JAST 2015 M.U.C.Women s College, Burdwan ISSN

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1 JAST 2015 M.U.C.Women s College, Burdwan ISSN a peer reviewed multidisciplinary research journal Vol.-01, Issue- 01 The Metallurgical Problems of Dokra Artisans of Bikna Village in Bankura: A Study Abstract Punarbasu Bose Bankura Christian College, Bankura West Bengal, India punarbasu_bose@yahoo.com An age old technique of metal casting called dokra still survives almost in its primitive form in Bikna village situated near the district town of Bankura. The technology of the Dokra artisans is essentially that of nomadic era and hence facing different problems these days. The artisans are fully aware of the shortcomings of their arrangement. The furnace is fuel inefficient. In addition it makes the casting faulty. Evaporation of metal is rising up the input costs. Non-uniformity of temperature is pushing up the rejection rate to an unacceptably high level. NISTADS worked for improving furnaces. Causes of different shortcomings from material science point of view will be discussed here, and possible solutions also have been suggested. Key words: Dokra, cire-perdu, open furnace, metal casting, NISTADS, material science 1. Introduction On the auspicious days of Bengali month of Bhadro (August-September) in every Bengali Hindu house in West Bengal goddess Lakshmi, provider of wealth is worshipped, and the altar of the goddess is decorated with small metal sculptures called Dokra [Fig.1 (a)]. Dokra the traditional metallurgical art form of Bengal is not only part of socio-religious culture of Bengal, but it has a great demand as collectable decorative item worldwide Historical Background The lost-wax or investment method of the traditional metallurgical art-form is an age old technique of metal casting known to mankind even from the 3 rd millennium BC, and it has sustained few changes since then. Its Indian version, Dokra traces back its existence in Indian sub-continent to the time of Indus valley civilization. The famous bronze figure of a dancing girl of Mohenjo-Daro [Fig.1 (b)] can be taken as one of the evidences in support of that which was crafted some 4500 years ago. Apart from India, this lost wax technique was known and used in ancient Egypt, Mesopotamia, China, Greece, France, Mexico, and so on. In French the technique is called cire-perdue. The ancient craft of dokra (cire-perdue, or lost wax, or investment) metal casting was once widespread throughout India, but is now restricted to a small number of groups of traditional artisans in widely dispersed locations. The name 'Dhokra' or 'Dokra' was formerly used to indicate a group of nomadic craftsmen, scattered over vast tract in the mineral-rich central Indian tribal belt covering modern day Jharkhand, Chhattisgarh, Madhya Pradesh, Orissa, and parts of Andhra Pradesh. They also settled in the adjoining alluvial districts of Bankura, Bardhhaman, Purulia, and Midnapur in the western part of Bengal. One significant nucleus of the craftsmen still exists among related groups of families in Bikna village (Bankura), West Bengal, India. The present study has been carried out there. Dhokra artisans are doing a great job by preserving this priceless heritage. [Article History: Received on , Accepted on ] [44]

2 The Metallurgical Problems of Dokra Artisans of Bikna Village in Bankura: A Study Author: P. Bose 2. Dokra or Lost Wax or Investment Technique of Metal Casting Theoretically the casting process is just like photography. An image is reproduced though the use of negatives and prints, but in casting negatives are moulds and our prints are castings. Lost-wax is a method of metal casting in which a molten metal is poured into a mould that has been created by means of a wax model. The process that the craftsmen at Bikna follow can be summarized in the following steps: Preparing the wax model Applying fine-grain clay (made up off fine sand, fine termite clay and cow dung over the wax surface Attaching the crucible to the mould with a mixture of clay and rice hux Firing the mould in the furnace Pouring metal into the mould Collecting the metal cast Polishing and finishing Step 1: A core is made using rather coarse clay vaguely resembling the end product. The clay core is hardened either by drying in the sun or by mildly firing in an oven. Step 2: Bees-wax mixed with dhuna, (a resinous gum of the Sal tree), and boiled in mustard oil. This mixture, also is called dhuna, is drawn into stripes and thin wires, the artist wraps them around the clay core to produce a replica of the final product. Step 3: The replica is coated with a very thin layer of very fine wet clay prepared by mixing of fine sand, termite clay, and raw cow dung. All the fine details of the wax replica are now impressed upon clay. This layer is sun dried, and further layers of clay are added. The mould is now ready [Fig.2 (a)]. A clay funnel is added for molten metal to flow inside the mould. Step 4: The mould is carefully heated so that the wax melts and is lost, leaving behind a cavity. Step 5: The cavity is filled with molten metal, and this step involves actual casting in a furnace [Fig.2 (b)].then the mould is left to cool down. Step 6: The clay mould is broken and the artifact is taken out for cleaning and polishing. And the end product is ready [Fig.2(c)]. JAST -2015, Vol.-01, Issue-01 [45]

3 JAST-a peer reviewed multidisciplinary research journal Vol.-01, Issue-01 Fig.1: (a) Decoration with dokra in worship of goddess Lakshmi (b) The dancing girl of Mohenjo-Daro Fig.2: (a) Preparation of mould, (b) Burning the moulds in open furnace, and (c) The finished product Fig.4: (a) a good cast, (b) a burnt out cast (visibly different surface texture of (a) and (b)) Fig.5: (a) Distortion and poor surface finish: nodules and ridges (b) Incomplete casting [46]

4 The Metallurgical Problems of Dokra Artisans of Bikna Village in Bankura: A Study Author: P. Bose 3. The Problems Defects in casting can generally be divided into: a) Distortion, b) Poor surface finish, c) fins / spines, d) nodules, e) veins and ridges, f) rough surface, g) Porosity, h) Voids, and i) Incomplete as well as inefficient casting. These imperfections are of general nature and part of any metal casting process as well as Dokra casting process followed by Bikna craftsmen. They can be eliminated by ensuring minimization of the common causes. But the main source of problems with the process followed by Bikna craftsmen is the age old primitive technique and the primitive furnace they are using. A crude furnace is built in a convenient open space, using loose bricks [Figure 2(b)]. The fire is made using cow dung cakes and bought charcoal. Completed moulds are laid in the fire, with the cup downwards. When the mould is judged to be ready, it is removed from the fire using tongs or a pair of green bamboo sticks. It is inverted, so that the metal cup is at the top, allowing molten brass to run down into the mould space. When the mould is cooled down the special weak panel in the metal cup is break opened with the help of a stick. Secondly, and perhaps most importantly from metallurgical point of view, Bikna artisans use a Cu-Zn alloy called pittal ( + Brass, Cu-40 Zn Wt. %) [Fig. 3 (a)] for casting, which they don t prepare on their own but buy from open market. To maintain the quality product that pittal should be of right quality. Pittal bought from open market most of the time is not scientifically standardized, and if there is an admixture of another Cu-based alloy called kansha (bell metal, Cu-22 Sn Wt%, or bronze) [Fig.3 (b)], which appears to be identical but are completely different alloys in crystallographic sense and as well as in composition; then the end product will be burnt out [Fig.4 (b)]. The surface texture of a burnt out cast is visibly completely different from that of a good cast [Fig.4 (a)]. Figure.3: (a) Cu-Zn phase-diagram, (b) Cu-Sn phase-diagram The traditional furnace [Fig.2(b)] is inefficient in two ways: Firstly it consumes much fuel. Each furnace is built for a single batch production only. Again fuel is wasted heating the furnace and the moulds to casting temperature, and there is no gain from multiple firing in the same oven, thereby conserving heat. The primitive nature of makeshift furnace introduces a variety of great many problems as it is more or less impossible to control the firing temperature of the furnace. Due to the nonuniformity of temperature the rejection rate was unacceptably high. Metal, particularly zinc, is lost by sublimation when the moulds were broken open. The loss of metal led to serious metallurgical degradation of the brass, as well as being another cause of cost inefficiency. Although the craftsmen are very much aware that metal is wasted in this process, they don t know how to prevent this. JAST -2015, Vol.-01, Issue-01 [47]

5 JAST-a peer reviewed multidisciplinary research journal Vol.-01, Issue-01 And, last but not the least, that evaporation of metals is causing severe health hazards is evident from the fact that many of the people of Bikna suffer from eye problems, probably due to heavy metal irritation. 4. Role of Material Scientists: Searching the Causes of Defects 4.1. Defects arising out of casting technique: The first two defects of metal casting as listed in general category above, i.e. a) Distortion, and b) Poor surface finish can be minimized by careful and proper manipulation of the wax and handling of the pattern properly. The defects of poor surface finish can be further divided in excessive surface roughness, unexpected surface irregularities, and inappropriate surface colour [Fig.5 (a)]. Surface roughness can be described as finely spaced surface imperfections, while the irregularities relate more to isolated imperfections (such as nodules). It should be accepted that the surface roughness of the casting is greater than the roughness of the wax pattern it is made of. This can be explained by a particle size of the investment which is such that it cannot reproduce the wax pattern in fine enough details. c) Porosity: Porosity of the casting can be spread within the casting and on its surface. The surface porosity adds to surface roughness, but can also be a sign of the internal porosity. The internal porosity might weaken the casting, cause discoloration if spread to the surface, and in extreme cases lead to a leakage. Main causes of the porosity of alloy castings are: 1) Solidification defects, 2) Trapped gases, 3) Residual air, and 4) Incomplete casting. 1) Solidification defects: Solidification defects can lead to two different manifestations of porosity; localized shrinkage-porosity, and Micro porosity. Localized shrinkage-porosity is caused by insufficient feeding of the alloy during solidification. As the alloy solidify, it also shrinks by over 1%, and a sufficient supply of molten alloy is required during this phase to counteract reduction in the volume caused by the shrinkage. If the sprue (a channel for molten alloy to reach the mould once the wax has been burnt) is not properly designed and implemented then it may solidify before the feeding is complete thus preventing a continuous supply of molten alloy. This type of defect usually occurs close to the spruecasting junction. Micro porosity is also caused by solidification shrinkage, but generally happens in fine grain alloys when the solidification is too rapid for the micro voids to segregate. This in turn causes the mould or casting temperature being too low. 2) Trapped gases: Many metals dissolve or occlude gases when they are molten. On solidification, these gases are forced out of the casting causing what is usually called pinhole porosity. These voids are rather small. Larger voids could be caused by the same mechanism, but also be caused by the gases mechanically trapped within the molten alloy during the casting procedure. 3) Residual airs: These voids occur on the inner surface of the casting and are caused by entrapped air which cannot escape through the investment. This can be prevented by adequate temperature during the casting, sufficiently high pressure, and correct liquid powder ratio. 4) Incomplete casting: If the molten alloy is prevented from fully or partially filling the mould it results in incomplete or even no casting at all [Fig.5 (b)]. The two most obvious reasons for this are insufficient venting of the mould and high viscosity of the molten alloy. If [48]

6 The Metallurgical Problems of Dokra Artisans of Bikna Village in Bankura: A Study Author: P. Bose the air in the mould could not be vented quickly once the molten allow is poured into the mould, its back pressure will prevent complete filling of the mould by the molten alloy. This point in direction of either insufficiently high casting pressure or that it has not been applied for a sufficiently long period of time. At the early stages of the casting the metal is still quite soft, but starts to solidify (and thicken) quite quickly. High casting pressure should be maintained sufficiently long after onset of solidification. If the elimination of the wax patter was not fully complete, the pores of the investment might become filled with the combustion products thus preventing venting of the air which can cause an effect similar to the above. Water film: Wax is a hydrophobic material (i.e. it is repellent to water). If the investment is not enclosing contact with the wax pattern a water film might be formed over the surface. This water might cause occurrence of small veins and ridges on the surface of the casting. Loss of contact between the investment and the wax pattern might be caused if the pattern has moved slightly, if it has been subjected to vibrations, or if the painting has not been properly applied to. Air bubbles: Occasionally, small air bubbles can become attached to the pattern during or following the investing procedure. During the casting, the bubble is filled with the casting material (alloy) and is manifested as a nodule. Occurrence of the air bubbles can be prevented if a vacuum investing procedure is adopted. Some other measures can be adopted to prevent this happening, too, such as use of a mechanical mixer with vibrations or application of a wetting agent in a thin layer Problems Caused by Primitive Furnace High heating rates: Adopting too high heating rates for the mould lead to occurrence of fins (or spines) in the casting. The mechanism for this is as follows: a) If the investment is subject to too high heating rate the outside layer becomes hot faster than the inner and the temperature difference between the outside layer and the center of the investment is increased. Consequently, the outside layer tends to expand more than the inner parts. However, the outside layer is held back by the inner, cooler part. Because of that the outside layer is subject to compressive stresses, while the inner part is subject to tensile stresses. Since the investment is a brittle material, it tends to crack under tensile stresses. In this case, the most typical cracks are radial, starting from the interior of the investment and spreading outwards. During casting, these cracks are filed by the casting alloy, manifesting as fins or spines. Low temperature: Too low a temperature may cause incomplete removal of the wax. Gases, formed when the hot alloy comes in contact the residues may cause porosity or voids in the casting. Too low a temperature can be caused by too short a heating time, or by if insufficient air is available in the furnace. Prolonged heating: Prolonged heating at too high temperatures could lead to a disintegration of the investment with a consequence of rough mould walls. In addition, products of the disintegration could contaminate the alloy causing surface defects. Alloy temperature: If the alloy temperature is too high it can attack the surface of the investment similar to the case of the prolonged heating. As a consequence, a similar surface roughness can occur. The other problems that can arise due to uncontrolled heating in primitive furnace are as follows: i) Excessive heat can change the composition of the alloy; ii) environmental gasses can become dissolved in the melt increasing its porosity; iii) some alloys such as base metal can be hardened and made brittle by excessive heat; and iv) oxidation of the alloys can also occur due to very high temperatures. JAST -2015, Vol.-01, Issue-01 [49]

7 JAST-a peer reviewed multidisciplinary research journal Vol.-01, Issue The Measures Taken So Far There has been some Government initiative in improving the situation when NISTADS (National Institute of Science, Technology and Development Studies) became involved to provide improved furnace for dokra metal casting at Bikna, Bankura. NISTADS under its TARA (Technological Assistance to Rural Artisans) programme funded Bengal Engineering College to design and develop a fuel-efficient permanent furnace under the management of Dr A. K. Mukopadhyay, NISTADS Resident Scientist in Bankura. As a result a new furnace becomes operative on 26 th September The improved process has effected substantial reduction in fuel consumption for melting brass in a low cost furnace, of capacity 8-12 kg /batch. NISTADS has helped Bikna artisans to obtain bank loans to avail modernized facilities. However, despite this knowledge, the Bikna families made no move to adopt the new technology 1, 2. Although a little advancement has taken place in modernizing furnaces, very little scientific study has been made so far to understand the micro structural nature for the defects of this art form. There is a wide and open scope of micro structural and mechanical characterization of the metal casts, and only that can bring out the root cause of the defect of such metal casts and save this traditional art form. Acknowledgement I am grateful to Mr Gautam Roy, owner of a show-room of dokra artifacts in Bankura town, for introducing me to some of the dokra artisans of Bikna village of Bankura, and Mr. Kamal Karmakar, artisan of Bikna village, for providing me necessary inputs for this study. References 1). Rajesh Kochhar, Dhokra The Traditional Art of Metal Casting, Chitrolekha International Magazine on Art and Design, Vol. 1, No. 2, August, ) David Smith, Rajesh Kochhar, TheDhokra Artisans of Bankura and Dariapur, West Bengal: A Case Study and Knowledge Archive of Technological Change in Progress. [50]