The process of going from
a clay model to a bronze sculpture is most perplexing to people who have never
visited a foundry. We know this to be true when a friend of The Horse calls
us and asks if we are ready to "bronze coat" the model. Though the
actual foundry process is somewhat complex and involves many steps, it is
not difficult to understand and is quite fascinating. We ask you to find a
comfortable chair, relax, and visualize what the words describe. In your imagination
you will see a Horse go from clay to bronze.
Recall that enlarging machines were used to construct a 24-foot model from
the final 8-foot model. Sculptor Nina Akamu and her seven assistants then
refined the muscle forms, surface details and nuances so that the 24-foot
was an exact copy.
The next step involved planning how many pieces would be cast in sand molds and how many would be cast by the lost wax method. Metal shims were inserted in the clay horse to indicate the boundaries of each mold section. Our Horse has about 60 sections, each of which is no larger than approximately four feet square.
Then, Tallix workers made a rubber mold of the completed 24-foot model by spraying liquid rubber mold material over the entire Horse to about a ¼ inch thickness.
This coating captured the most minute details of the clay sculpture, and was the start of all succeeding steps. Workers applied a half-inch layer of polyester resin and fiberglas to the outside of the rubber coating to provide a sturdy backing called the "mother mold." When it hardened, they removed all the sections from the model, the parting lines having been established by the metal shims.
The majority of The Horse was cast in sand molds. A positive plaster was cast from the inside of the mother molds. One by one, each plaster pattern was placed in a heavy 8-foot square steel molding box where a mixture of sand and binder was rammed against it. After that mixture set, upper and lower parts of the molding box were separated, the plaster removed (leaving a negative image in the sand), and runners and gates were cut into the surrounding sand to provide channels for the bronze to flow. The upper and lower parts of the molding box were closed and molten bronze (at about 2,000 degrees F) was poured through pour spouts. The bronze flowed into the space between the upper and lower sand molds creating a bronze section of The Horse about ¼ inch thick. Once the molds had cooled enough to be handled, the sand was knocked apart to reveal a section of The Horse. Foundry workers repeated this process daily, piece-by-piece, until all sand-cast pieces were completed.
A few sections were cast by the lost-wax process, however, which is more applicable to areas where many minute details are grouped together, or there are undercuts in the sculpture. The mane, forehead (and forelock), ears and tail were cast in this method. Recall the rubber molds from our earlier step. In the lost wax process, successive coats of hot wax were painted directly into each mold, while the resin mother mold cradled it for support. The wax was forced into every nook, crevice and undercut of the mold to capture every detail. A pour spout was added, the mold closed, and liquid wax poured in and sloshed around to create a uniform inner thickness against the rubber. The excess was poured out. When the wax cooled, the mold was parted from the wax, which was then a positive copy identical to the original clay ¾ remember, use your imagination ¾ complete with seams and surface details.
In the wax department, Tallix artisans re-worked any mold lines or imperfections with heated tools, and added pour spouts. Then an elaborate gating system, the "circulatory system for bronze," was added. Gates and runners of wax rods were attached to the wax sculpture. When those shapes are "burned out" later, molten metal will use those paths to flow into the mold. But more on that later.
Next, foundry workers applied a liquid slurry of ceramic material over all the wax surfaces by brushing and dipping in vats. Several coats were applied over a period of days, creating a shell of ¼" to 5/8" thickness. That fragile shell was baked at 1350-1450 degrees F. in a furnace, during which the wax melted and ran out (was "lost"), which explains the "lost wax" name.
That hardened refractory
shell was placed in a deep bed of sand and covered, leaving the pour spouts
protruding, and molten bronze was poured in. Any area formerly occupied by
wax was replaced with bronze. The liquid bronze was distributed by the gating
system previously mentioned. When the bronze and shell had cooled, the shell
was "cracked" to expose the latest section of The Horse. This process
having been repeated several times, the result is a number of new castings
which, when assembled with the pieces done in sand casting, will form the
complete bronze Horse.
Meticulous Fabrication
The next step could be described as "meticulous fabrication." After edges of the bronze castings have been debarred and trial-fitted to adjacent mates. Tallix workers carefully clamped and temporarily welded ("tacked") sections together. In order to insure that the final bronze Horse was an exact cop of the 24-foot clay, the fabricators constantly took measurements from it and compared the bronze sections being assembled. Cranes hoisted larger pieces together as workers constructed, in effect, a giant three-dimensional jigsaw puzzle.
Corrections were made along the way. As the body of The Horse (the "barrel") was taking shape, for instance, workers could compress or stretch joints with portable hydraulic jacks to get exact fits. Only when large sections were tacked together and measurements checked were seams finally welded, and even that process was over small areas at a time so that the heat of the welding did not warp or deform the bronze. It was a laborious process requiring much patience.
So was the second process being undertaken simultaneously: the fabrication of a stainless steel armature that runs up the legs and through the body of The Horse.
Workers plasma-torched pieces of type 304 stainless steel to exact curvilinear fits along the inside of the body at upper and lower levels, sort of like horizontal ribbing, and these plates were connected via 3" diameter stainless steel tubing to match plates. These plates are located both at the top of each leg, and the bottom of each "socket" into which the legs fit, creating a method of bolting each leg in place. Similar match plates were created to hold the head and tail in place. In case you're wondering how the worker who's tightening these bolts inside The Horse can get outside, there is a trap door in the belly which will get welded shut after The Horse is reassembled in Italy.
Certainly the most exciting engineering aspect of The Horse is what allows it to stand two legs. According to the engineer's plans, workers welded 8-inch diameter, 1-inch thick walled stainless steel tubing inside the legs. The tubes protrude about 18 inches below the hoofs and consequently will be welded to steel match plates in the pedestal and imbedded in concrete.
Technically, the bronze "skin" of The Horse is not holding it up. All the weight is transferred through the ribs and body tubing to the match plates to the tubes in the legs and down to the pedestal, an engineering feat at which Leonardo would have marveled. Once The Horse was almost fully assembled, it was stood up on its legs and plumbed measurements taken from the clay horse to the bronze horse to check for leg alignment. No changes were needed so final welding proceeded.
The only seams not closed, of course, were where the seven subsections of The Horse disassembled. These seams were welded shut, ground smooth and a patina applied after reassembly in Italy by Tallix fabricators, welders and patination experts.
As viewers at the Buon Viaggio party noticed, most of the patina work was completed on The Horse because it is easier to control quality at home at Tallix.
We wish we could have
sent every reader a packet of 100 slides so that you could have enjoyed seeing
the fascinating foundry process, but, that being impractical, we hope these
word-pictures have helped you to appreciate just what effort is needed by
talented artisans to take a sculpture from clay to bronze.
