The mastery of the early craftsmen was looked upon in a mystic fashion for onlookers, to whom it must have seemed magical. The bronze-casters had the talent for developing a strong, brilliant material from the relatively useless raw ores that were abundant. He would envelop a wax model in clay and offer it to the fire. A few clumps of rock or powdery earth would be fired separately and would gradually bubble into a glowing red fluid. Adding this to the prepared mold that was now buried into the earth, the craftsmen would likely speak enchantments or offer prayers. Finally, after a few hours the mold would be recovered from the earth and cracked to reveal the awe-inspiring shiny product of earth and fire (Getty, 1990: 145-146).
While less mystically viewed in modern times than in ancient times, the technique has not been greatly enhanced by technology or experience over the centuries. While more elaborate approaches are now employed for larger bronze sculpture, the method for smaller wax models differs merely in the heating material. The original charcoal from ash wood fuel has been replaced by oil, gas, and electricity (Getty, 1990: 145).
The composition of the bronze as a mixture of metals varies quite a bit among cultures, affected by the resources available, the desired strength, hue, and tone, as well as the whim, or "trademark", of the individual bronzeworker. Typical proportions of the primary components, copper and tin, are ninety percent (90%) and ten percent (10%), respectively. In addition, other components are often added, especially tin, zinc, and lead. Each of these allows adjustments of various aspects of the mixture. Tin lends hardness to the copper, while zinc aids in the release of gases from the cast. Lead adds malleability to the cast, easing clean-up and chasing (Getty, 1990: 147).
The lost-wax process begins with the development of a wax model that is created anew or duplicated from a previous sculpture. The material of the base sculpture varies according to the preference of the individual sculptor. Modern sculptors tend to prefer flexible molds because reproducible hollow waxes are relatively simple to develop from them. The flexible mold is then protected by a shell of a harder material, often plaster of paris, that keeps the mold rigid and in place until the wax cast is removed.
When choosing a wax the desired characteristics are softness, plasticity, and strength, which favors the use of bees wax. Yet the rarity and expense for bees wax prohibits its use as a standard, thus requiring substitutes that offer like properties. To that end, two waxes are usually combined to produce the model from the flexible mold. The first is a finer wax that is painted into the mold with a brush which would cling well to the intricate surface of the mold. After then closing the mold, the second, more generic wax is poured into the mold until it fills it. The wax is retained in the mold until the appropriate level of wax buildup is attached to the inside wall, after which the remaining liquid wax is poured out. Thus producing a hollow wax, the founder knows the appropriate amount of bronze to prepare while saving a large portion through the usage of a core of simpler, cheaper base material.
At this point, the wax model is "retouched" to remove the marks left by the flexible mold seams and clean up any obvious imperfections. The core is then filled and long steel nails, known as "chaplets", are inserted into the wax model to hold the core in the appropriate position relative to the outer mold. "Gates" and "vents" (Getty, 1990: 150) made of flexible wax rods are added to the wax model. The former will later be used to introduce the molten bronze into the mold, while the latter offer escape passages to prevent air from becoming trapped inside.
The finished wax model is surrounded by a refractory material that may be a mixture of substances such as plaster of paris, brick powder, ceramic grain, ground sands, etc. This "investment" (Getty, 1990: 151) is then heated to 1250 degrees Fahrenheit, at which temperature the water and remaining wax residue are driven from the mold. The normal time for this burn-out is twenty-four hours for a life-size head or smaller cast. For larger sizes, a week is not irregular.
The conclusion of the process involves pouring the molten bronze at a constant rate into the mold and letting it dry for a few hours. After such time, the investment is then cracked open and the bumps left by the gates and vents are filed down as much as possible. At this point, the individual cast is finished (Getty, 1990: 147-154).
Once the cast has been made it may serve as a completed piece of art or may be a mere component of a larger work. Often the latter was the case in the ancient efforts at bronze-work. This is clear when one looks at two examples of the period, labelled "statuettes of riding youth" (Getty, 1990: 28). In these statuettes, labelled "rider A" and "rider B" respectively, came from the same sanctuary. What is intriguing about them is the closeness of their forms. They have been measured and proven to be identical in size and form in the main body and head. Thus, it is likely that they were both cast from a master mold, some "rider X". Yet, the other parts of the figures differ. The logical conclusion is that, while the heads and main body were formed from the same mold, at least one of the two figures obtained its limbs (arms and legs) from another cast. While the headresses are different, this can be reasonably accepted as a consequence of the handling of the wax models prior to casting. Nevertheless, it becomes clear that in at least one of the figures multiple casts were assembled together to produce a single figure (Getty, 1990: 29).
Further evidence for the fact that this was common practice is also evidenced in the X-radiograph of Roma (Getty, 1990: 40). Before further examination it should be clarified that the x-ray image is oriented as if the statue were facing us. From this perspective, it is clear that one hollow cast was used to make the head, the torso, and a small part of the left leg. The remainder of the left leg and essentially all of the right leg appear to have been affixed to the main cast from pre-cast components that were made independently. Clearly, the sculptors were knowledgeable of the possibilities available through joining separate casts (Getty, 1990: 39-40).
Bronze serves well as a material for casting, for it expands as it solidifies--forcing itself into the nooks and crannies of the mold and shrinks somewhat when cooling. Thus, the wax model is properly imitated by the bronze with which it is replaced and the finished bronze cast separates fairly easily from the mold when ready (Savage, 1968: 17).
The cire perdu, or lost wax, method of casting offers opportunities for artistic expression which are not as accessible in other media. There is great flexibility in the modeling materials that can be used for the core of the model, allowing the base image to be carved, molded, or sculpted in a variety of fashions. Additionally, bronze itself offers a strong material for sculpting, while it can also be easily thickened, by changing the relative proportions of the component substances. When the demands of the target object are greater than can be accommodated easily, the image can be easily adapted when the cire perdu method is being employed. Thus, not only can strength be offered by the material but also the ability to portray the desired artistic effect (Savage, 1968: 61).
For an example of the advantages of bronze, see Giovanni da Bologna's Mercury (Savage, 1968: 173). Here the beauty of bronze can be seen in the grace portrayed in the image, the apparent lightness of the figure. The smoothness and purity of the figure are readily apparent. Yet, when one looks more closely the cire perdu advantage is even more apparent. For the extended (right) arm, the metal should be thicker under the shoulder than it is in the armpit for a realistic portrayal. The process combined with the characteristics inherent in bronze sculpture allow the artist the necessary freedom to portray the desired image.
Mitten, David Gordon and Doeringer, Suzannah F.
1967 Master Bronzes From The Classical World
Mainz: Phillipp von Zabern.
Savage, George
1968 A Concise History of Bronzes
London: Frederick A. Praeger, Publishers.
