The Metallurgical Problems of Dokra Artisans of Bikna Village in Bankura: a Study
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JAST © 2015 M.U.C.Women’s College, Burdwan ISSN 2395-4353 -a peer reviewed multidisciplinary research journal Vol.-01, Issue- 01 The Metallurgical Problems of Dokra Artisans of Bikna Village in Bankura: A Study Punarbasu Bose Bankura Christian College, Bankura West Bengal, India [email protected] Abstract 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. 1.1. 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 3rd 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 10.03.2015, Accepted on 14.05.2015] [44] 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] 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] 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 non- uniformity 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] 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.