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TAMIL HERITAGE...
the Tamils are an ancient people
Wootz Steel - an Advanced
Material of the Ancient World
S. Srinivasan
and S. Ranganathan
Department of Metallurgy, Indian Institute of
Science, Bangalore
Abstract
The development of ancient Indian wootz steel is
reviewed. Wootz is the anglicized version of ukku in
the languages of the states of Karnataka, and Andhra
Pradesh, a term denoting steel. Literary accounts
suggest that the steel from the southern part of the
Indian subcontinent was exported to Europe, China,
the Arab world and the Middle East.
Though an ancient material, wootz steel also
fulfills the description of an advanced material,
since it is an ultra-high carbon steel exhibiting
properties such as superplasticity and high impact
hardness and held sway over a millennium in three
continents- a feat unlikely to be surpassed by
advanced materials of the current era.
Wootz deserves a place in the annals of western
science due to the stimulus provided by the study of
this material in the 18th and 19th centuries to
modern metallurgical advances, not only in the
metallurgy of iron and steel, but also to the
development of physical metallurgy in general and
metallography in particular.
Some of the recent experiments in studying wootz
by re-constructing composition, microstructure and
mechanical behaviour, along with some recent
archaeological evidence, are described.
Wootz, High-carbon Steel, South India,
Superplasticity, Crucibles, Analyses
1. Introduction
India has been reputed for its iron and steel
since ancient times. Literary accounts indicate that
steel from southern India was rated as some of the
finest in the world and was traded over ancient
Europe, China, the Arab world and the Middle East.
Studies on wootz indicate that it was an ultra-high
carbon steel with 1-2% carbon and was believed to
have been used to fashion the Damascus blades with a
watered steel pattern. Wootz steel also spurred
developments in modern metallographic studies and
also qualifies as an advanced material in modern
terminology since such steels are shown to exhibit
super-plastic properties. This paper reviews some of
these developments.
2. History of wootz steel
There are numerous early literary references to
steel from India from Mediterranean sources including
one from the time of Alexander (3rd c. BC) who was
said to have been presented with 100 talents of
Indian steel, mentioned by Pant [1]. Bronson [2] has
summarised several accounts of the reputation of
Indian iron and steel in Greek and Roman sources
which suggest the export of high quality iron and
steel from ancient India. Srinivasan [3], Biswas [4]
and Srinivasan and Griffiths [5] have pointed out
that the archaeological evidence from the region of
Tamil Nadu suggests that the Indian crucible steel
process is likely to have started before the
Christian era from that region. Zaky [6] pointed out
that it was the Arabs who took ingots of wootz steel
to Damascus following which a thriving industry
developed there for making weapons and armour of this
steel, the renown of which has given the steel its
name. In the 12th century the Arab Edrisi mentioned
that the Hindus excelled in the manufacture of iron
and that it was impossible to find anything to
surpass the edge from Indian steel, and he also
mentioned that the Indians had workshops where the
most famous sabres in the world were forged, while
other Arab records mention the excellence of
Hinduwani or Indian steel as discussed by Egerton
[7].
Several European travellers including Francis
Buchanan [8] and Voysey [9] from the 17th century
onwards observed the manufacture of steel in south
India by a crucible process at several locales
including Mysore, Malabar and Golconda. By the late
1600's shipments running into tens of thousands of
wootz ingots were traded from the Coromandel coast to
Persia. This indicates that the production of wootz
steel was almost on an industrial scale in what was
still an activity predating the Industrial Revolution
in Europe.
Indeed the word wootz is a corruption of the word
for steel ukku in many south Indian languages. Indian
wootz ingots are believed to have been used to forge
Oriental Damascus swords which were reputed to cut
even gauze kerchiefs and were found to be of a very
high carbon content of 1.5-2.0% and the best of these
were believed to have been made from Indian steel in
Persia (Figure 1) and Damascus according to Smith
[10]. Some of the finest swords and artefacts of
Damascus steel seen in museums today are from the
Ottoman region i.e. Turkey.
In India till the 19th century swords and daggers
of wootz steel were made at centres including Lahore,
Amritsar, Agra, Jaipur, Gwalior, Tanjore, Mysore,
Golconda etc. although none of these centres survive
today. Different types of Damascus sword
patterns have been identified, described in some
depth by Pant [1], who also identified a new design
from blades kept in the collection of the Salar Jung
Museum in Hyderabad.
It may be mentioned however that the term Damascus
steel can refer to two different types of artefacts,
one of which is the true Damascus steel which is a
high carbon alloy with a texture originating from the
etched crystalline structure, and the other is a
composite structure made by welding together iron and
steel to give a visible pattern on the surface.
Although both were referred to as Damascus steels,
Smith [11] has clarified that the true Damascus
steels were not replicated in Europe until 1821.
3. Role of wootz steel in the development of
modern metallurgy
The legends associated with the excellent
properties of the wootz steel and the beautiful
patterns on Damascus blades caught the imagination of
European scientists in the 17th-19th centuries since
the use of high-carbon iron alloys was not really
known previously in Europe and hence played an
important role in the development of modern
metallurgy. British, French and Russian metallography
developed largely due to the quest to document this
structure. Similarly the textured Damascus steel was
one of the earliest materials to be examined by the
microstructure. Smith [10, 11] has fascinatingly
elucidated this early historiography of the interest
in the study of wootz steel and its significance to
the growth of metallurgy.
Although iron and steel had been used for
thousands of years the role of carbon in steel as the
dominant element was found only in 1774 by the
Swedish chemist Tobern Bergman, and was due to the
efforts of Europeans to unravel the mysteries of
wootz. Tobern Bergman was able to determine that the
compositions of cast iron, steel and wrought iron
varied due to the composition of 'plumbago' i.e.
graphite or carbon. As suggested by Smith [11] the
Swedish studies received an impetus following the
setting up of a factory to make gun barrels of welded
Damascus steels, and it was on observation of the
black and white etching of the steel and iron parts
that a Swede metallurgist guessed that there was
carbon in steel, and interest in replicating true
Damascus steels followed.
In the early 1800's,following the descriptions of
crucible steel making in south India by the European
travellers, there was a spurt in interest in Europe
in investigating south Indian wootz steel, from which
the fabled Damascus blades were known to be made,
with the aim of reproducing it on an industrial
scale. Mushet's [12] studies in 1804 were one of the
first to correctly conclude that there was more
carbon in wootz than in steel from England, although
this idea did not gain currency until later. Michael
Faraday [13], the inventor of electricity and one of
the greatest of the early experimenters and material
scientists, as pointed out by Peter Day [14], was
also fascinated by wootz steel and enthusiastically
studied it. Along with the cutler Stodart, Faraday
attempted to study how to make Damascus steel and
they incorrectly concluded that aluminium oxide and
silica additions contributed to the properties of the
steel and their studies were published in 1820 [15].
They also attempted to make steel by alloying nickel
and noble metals like platinum and silver and indeed
Faraday's studies did show that that the addition of
noble metals hardens steel. Stodart [16] reported
that wootz steel had a very fine cutting edge.
Following this the interest in Damascus steel
moved to France. Wadsworth and Sherby [17] have
pointed out that Faraday's research made a big impact
in France where steel research on weapons thrived in
the Napoleonic period. The struggle to characterize
the nature of wootz steel is well reflected in the
efforts of Breant [18] in the 1820's from the Paris
mint who conducted an astonishing number of about 300
experiments adding a range of elements ranging from
platinum, gold. silver, copper, tin, zinc, lead,
bismuth, manganese, arsenic, boron and even uranium,
before he finally also came to the conclusion that
the properties of Damascus steel were due to
'carburetted' steel. Smith [10] has indicated that
the analysis of ingots of wootz steel made in the
1800's showed them to have over 1.3% carbon. The
Russian Anasoff [19] also studied the process of
manufacturing wootz steel and succeeded in making
blades of Damascus steel by the early 1800's.
In the early 1900's wootz steel continued to be
studied as a special material and its properties were
better understood as discussed further in the next
section. Belaiew [20] reported that blades of such
steel to cut a gauze handkerchief in midair. In 1912,
Robert Hadfield [21] who studied crucible steel from
Sri Lanka recorded that Indian wootz steel was far
superior to that previously produced in Europe.
Indeed in the 18th-19th century special steels were
produced in Europe as crucible steels, as discussed
by Barraclough [22].
4. Investigations of superplasticity and other
mechanical properties of wootz steel
Some European scientists were successful in
replicating and forging wootz and Stodart who used it
in his cutlery business found that wootz steel had a
superior cutting edge to any other, while Zschokke in
1924 found that with heat treatment this steel had
special properties such as higher hardness, strength
and ductility, mentioned by Smith [10]. By 1918 an
important finding concerning Damascus steel was made
by Belaiew [20] who was probably the first to
attribute the malleability of Damascus steel to the
globulitic (i.e. spheroidised) nature of the forged
steel and to recognize that this occurs during
forging at a temperature of red heat (i.e. 700-800 0
C).
Panseri [23] in the 1960's was one of the first to
point out that Damascus steel was a hypereutectoid
ferrocarbon alloy with spheroidised carbides and
carbon content between 1.2-1.8%. Recent studies have
indicated that ultra-high carbon steels exhibit
superplastic properties. As pointed out by Wadsworth
and Sherby [17], by 1975 Stanford University had
found that steels with 1-2.1% C i.e. ultrahigh carbon
steels could be both superplastic at warm
temperatures and strong and ductile at room
temperatures. It was only subsequently that it came
to the authors' notice that these steels were in fact
similar in carbon content to the Damascus steels.
Superplasticity is a phenomenon whereby an
elongation of several hundred percent can be observed
in certain alloys in tension, with neck free
elongations and without fracture. By contrast most
crystalline materials can be stretched to no more
than 50-100 per cent. Superplasticity occurs at high
temperatures and superplastic materials can be formed
into complex shapes. For superplastic materials the
index of strain rate sensitivity (m) is high, being
around 0.5. At ideal m=1 flow stress is proportional
to strain rate and the material behaves like a
Newtonian viscous fluid such as hot glass.
Superplasticity occurs only above 0.3-0.4 Tm K where
Tm is the melting point. Another feature is that once
super-plastic flow is initiated the flow stress
required to maintain it is very low. Superplastic
material essentially comprises of a two-phase
material of spherical grains of extremely fine grain
size of not more than 5 microns at the working
temperature. Such ultrafine grained materials exhibit
grain boundary sliding yielding superplastic
properties.
Contemporary studies by Wadsworth and Sherby [17]
and Sherby [24] indicated that UHCS (i.e. ultra-high
carbon steels) with 1.8% C showed a strain-rate
sensitivity exponent nearing 0.5 at around 7500 C
(Figure 2) suggesting that Damascus steel could well
have exhibited superplastic properties and a patent
was awarded for the manufacture of such UHCS.
The explanation of the superplasticity of the
steel is that the typical microstructure of
ultra-high carbon steel with the coarse network of
pro-eutectoid cementite forming along the grain
boundaries of prior austenite (Figure 3 a, b), can
lead to a fine uniform distribution of spheroidised
cementite particles (0.1 m m diam.) in a fine grained
ferrite matrix. This spheroidisation of cementite is
described in Wadsworth and Sherby [17], Sherby [24]
and Ghose et al. [25]. Such steels are also found to
have strength, hardness and wear resistance.
Such steels had to be forged, however, in a narrow
range of 850-6500 C and not at the white heat of
12000 C to get the desired fine grain structure and
plasticity. In fact as pointed out in an appraisal of
Indian crucible steel making by Rao [26], and in a
review of ancient iron and steel in India by Biswas
[4], the early European blacksmiths failed to
duplicate Damascus blades because they were in the
practice of forging only low carbon steels at white
heat, which have a higher melting point. Biswas [4]
mentions that the forging of wootz at high heat would
have led to the dissolution of the cementite phase in
austenite so that the steels were found to be brittle
enough to crumble under the hammer.
Moreover, attractive combinations of strength and
ductility were found to be achieved by Wadsworth and
Sherby [17] and Sherby [24] when the ultra-high
carbon steels were in spheroidised conditions with
high yield strengths varying from 800 Mpa to 1500 Mpa
with increasing fineness of spheroidised carbides,
while the steel with coarsely spheroidised carbides
was especially ductile with up to 23% tensile
elongation.
While it is not yet known how fully the
superplastic or superformable properties of this
steel were exploited by the ancient blacksmiths of
West Asia and India, accounts indicate that they were
certainly able to manipulate the alloy with a skill
that could not be easily replicated by the European
experimenters of the 19th century. Indeed the swords
of Damascus steel were reported to have high strength
and ductility. Nevertheless, whereas the links
between the patterns on the traditional Damascus
blades and the crystalline structure of ultra-high
carbon steels have been better established, the
mechanical properties of the traditional Damascus
blades and the degree of exploitation of the unique
properties of the steel are less well understood.
Verhoeven [27] and Verhoeven et al. [28, 29] have
attempted to 're-invent' the Damascus steel and
blades as it were with replication experiments based
on historical studies of Damascus blades and
composition of wootz ingots. Verhoeven et al. [29]
used two methods by which the ingots were made, one
of which consisted of melting iron charge in a small
sealed clay graphite crucible inside a gas-fired
furnace with the ingot formed by furnace cooling.
These were made by rapidly heating the charge and
holding it for a period of 20-40 minutes between
14400 C-14800 C followed by cooling at furnace
cooling rates or faster. The composition of the
charge was chosen to match that of genuine Damascus
blades of about 1.6% C and 0.1% P. However the fairly
high level of phosphorus made the blades very hot
short and difficult to forge. To overcome this
problem the ingots were held at 12000 C in iron oxide
to produce a protective rim of pure iron around the
ingot which was ductile so that the ingot could be
forged. Ingots were also made with the phosphorus
levels reduced to the point where the ingots were not
hot short which eliminated the need for the rim heat
treatment. Verhoeven et al. [29] also made ingots by
a process of vacuum-induced melting whereby the
charge was melted by heating to around 10000 C,
backfilling with nitrogen gas, heating to about 15800
C and then outgassing for around 5 minutes so that
cooling rates at arrest temperature were around 5-100
C/minute.
It may be commented however, that although the
structures of the ingots so produced do simulate
those of Damascus blades, the methods used by
Verhoeven et al. [29] are not strictly experimental
re-constructions of the traditional processes, but
rather laboratory simulations of the process, since
the methods used do not really replicate conditions
related to traditional or archaeological processes.
For instance the charge is fired in both the methods
described above in a very short time and the melt is
cooled very rapidly under modern industrial
conditions which could not have been achieved
traditionally, while the 19th century descriptions of
the wootz process suggest a very long firing cycle
for the charge. In fact the eye witness descriptions
of Voysey [8] and Buchanan [9] lay emphasis on the
fact that the prolonged heating of the charge and its
slow cooling were essential for obtaining the optimum
results in the wootz process.
However the experimental simulations by Verhoeven
et al. [29] served to monitor in detail the thermal
cycles and cooling curves and composition so as to be
able to arrive at a final product which matched that
of Damascus blades and to understand the mechanism of
formation of the pattern of aligned bands on the
blades, which is reported by them to be produced by a
carbide banding mechanism which was found to be
assisted by the addition of P, S along with V, Cr,
and Ti. Moreover their experiments are amongst the
few comprensive studies on the general process of
manufacture of the ingots themselves.
5. Archaeological and analytical evidence
Some of the archaeological and analytical evidence
for crucible steel production is discussed covering
the investigations of Rao [30], Rao et al. [31], Lowe
[32, 33], Srinivasan [3] and Srinivasan and Griffiths
[5]. These indicate that the crucible processes for
steel production were spread over large parts of
south India. Lowe's investigations have concentrated
mainly on surveying and studying numerous sites from
the Hyderabad region or the Deccani crucible steel
process while pioneering investigations by Rao et al.
[31] have covered other parts of south India such as
the Mysore region and Salem district of Tamil Nadu.
Field and analytical investigations were made by
Srinivasan in 1990, whereby she was able to identify
some hitherto unreported sites of crucible steel
production in South Arcot, Tamil Nadu and from
Gulbarga, Karnataka, reported in Srinivasan [3] and
Srinivasan and Griffiths [5]. Figure 4 gives a view
of a dump for wootz crucible steel production from
South Arcot, Tamil Nadu and Figure 5 of fragments of
fired wootz crucibles from Gulbarga identified by
Srinivasan.
Srinivasan [3] has pointed out that whereas the
process documented by Lowe [32, 33], the Hyderabadi
or Deccani process, involved the co-fusion of cast
iron with wrought iron, the crucibles from sites
reported by Srinivasan from Tamil Nadu and Karnataka
pertained to the carburisation of wrought iron in
crucibles by packing it with carbonaceous material.
Analytical investigations made by Rao et al [30],
Lowe [32, 33], Srinivasan [3], Craddock [34] and
Srinivasan and Griffiths [5] on crucibles from
production sites are briefly summarized.
The details of the furnace described and sketched
by Buchanan [8] indicate that crucibles were packed
in rows of about fifteen inside a sunken pit filled
with ash to constitute the furnace which was operated
by bellows of the buffalo hide, fixed into a
perforated wall which separated them from the furnace
probably to minimize fire hazards (Figure 6). The
fire was stoked from a circular pit which was
connected to the bottom of the ash pit. The crucibles
themselves were conical and could contain up to 14
oz. of iron, along with stems and leaves. The wootz
steel process in general refers to a closed
crucible
process and Lowe [32] has remarked that the
processing of plant and mineral materials in closed
crucibles is often described in Indian alchemical
Sanskrit texts of the 7th-13th c. AD.
Investigations by Craddock [34] indicated the
wootz ingot itself had a dendritic cast structure.
Lowe [32, 33] has investigated particularly well the
refractory nature of the crucibles of the crucibles
which indicate that they were robust enough
refractories to withstand the long firing cycles of
up to 24 hours for the process. The formation of
mullite and cryistobalite was detected in the
crucible fragments studied by Lowe [32, 33]
suggesting they had been well fired to high
temperatures of over 1300-14000 C, while Rao et al
[31] also observed the formation of mullite and
cryistobalite in crucibles.
However the microstructures investigated by Lowe
[32] of the metal remnants within the particular
Deccani crucibles studied by her from Konasamudram
could only be related to a failed process of crucible
steel production at that particular site or context
since they related more to white cast iron, a brittle
and not very malleable material formed by
over-carburisation, rather than ultra-high carbon
steel. In fact based on these findings Lowe [32] has
preferred to cautiously aver that it was a white cast
iron ingot that was produced by the Indian crucible
process. Craddock [34] has also opined that the
product of the Indian crucible steel process was
probably a general homogenous steel rather than
specifically a high-carbon steel.
On the other hand investigations by Srinivasan [3]
and Srinivasan and Griffiths [5] indicated the
presence of solidified metal droplets in the
crucibles with a typical micro-structure and
micro-hardness corresponding to a good quality
hypereutectoid steel with the formation of hexagonal
grains of prior austenite with fine lamellar pearlite
within the grains, with the precipitation of
pro-eutectoid cementite along the grain boundaries of
prior austenite: which is in fact the classic
structure of ultra-high carbon steels of about 1.5% C
which were made under laboratory conditions by
Wadsworth and Sherby [17}and Verhoeven et al. [29].
The findings reported in Srinivasan [3] and
Srinivasan and Griffiths [5] are hence significant in
that they prove beyond doubt that high-carbon steels
were indeed made by crucible processes in south
India. Studies by Srinivasan and Griffiths [5] also
indicated that temperatures of over 14000 C had
indeed been reached inside the crucibles to melt the
wrought iron and carburise it to get a molten
high-carbon steel with the typical hypereutectoid
structure on solidification.
Conclusions
The above review indicates that the reputation of
wootz steel as an exceptional and novel material is
one that has endured from early history right into
the present day, with the story of the endeavours to
study it in recent history being nearly as intriguing
as the story of its past. The archaeological findings
indicate that crucible steel does have an ancient
history in the Indian subcontinent where it took
roots as suggested by literary references, while the
analytical investigations indicate that a high-grade
ultra-high carbon steel was indeed produced by
crucible processes in south India. Recent
investigations on the properties of the ultra-high
carbon wootz steel such as superplasticity justify it
being called an advanced material of the ancient
world with not merely a past but also perhaps a
future.
Acknowledgements
The authors would like to acknowledge the Indian
National Academy of Engineering. Srinivasan would
like to acknowledge the support of British Council,
New Delhi for a British Chevening Scholarship for
doctoral research, and the interest of Dr. D.
Griffiths, Institute of Archaeology, University
College London, Dr. J. A. Charles, Cambridge
University, late Dr. C. V. Seshadri,
founder-President, Congress of Traditional Science
and Technology, and Hutti Gold Mines Ltd. for
assistance with fieldwork and the support of the Homi
Bhabha Research Council.
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