10.4. Crucible Steel
10.4.1 The Making of Crucible Steel in Antiquity
|Debunking the Myth of "Wootz"|
|Before I start on "wootz" I
want to be very clear on how I see this rather confused topic:
|If you are dreaming of wielding that wootz super-sword one day, enabling you to beat the competition, kill the dragons and win the heart and more of all those damsels out there - sorry for letting you down. My advice is to go see a shrink about the possible meanings of those dreams.|
|Now let's see what one can find out about crucible steel. The swords made from crucible steel will be dealt with in some detail in the next chapter.|
|What is Crucible Steel?|
|Can you produce iron in a crucible? Well - why not? Take a crucible, fill it with a mixture of (powdered) iron ore and charcoal, and heat it up. There is something like a direct reduction process between iron oxide and carbon that produces iron and carbon monoxide. I have described that process in some more detail in a science module. If something is possible, chances are that somebody did it. So did our remote ancestors make iron and steel in a crucible?|
|No - they didn't! It's not as simple
as it sounds. There are a few catches:
|I'm starting a touchy business here and want to be very clear. Whenever I use the assignation " crucible steel" in what follows, I mean:|
|Why is crucible steel a touchy
business? There are multiple reasons. Here is the list:
|Let's start by assuming that you,
personally, have a furnace that can go up to 1350 oC (2462
oF), and a crucible that can take the heat. You have build and run a
bloomery just for the fun of it - just like
Claes Vahlberg and his
buddies - and now you have some normal bloomery steel or wrought iron at your
disposal. In other words, you have an inhomogeneous mass of iron with an
average carbon concentration of around 0.3 %, slag inclusions, and possibly
some phosphorous. This is now your starting material for making crucible steel.
Two thousand years ago some of our ancestors could muster about the same equipment and materials. But you, in contrast to your ancestors, also know all about the science and engineering of steel! You decide to use this knowledge for turning your bloomery iron into good steel.
It is absolutely clear to you that you must melt the stuff. This is the only way to release all the entrapped slag and homogenize the carbon concentration plus whatever else is in the iron. You might also purify the molten iron a bit, along the lines used for copper purification or for making steel out of cast iron.
|You look a the iron-carbon phase diagram, of course. It will tell you what kinds of liquid you might be able to get at the highest temperature your furnace can produce. Just look:|
|Obviously, you have to have at least
about 2.1 % carbon in your steel if it is to melt at 1350 oC
Now you know what you have to do. You need to supply just the right amount of carbon to raise the level from the average carbon concentration in your bloom (about 0.3 %) to at least the required minimum of 2.1 %. It's easy enough to figure out how much carbon that will be per kg of bloom iron. So you take your (pieces of the) bloom, put them into your crucible, add the proper amount of carbon, cover it with a lid (with a hole so gases can escape), and put the filled crucible in your furnace. Run it up to the maximum temperature, keep it there for the time it takes to have a few beers with the boys, then let it cool down.
What you will find is a steel ingot snug in the bottom part, covered with a bit of slag. The slag comes from the slag still contained in the bloom, and because you might dissolve some of your crucible material. At high temperatures the slag will liquefy and sit on top of the far denser liquid steel, protecting it to some extent.
|Now to your questions. The first,
quite obviously, is:
|Modern scientists and smiths have
made crucible steel as described above; I will get to that in due time. Here I
only want to impress on you that making
crucible steel involves neither a secret nor is some magic required. It is
rather straight forward. Precise recipes for the process have been published by
several authors from 400 AD onward.
There are a few documents that were written before 400 AD and allude to iron / steel in ways that could mean crucible steel from India. Maybe, but probably not.
More important than a few very vague early writings that may or may not allude to crucible steel are the large amounts of early writings, in particular in India, that never mention crucible steel. We are justified in concluding that if crucible steel existed before 300 AD or so, it was of no importance.
|So let's assume that
(Indian) crucible steel appeared "on the market" around
300 AD. The first
surviving good recipe for making it dates to about 380 AD. It was written by
one Zosimos (ca. 350 AD - ca. 420 AD), a
kind of early alchemist in Alexandria. Refer to the special module for details
The problem we have with ancient crucible steel from around 300 AD to 1700 AD is not to figure out how it was made but:
|How good is (good) crucible steel? It
is certainly better in one respect than run-of-the-mill bloomery steel. Since
good crucible steel was once molten, it is homogeneous on a macroscopic scale
and free of slag or other inclusion. That makes automatically for far better
toughness properties. That is the bright side of crucible steel.
As always there is also a dark side: Ancient crucible steel is always high-carbon steel and thus rather hard but perfectly brittle, just like cast iron. In contrast to cast iron, however, it could be forged - with special skills, love, and tender care. That allowed to make useful objects like razor blades, surgical instruments, or files. For those products extreme hardness is good (e.g. for making very sharp edges that stayed sharp) and the still large degree of brittleness is tolerable because these objects were not exposed to high-energy impacts like swords.
One also could make crucible-steel swords with very hard and sharp blades that could be bend without fracturing to some large degree. Thise swords, however, were still rather brittle since the grains were encased in cementite as shown here, and possibly cold-short since crucible steel like any early steel often contained phosphorous with all the concomitant problems. But then there are also what I will call "wootz" swords, showing a water pattern on theire surfaces. This pattern is due to "spheroidized" cementite, made in rather tricky ways (just wait), that gives some ductility to the steel.
|Looking into these points in some detail will be a substantial amount of work. Let's start with the when and where.|
|Extremely Short History of Early Crucible Steel|
|What do we know about the early history of crucible steel? We have essentially
three sources we can exploit:
Let's start with the evidence from dug-up crucible steel work shops.
|There is one thing we know for sure
about crucible steel work shops:
we haven't yet discovered most of them. Until about 20 years ago, the only
known early places for crucible steel were in Sri Lanka and South India.
(plus many others), put the Merv area in present Turkmenistan on the
"crucible steel" map, in particular in her (unpublished but
available) PhD thesis work (2002 Univ London) and in later publications 1). Meanwhile we also have Akhsiket in
Uzbekistan 2) and a few places in
(South) India 3).
However, the crucible steel workshops in Central Asia that were dug up and investigated by an international team date to the early Islamic period, i.e. the late 9th - early 10th century AD. That is, maybe, still "early" but not very early. Anna Feuerbach, however, has reasons to believe that crucible steel technology in Central Asia goes back at least 500 more years.
The map below shows the known early places (plus Damascus) and makes clear that there is definitely a lot of room for more finds.
|I emphasize "early places" here because crucible steel has
been made for a long time - from at least 300 AD up to the 19th century in
India and Central Asia. Of course we know a lot more about later places than about the more interesting early
Contrariwise, there is presently no evidence at all that crucible steel has been made in Damascus / Syria or, more generally, in what one might call the "Arabic" countries, not to mention the "West" at large. There are a few written hints that some crucible steel was made in Syria and Egypt but nothing has been found yet. On the other hand we know for sure that "India" exported crucible steel on a large scale to the Arabic / Muslim countries for 1000 years or more.
Once more, this doesn't mean that the "Arabs" didn't know how to make crucible steel (or bloomery iron / steel). It just was easier and cheaper to import the stuff - considering that the Arabian peninsula, Egypt, and so on had neither much iron ore nor much wood for charcoal on their territories. India, in contrast, had plenty of both. In North Germany we do import our wine even so we know how to make (and drink) it.
|If one digs up an old crucible steel
work shop one tends to find parts of used crucibles, remains of furnaces, and a
bit of slag. The very few iron / steel objects left over only exist as rust or
small "prills" embedded in the slag or sticking to crucible shards.
To the best of my knowledge, no good ancient "wootz cake" or "pulad
egg" (I'll get to that) has ever been found so far. Dating the finds is
difficult, and interpreting what they mean not always easy. For example many
crucibles dating to 300 BC - 300 AD have been found in Kodumanal in India.
These crucibles may or may not have been used for making crucible steel. You
also need crucibles for the much older techniques of melting and casting
copper, bronze and gold. Crucible remains from other (Indian) places definitely
have been used for making steel (they might, for example, be encrusted with
steel prills) but no dating was possible.
In other words, we are a far cry from knowing who, where and when made the first crucible steel. We do know, however, that whoever did it, had to be able to run a bloomery and to make some pretty good ceramics, far more heat resistant than the normal pottery of those days. In addition they needed a furnace or kiln that could produce very high temperatures for many hours in a reducing environment. Moreover, without a good knowledge of forging technologies, crucible steel would have been of little value.
|All things considered, crucible steel could have been made as early as 500 BC or so - it's just not likely. If so - and personally I'm not convinced of that - it either was a small-scale operation or we have not yet found major centers. What we need to bear in mind is that almost anything that is known or assumed to be known about crucible steel and the swords made from it comes from around 300 AD or later, much later in fact. We simply do no know much about very early (= before 300 AD) crucible steel making at present. There are good reasons to believe that there might not have no been any. To quote Bennet Bronson, whose 1986 paper 4) in my opinion still sets the standards:||
pillar from around 400 AD, by the way, was made from bloomery iron. That
was the only way since the high-carbon crucible steel cannot be hammer welded,
it is said.
What we do know about crucible steel and most of the mythology surrounding it and those "true damascene swords" comes from its "late" history (after 1700 AD, say). The special module gives a short account of the late (and very confused) history of crucible steel.
|No let's look at old artifacts. For old steel artifacts, in particular swords, a detailed metallographic analysis can give clear indications if the object was made from bloomery or crucible steel. Crucible steel is free of slag, inclusions of which are always found in bloomery steel. Early crucible steel should always be hypereutectoid high-carbon steel, and thus contains a lot of cementite in one form or other. The concentration of carbon and other elements is rather uniform throughout the material, in contrast to bloomery steel.|
|Anna Feuerbach has analyzed a number of old blades and reported the results in her 2002 PhD thesis and in a paper from 2005 1). She found a few blades that have been made from crucible steel: "One of these blades (from 3rd4th century A.D. burials in the Russian Northern Caucasus) has aligned spheroidal cementite, the metallographic feature that produces the visible pattern. This is the earliest known crucible Damascus blade". If we assume that Anna is right, crucible steel and the forging technology going with it must have existed at least as early as 300 AD; a date we are familiar with by now.|
|More recently, Sharada
Srinivasan, an Indian authority on
crucible steel, reported on metallographic investigation of a (heavily
corroded) ring (see picture) form the South Indian site of Kadebakele, securely
dated to to 800 BC 440 BC. It is: "one of the
earliest known examples of pearlitic or higher carbon steel found anywhere in
the world. The structure is not inconsistent with that found in the crucible
fragment from Mel-siruvalur, which is confirmed to be from high carbon crucible
steel production" 3).
That is certainly true but the structure is not inconsistent with high-carbon bloomery steel either, something that ancient smiths could turn into small things like rings on occasion. Other iron artifacts from the same place and time were definitely bloomery iron, says Sharada.
|As far as early authors go, I already mentioned that
above. There are no definite statements from
the time before the Alexandrian historian
Zosimos wrote a detailed
description of crucible steel production during the 3rd century AD. He stated
that it was being used in India and Persia. This would date crucible steel
making to at least 300 AD once more.
Just to give a taste of what else is around: Alexander the Great reportedly received some special "white" steel from Indian rulers in 326 BC. This could have been crucible steel, putting the date of its first making before 350 BC or so - but this is just guessing. More to that in this module.
|What emerges once more is that the
making of crucible steel might go back to
about 500 BC but that there is no incontrovertible evidence for that. We know
that bloomeries could produce
cast iron and
thus also high carbon steel. Mrs. Srinivasan's ring thus might have been made
from a high-carbon steel piece that was produced in a bloomery. Whatever. If
people actually made crucible steel before about 300 AD is doubtful, and even
it they did we can simply ignore it at present for lack of data.
While the basic technique for making crucible steel never changed, we can be sure that the technology developed and that late processes were far more advanced and better controlled than the early ones. We do know this, actually. Many Westerners have seen crucible steel being made in India between 1795 and 1900. Some wrote about that and we still have about 20 written eyewitness accounts, some from people with a scientific training or a deep knowledge about iron and steel. Look at the "Late History" module for some more details
|The "West", however, never
caught on, and never made crucible steel of its own (with the
exception of the
Russians in the first half of the 19th century).
Benjamin Huntsman; if you
wonder about that statement, just melted steel in crucibles in the 18th
century; he didn't add carbon.
This is a puzzle of sorts. Overfunded spy organizations like the CIA or NSA are rather recent inventions but it is nevertheless not likely that nobody was able to steal the secret recipes, in particular because they weren't all that secret to start with. Plenty of written recipes were open to all that could read since 300 AD and certain ways for getting a working recipe would have worked just as well then, as getting big sports events to Qatar now, for example.
As always, it might have been plain old stupidity or religious / social bias (same thing anyway) that kept crucible steel making out of Western Europe. That attitude keeps equal rights for females out of many countries at present, for example. But, maybe, the real reason was that Western steel and Western swords were just as good as their Eastern crucible-steel counterparts. The Crusaders, after all, fought their way deep into the lands of crucible steel swords late in the 11th century and most of the 12th century with their bloomery steel swords, while the other side never quite made it into Northern Europe - and they sure tried for many centuries!
|Some Points about Making Crucible Steel|
of all you need a crucible that can take the heat, at least 1350 oC
(2462 oF) and higher is better! You can't make it from
"regular" clay because your crucible then would
start to melt around 1200 oC (2192 oF). So use
"special" clay and strengthen it with roughly equal amounts of
(silicon dioxide rich) rice chaff. Or find some other way, e.g. by using
"China clay" or stuff containing a lot of aluminum oxide.
Different cultures at different times found different solutions. The Sri Lanka / Indian crucibles, for example are quite black whereas the Central Asia stuff is rather on the white side - like modern porcelain.
I don't want to belittle crucible technology. It is absolutely crucial to the development of metallurgy, and much could be said about it. It's just just not crucial to the topics here. If you can make one that can take the heat, you can make crucible steel. If you can't, forget it. The schematic drawing shows what old crucibles looked like in almost natural size.
you need a furnace that can produce at
least 1350 oC (2462 oF). You can easily get
that kind of temperature close to the tuyere of a well-constructed and
vigorously blown bloomery but here you need to heat a crucible inside a kind of
is not so easy to do.
In Merv some remains of kilns from around 900 AD have been found. They were rather advanced constructions, certainly outperforming their counterparts from 500 BC or 100 AD. Several crucibles were put in a bed of charcoal inside a domed kiln and a lot of air was blown in from below.
|But nothing helps. If you want to make crucible steel with at most 2 % carbon, possibly a bit less, you must keep up a temperature of at least 1350 oC for hours and within a not too small area. One might speculate what you might get at lower temperatures, where you find yourself inside the "g + liquid" phase field. There you have a mixture of carbon-lean solid austenite and a carbon-rich liquid. This probably has happened a lot, and than you get "bad" crucible steel that is rather inhomogeneous.|
you need some bloomery iron and a source of carbon. For the latter you might
use those magic plants the shaman recommended or just about any organic waste.
Some plants might work better than others because they might supply a bit more
of "good" trace elements but we do not know much about that. The
decisive point is to add just the right
amount of organic matter. Since even you, the modern crucible steel maker, do
not know the precise amount of carbon contained in your bloomery iron and also
not exactly how much carbon will remain from the plants after pyrolyzing (some
of the carbon will escape as CO2 or CO), this is simply a matter of
trial and error, culminating eventually in a
Your iron needs to be crushed into suitable lumps. Note that for an "ideal" crucible steel process, you may want to have your iron more or less as dust or powder - but that is not something easily done with ancient technology.
For added value, you might also want to add a bit of "flux". It should be the right kind and the right amount, of course. Some manganese containing stuff would be good since a bit of manganese in your steel is always beneficial.
Then you might consider how to stack the stuff inside your crucible. Iron - flux - leaves? The other way around? Several layers in succession? There is no obvious good reason why this should matter but it is an issue with modern crucible steel makers.
Now put a lid on and maybe secure it by smearing some clay around the rim. Or seal your crucible in some other way. You need to do this for the reason mentioned above and for simply keeping stuff from falling in. You might want to put a small hole in the lid, though, because gases will develop with your particular mixture for almost sure. You should allow them to escape lest they blow up your crucible.
organic matter and not charcoal? My guess is that the decomposition of the
organic stuff at high temperatures
hydrogen (H2) and carbon monoxide (CO) among other stuff, gases
that will not only prevent the oxidation of the iron but remove the oxides
already there. The carbon produced by pyrolysis of leaves etc. will also be a
fine dust that can more easily coat the iron and penetrate it than lumps of
charcoal. The ash produced might help in slag formation.
But that's only a guess. There are reports that in more modern times charcoal was actually used as a source of carbon. If done just right there shouldn't be a problem.
|All you need to do next is to stew
your charge for a while - several hours, maybe a day or two, and then let it
cool down. You have two options for that:
|This is a rather important issue
since the cooling speed determines the general microstructure obtained at room
temperature, including in particular the amount of macro and microsegregation
of whatever impurities are contained in the iron. If a "water"
pattern can be formed in swords forged from the steel, and what it will look
like, is possibly determined by this structure.
I will come back to this in detail, meanwhile it would be a good idea for you to read up on those topics a bit.
Go ahead, Give it a try. Don't drink beer. Have your girl friend support you if necessary.
|Sorry about that but it's good to
have you back.
Now you can remove your crucible steel from the crucible (which you have to destroy in the process). You will have produced either
|The somewhat unusual identifier
"wootz" was and still is the
commonly accepted name for crucible steel in the West. Lengthy papers have been
written about its origin. "Wootz",
writes Sharada Srinivasan in 2009 3), "is thought to be a
European corruption of the south Indian Telegu/Kannada word for steel,
ukku". Ukku may derive from the
related Tamil word uruku, meaning to make
molten. It could also be related to ekku,
the old Tamil word for sharpness/ sharp spear. And so on.
As far as I'm concerned one needs to imbibe a lot of beer before one is able to pronounce "ukku" like "wootz". Maybe the British just didn't want to be outdone by the French in the art of mispronouncing foreign words.
|Pulad, Fuladh, Bulat, Bolat and so on simply is the (much older) term for "wootz" in most "non-Western" countries. It is also the word used by the old writers like Al-Kindi and Al-Biruni Maybe it relates to the Persian word "pulad" = steel. Or to some other words in some other languages.|
|While I'm at it: Damascene or damask, one of the many names for blades with a "water" pattern, could mean coming from the city Damascus, the capital of present Syria, that a smith named "Damasqui" has made the sword, or from the Arabic word damas = water.|
|The point is that both words are a
bit ambiguous. They do not always mean just "crucible steel". The
term "wootz sword", for example, denotes only a sword with a visible "water" or
"damask" pattern. While all
"water -patterned" swords were made from a wootz cake or bulad egg,
there are crucible-steel swords without
And now we finally can ask the most difficult question in the context of crucible steel:
|That is the question. It is a tricky question because an
ancient blade with a water pattern unambiguously proves that its maker could
manipulate his wootz / pulad in a way that produced the pattern. One might
speculate if somebody somewhere and sometime was able to produce a
"water"-patterned blade from a steel that was not a crucible steel? The answer seems to be no - as
long as we do not look at modern 20th century steels. I will get back to this
topic in a somewhat different context later.
Blades without a water pattern, however, do not prove anything directly. There could be several reasons for the lack of a well developed "water" pattern:
|Let's make clear what I mean with
"water" pattern. It means a special kind of a layered distribution of
cementite particles in the steel because, after some etching, a nice pattern visible to the naked eye is produced.
If, by contrast, the unavoidable cementite particles are distributed uniformly,
there is no nice pattern; at best the blade looks slightly mottled. I will go
into this in far more detail in the
Just to make sure we understand each other, here is a picture illustrating a few quick points about patterns on a blade:
|The left part shows the
"water" pattern of parts of a "wootz" blade. People gave it
many names and I don't care how you like to
call it - true damascene, perhaps. Important is only that the pattern results
from an inhomogeneous distribution of cementite
particles. You do not see the individual cementite particles but
their arrangement in the fuzzy white bands.
On the right is the center of a (modern) pattern-welded blade. Two kinds of steel are involved here. One that appears bright after etching, and one that appears dark.
The center shows a Japanese blade with a distinct "hamum". The blade actually consists of at least two kinds of steel but you see only one. The pattern is due to a different microstructure of one kind of steel. The bright parts are mostly martensite whereas the darker parts are bainite / pearlite.
I do not show a fourth variety; bands of pearlite grains separated by ferrite grains. This is observed in modern steels (with a microscope) and Anna Feuerbach mentions it in her thesis as a possible "wootz" pattern. I have yet to see this in an old steel, so there is no picture.
|Finally we are at the heart of all
the excitement and emotions about "wootz" steel and blades. The tough
questions to address now are:
|I'm talking more and more about sword blades now. Since this is the topic of the next and final chapter, I will stop right here and go on with a quick look at modern ways to reproduce crucible steel and use it for blades.|
|1)||Anna Maria Feuerbach:
PhD thesis work (2002 Univ London; Inst. Archaeology)
Ann Feuerbach: "PRODUCTION AND TRADE OF CRUCIBLE STEEL IN CENTRAL ASIA", Indian Journal of History of Science, 42.3 (2007) 319-336;
A. Feuerbach, D.R. Griffiths, and J.F. Merkel, Crucible Steel Manufacturing at Merv, Mining and Metal Production through the Ages", ed. P. Craddock and J. Lang (London: British Museum, 2003), pp. 258 266.
Ann Feuerbach: "An investigation of the varied technology found in swords, sabres and blades from the Russian Northern Caucasus"; iams 25 for 2005, p. 27-43 (Institute for Archaeo-Metallurgical Studies Newsletter).
|2)||Thilo Rehren: "As similar as black and white: steelmaking crucibles from South and Central Asia"; Archaeology International, open access, 23 October 2002, p. 37.|
|3)||Sharada Srinivasan, Carla M. Sinopoli, Kathleen D. Morrison, Rangaiah Gopal and Srinivasa Ranganathan: "South Indian Iron Age iron and high carbon steel: with reference to Kadebakele and comparative insights from Mel-siruvalur", in: "Metallurgy and Civilisation: Eurasia and Beyond" Archetype, London 2009, ed. J. Mei and Th. Rehren.|
|4)||Bennet Bronson: The making and selling of wootz, a crucible streel of India", Archeomaterials, Vol1, No. 1 (1986) p. 13 - 51.|
© H. Föll (Iron, Steel and Swords script)