The Thai tradition of bronze metallurgy is as ancient and sophisticated as
those that originate in Egypt and China. Scholars still dispute whether bronze
technology was invented in the Middle East and spread easterwardly, eventually
diffusing throughout the ancient world; or whether technological flowering of
the “Bronze Age” was the product of independent discovery and development
at various points, Thailand among them, from the Mediterranean to the South
China Sea.
Nonetheless, not only is there archeological evidence for bronze production
in the north-eastern plateau region of Thailand that dates back to the middle
of the second millennium B.C., but a very substantial portion of the metal artifacts
produced over the millennia of this archeological record consist of ornaments
cast by the hollow lost-wax method.
By the Dvaravati period in the 7th century A.D. these lost wax techniques were
being used in the Menan delta of Thailand to cast remarkable effigies of the
Buddha, bringing into Buddhist art an entirely original image, produced for
the first time in bronze. By the Sukhothai period the properties of the bronze
medium were fully exploited to express the new aesthetic of Thai art.
Today, the casting of Buddhist images by the lost-wax method continues as a
direct heritage of the techniques employed in Buddhist art since the Dvaravati
period. Despite some modern adaptations - the use of electric bellows, plaster
of Paris, and steel pins as chaplets, for example - the artisans still employ
the principles of bronze metallurgy used in Thailand for over a millenium.
The Direct Lost Wax Method
Basic metal casting, technically known as the direct lost wax method, is quite simple. To begin with, a positive image of the object that is ultimately to be reproduced in bronze is fashioned in wax to create what is called the "master model."
A wet cast is then formed around the master model which, when it hardens, forms the "master mold." The cast picks up the negative image of the master model so that now there are a positive (model) and negative (mold) of the same image. This image transfer from the positive to its negative (and back again) provides the logical key to bronze casting, no matter how complex the process becomes.
Molten bronze is then poured into the mold, transferring the mold negative image to a metal positive. The bronze cools, the mold is shattered and, finally, a faithful reproduction of the master model now emerges miraculously in stunning bronze.
There are, however, two problems with this direct lost wax method. First, the physical properties of bronze do not allow large solid casting, thus limiting the technology of the ancient craftsmen to the reproduction of small items such as votive tablets, arrow heads, small weapons, or ornaments.
And, secondly, both the master model (the positive image in wax) and master mold (the negative image of the clay cast) are destroyed in the process of bronze casting. The wax is melted to make way for the bronze; and the cast is shattered to free the metal image.
Each bronze produced by direct casting is therefore unique; and serial reproduction is impossible. Thus the ancient craftsman was obliged by the direct casting method to produce both a fresh master model and new master mold for each bronze copy.
Image Transfer and the Hollow Cast Indirect Lost Wax Casting Method
The hollow cast, "indirect" method of lost wax bronze reproduction however solved these problems. The new method introduced two simple new "layers"– one inside the wax core to form an armature and the other outside the cast to form a "mother-cast"– that triggered the spectacular Bronze Revolution in the arts of antiquity.
The first layer, inside the core, solves the problem of scale by making possible the casting of large-sized hollow bronze sculptures that were theretofore inconceivable. The bronze image is exactly the thickness of the wax it replaces. Therefore, large hollow bronze objects could only be cast by replacing the solid wax core of the direct method with an armature covered by a thin layer of wax.
The armature thus resembles less the candle-like solid wax core of the direct method and more a thin dress draped over a light manikin. Now, with its double-layered armature, the indirect method could produce life-size art objects in bronze, the production of which had previously been restricted to wood or stone.
The other seminal innovation of the indirect method – covering the master mould with a new layer, called the "mother mould"– made possible, for the first time, the serial reproduction of the same image in several copies. By wrapping the master cast in a mother cast, in effect creating a cast within a cast, the indirect method preserves for later use the master cast that is otherwise destroyed when casting bronze directly.
These technical innovations also had the consequence of doubling the number of steps in the casting process so that, at first glance at least, the indirect method appears unnecessarily complicated.
The entire casting sequence – from the first production of the positive model to its final reproduction in bronze – increased the number of image transfers from the three (as in the direct method) to the six separate transfers that are required by the indirect method. The logic of casting remains unchanged: it is essentially the three step image transposition – from the positive of the master model, to the negative of the master cast and, finally, to the positive of the bronze image – of the direct method described above.
However, the indirect method is designed to preserve for later use both the master model (positive) and master mould (negative), which are destroyed when casting bronze directly. As a result, extra steps are necessary to create duplicates of the model and mould that can be destroyed in the final casting stage, preserving the masters for later use.
The new steps of the indirect method extend the casting sequence thus: from a positive (in clay) to a negative (in plaster) to a positive (in wax) to a negative (in six layers of clay) and, finally, to a positive (in bronze). Let’s follow the sequence step by step.
Six Step Process of Indirect Bronze Casting
First a master model is produced in a malleable material, usually clay. This is the positive image that the cast bronze will eventually reproduce. A plaster piece-mold is taken from the clay master model to receive the first image transfer into the negative.
It is called a "piece" mold because it is constructed around the model rather like an orange peel that has been removed in sections and then reassembled in its original shape. This is done to preserve undercuts that occur in finely sculpted pieces with delicate details such as ears, nose or chin, etc. that would be destroyed unless the cast itself is either shattered (as in direct method with a one-piece mold) or removed in pieces (as in the indirect method with a piece-mold in sections).
The piece-mold is removed from the master model in sections so as to leave the model undamage and ready for reuse. Its pieces are then reassembled inside the mother mold – the second innovation of the indirect method described above – much as the pieces of a walnut might be placed back into the matching halves of its walnut shell. The walnut shells of the mother mold are then closed and bound, holding the sections of the piece-mold in place, so that an image of the original positive now exists accurately in a stable construction, both in the negative and in the round.
Molten wax is then poured into the piece mold and allowed to harden into a thin shell. The bronze metal will eventually take the place of the wax, so the thickness of the wax determines the gauge of the bronze itself, usually a quarter inch thick. The wax cools. Now, to provide the second “wall” to control the molten bronze, the inside core of the armature is created. This is done by pouring a thin clay mixture into the hardened wax shell and then allowing it to dry.
When wax and core have set, the mother mold is opened and the sections of the piece mold are carefully removed from the surface of the wax. The wax has received the negative image from the mold, duplicating the master model; it represents the third image transfer; the bronze image will be the fifth.
Investment Cast – The Throwaway Mold
However, first the wax is tooled to achieve a good finish, sharpen details and remove any marks of the piece-mold. Next a cast, called an investment, is made of this wax positive. This investment cast is a duplicate of the master cast and forms the mold into which the molten bronze is poured. Like the wax image of which it is the negative, the investment cast is expendible and will be destroyed in the final bronze casting.
Before investment begins small wax tubes called "headers" are attached to the wax model to direct the inward flow of the molten metal over the surface of the mold. With the headers in position, a mixture of cow dung and water is painted over the surface of the wax image to form the first layer of investment.
This is a very fine-grained solution that will pick up all the detail present in the wax. Iron nails are stuck through this first layer of the investment, through the wax and into the hardened clay core, in order to stablize the construction and prevent any misalignment from occurring when the wax is melted out and the bronze poured in.
After the second layer of clay is applied, wax tubes, or sprues, are connected to the headers to serve as channels for the metal. After the third layer of clay, the mold is left to dry in the sun. Iron wires are attached to prevent it from cracking under the stress of the later stages.
Three or four more layers will be necessary to complete the invement mold. While these last layers of clay are being applied, large wax headers, called "gates", are attached to the wax tubes at the base and shaped into cups. These cup-shaped gates serve as funnels to direct the molten bronze as it is poured into the the inverted mold.
After drying, the mold is turned up-side-down and heated so that the wax can run out, leaving room for the metal. In the meantime, the furnace for melting the bronze is built, crucibles and wood put in, the fire stoked, and the metal brought up to the casting temperature.
Molds are placed bottom-up in a steel frame, and the metal is poured in from the crucible. Since the bronze shrinks during solidification, a steel spoon is used to it top up as it is poured. A number of images are poured at the same time.
After the metal is solidified, the (investment) mold is broken off, and the surface of the image is tooled. Parts that were cast separately are joined and the whole image is polished and, if desired, a patina (often black) is chemically applied.
With the exception of a few modern devices, this method could have been used by artisans of the Dvaravati period. It is quite similar to lost-wax casting in other parts of the world; but some features such as the attachment of wax tubes after partial vestment seem to be unique to Thai practice. Now, as in ancient times, in the hands of skilled artisans, the lost wax process can produce images of great beauty and excellent technical quality.
