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UNDERSTANDING ALLOY DESIGN PRINCIPLES AND CAST METAL TECHNOLOGY IN HOT MOLDS FOR MEDIEVAL BENGAL

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Indian Journal of History of Science, 45.1 (2010) UNDERSTANDING ALLOY DESIGN PRINCIPLES AND CAST METAL TECHNOLOGY IN HOT MOLDS FOR MEDIEVAL BENGAL BARNALI MANDAL *, PRANAB K. CHATTOPADHYAY **,
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Indian Journal of History of Science, 45.1 (2010) UNDERSTANDING ALLOY DESIGN PRINCIPLES AND CAST METAL TECHNOLOGY IN HOT MOLDS FOR MEDIEVAL BENGAL BARNALI MANDAL *, PRANAB K. CHATTOPADHYAY **, DIPTEN MISRA + AND PRASANTA K. DATTA ++ (Received 9 September 2009; revised 17 November 2009) Two beautiful bronze images of medieval period unearthed in South Bengal have been investigated. After laser cleaning, material characterization was undertaken to reconstruct the technology as well as the knowledge base of metal workers. The bronze items were found to be cast product whose chemistry belongs to copper zinc lead system with small addition of tin. The use of copper-zinc alloy in place of copper-tin with addition of lead becomes quite significant in this respect and points towards developing a working knowledge for copper-alloy design. The analysis of microstructures signifies towards a casting technology, where, hot molds were used. For confirmation of the molds, some tribal metal casting operations were also investigated. The technology was then reconstructed in the laboratory and some casting as the facts were produced. Microstructures of all three types look almost similar leading to the conclusion that coppersmiths used hot clay molds for bronze production. Key words: Alloy-design, Bronze casting, Hot molds, Laser cleaning INTRODUCTION East Indian subcontinent has got a long history of using different metals from ancient time and a number of archaeological sites have been * Research Scholar: Department of Metallurgical & Material Engineering, Jadavpur University, Kolkata India, ** Centre for Archaeological Studies & Training, Eastern India, 4 Camac Street, Kolkata India, + Director: School of Laser Science & Engineering, Jadavpur University, Kolkata India, ++ Professor: Department of Metallurgical & Material Engineering, Jadavpur University, Kolkata India, 102 INDIAN JOURNAL OF HISTORY OF SCIENCE excavated which produced a large number of metal images used by the ancient population. From the days of Pa ṇḍu Ra ja r Dhibi 1 to the recent excavation of Tilpi in South Bengal it proves the occurrence of the metal artefacts in continuum. Two copper- alloy images one of Ra dha and Kṛṣṇa (referred here as bronzes) were locally (Baruipur) collected by a Museum, Sundarban A n chalik Saṅgrahasƒa la, Baruipur, South 24-Parganas, West Bengal, India, for conservation. Bronzes had been in heavily corroded state and needed restoration for display at the museum gallery of Sundarban A n chalik Saṅgrahasƒa la. Both the metal images incidentally do not hold any stele behind them. As per conventional museum practice, normal cleaning work was attempted but failed to provide desired result. Jadavpur University was requested to help in the restoration work of these specimens. For preservation, the bronze items were cleaned by application of a Q-switched Nd: YAG laser keeping the patina, as intact as possible, for making them ready as exhibits. Small metal segments were made available by the permission of the Museum authority for metallurgical investigation using common metallurgical tools. The material investigation reveals the bronze items as cast product and the casting technology as well as alloy design of the metals yielded valuable information about medieval metal forming technology of Bengal History of Metal Icons 1. HISTORY AND CLEANING OF BRONZES Metal images of Kṛṣṇa (Fig.1) (Ht. 145 mm and Wt. 378 gm) and Ra dha (Fig.2) (Ht. 150 mm and Wt. 716 gm) were recovered from the area of Karergaṅga, Rajpur Ba za r near Baruipur which falls on the Eastern side of dried up A digaṅga (later down South became a part of Mani-river), along with archaeological debris of bricks, stone artefacts, Sƒivaliṅga, coins, etc. This South Western Sundarban area holds a number of important archaeological sites of Buddhist, Jain and Bra hmanical origins at Boral 2, Sitakunda Atghora, Deulpota, Harinaryanpur, Nalgora, Kankandighi, Jatar Deul, Baishhata and others. Close to Baruipur at Ramnagar many Bra hmanical images of Mahiṣa surmardini, Gaṇesƒa, Su rya and at Baishhata Viṣṇumu rti images are hallmark of a new style of bronzes for Bengal art 3. UNDERSTANDING ALLOY DESIGN PRINCIPLES 103 Fig. 1. Icon Kṛṣṇa Fig. 2. Icon Ra dha A prolific number of East Indian bronzes, were recovered from different sites of Bengal and Bangladesh, and some even were found in hoards. Mitra and Bhattarcharya 4, Neogi and Schroeder, Sahai and Manowar Jahan, Datta and Chattopadhyay, and others analyzed some of these images. A part of that booty, a large number of metal images were also unearthed in 24-Parganas, in South Bengal in those areas mentioned in earlier paragraph. S. K. Saraswasti 5 recorded a few of them which were found at Manir Tat (a mariaci image) and Nalgora (an Ambika image), near Baruipur. As recent as 2006, new finds in 24-Parganas, of South Bengal were excavated by Directorate of Archaeology, Government of West Bengal at a village Dhosa, near Jaynagar close to District headquarter Baruipur. Structural evidences were exposed at that site and there was suggestion of existence of stupa during 1 st and 2 nd century BC. Close to that site lies Tilpi 6 (22 15 N, N), in 24-Parganas, where bronze ingot, crucibles, hearth, slag were excavated and that indicates continuous metal production and human habitation near and adjacent regions of South Bengal, throughout historical periods - though ravaged or washed away, by flood, or nor-wester and changing courses of rivers. So is there, the recovery of metal images, on a regular basis in this area of 24-Parganas, from different periods of history, be it Gupta or Pa la- 104 INDIAN JOURNAL OF HISTORY OF SCIENCE Sena or Medieval or pre-modern ages. In close proximity, also inhabit a large number of famous archaeological sites of Chandraketugarh, Balaidhap (near Mahasthan), Sƒa gardi ghi, etc. The excavated materials proved to be remnants of a lost civil society of this area. From the iconographic angle, Sri Hemen Majumdar, Secretary of the Museum, suggested in consultation with many art historians, that the excavated images represent icons of Ra dha and Kṛṣṇa, following Vaiṣnavaite tradition of Madanapa la (12 th century) period. Those two metal images are of very miniature form 7. The images also do not belong to the same pair, because Ra dha stands taller than Kṛṣṇa, which is most unlikely from the iconographic angle. Probably, the images were worshipped in separate enclosures, following Sƒaivate tradition of Pa la-sena period like Kevalacandrasƒekhara 8 fashion. The probable reasons of small sizes of icons may be explained as follows. South Bengal ran a vibrant economy of trade and agriculture in historical periods and produced enough surpluses to indulge in costly metal culture. But, being deltaic and fragmented hinterland, the area (maṇḍala) never had a large political hegemony of a kingdom and was only capable of supporting little affluence of producing tiny metal images for village temples as iṣṭadevata. So, Bengal bronzes, in comparison with South Indian bronzes predominantly are small in size. These bronzes from stylistic angle followed the proto-australoid anatomy of round geometry as generally depicted by Jamini Roy s folk women or Konarak s dancers. As per physiognomy, Ra dha looks with wide open eyes made by incision in the shape of angular ends (paṭalcera cokh) with a plump face of Pa la-sena female icons. She possesses features of an ideal Bengali woman, having short stature, voluptuous breast, and plump hip, supported by heavy buttocks, round hands and legs. Similar to Hara Pa rvati bronze at Nalgora (7 th - 8 th century AD) 9, a tṛratha pedestal of a full blown lotus, a derivative of Pa la-sena period, carry the image. She stretches out her palms probably to hold a lotus or a flower. Conversely, Kṛṣṇa has a chiseled frame, almost lifted from the figure of Cola bronze, with tṛbhaṅga pose, playing his flute. He wears a pointed crown over his head and postures close to a nṛtya mu rti (dancing pose), exposing graceful radiance of romance, unlike soft Buddha image of Pa la-sena style. His exuberant mood was further enriched by a flat lotus at his knee with flying UNDERSTANDING ALLOY DESIGN PRINCIPLES 105 robes at his belt with a full-blown lotus pedestal under his feet, where petals are projected outside at second layer of tri-ratha foundation. Kṛṣṇa image again was better crafted and fashioned with sleek physique having sunken eyes, and can pose a challenge to earlier Pa la-sena bronzes in beauty. All these iconographic features, with a free-wheeling style of over-flowing emotions, after Buddhist period of Pa la-sena age, hold many sharp features of earlier bronzes acquired through generations. Samatat area or Mani-river basin are dated between 8 th - 12 th century AD and many areas are variously dated from as early as Kuṣa ṇa period (Kuṣa ṇa coins at Jatardeul) 10 to as late as 13 th century AD. Considering Bra hmanical nature of icons, those two images could be placed between 13 th -14 th century AD Laser Cleaning Operation of Bronzes Both the bronzes before cleaning had hard dark corrosion coating over the surface. The dark coating was removed by laser cleaning method with Q-switched Nd:YAG laser. Bronzes were not subjected to chemical cleaning rather laser cleaning were adopted with a very slow removal rate, by Quanta System Laser (Model: PALLADIO, Serial No. PLLOOI-0207). During laser treatment bronzes got overheated which was countered by cooling the items under water. It was also observed during Laser treatment, that cleaning of the artefacts under wet condition yield better contaminant removal rate than dry condition. By repeated laser cleaning for two hrs per seating it took 4 to 5 seating, for arriving at a dull but clear appearance. The dull appearance comes from the presence of a thin layer of patina. The patina is deliberately left on the artefacts as it helps in protecting the substrate against further degradation. Details are shown in Table - I. After removal of the coating, Bronze castings showed dull yellow color with shallow irregular poke marks over some areas. The cleaning sequence had been documented with photographs (Fig. 3 and Fig. 4) after each stage for the items of Kṛṣṇa and Ra dha, respectively. 2. RESULTS OF THE LABORATORY ANALYSES Two smaller inner fragments of the bronzes were collected and then were investigated using common metallurgical tools for Microstructure study, Scanning Electron Microscopy with EDX and X-Ray Diffraction Analyses. 106 INDIAN JOURNAL OF HISTORY OF SCIENCE Table 1: Details of the Laser Cleaning Operation by Q-switched Nd: YAG Laser Experimental Sample Time (Second) Frequency(Hz) Energy(m J) Run No. 1. Bronze* Bronze Bronze Bronze Bronze Bronze Bronze Bronze *Bronze here signifies copper-alloy items as is the convention of archaeologists and does not represent the scientific copper-tin alloy understood by metallurgists. Fig. 3. Icon Kṛṣṇa Fig. 3(a) Half Cleaned Fig. 3(b). Totally Cleaned (as received) UNDERSTANDING ALLOY DESIGN PRINCIPLES 107 Fig. 4. Icon Ra dha Fig. 4 (a) Half Cleaned Fig. 4 (b) Totally Cleaned (as received) 2.1. Chemical Compositions by SEM-EDX The average chemical composition analyzed using Energy Dispersive X-Ray Analysis (EDX) of Scanning Electron Microscopy has been given in Table-II. From the given compositions, bronzes Ra dha and Kṛṣṇa are found to belong to Cu-Zn system and can be termed as Leaded-Brass. Art historians generally call all copper alloy artefacts as bronzes but in actual metallurgical terminology Cu-Zn alloys are called as brass. When brass contains up to 30 wt. % Zn, the brass is called α-brass or 70/30 brass as per British practice 11. In American practice 12 when brass contains good percentage of zinc and low amount of tin and lead, those are called Leaded Semi-Red Brass. Table II: Chemical Compositions of Bronzes Elements (Weight%) Ra dha Kṛṣṇa Zn Pb Sn As Sb Fe Cu 108 INDIAN JOURNAL OF HISTORY OF SCIENCE Considering, (i) Lead is soluble in copper at higher temperature having monotectic α + L 1, (ii) But, lead is insoluble in copper at room temperature and get precipitated beyond the Cu-Zn system, (iii) Although, lead forms eutectics with tin 13 at 183 C and so, Lead becomes sparingly soluble in Cu-Zn-Sn system and let us assume, 80 % of the lead, to be out of the alloy system. Therefore, 20% lead will be soluble in the Cu-Zn-Sn system. For example, in icon Ra dha, percentage of Lead insoluble and outside the system: 6.16 x 0.8 = 4.93 wt.%, percentage of Lead: 6.16 x 0.2 = 1.23 wt. % soluble in the system. Total elements remaining in the solid Cu + Zn + Sn + As + Sb + Pb = = wt.% In weight percentage, the actual composition achieved by the alloy is, Cu = = %, Zn = = %, Sn = 3.85 = 4.04%, As = = 0.85 %, Sb = = 0.92 %, Pb = 1.23 = 1.29 % With this alloy for calculation* of Zn-Equivalent 14, the zinc will be, (Zn) + 2 x 4.04 (Sn) + 1 x 0.85 (As) + 1 x 0.92 (Sb) + 1 x 1.29 (Pb) = 28.9 % For Zn-Equivalent calculation the total comes to (Cu) = * The equivalents, due to Guillet, Element Sn Pb Fe Equivalent Zn UNDERSTANDING ALLOY DESIGN PRINCIPLES 109 Zn-Equivalent will be = wt.% and Cu will be wt.%. Similarly, in icon Kṛṣṇa, percentage of lead insoluble and outside the system: 9.75 x 0.8 = 7.8 wt.%, percentage of lead: 9.75 x 0.2 = 1.95 wt.% soluble in the system. Total elements remaining in the solid Cu + Zn + Sn + As + Fe + Pb = = 92.2 wt.% In weight percentage, the actual composition achieved by the alloy is, = Cu = = %, Zn = = %, Sn = = 2.02 %, As = = 1.03 %, Fe = = 2.76 %, Pb = = 2.11 %. With this alloy for calculation* of Zn-Equivalent, the zinc will be, (Zn) + 2 x 2.02 (Sn) + 1 x 1.03 (As) x 2.76 (Fe) + 1 x 2.11 (Pb) = % For Zn-Equivalent calculation the total comes to (Cu) = Zn-Equivalent will be wt.% = wt.%. and Cu will be Therefore, taking the composition as binary Cu-Zn system, the bronze items can be approximated as α-brass from calculation of Zn-equivalent in copper as shown in Table - III. According to that convention, both Ra dha and Kṛṣṇa icons can be termed as α-brass, having constitutionally a single phase structure, as shown in copper-zinc phase diagram. Following American convention both can be termed as Leaded semired brass having small percentage of tin in both bronzes. Those can be = 110 INDIAN JOURNAL OF HISTORY OF SCIENCE Table III: Calculated Zn-Equivalent of Bronzes (wt.%) (excluding undissolved Lead) Radha Kṛṣṇa Zn Equivalent wt.% wt.% Cu wt.% wt.% (Pb) ( wt.%) (+ 7.8 wt.%) further classified as Naval brass or a variety of Gunmetal where the basic alloy constituents are Copper-Zinc-Tin-Lead system, as per British practice. Further to note that Bengal copper smiths introduced tin and lead into low zinc brasses (zinc content, wt.%). Metallurgically, the use of Cu-Zn system with large amount of lead and low amount of tin though is not very conventional, but this complex combination of alloying elements are something notable for Bengal copper smiths. This may be due to nonavailability of Tin in lower Bengal region or may be a borrowed practice from itinerant metal workers of copper hoard culture. Some of them still exist as migratory Dokra s at present. The similarity of metal culture between the coppersmiths of Bengal and migratory Dokra s is noteworthy. On the basis of this discussion, it is well understood that Bengal metal smiths probably had thoughtfully designed the alloy of bronzes. Therefore, the alloy designs of the two bronzes signify many notable metallurgical understanding Microstructure of Bronze Icon Ra dha Optical Microscopy The microstructure of bronze (Fig.5) Ra dha, in chemical composition, can be pronounced as a Leaded semi-red brass, Cu-Zn-Sn-Pb system, and shows typical cast brass structure with clear copper rich dendrites arranged in fir-tree pattern (white areas) as α-cu phase. Black areas remain as lastto-freeze solute Zn-Sn rich β-cu phase. Black round spots, in between dendrite, or sometime within dendrites indicate low melting point insoluble Lead 15, which does not dissolve in copper and present itself dispersed as uniformly distributed globules in the matrix and some time occupy the spaces of micro-voids or simply get entrapped. In such castings, lead effectively UNDERSTANDING ALLOY DESIGN PRINCIPLES 111 Fig. 5. Magnification: 50 X; Etchant: FeCl 3 in HCl. For icon Ra dha, white areas show Copper rich dendrites. Dark areas indicate solute rich (Sn, Zn etc.) last solidified β-cu phase. Round spots reveal Lead. (R.H.S. in Fig. 5.a) The morphology of dendrites has been shown schematically. seals the pores and helps to produce castings without micro-porosity. The typical Cast structure proves conclusively that the icon Ra dha had been produced by metal casting process in a foundry Scanning Electron Microscopy For understanding distribution of elements, the specimen was further investigated by SEM & EDX method. From the results of the Scanning Electron Microscopy the structure (Fig. 6) clearly reveals serial orientation of Cu-rich α-phase dendrites (black) as matrix. Last-to-freeze solute rich, β- phase can be seen as shaded areas (white to grey). Lead particles are shown as white round or roundish spots. The typical cast single phase, the microstructure of Gunmetal, showing dendritic morphology predominates the photograph. The composition analyses of many areas conclusively prove the above observation. The chemical analysis of matrix α-phase (black) proves Cu-rich primary grains of dendrite. The round spots analyzed show the presence of lead as major constituent (white). Also the presence of (Zn, Sn, Pb, Cu) S sulphide (2 nd left, Fig.6) concentration points towards the origin of copper as sulphide ore like copper pyrites, sourced from Singhbhum copper belt. 112 INDIAN JOURNAL OF HISTORY OF SCIENCE Fig. 6. Magnification: 100 X, Etchant: FeCl 3 in HCl, all compositions are given above in wt.%. The microstructure is of same material as of Fig. 5, but the contrast is different to indicate the matrix of α-cu phase. β-cu phase is white and round insoluble Pb particles are of the same color. The minor phases obviously contain less volume fraction. Note the serial nature of dendrites at the left hand portion shown by parallel (Q%) lines, with sectional cut-off polygonal rounds at the right hand side (schematically later shown in Fig. 10). SEM-EDX analyses the material compositions of respective phases. During non-equilibrium cooling 16, segregation ratios k, C k = C S L (1) calculated by the minimum solute composition to the maximum solute composition, gives the idea of non-equilibrium cooling, during solidification. C S = Concentration of solute (Zinc) in solid phase, wt.% (First to solidify), C L = Concentration of solute in liquid phase, wt.% (Last-to-solidify) (see Fig. 7). For icon Ra dha, Zn: : = 0.32, which is different form, equilibrium CS 16.5wt.% Zn diagram values of, k = = = 0.83 obtained from Cu-Zn phase C 20wt.% Zn L diagram. 1000ºC, C S = 16.5wt.% Zn, C L = 20wt.%Zn]. UNDERSTANDING ALLOY DESIGN PRINCIPLES 113 Fig. 7. A part of Cu-Zn phase Diagram 17 This indicates deviation from phase diagram compositions and occurrence of the non-equilibrium cooling of liquid metal. In case of cellular dendrites Segregation ratios (k) generally change significantly 18. The cellular dendrites and Segregation Ratio (C S :C L ), also provide information about slow cooling rate of the casting where, as only low liquid temperature gradient can exist at that condition. From the classical solidification condition for plane front solidification 19, G mc (1 k) R Dk Where, G = Temperature gradient of cooling liquid metal, K/cm, R = Rate of solidification front advancement, cm/sec, m = Slope of the liquidus line in Cu-Zn Phase Diagram, 0 k = Partit
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