Every time I see the great triceratops skull looming out of foliage in front of the National Museum of Natural History in Washington, D.C., I think about all that went into it. The idea for a bronze of the enormous skull was born when the museum began refurbishing its dinosaur exhibit in 1998 and curators decided that an outdoor sculpture would serve as a terrific logo, not just for the exhibit but for the museum itself.
The exhibit’s prized triceratops—a gigantic plant-eating dinosaur with three horns and a huge, saddle-shaped skull—had been discovered in Wyoming in 1891, hauled out by horse-drawn wagon and shipped East on the new transcontinental railroad. Since the skeleton wasn’t complete, bones from 14 other triceratops were used to create the world’s first mounted triceratops. It went on display at the Smithsonian in 1905. As the years passed, some bones developed cracks or showed signs of pyrite disease, a buildup of minerals that causes internal fissures. (Of course, at 65 million years of age, a body is entitled to a touch of arthritis.)
Equipped with the latest technology, experts set out two years ago to create a more perfect triceratops. The 1905 body was too large for its head, so scientists used a computer to produce a new skull 15 percent larger than the original. Using three-dimensional laser-scanning techniques unknown even five years ago, conservators also created an anatomically correct skeleton made of resin, plaster and fiberglass. The left shoulder blade, for instance, which had originally been sculpted by guesswork, was replaced with a mirror image of the right blade. The finished dinosaur, some 20 feet long and 8 feet tall, is what you see in the newly renovated Dinosaur Hall at Natural History. (A computer image can be viewed on-line here.) Triceratops’ original skull and a few other carefully preserved bones are also on display alongside the replica. Diceratops, the skeleton of a baby styracosaurus, centrosaurus and various dinosaur relatives are there too.
But getting back to that big bronze head outside the museum, which was cast from the same computer-generated model as the skeleton inside: to make it, the Smithsonian chose sculptors at Millersville University, near Lancaster, Pennsylvania, and sent them a rubber mold of the new skull, sectioned into eight pieces for transport. The sculptors reassembled the pieces, which averaged eight by six feet, and poured several layers of molten wax into the mold. Making a hollow wax head is complicated. Because the wax version of triceratops was produced from a laser-generated model, it lacked the exact markings of the original fossil. Artists spent more than a thousand hours sculpting in the details. "We used photographs and visited the museum to see the fossil firsthand," says George Mummert, one of the three sculptors in residence at Millersville.
The finished wax head was only a quarter-inch thick but weighed 200 pounds and was too fragile to hold its shape without support. So the Millersville artists built a cradle of plywood and plaster reinforced with wire and burlap to hold the head. Then they cut it up again. "The thing was so big we had to break it into 30 to 35 pieces," Mummert adds.
One by one, the enormous platter-shaped chunks of the wax head were fitted with sprues, wax conduits for molten bronze. Then the pieces were dipped into a vat of ceramic slurry—a suspension of silica flour and liquefied plastic. Over several days, the artists, wearing respirators and protective clothing, dusted the pieces with zircon and silica sand after each bath to thicken the ceramic shell.
After drying thoroughly, the unshapely sections went in a kiln, heated to 1,500 degrees Fahrenheit. The wax melted out as the ceramic was fired into rock-hard receptacles able to withstand molten metal.
To pour the bronze, three artists donned heavy silvery fireproof suits: gaiters, aprons, coats, gloves and welder’s masks. After lowering the crucible into the furnace, one sculptor loaded it with several 25-pound bronze ingots. Someone lit the furnace, creating a deafening roar. Melting the bronze took almost an hour. A sculptor dipped a pyrometer, a long metal thermometer, into the liquid bronze to determine if it was hot enough—2,150 degrees F—to pour.
With huge iron tongs attached to a chain hoist, a sculptor lifted the glowing red-hot crucible from the furnace and carefully set it into a steel frame. Working quickly but methodically, two other artists poured a stream of liquid bronze into a ceramic mold. Hot metal rushed through the sprueing system, filling each hollow casing.
After a mold had cooled for several hours, the crew used hammers and chisels to crack the ceramic shell, revealing the bronze casting, still festooned with sprues. Carefully applying saws and grinders, the sculptors removed the extraneous parts; what remained was a bronze copy of a single chunk of the triceratops skull.