1000 degrees Celsius), the carbon dioxide reacts with more carbon to form carbon monoxide (CO2 + C = CO). The carbon monoxide gas reduces the copper ore to copper. This reaction is simplified if we treat the copper ore as copper oxide only (CuO + CO = Cu + CO2). A mixture of metal and slag (from the non-metallic impurities) was formed in the furnace, and this was subsequently broken up and the copper melted in crucibles to purify it.
Unlike those of copper, iron ores are very widespread, but the extraction of iron is not so simple, due to its higher melting point (around 1550 degrees C)[xiii]. An attempt to smelt iron in a copper-smelting furnace would result in a useless mixture of iron and slag. Even if the iron is exceptionally pure, there is still sufficient silica (silicon dioxide, SiO2) present in the stones and clay which make up the wall of the hearth to react with part of the iron ore and form a slag (FeO2 + SiO4 = Fe2SiO4, iron silicate)[xiv].
Excavations in Sinai show that the Egyptians used a sophisticated smelting technology around 1200 BCE. By reducing the copper ores in a bowl-shaped hearth with charcoal, assisted by the blast of bellows, and using iron oxide, manganese oxide, or limestone as fluxes, the liquid slag could be “tapped off” to separate it from the copper[xv]. These furnaces resembled those later used for smelting iron; in fact, both types produced mostly iron silicate slags under free-running temperatures around 1200 degrees.
Iron ores reduced under these conditions can produce iron free from most of the slag, which when it liquefies, runs down away from the still solid iron, the particles of which would be left sticking together as a lump (or “bloom”), porous in form and containing very little dissolved carbon but much entrapped slag. Such furnaces are called “bloomery hearths”, and their produce “bloomery iron” or “wrought iron". Repeated heating and forging is necessary to remove the remaining slag and consolidate the bloom[xvi].
The product of the bloomery might be a lump containing widely dissimilar elements, some of which might be of higher carbon content than others. Early smiths would have found that some samples of “iron” were harder than others, but whether they could be deliberately produced was another matter. The simplest way of obtaining steel is to simply make a large bloom, break it up, and separate out the hardest fragments. These fragments would then have to be forged back together to make anything but the smallest item. A similar technique has been used by Japanese swordsmiths for centuries[xvii]. Though techniques such as this one, as well as case-carburising, and forge-welding steel to iron were used, it would be a long time before the production of steel could be anything but incidental. The abundance of iron ores, however, meant that iron tools and weapons could be made much more cheaply than those of bronze, and would therefore be available to many more people once the techniques of smelting and forging were widely known[xviii]. So, for many users, stone tools and implements were succeeded not by bronze, but by iron ones, even if those early iron tools were little better (if at all) than those of bronze[xix]. Indeed, bronze remained in use alongside iron for many centuries.
Iron weapons and armor did not become superior to bronze until the discovery was made that quenching (plunging the red-hot metal into cold water) after carburization resulted in a dramatic increase in hardness. The process is a difficult one to manipulate, however, as the hardness is due to the formation of martensite, an excess of which causes the steel to become brittle[xx]. Quenching is first mentioned by Homer in the Odyssey (IX, pg 459), and quenched edges have been detected on excavated specimens from the 10th c BCE onwards, but the difficulty of controlling the carbon content of steel meant that quenching was to remain a hit-and-miss process (and therefore avoided by many smiths) for a long time to come[xxi].
If steel is allowed to cool in air after forging, then equilibrium conditions will prevail. The carbon which was dissolved in the iron above 900 degrees C comes out of solution as a combination of iron carbide and ferrite called “pearlite”, which has a distinctive layered appearance under a microscope[xxii].
The scale of hardness commonly used is the Vickers Pyramid Hardness scale (VPH), and its units are kg/mm[xxiii].
Pure ferrite has a hardness of around 80 VPH, but since medieval iron contains slag as well as ferrite, and traces of other elements, then its measured hardness may be around 100 to 180 VPH. The presence of iron carbide makes pearlite much harder, so steel may have a hardness of anywhere from 180 VPH (for 0.2% C) to 260 VPH (for 0.6% C). Grain size will also affect hardness; a finer grain is harder, as smaller crystals are more difficult to deform.
However, if the steel is cooled more rapidly than it cools in air, and equilibrium is not attained, then other crystalline products may form, such as pearlite, bainite (a needle-like material), and martensite (a material of great hardness). Much depends on the carbon content and size of the object being treated, but quenching in water generally results in such rapid cooling that an all-martensite structure is obtained. This is called “full-quenching”. Quenching in oil, molten lead, or some other less drastic coolant than water may allow other crystalline structures to form as well as martensite – as may an interrupted or delayed quench. Such procedures are collectively known as “slack-quenching”, and while avoided nowadays, seem to have been widely practiced in the Middle Ages[xxiv]. Slack quenching will increase the hardness to between 300 – 400 VPH, depending on the proportions of martensite, pearlite, etc. Full-quenching increases the hardness enormously (depending on carbon content), up to 800 VPH or more (for 0.6% C). It is also worth noting that a higher carbon content will lower the temperature at which martensite forms[xxv], so steel with much more than 0.6% carbon may not show a proportionate increase in hardness, because the transformation to martensite will not have been completed before room temperature has been reached.
The appearance of martensite as a microconstituent does not, of course, prove any deliberate steel production on the part of the smith. He may well have quenched everything he made on the grounds that it might improve the tool or weapon, and was unlikely to do much harm, at the low carbon contents prevalent back then[xxvi].
The customary modern procedure for
hardening a plain carbon steel is to fully quench it, then carefully reheat it
to “temper” it. Tempering reduces the hardness of martensite somewhat, but by
removing most of the internal stresses, it reduces its brittleness and hence
increases its impact strength. Tempering may reduce the hardness to 400 or 500
VPH and improve toughness by removing stresses which lead to cracks, but its
success is dependent on having steel of constituent composition, and the
accurate control of both time and temperature, neither of which could be
measured with any precision during the Middle Ages. So, it is not surprising
that many medieval armorers preferred slack-quenching, with some small margin
of error, to the combination of full-quenching and tempering,even if the
slack-quenched product was of inferior strength[xxvii].
So that does it for our beginner’s look at metallurgy. A lot to take in, isn’t it? Next month, we’ll look at how all this pertains to maille armor, and the month after, we’ll explore the medieval armoring industry’s rise and fall.
Thanks for joining us for another month of Blackmaille! As usual, any questions, comments, fan mail or hate mail can be sent to:
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See you next month!
[i] McCreight, Tim “The Complete Metalsmith” pg 2 Davis Publications, 1991
[xii] Williams, Alan “The Knight and the Blast Furnace” pg 3, 2003 Brill – Leiden, Boston
[xiv] Ibid, pg 4
[xvii] Kapp, L Kapp, H. & Yoshihara, Y. “The Craft of the Japanese Sword” (Tokyo, 1987)
[xviii] Williams, Alan “The Knight and the Blast Furnace” pg 8
[xix] Smith, C.S. “Metallographic examination of some fragments of Cretan bronze armor from Afrati” Appendix III in “Early Cretan Armorers” ed. H. Hoffmann, Fogg Art Museum (Cambridge, Mass. 1972) 54.
[xx] Williams, Alan “The Knight and the Blast Furnace” pg 8
[xxii] Ibid, pg 17
[xxiii] McCreight, Tim “The Complete Metalsmith”
[xxiv] Williams, Alan “The Knight and the Blast Furnace” pg 17
[xxv] Honeycombe, R.W.K. “Steels: microstructure and properties” (1981).
[xxvi] Williams, Alan “The Knight and the Blast Furnace” pg 18
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