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Delhi Iron Pillar: New Insights

Review: Delhi Iron Pillar: New Insights. Balasubramaniam, R. 2002. Delhi: Aryan Books International. Pp.168, figures 33. Price Rs.1800/- ($40/-).
by D.P. Agrawal

Delhi’s iron pillar (DIP) in the Qutub area has attracted the attention of archaeologists, laymen and metallurgists alike for its rust less property. Various theories have been propounded about its rust free iron. Only recently Balasubramaniam has carried out more rigorous analyses using latest techniques, as a professor of metallurgy in Indian Institute of Technology, Kanpur, these facilities were available to him.

Balasubramaniam attributes these anticorrosion properties to significant presence of Phosphorus (P) in the DIP. We will presently see the technical details of his analysis. He also shows that the maximum corrosion on the DIP occurs near the joint, mainly because of presence of lead.

The book is divided into seven chapters, with one appendix on the technical details on the presence of phosphorus in ancient Indian irons. The first chapter introduces the DIP and its composition. The second chapter deals with the history of DIP which is quite authentic. Balasubramaniam gives even the transcripts of the inscriptions, a discussion of the history and association of DIP with Chandragupta II Vikramaditya, which are quite convincing. Balasubramaniam also discusses details of the palaeography of the inscriptions. He agrees with the date of 410 AD given by the famous epigraphist Hoernele. He also shows that the original location of DIP was actually in Madhya Pradesh, in the Udayagiri hills which have been identified with Vishnupadgiri. As Chandragupta II was a great devotee of Vishnu, the pillar was erected in the honour of his favourite God.

In Chapter 3, Balasubramaniam gives the structural details of the DIP; he also shows that the capital was joined with the pillar by using lead as the joining metal. He also gives a brief history of lead metallurgy in ancient India.

Chapter 4 discusses the decorative bell capital on the pillar. He also discusses the technology of shrink fitting methodology as to how the bell capital was fitted to the iron pillar. The author has illustrated the reconstruction of techniques.

In Chapter 5, Balasubramaniam has reconstructed the manufacturing techniques of the pillar, which was done through forge welding, and use of inserts. In chapter 6, the author discusses the technical details of corrosion resistance of DIP. He has illustrated the discussion with a number of x-ray diffraction and FTIR spectra and diagrams. The author also did Mossbauer spectroscopy of the rust samples. He shows that the phosphate was crystalline iron hydrogen phosphate hydride. The author also pleads for more detailed scientific studies on various aspects.

In Chapter 7, the author gives the summary of his analysis of DIP. In this chapter he also discusses the Dhar iron pillar and the other iron pillars at Chadira hills, Mandu hills, Mount Abu and also some Mughal canons. In the appendix the author has given the technical details and thermodynamic models of the origin of high phosphorous contents in ancient iron pillars. He has also compared here the ancient and modern slag. In the appendix he has shown that ancient Indian iron always produced higher phosphorous than modern iron at all temperatures. He attributes the higher P content in ancient iron to lack of use of lime (CaO) in the flux.

Balasubramaniam summarises his observations as follows:

Several new insights on the Delhi iron pillar have been presented in the present monograph. The subject of iron extraction that was practiced in ancient India was briefly discussed and specific attention was focused on the composition and microstructure of iron of the Delhi pillar. The origin of high P content in the Delhi iron pillar, in particular, and in ancient Indian iron, in general, has also been addressed.

The identity of king Chandra of the Delhi iron pillar Sanskrit inscription has been critically addressed. The name Chandra firmly establishes that the king was Chandragupta II Vikramaditya. Numismatic evidence for the short name of Chandragupta II Vikramaditya being Chandra has been provided for the first time by comparing the archer gold coin types of all the Gupta monarchs. Arguments have been provided to show that the inscription was not posthumous in nature. The conquests of Chandra corroborate the conquests of Chandragupta II Vikramaditya. Numismatic and archaeological find spots have been analyzed to provide support to Chandragupta’s conquests. The personal religion of Chandragupta II also lends strong support to his identification as Chandra. The identification of Chandra with Chandragupta II Vikramaditya poses the least contradictions. The locations of Vahlika and Vishnupadagiri have been critically analyzed. It is proposed, based on archaeological and historical evidence, that Udayagiri could be favorably considered as ancient Vishnupadagiri, where the iron pillar was originally erected. Careful archaeological excavations are necessary at Udayagiri to firmly confirm the original location of the iron pillar.

The various aspects related to the structural features of the pillar have been addressed. A detailed analysis of the dimensions of the pillar and its decorative bell capital has been presented. The presence of lead in various regions of the pillar has been addressed, along which the construction of the pillar has been explored in detail starting from the pillar bottom. The presence of lead in several regions of the pillar has been described and the possible implication for lead presence on the corrosion of the pillar has also been discussed. A brief discussion on the status of lead metallurgy in ancient India has been provided.

The various components that comprise the decorative bell capital have been addressed in detail and the joining methodology of the capital parts as well as the capital to the main body of the pillar has been established. Insights on the possible image of garuda, which was originally placed on the top of the capital, have been provided. The decorative capital of the Delhi iron pillar has been fabricated from individual pieces (that were produced by forge welding and not casting). The individual pieces that constitute the iron pillar’s capital have been intelligently shrunk fit on a hollow cylinder in an artistic and aesthetic manner keeping sound engineering principles in mind. It is important to perform careful ultrasound measurements on all the various sections of the decorative capital in order to obtain further insights into the shrink fitting methodology.

The manufacturing methodology employed to construct the main body of the Delhi iron pillar has been elucidated. The vertical and horizontal methods of forging for manufacturing the main body of the pillar have been critically compared. Several aspects of the manufacturing methodology (hammering method, heating method, use of inserts, use of dies, possible handling method and surface finishing operation) have been discussed. Visual evidences suggest sideways addition of metal lumps with the aid of hand-held hammers with the pillar in the horizontal position. The addition of iron lumps on to the side of the pillar, with the pillar placed in the horizontal direction, appears the likely method of the manufacture of the pillar. The nature of the iron lumps that were forge welded on to the body has been discussed. The use of hand held hammers for the forging operation is also established. The ingenious method employed to handle such a large object has also been illustrated. The to-and-fro motion of the pillar during the forging operation must have been possible with the use of handling clamps on the pillar. The rotational motion of the pillar (and also handling) must have been aided by the use of rotating pegs inserted in the bottom and top cross sections of the pillar, and also on the sides of the pillar. The final surface finishing operations (hot hammering, chiselling and burnishing) produced the smooth surface and taper of the cylindrical pillar.

The current theories (environmental and material) for the corrosion resistance of the pillar have been critically reviewed. The apparent anomaly of a two-phase (iron and entrapped slag inclusions) heterogeneous structure of wrought iron of the Delhi pillar possessing superior corrosion resistance has been understood by mixed potential theory analysis. The nature of the protective passive layer on the corrosion resistant Delhi iron pillar has been addressed based on a detailed characterization of its rust. The rust is composed of iron hydrogen phosphate hydrate (FePO 4.H3PO 4.4H2O) in the crystalline form in addition to a-, y-, o-FeOOH and magnetite, all in amorphous form. The process of protective rust formation on DIP iron has been outlined based on the rust analysis. The passive film formation on the Delhi iron pillar has been contrasted with rusting of normal and weathering steels. The critical factor aiding the superior corrosion resistance of the Delhi iron pillar is the formation of crystalline iron hydrogen phosphate hydrate, as a thin layer next the metal-scale interface, which drastically lowers the rate of corrosion due to its low porosity content. The formation of protective crystalline phosphate is aided by alternate wetting and drying cycles, which is the important contribution of the atmosphere to the pillar’s corrosion resistance. Therefore, the corrosion resistance of the Delhi iron pillar is due to both Delhi (the environment providing alternate wetting and drying conditions) and iron (with its high P content conferring protection by the formation of the crystalline iron hydrogen phosphate).

Suggestions have been provided at the end of each of the previous chapters on the studies that need to be conducted on the Delhi iron pillar. Scientists from the Indira Gandhi Centre for Atomic Research have conducted several scientific studies (in-situ metallography, radiography, sonography and surface potential measurements) on the Delhi iron pillar in 2001 (IGCAR 2001) and these studies should shed valuable insights. Most importantly, there is an urgent need to replace the lead sheet covering the surface of the pillar in the buried underground regions. When the pillar was re-erected by Beglar in the 19th century AD, the stone platform was constructed and a coating of lead was provided on the buried underground surface of the pillar. This uneven coating of lead (of about 3 mm in thickness) was found to be in an excellent state of preservation when the buried regions of the pillar was again excavated in 1961 on the eve of the centenary of the Archaeological Survey of India. However, the buried portion was found covered with rust layers ranging from a few mm to 15 mm. After removal of the rust scales (Pl. 36a), numerous cavities and corrosion pits were observed on the surface. The preliminary treatment of the pillar comprised of elimination of rust, earthy accretions and water- soluble salts resulting in the structure shown in Pl. 36b. The holes, cracks and cavities were consolidated and provided a rust preventive treatmen. The lead sheet coating was again provided to the surface for protecting the pillar from direct contact with mortar and the saline soil on the recommendation of the structural conservators and archaeological engineers. This new lead coating was provided much against the wishes of the Chief Chemist of ASI (Lal 1996). Therefore, the present corrosion rate of iron in the buried regions is much more than that of the exposed surface due to galvanic action with the lead layer, as lead is cathodic with respect to iron (Balasubramaniam 1999b). The iron in the buried underground region is currently subjected to intense galvanic corrosion. It has been suggested, first by Lal (1996), and recently by Anantharaman (1996) and Balasubramaniam (1997b), that the lead coating be removed and replaced with a zinc coating, because, unlike lead, zinc is anodic with respect to iron and therefore would sacrificially protect the iron underneath. However, this may not be appropriate because zinc corrodes rapidly in saline soils and the soil around the pillar, in its current location, is loaded with chlorides and sulphates (LaI1996). It is important to replace this lead coating (Pl. 38) with another suitable coating (epoxy-based coating, especially if the soil is saline, and maybe combined with cathodic protection) for proper preservation of this important cultural and scientific object (Pl. 39). The replacement of lead coating with a suitable coating must be addressed at the earliest by excavating the bottom regions of the pillar.

Although the Delhi iron pillar has been the focus of attention in the present monograph, it must be realized that there are several other large ancient iron objects in India. Some of these objects would be briefly reviewed here. The study of these objects has not been taken up on a large scale, with the reason probably being the lack of knowledge of these objects. It is anticipated that serious studies in the future will address these objects.

The bibliography is quite up-to-date and exhaustive, as also the index. I am sure, archaeologists, archaeometallurgists and the layman alike would welcome this book by a competent metallurgist. The profuse illustrations, both in black and colour, detailed line drawings, graphs and tables make it a very convincing case for explaining the anticorrosion properties of phosphorous, the higher content of which is due to the lack of the use of lime in the flux.

An interesting book on all counts.