Wootz steel: an advanced material of the ancient world (Srinivasan S & S. Ranganathan, 1997) by kalyan97

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									                                                           Iron & Steel Heritage of India
                                      Ed. S. Ranganathan, ATM 97, Jamshedpur, pp. 69-82


   WOOTZ STEEL : AN ADvANCED
' AT     OF THE ANCIENT WORLD

               S. SRINIVASAN and S. RANGANATHAN
          Departrnent 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 super-
   plasticity 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 metal-
   lurgy of iron and steel, but also to the development of physical metallurgy
   in general and metallography in particular. Some of the recent experi-
   ments in studying wootz by re-constructing composition, microstructure
   and mechanical behaviour, along with some recent archaeological evi-
   dence, are described.

   Keywords : Wootz, High-carbon Steel, South India, Superplasticity, Cruz
             cibles, Analyses

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

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                      S. SRINIVASAN and S. RANGANATHAN



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.

HISTORY OF WOOTZ STEEL

     There are numerous early literary references to steel from India from Medi-
terranean sources including one from the time of Alexander (c. 3rd BC) who was
s4i-d-to have been presented with 100 talents of Indian steel, mentioned by Pant
111. Bronson `21 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 141 and Srinivasan and
Griffiths t5i 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 161 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 171.

    Several European travellers including Francis Buchanan 181 andVoysey 191 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 Fig. 1 and Damascus
according to Smith [101. 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

                                         70
               • WOOTZ STEEL : AN ADVANCED MATERIAL




    Fig. 1 : Detail of 17th century Persian blade of Damascus steel or Wootz steel
     showing typical etched crystalline structure of high-carbon steel (Smith 1")). •

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
Pane", 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 oil has clarified that the true Damascus steels were not replicated in
Europe until 182.1.

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 alloy, was
not really known previously in Europe and hence played an important roTejin 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 l' "1 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

                                          71
                      S. SRINIVASAN and S. RANGANATHAN



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 m 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 1121 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 "31, the inventor of electricity and one
of the greatest of the early experimenters and material scientists, as pointed out
by Peter Day t"I, 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 1 "i. 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 [161 reported that wootz
steel had a very fine cutting edge.

    Following this the interest in Damascus steel moved to France. Wadsworth
and Sherby 1171 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 "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 [101 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 1191 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

                                         72




                                                                                         mot
                    WOOTZ STEEL : AN ADVANCED MATERIAL



and its properties were better understood as discussed further in the next section.
Belaiew (2°1 reported that blades of such steel to cut a gauze handkerchief in
midair. In 1912; Robert Hadfield (211 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 Barraciough 1221.

INVESTIGATIONS OF SUPERPLASTICITY AND OTHER MECHANI-
CAL 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 t". By 1918 an important finding concerning
Damascus steel was made by Belaicw 121)1 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°C).

    Panseri ("tin 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(171, 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 elegations
and without fracture. By contrast most crystalline materials can be stretched to no
more than 50-100%. 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

                                         73
                           S. SRINIVASAN and S. RANGANATHAN

         103                 I   I   1   I   1   1 1    1    i   I I l     T   i I I


                   UHCS-1.8C with DAMASK
                   750"C




                             I II    I       I I   I    I        I I   I   1   I   I   I
         100
           io-15              10-4             104               . 10-2                10-1
                                         Strain tato (s-11



   Fig. 2 : The flow stress-strain rate response of ultra-high carbon steel at 750°C
    illustrates that the stress-strain rate curve has a slope showing a strain-rate
   sensitivity exponent of 0.43 indicating it is a superplastic material (Sherby 1241)

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 1241 indicated
that UHCS (i.e., ultra-high carbon steels) with 1.8% C showed a strain -rate
sensitivity exponent nearing 0.5 at around 750°C Fig. 2 suggesting that Dam-
ascus steel could well have exhibited superplastic properties and a patent was
awarded for the manufacture of such UHCS. The explanation of the superplastic-
ity 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 Fig. 3 a, b, can lead to a fine uniform distribution of spheroidised
cementite particles (0.1 mm diam.) in a fine grained ferrite matrix. This
spheroidisation of cementite is described in Wadsworth and Sherby Ern, Sherby [241
and Ghose et al. 1253. Such steels are also found to have strength, hardness and
wear resistance.

    Such steels had to be forged, however, in a narrow range of 850-650°C and not
at the white heat of 1200°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 1261,
and in a review of ancient iron and steel in India by Biswas 141, the early European

                                                 74
                    WOOTZ STEEL : AN ADVANCED: MATERIAL




 Fig. 3 : a) Photomicrograph of ultra-high carbon steel witlz I .8%C, showing coarse pro-
 eutectoid carbide (cementite) network (Sherbytul) b) Photomicrograph of same structure
   at high magnification shows iron grains with fine spherozdised carbides (Sherbyin.


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 [41 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
                                                     1
achieved by Wadsworth and Sherby 1173 and Sherby 1" 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 u-plo 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

                                           75
                      S. SRINIVASAN and S. RANGANATHAN



Damascus blades and the degree of exploitation of the unique properties of the
steel are less well understood.

    Verhoeven 1271 and Verhoeven et al. [28. 291, have attempted to 're-invent' the
                                             2

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., 12)1 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 1440°C-1480°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 1200°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 1000°C, backfilling with
nitrogen gas, heating to about 1580°C and then outgassing for around 5 minutes
so that cooling rates at arrest temperature were around 5-10°C/minute.

    It may be commented however, that although the structures of the ingots so
produced do simulate chose of Damascus blades, the methods used by Verhoeven
et al. (291, are not strictly experimental re-constructions of the traditional pro-
cesses, but rather laboratory simulations of the process, since the methods used
do not really replicate conditions related to traditional or archaeological pro-
cesses. 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 181 and Buchanan (91 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.1291, 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

                                        76
                     WOOTZ STEEL : AN ADVANCED MATERIAL




    Fig. 4 : View of newly identified old dump for high—carbon wootz crucible steel
      production from South Arcot, Tamil Nadu (photographed by S. Srinivasan).

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 comprehensive studies on the
general process of manufacture of the ingots themselves.

ARCHAEOLOGICAL AND ANALYTICAL EVIDENCE

    Some of the archaeological and analytical evidence for crucible steel produc-
tion is discussed covering the investigations of Rao 13"1, Rao et al., 1311, Lowe 132.331,
Srinivasan 131 and Srinivasan and Griffiths 151. 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. 13 '1 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-izttntify
some hitherto unreported sites of crucible steel production in South Arcot, Tamil
Nadu and from Gulbarga, Karnataka, reported in Srinivasan 131 and Srinivasan and
Griffiths 151. Fig. 4 gives a view of a dump for wootz crucible steel production
from South Arcot, Tamil Nadu and Fig. 5 of fragments of fired wootz crucibles
from Gulbarga identified by Srinivasan.

    Srinivasan 131 has pointed out that whereas the process documented by Lowe
131331, 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

                                           77
                      S. SRINIV'ASAN and S. RANGANATHAN




                                   GS'S"   1111111:-.'   • RIM



             Fig. 5 : Fragments of newly identified remains of fired wootz
        crucibles from Gulbarga, Karnataka (photographed by S. Srinivasan).


and Karnataka pertained to the carburisation of wrought iron in crucibles by
packing it with carbonaceous material. Analytical investigations made by Rao
et al. ("I, Lowe [32331, Srinivasan 13), Craddock (341 and Srinivasan and Griffiths 151
on crucibles from production sites are briefly summarized.




        Fig. 6 : Furnace for production of crucible steel production sketched by
      Buchanan (1807) during his travels,. indicating that crucibles were packed
   in a pit with the furnace being operated by bellows of buffalo hide (reproduced
                       from K.N.P. Rao, unpublished monograph).

                                           78
                    WOOTZ STEEL : AN ADVANCED MATERIAL


    The details of the furnace described and sketched by Buchanan ESl 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 Fig. 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 process
generally refers to a closed crucible process and Lowe 1321 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 [341 indicated the wootz•ngot itself had a dendritic
cast structure: Lowe 132.331 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, 331 suggesting they had been well fired to high temperatures
of over 1300-1400°C, while Rao et al. 1311 , also observed the formation of mullite
and cryistobalite in crucibles.

    However the microstructures investigated by Lowe 1321 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 1321 has prefer'red 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 131 and Srinivasan' and
Griffiths 151 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: whic.h 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 1171 and Verhoeven
et al. 1291. The findings reported in Srinivasan 131 and Srinivasan and Griffiths 151
are hence significant in that they prove beyond doubt that high-carbon steels were

                                          79
                      S. SRINIVASAN and S. RANGANATHAN



indeed made by crucible processes in south India. Studies by Srinivasan and
Griffiths 151 also indicated that temperatures of over 1400°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 solidifica-
tion.

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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[4] A. K. Biswas, Iron and steel in pre-modern India- a critical review,-Indian Journal
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                                          80
                      WOOTZ STEEL : AN ADVANCED MATERIAL



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                                            81
                          S. SRINIVASAN and S. RANGANATHAN



         Indian Institute of Science, Bangalore (1997).
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