When curators at the Fernbank Museum of Natural History wanted to build a full scale replica of the largest plant-eating dinosaur ever discovered—the 126-foot-long Argentinosaurus—they turned to Train Hall, a Toronto sculptor who specializes in recreations of prehistoric creatures for museums and theme parks. Hall immediately broke out the fiberglass and silicone.
Working over an 18-month period, Train produced more than 200 gel-coated fiberglass bone replicas, ranging in size from just three inches long to a monster scapula measuring 9 × 5 feet. "In the original bone material the scapula alone would have weighed over two tons," Train reports. He started in Argentina by copying 13 Argentinosaurus fossils, using each one as a master for a silicone mold. "After we cleaned the surface of each bone, we covered it with liquid silicone rubber while it was still wet," he says. As that first layer of silicone cured, Train backed it up with successive layers, and finished the mold with a layer of fiberglass for structural reinforcement.
With only 13 fossils found and 200 bones needed for the skeleton, Train then turned his attention to filling in the gaps in the fossil record. He used an MD Robotics laser scanner to capture images of real fossils—from the Argentinosaurus and from smaller related species. By scaling up the smaller bones and integrating them with existing fossils, he created a computer model of the entire Argentinosaurus. Each bone in the computer model was ultimately sculpted into a clay master for another set of silicone molds, similar to those created directly from the fossils.
For a moldmaking material, Train picked an HS III RTV silicone from Dow Corning (Midland, MI). With a shrink of 0.1% after seven days at room temperature, the material is not intended for precision prototypes or other jobs needing zero shrink. But for the dinosaur application, it had just the right balance of properties.
For one, HS III cures in six to eight hours at room temperature—and does so even when that temperature varies quite a bit. Train points out that much of the moldmaking work in Argentina took place in a desert climate with blistering hot days and freezing nights. "So we needed a material that wasn't sensitive to temperature swings," he says.
HS III also provided the right mechanical property requirements. Train points out that large bones need even larger molds. What's more, it has to be durable enough for multiple uses, since Train creates copies of the dinosaur. "We have to yank the molds apart so we can use them again and again," he says. So whatever moldmaking material Train used had to have enough strength to contain large, repeated fiberglass lay-ups. HS III fit the bill on both scores. Intended for applications needing high strength, it has 300 psi tensile strength, reports Brian Reno, Dow Corning technical service engineer.
Finally, the material did a good job picking up surface texture and detail, which Train needed in order to match the appearance of the fossils' replicas with his sculpted recreations. "The silicone capture fine detail exceptionally well," he says, noting that the exhibit has to endure up close and personal inspections. "You can walk right up and grab a big toe," he says. A very, very big toe.
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