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17
2004 January • JOM
Archaeotechnology
Feature
Over the last 40 years, there has been a discernible increase in the number of scholars who have focused their research on early industrial
organizations, a fi eld of study that has come to be known as Archaeotechnology. Archaeologists have conducted fi eldwork geared to the study
of ancient technologies in a cultural context and have drawn on the laboratory analyses developed by materials scientists as one portion of their
interpretive program. Papers for this department are solicited and/or reviewed by Michael Notis, a professor and director of the Archaeometallurgy
Laboratory (www.Lehigh.edu/~inarcmet) at Lehigh University.
In this article, metallurgical aspects
of a 17th century forge-welded iron
cannon at Thanjavur are addressed,
including an analysis of manufactur-
ing methodology based on careful
observation of its constructional details.
Microstructural examination of iron
from the cannon reveals that the iron
was extracted from ore by the direct
process. Thus, the cannon was fabricated
by forge welding and not by casting.
Electrochemical polarization studies
indicate that the corrosion rate of the
cannon iron can be compared to that of
0.05% carbon mild steel under complete
immersion conditions. However, the
atmospheric corrosion resistance of
the cannon is far superior to that of
modern steel and can be attributed to
the formation of an adherent protective
passive fi lm. It is concluded that this
cannon constitutes a marvel of medieval
Indian metallurgical skill.
INTRODUCTION
The high status of iron and steel
technology in ancient and medieval
India is refl ected in the manufacture
and use of numerous large iron objects,
including forge-welded cannons.1–4 Such
cannon, found at Nurwar, Mushirabad,
Dacca (in Bangladesh), Bishnupur, Bija-
pur, Gulbarga, and Thanjavur, exemplify
the medieval Indian blacksmith’s skill in
the design, engineering, and construction
of large forge-welded iron objects. The
wrought-iron cannons found in different
parts of India were manufactured from
individual iron rings that were forge-
welded together. Medieval blacksmiths
continued to use this technique in the
fabrication of small and large iron
objects, such as the Delhi and Dhar iron
pillars.5,6 The forge-welded cannons
have not been properly catalogued in
the literature, unlike their European
counterparts.7–9 The massive cannon
at Thanjavur in Tamil Nadu will be
discussed in this article.
Based on its weight and size, the
cannon (Figure 1) at Thanjavur, earlier
known as Thanjai or Tanjore, must be
regarded as one of the largest forge-
welded iron cannons in the world.
According to a recent authoritative his-
tory of the ancient city of Thanjavur,10 the
cannon was manufactured in Thanjavur
during the regime of Raghunatha Nayak
(1600–1645 A.D.). Thanjavur was by
this time a very important center of
Hindu architecture (as exemplified
by the Brihadiswara Siva Temple),
literature (with thousands of palm leaf
manuscripts preserved at the library
of the Saraswati Mahal Museum at
Thanjavur), and metallurgical skill
(as shown by the numerous bronze
sculptures executed by the lost-wax
process). The Thanjavur cannon was
forged as a component of a defense
barricade that protected the city, then
already a few centuries old. The cannon,
located at the eastern entrance of the
ancient city, is referred to as Rajagopala,
according to local traditions.
There is no specifi c recorded history
of the cannon. However, a Thanjavur
palace novel that describes Nayak’s rule
mentions the presence in the Thanjavur
fort of an object referred to as a
“fi re-breathing barrel shaped weapon.”
The cannon is believed to have been
constructed in the Manojipatti area of
Thanjavur, famous for iron working.10
Figure 1. The historical forge-welded iron cannon at Thanjavur.
R. Balasubramaniam,
A. Saxena,
Tanjore R. Anantharaman,
S. Reguer, and P. Dillmann
A Marvel of
Medieval
Indian
Metallurgy:
Thanjavur’s
Forge-Welded
Iron Cannon
JOM • January 2004
18
DESIGN
The Thanjavur iron cannon rests on three concrete supports, about 60 cm thick, 120 cm
high, and 2.25 m apart from each other. The cannon is a muzzle-loading type, wherein the
gunpowder and the projectile object are loaded from the muzzle (i.e., front end). The cannon
is 751.5 cm in length from end to mouth, including the 31.5 cm projection at the end of the
barrel. The outer and inner diameters of the gun barrel are 93 cm and 63 cm, respectively.
All the dimensions in this article are in centimeters. However, the cannon’s dimensions are
closely related to the inch system of measurement, which was the unit of measure in ancient
India.6 In this context, the entire length of the cannon is 25 feet and the rear portion is 1 foot
long. The inner and outer diameters of the cannon barrel are 25 inches and 37 inches,
respectively, with each ring approximately 2 inches thick.
Assuming the hollow cylinder of the cannon barrel to extend to the complete end of the
cylindrical barrel (length of 720 cm), the minimum weight of the cannon estimated from the
known thickness of the barrel (i.e., 15 cm) is 20.6 t. The size of the solid portion in the rear
of the cannon is not known with certainty and therefore, this estimate is a lower-bound value
because the solid will add weight to the cannon. The distance from the fuse hole to the end
of the barrel is 36 cm. It is reasonable to assume that the fuse hole represents the rear end
of the hollow section of the cylindrical barrel, as is usually the case with medieval
wrought-iron cannon designs.7 Therefore, the rear solid portion of the cannon will add
approximately another ton to the estimated weight. Counting the weight of additional
supporting external rings, the minimum weight of the cannon is more than 22 t.
The front end of the cannon indicates that 39 iron strips were folded out from inside the
cannon. Each strip is about 1.5 cm thick and 5 cm wide. These iron staves continue
longitudinally through the length of the inner bore of the barrel. Their purpose appears to
have been to provide a smooth inner surface to the cannon barrel. The front end also reveals
that concentric layers of iron rings were used to construct the barrel of the cannon. Four
concentric rings are clearly visible in the front plane of the cannon barrel. (The iron strips
and iron rings are addressed in the construction methodology section of this article.) The
complete barrel is made up of three rings, hooped over the iron staves.
A detailed dimensional analysis found that the width of the individual rings along the
length of the cannon was not constant. Generally, rings of smaller widths were also located
along the length of the cannon. An example
from just behind the cannon front face is shown
in Figure A. It has been suggested that the
smaller rings might have been placed for fi lling
the gaps or for sealing the cannon barrel.3 This
may be true for some of the smaller rings. In this
regard, it is also important to note the systematic
placing of smaller rings between larger rings at
two specifi c locations, just behind the muzzle
of the barrel and in the middle of the cannon. In
these locations, the smaller rings seem to have
been placed in a very calculated manner.
Therefore, the design of the cannon required
the use of smaller width rings not only to close
the gaps between the larger width rings, but
also to ensure greater toughness for the barrel.
At periodic intervals along the length of
the cannon, additional external rings are on the
external surface of the cannon. These raised
locations can be noticed in Figure 1. Seven
(See sidebar for design details.) The
gun is still standing at the same location
in Thanjavur, facing east. Its location
within Thanjavur is now known as
beerangi-maedu in Tamil, literally mean-
ing the cannon-mound. The cannon is a
protected monument of the Archaeologi-
cal Survey of India.
CONSTRUCTION
A high level of engineering skill was
involved in the construction of this
cannon. Some insights on its possible
method of manufacture can be obtained
from a detailed study of its structural
condition, described in the sidebar.
Ultrasonic measurements conducted by
Roessler3 on the wall of the cannon
indicated three layers of rings, each
of them about 5 cm thick, which were
fi tted around each other. The iron rings
appear to have been joined by hooping
and later by forge welding. Only three
ring layers were used to construct the
length of the cannon barrel, as evident
from the measured diameter of the
cannon at the front face and along the
gun barrel. The front face, where the
three inner iron rings can be discerned,
shows the presence of an additional outer
ring (see also Figure A) that provided
further strengthening. Additional outer
rings are also seen at the clamping
locations (Figure B). The front view
of the cannon further indicates an
additional layer of 1.5 cm thick iron
strips tightly placed in the inner pipe
along the innermost ring. These strips,
which progress across the length of the
cannon, were bent on the front side of
the gun and tightly placed to hold the
whole structure together.
Although medieval Indian black-
smiths successfully used casting in the
manufacture of intricate bronze objects,
available evidence indicates that few
practiced iron-casting techniques.11 The
blacksmiths’ lack of interest in casting
was likely due not only to the high
temperatures required for casting, but
also to their mastery over the forge-
welding technique to produce large iron
objects. It is certain that the iron cannon
was not cast, implying that the cannon
is made of wrought iron. The cannon
was fabricated by forge welding (i.e.,
forging together rings of forge-welded
iron). Forge welding was also used
to join the layers of secondary and
Figure A. A close-up view of
one of the forge-welded joint
regions on the outer surface
showing that a smaller ring
has been used to join the
gaps between larger rings.
The provision of a handling
hole on the smaller ring
should also be noted.
Figure B. The rear portion of the barrel showing the additional
outer rings provided as three-ring assemblies. These additional
ring assemblies would have provided further strengthening
to the cannon.
19
2004 January • JOM
such locations can be identifi ed along the length of the barrel. These additional rings usually
are present as three-ring assemblies (see Figure B). At four locations along the length of the
cannon (i.e., the fi rst, third, fourth, and sixth three-ring assemblies assuming the fi rst one to
be closest to the muzzle), the outer forged central ring, of larger diameter, ends in a 2.5 cm
thick plate (Figure C). Each of these four plates is provided with arrangements for holding
two handling rings. All but two of the original rings are missing. The rings are 40 cm in
diameter and the diameter of the cross section of the ring is 4 cm. These rings were provided
to manipulate the cannon’s direction and also, possibly, to aid its movement and transportation.
Similar iron rings can be noted on the large forge-welded cannons at Mushirabad and
Gulbarga.1 Long iron rods or wooden beams, inserted through these clamp rings, would have
aided in positioning of the cannon during its use. The method by which the gun was moved
using these clamping rings is not known, but it must have involved lifting by means of either
a chain-and-pulley arrangement or manual methods. The former method appears likely
given the enormous weight of the cannon. Trunnions (i.e., supporting cylindrical projections
on the sides of the cannon) like those usually found on smaller cannons are not provided.
Trunnions were used to house the cannon on wheels, thereby aiding its easy transportation.
The absence of such a device on the Thanjavur cannon indicates that it was not meant for
mobile use.
The rear end (Figure D) is not fl at but consists of successively smaller diameter circular
iron rings, presumably to provide impact resistance to the rear section of the cannon. A
fuse hole of 10 cm diameter, located on top of the cannon near the rear end, was used
for ignition of gunpowder. Based on the location of this fuse hole and the measured
dimensions of the cannon (i.e., cross-sectional area of 630 mm2 and length of 500 mm),
Roessler estimated that at least 155 L of gunpowder must have been utilized to fi re the
cannon.3 However, this estimate is not reliable because Roessler assumed the rear portion
of the cannon barrel to be hollow, whereas it is known that the rear end of the cannon up to
the location of the fuse hole is solid. The amount of gunpowder that was packed to create
the explosion must have depended on the type, number, size, and nature of the projectile
material. Therefore, it would not be possible to speculate on the gunpowder volume used
to fi re this cannon. It is obvious, however, that this must have been quite a signifi cant amount
based on the dimensions of the cannon. It is interesting to note that an additional three-ring
assembly just behind the fuse hole location (see
Figure B) was provided for strengthening and
additional impact resistance.
The actual size of the cannon ball fi red from
this cannon is not known with certainty.
Assuming the diameter of the spherical cannon
ball to be slightly smaller than the inner diameter
of the cannon barrel, the weight of the ball can be
estimated as 1,000 kg if made of iron or 300 kg
if made of stone.3 The type of balls used, however,
is not known. Cannon balls were usually made
of iron and not of stone, as recent discoveries of
cannon balls in Thanjavur testify. The amount of
gunpowder that must have been required to
force a 1 t iron cannonball must have been
enormous and moreover, the impact from the
explosion of such a large amount of gunpowder
must have been very severe. Therefore, the
cannon ball must have been much smaller than
the actual inner diameter of the cannon barrel.
tertiary rings that further strengthened
the entire structure. The manufacturing
technology of the Thanjavur cannon
can therefore be classifi ed under forge
welding of pre-forged iron rings, hooped
over longitudinally placed iron staves,
with correct positioning and alignment.
Available evidence and examples from
medieval European wrought-iron can-
nons7 indicate that they were divided
into two distinct parts—the chamber
and the barrel. The function of these
parts and, hence, the material and design
requirements, are also different. For
example, the barrel’s main function was
to contain the lateral exhaust of the gas
(from the explosion of the gunpowder)
and, in this process, the projectile was
pushed out. The barrel needed to be
tough and not deformable. It also needed
to possess a smooth inner surface.
On the other hand, the chamber was
exposed to much higher gas pressures
than the barrel due to the exploding
gunpowder at the point of ignition. The
rear side had to be tightly closed to
withstand the gunpowder explosion.
Therefore, this part had to be impact
resistant.
External observations of the cannon
surface indicate that the same type
of material (i.e., wrought iron) and
manufacturing methodology (i.e., forge
welding) was used for the chamber
and the barrel. Interestingly, while
the wrought-iron cannons were manu-
factured by separately fabricating
the chamber and the barrel and later
joining them together, the cast guns
were fabricated as one piece. The
manufacturing methodology of the barrel
can be deduced from the appearance
of the barrel. However, the manufactur-
ing method for the chamber is not
known with certainty and the proposals
presented here are based only on the
features clearly evident on the cannon
surface.
The solid part of the cannon, from
the extreme rear end to the fuse hole,
could have been built using forged iron
plates or rings over a cylindrical central
solid iron shaft. Some details of the
methodology can be gleaned from the
appearance of the rear portion of the
cannon (Figure D). It appears that iron
rings were forge welded over a solid
cylindrical shaft that made the rear
portion. The solid cylinder’s outline can
Figure C. Details of one of the handling clamps.
Figure D. The rear of the
cannon.
JOM • January 2004
20
be seen in the extreme rear section of
the cannon. The depth of the hollow slot
of the cannon could not be determined
for want of a suffi ciently long pole.
However, based on the design of the
forge-welded-iron cannon at Bishnupur12
and also based on the design of cannons
in general, the solid portion of the rear
of the cannon should extend up to the
fuse hole location. This portion appears
to have been constructed of rings that
were successively forged over each
other. It appears that the medieval
engineers were familiar with the idea
of structural design for improved
fracture toughness because the solid
structure created with successively larger
diameter rings would have possessed a
better impact resistance compared to a
single solid piece of wrought iron. The
solid rear portion of the cannon was
constructed by forge welding iron rings
of different diameters over a central
solid cylindrical shaft.
The barrel must have been fabricated
separately from the chamber. Initially,
the long iron strips were placed on a
mandrel in order to provide it support
and to aid manufacturing operations
that followed. Pre-fabricated iron rings
were expanded and then shrunk fi t over
the long iron strips. The rings of the fi rst
layer were brought from the front end.
After the fi rst layer was forge welded,
the other two layers were subsequently
built up. Roessler suggested that after
the rings of the fi rst layer were forged
welded, the rings in the second layer
were positioned in such a manner that
the middle of each ring closed the gap
between the rings of the fi rst layer.
Similarly, the rings of the outermost
layer were proposed to close the gaps in
the second layer. This hypothesis has to
be verifi ed by careful non-destructive
studies. Once the barrel had been
fabricated, the protruding iron strips
on the front face were folded up. The
protruding iron strips at the rear end
must have been connected at appropriate
locations to the solid cannon rear section,
and the complete cannon realized. The
chamber and the barrel must have been
joined using the protruding longitudinal
staves of the barrel and by strengthening
this joint area externally with additional
rings.
The total number of rings counted on
the cylinder barrel surface is 95, with
six rings visible in the rear end. Three
rings are found across the thickness of
the barrel, while the number in the solid
rear portion is not known. Therefore,
there is a minimum number of 291 iron
rings used in the construction of the
cannon. It is important to realize that the
iron rings had to be engineered to exact
dimensions to allow for expansion and
shrinkage on heating and cooling while
the rings were laid on each other to
form the fi nal three-layer structure over
the strips.
The method by which the entire
cannon was handled during its manu-
facture is not known, but the method
must have been ingenious because of
the additional complications due to the
high temperatures involved in the forge-
welding operation. The cannon must
have been handled with an arrangement
similar to that utilizing the iron handling
rings. A careful non-destructive study
will offer further insights into the
manufacturing methodology, especially
in the rear solid portion of the cannon.
Interestingly, iron cannons were also
manufactured by forging together
round, solid pancakes of iron and later
punching out holes in the center using
chisels.11 There is mention of iron
cannons fabricated in this manner in
North India during the reign of Akbar
(1556–1605 A.D.).11
MATERIAL
CHARACTERIZATION
An extremely small iron sample
was extracted from the plate, at the fi rst
clamping location, for analysis. It was
used for all the scientifi c studies reported
here. The chemical composition of
the iron, determined by a Jobin Yvon
JY-38S inductively coupled plasma
atomic emission spectrometer, is 93.4
wt.% Fe, 0.01 Cr, 0.003 Al, 0.026
Ni, 0.003 Mo, 0.042 P, and 0.411 C.
A separate analysis found the carbon
content to be 0.419%, while a separate
analysis for sulfur content revealed
that it was less than 500 ppm. The low
amount of phosphorous at this location
is not typical of ancient and medieval
irons,5,6 because limestone was not added
in the charge of bloomery furnaces and
therefore, a higher amount of phospho-
rous was retained in the metal at the
time.13 The unusually low phosphorous
content could be due to a lower amount
of phosphorous in the particular sample
that was analyzed. Metallographic
investigations revealed that the iron
sample extracted contained a relatively
high slag volume fraction and this could
Figure 2. (a) and (b) Optical micrographs showing slag inclusions in ferrite matrix, and (c) a SEM micrograph showing the same features.
a 200 µm b 200 µm c 10 µm
21
2004 January • JOM
Table I. Parameters Measured from Tafel Extrapolation Studies
βa βc icorr
Material pH (V/dec) (V/dec) (µA/cm2) µm/y
Thanjavur iron 7.62 0.110 0.146 0.129 1.499
Thanjavur iron DDW* 7.00 0.223 0.210 0.244 2.827
0.05%C steel 7.62 0.237 0.075 0.346 4.010
* DDW indicates double-distilled water.
be one reason for the lower amount of
phosphorous. The microsegregation of
phosphorous in archaeological iron is
quite different from macrosegregation
that is observed in modern iron- and
steel-making practices. The microseg-
regation is strongly influenced by
the presence of entrapped slag inclu-
sions and also by high-temperature
metallurgical phase transformations.14
Another reason for the low phosphorous
(and the relatively high carbon) could
be the deliberate heat treatment (by
carburization) of the iron used in the
location from where the sample was
obtained. Metallographic investigations,
to be discussed in the following, did
not reveal any deliberate carburized
structure. Therefore, some of the carbon
in this analysis may have arisen from
entrapped cinder in the iron sample.
The total elemental composition
added up to 93.895 wt.%, thereby
indicating that the remainder of the
material used for the chemical analysis
must have been the entrapped slag
inclusions. The presence of those inclu-
sions was verifi ed by volume fraction
analysis using an optical microscope
(Figure 2a and b). These inclusions
were not uniformly distributed. At some
locations, there was a much larger
fraction of these inclusions compared to
other locations (Figure 2c). In addition to
their distribution inside the ferrite grains,
slag inclusions were also observed to
coat some of the grain boundaries. The
grain size of the sample was not uniform.
These observations coincide with the
general characteristics of ancient Indian
irons.5,6 The volume fraction of the
entrapped inclusions was determined
by the grid intercept method, based on
a large number of fi elds of view. The
volume fraction of entrapped slag was
found to be 6.07%, slightly greater
than the 2–4 vol.% slag inclusions
generally observed in ancient Indian
irons. Microstructural analysis revealed
that the material of construction was not
a cast structure, thereby fi rmly verifying
that the cannon was manufactured by
forge welding of wrought iron and not
by casting.
ELECTROCHEMICAL
CHARACTERIZATION
In order to conduct electrochemical
studies, the iron sample from the
Thanjavur cannon was mounted, with
an electrical connection on the back-
side. The surface area exposed for
electrochemical studies was precisely
maintained by a protective layer coating
at the edges. Microstructural analysis
of the area exposed for electrochemical
studies indicated that the volume fraction
of slag inclusions at this location was
low. The same sample was used for all
electrochemical experiments, with the
surface prepared to 4/0 emery paper
fi nish before every experiment. The
sample surface was also thoroughly
cleaned and degreased before each
electrochemical experiment. Electro-
chemical polarization studies were
conducted in double-distilled water
containing 0.005 M Na2SO4 (to aid
solution conductivity) of pH 7.00 and a
borate-buffered solution (0.01 M KNO3,
0.5 M H3BO3, and 0.05 M Na2 B4O7.10
H2O) of pH 7.62. For comparison
purposes, a 0.05% C mild steel (the
composition determined by wet chemi-
cal analysis was found to be 0.062 wt.%
C, 0.005 Si, 0.006 P, 0.02 Ni, 0.004
Co, 0.185 Mn, 0.042 Cr, 0.005 Mo,
0.024 Cu, 0.0007 Ti, 0.032 Al, and
0.012 S) was also investigated. The
electrochemical studies were conducted
on a Model 263A EG&G Princeton
Applied Research potentiostat. A round-
bottom electrochemical cell was used
for the studies, with a saturated calomel
electrode (+0.242 volts versus standard
hydrogen electrode) as the reference
electrode and graphite as the auxiliary
electrode. All the polarization experi-
ments were performed after stabilization
of free corrosion potential.
The potentiodynamic polarization
behavior of the Thanjavur cannon iron
has been compared with that of mild
steel in Figure 3. Both the irons exhibited
active behavior in pH 7.00 solution
and stable passive behavior in the
pH 7.62 solution. The polarization
behavior of the irons was comparable.
The breakdown potential for both the
samples in the borate-buffered solution
was similar, thereby indicating that the
slag inclusions did not affect passive fi lm
breakdown. The corrosion rates were
determined by the Tafel extrapolation
method as per ASTM standards. The
results are tabulated in Table I. The
corrosion rates of the ancient and
modern irons were comparable and of
the same order of magnitude.
–12–14 –10 –8
log i (log (A/cm2))
Thanjavur Cannon (pH 7.62)
Thanjavur Cannon (pH 7.00)
0.5% C Steel (pH 7.62)
E(mV vs SCE)
–6 –4 –2
–1,000
–800
–600
–400
–200
0
200
400
600
800
1,000
1,200
1,400
1,600
Figure 3. The potentiody-
namic polarization curves
for Thanjavur cannon iron in
borate-buffered solution and
double-distilled water com-
pared with that of 0.05% C
steel in borate buffered
solution.
JOM • January 2004
22
Table II. Identifi cation of Major and Minor Phases from µXRD Analysis
at the Locations Analyzed
Distance from
Environment Pattern
Interface (µm) Reference No. Major Phases Minor Phases
140 TG05 Iron, goethite, magnetite Lepidocrocite
120 TG06 Goethite, magnetite Lepidocrocite, akaganeite
100 TG13 Goethite, magnetite Lepidocrocite, akaganeite
50 TG12 Goethite, magnetite, lepidocrocite Akaganeite
Rust thickness can be predicted from
atmospheric corrosion rates of iron
in several environments:15 4–45 µm/y
in rural environments, 26–104 µm/y
marine, 23–71 µm/y urban, and 26–175
µm/y industrial. Assuming the Thanjavur
weather to be rural, the estimated cor-
rosion product layer over 350 y should
be between 2,800 µm and 31,500 µm.
Utilizing the corrosion rate measured
in the polarization testing of immersed
sample, the total approximate corrosion
suffered by the Thanjavur cannon iron
must be 350 y × 2 µm/y = 700 µm.
When converted to rust, it should have
resulted in a rust thickness of 1,400 µm.
This has certainly not been the case
because the Thanjavur cannon does
not show any evidence of signifi cant
rusting (Figure 1). Measurement of
the rust thickness at one location by
cross-sectional microscopy indicated a
maximum thickness of about 140 µm.
As the surface was not signifi cantly
corroded, the surface apparently was
protected against atmospheric corrosion
by a protective passive fi lm. Further
ideas about the atmospheric rust nature
were, therefore, obtained by rust
characterization studies.
RUST CHARACTERIZATION
Samples of atmospheric rust were
scraped out from the atmosphere side
of the Thanjavur cannon. This rust was
used to identify the constituents of the
atmospheric rust by x-ray diffraction
(XRD) and Fourier transform infrared
spectroscopy (FTIR). Fourier transform
infrared spectroscopy is a powerful
technique to identify the iron oxides and
oxyhydroxides, even if they are present
in the amorphous form. Therefore,
analysis of any rust in general by XRD
would provide information about the
crystalline phases, while the FTIR
spectroscopy, in addition to confi rming
the results of the XRD analysis, would
also provide information about the
amorphous phases. The presence of
lepidocrocite (γ-FeOOH), goethite
(α-FeOOH), magnetite (Fe3–xO4), and
δ-FeOOH is confi rmed by peaks in the
FTIR spectrum at 1,104.10 cm–1 and
797.38 cm–1 (γ-FeOOH), 887.72 cm–1
(α-FeOOH), 559.05 cm–1 (magnetite),
and 455.01 cm–1 (δ-FeOOH).16 The
spectrum shows a shoulder broadening
at 1,000–1,200 cm–1, which may be
attributable to the presence of ionic
phosphates.16,17
The XRD pattern obtained from the
rust on Thanjavur cannon iron was
compared with the JCPDF database
using the Diffrac+ program. Sharp
diffraction peaks were not observed
from iron oxyhydroxide and oxide
phases, presumably due to the low
thickness of the surface oxide. Some
phases identifi ed were lepidocrocite,
iron phosphate, and magnetite.
Microdiffraction (µXRD), which
is XRD analysis performed on small
samples or small areas of large samples,
is the technique of choice when samples
are too small for optics in conventional
diffraction instruments. A microbeam is
used as an x-ray probe so that diffraction
characteristics can be mapped as a
function of sample position. With the
ability to accurately and precisely
position the x-ray beam on a sample
surface, the information can be plotted as
a diffraction function map. Diffraction
data can contain information about
compound identification, crystallite
orientation (texture), stress, crystallinity,
and crystallite size. In this regard, the
unique features of synchrotron radiation
renders possible the investigation
of materials in a way not feasible
with conventional instrumentation. In
particular, wavelength tunability gives
control over penetration depth as well
as for spectroscopy measurements.
Microdiffraction experiments were
performed at several different locations
in one area of the Thanjavur cannon rust.
The µXRD experiments were conducted
on the D15 beamline at Laboratoire
pour l’Utilisation du Rayonnement
Electromagnetique at Orsay, France.
The entire experimental procedure is
outlined elsewhere.18 Photons centered
around 14 keV (λ = 0.8857 Å) were
Figure 4. An x-ray diffraction pattern from rust, confi rming the presence of
lepidocrocite, goethite, and magnetite.
20 30 40 50 60 70
20 (degrees)
80
Iron, Fe (JCPDF 060696)
Magnetite-Fe3–xO4 (JCPDF 190629)
Lepidocrocite - FeOOH (JCPDF 441415)
Iron Phosphate (JCPDF 702075)
90 100 110 120
0
0.4
Intensity (arb. units)
0.8
1.2
1.6
23
2004 January • JOM
focused down to a 10 µm2 × 10 µm2 beam.
The diffraction patterns were collected
with an image plate downstream from
the sample. One-dimensional diffraction
patterns were obtained by circularly
integrating diffraction rings using
the FIT2D software developed at
the European Synchrotron Radiation
Facility. The spectra were compared
with the JCPDF database using the
Diffrac+ program.
The rust was approximately 150 µm
thick at the location investigated. The
µXRD patterns were analyzed and the
results of the analysis are provided in
Table II. The major phases identifi ed in
this pattern have been indexed. Notice
that the inner region of the rust is
primarily composed of magnetite and
goethite while lepidocrocite appears as
a major phase only toward the rust-
environment interface. The identifi cation
of akagaenite (β-FeOOH), which forms
in the presence of chloride ions, indicates
some chloride has been present in the
environment, either from local sources
or from the ocean (which is situated
about 50 km east of Thanjavur).
Phosphates were not identifi ed in the
rust location studied by µXRD and this
must be related to the low phosphorous
content in the iron matrix underneath, as
revealed by the compositional analysis.
However, the color of the surface of the
cannon is quite indicative of the
enrichment of phosphorous in the
atmospheric rust of the cannon. The
phosphorous content in rusts on ancient
Indian iron generally follows the
mesoscopic variation of phosphorous
contents in the iron matrix.19
ACKNOWLEDGEMENTS
The authors thank the Archaeological
Survey of India for its cooperation and
assistance.
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R. Balasubramaniam is a professor with the
Department of Materials and Metallurgical
Engineering at the Indian Institute of Technology,
Kanpur. A. Saxena is a Master of Engineering
student in the School of Physics and Materials
Engineering at Monash University, Australia.
Tanjore R. Anantharaman is a retired professor
currently with Ashram Atmadeep. S. Reguer is
a Ph.D. student and P. Dillmann is an engineer
at CEA/CNRS.
For more information, contact R. Balasubramaniam,
Indian Institute of Technology, Kanpur, Department
of Materials and Metallurgical Engineering, Kanpur
208 016, India; e-mail bala@iitk.ac.in
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