ByUdit Chaudhuri
We re-visit thetechnique of lost-wax investment castings, which is ironically associated withspace-age technology but has been traditionally adapted and perfected overcenturies.
Intricate Yabyum bronze forms from Nepal, Dhokra castings by Adivasis from the Bastar region and bell-metal castings by Moosaris of Kerala are increasinglybecoming rare as collectors’ items. It is fascinating to note that these and several intricate Natarajas, Ganeshas etc., resplendent with decorations and ornaments in suchfine detail, are made by tribals living far away from urban industrialcivilisation. Will anyone believe that this is the same process as that used byspace-age technologists to produce precise parts for prosthetics, hydraulics,weapons, automobiles and yes, spacecraft too?
Thereis evidence going back as far as the 4th Millenium BC, of investmentcastings during the Han Dynasty in China, Benin Kingdom in Africa, and Aztecsof Mexico. Cross-cultural trade and movement may have contributed to itsdissemination. In the West, Renaissance sculptor Benvenuto Cellini cast hismasterpiece “Perseus and the head of Medusa” using this process inabout 1545. However, investment casting was ignored until the early 20thcentury, when dentists began to cast inlays and crowns. Perhaps this techniqueentered the urban Indian Art scene via colonial institutions, apparently inoblivion to the masterly perfection of this technique by tribal craftsmen here,until anthropologists, archaeologists and art historians found evidences oftotally indigenous processes along the same lines spread across thesub-continent, from Tibet to Bastar in Chattisgarh, to Kerala.
The essence ofinvestment castings lies in the strength property of wax to retain a basic formyet allow the lightest forces, like the touch of a finger to indent its surfaceand release it cleanly. Wax can be shaped by hand or simple tools into the mostintricate form and yet, its surface can be as fine as polish with a blast of hotair or flame of a blow-torch. Besides, wax evaporates cleanly, leaving notraces. Bees wax is the most common, abundant in forests. Another salientfeature of this process is in the pre-heated mould cavity, which does not chillthe molten metal surface in its contact to form ‘nuclei,’ as happens in othercasting processes.
The casting technique involves several iterations. Thewax form itself may be cast from a clay or plaster pattern, depending upon theform. A core of brick-mortar and clay along with lime-plaster is also used formaking hollow castings. When not cast, the pattern is assembled from waxitself. Here, the main form or torso may be shaped by hand or coil-formed byrolling wax into long and thin strings and winding them over a former.Additional components are separately made, either by shaping the wax strings,by rolling out perfectly flat sheets and cutting ornate profiles or by dunkingformers into molten wax. This is a highly delicate operation, akin to plasticsurgery since the surfaces and shapes need to be made ‘just so.’ Any chance tostain, dent, scratch and wilt must be prevented. On completing the assembly,another set of touches ensures the requisite finish, though the task is farfrom over.
The mould-making process needs to consider thegeometry of the statuette and the pouring properties of the metal used.Although the technique is also used for gold pendants and the like, suchsculptures are usually cast in bronze or a variant like bell-metal. Theorientation of the pattern and method of pouring molten metal also needsconsideration. Thus, wax columns are joined to the pattern, to become runnersfor molten metal to flow in and for risers and vents to displace hot air andgases as the level of molten metal rises. The entire assemblage of pattern,core if any, runners and risers is then placed in a position ready for pouringof molten and covered with a patent cement-like mixture of brick mortar orpulverized burnt clay and lime or plaster, exposing only the gates to receivemetal and gates to release vapour and air. Thus a shell-mould is made.
Now for the main casting process: A coal hearth fannedwith animal-skin bellows slowly bakes this shell, evaporating the wax, while acrucible containing an adequate quantity of metal is similarly heated to meltthe bronze. During this time, all wax evaporates, leaving a precise hollowchamber. A pair of iron togs is used to lift the crucible and pour metal. Anycrust in the surface is tapped. In modernized processes, potassium metal orde-gassing compound is tossed into the crucible at this stage. This alsoremoves any dissolved oxygen from the metal. The crucible is then carefullymaneuvered so as to pour molten directly into the gate of the shell-mould. Therate of flow should be adequate to fill up the cavity without cooling and yetnot as fast as to cause a backlash of air and gases expanding from the ingressof hot metal. Pouring stops when metal is seen rising to the top of the risersand close to the vents.
On completion of pouring the metal, the cast is leftto cool. This may take as much as 12-14 hours, since the metal and the mouldare both at high temperatures. A hot cavity and slow cooling are the nextreason for the fine surface properties of the casting. Once cooled, the cast isfettled by slowly chipping off the mortar-mix and after this, chiseling off therunners and riser columns form the casting. This is followed by brush-cleaningor at times, by treatment with a mildly acidic substance to provide luster tothe metal. The cast is now a finely finished statuette.
We bring you Sushil Sakhuja’s video on the process oflost-wax investment casting. Sakhuja is a Bastar artist and has extensivelypropagated this technique.
Great article. One never talks enough about the technic involved in art. I am always mesmerized when I look at my own bronze/metal collections, not only for the aesthetics but also for the great craftsmanship involved.
ReplyDeletePosted by Pascal on linkedin Group: ArtCollector.