"A Big Bone to Pick", Input/Output, ME Departments, July 2004

input/output

by Joyce Laird

A Big Bone to Pick

Industrial microblasting uses a blast of air and abrasive mixture to debur, clean, and prepare surfaces of machine parts. Paleontologists find the process gentle enough to clean fossilized bones.

This story begins with the Burpee Museum of Natural History in Rockford, Ill., which raises funds by hosting three week-long paleontological expeditions a year to Montana in search of dinosaur remains. They're called the Highway to Hell Creek Expeditions, and if you're a would-be fossil hunter, the name won't put you off. Hobbyists get to join professionals in the search for specimens, which are displayed at the museum. Each discoverer is given credit for a find.

On the last day of an expedition in 2001, several hobbyists found something very large, hidden in the side of a hill. Michael Henderson, curator of earth science for the museum and leader of the expedition, determined that it was the almost complete skeleton of a mystery dinosaur from the carnivore family.

A preparator uses a microblasting tool inside a desktop enclosure to clean a detail of the Burpee Museum's mystery dinosaur.

Henderson and Scott Williams, chief preparator at the Burpee, consulted Peter Larson, president of the Black Hills Institute of Geological Research, and an expert in fossil excavations. Larson told them to prepare to move the giant by surrounding it in a jacket of plaster. The dinosaur was named Jane and trans- ported, in her two-ton jacket, to the preparator's lab at the Burpee Museum. The next challenge was how to come up with a practical way to prepare the specimen.

Fossils are very fragile and must be carefully cleaned of debris. They are then hardened by painting with many thin coats of a vinyl-acetone mixture. The mixture penetrates the porous fossil bone and fills any tiny cracks and voids in it.
Once the fossils are safe to handle, a silicone rubber mold is made of each bone, and plastic casts are made from the molds. Copies of the skeleton are made available for distribution to other museums and to researchers around the world.

Scott Williams, in charge of the Jane restoration project, called Larson again.

Larson suggested a call to William Simpson, the collections manager at the Field Museum in Chicago. Simpson was the chief preparator when the Field Museum restored the Tyrannosaurus rex known as Sue. Williams recommended microblasting as a fast and safe way to clean the encased bones, and recommended the model he had used, a MicroBlaster from Comco Inc. in Burbank, Calif.

Microblasting offers a variety of blasting media from highly abrasive to very gentle, so the process lends itself well to fossil cleaning and restoration. Some bones can stand only 30 psi, and denser ones can take 80 or 90 psi.

Once fossils are cleaned, they can be used as the basis for full-scale display replicas like this one.

The work must be done inside an enclosed work chamber. Traditional microblasting workstations are designed to handle small parts, not an entire dinosaur skeleton. The Burpee Museum's solution was building two different workstations and connecting each to a portable vacuum unit for dust collection. The first workstation is large enough to process substantial fossil pieces. The chamber, built of wood, has a slanted glass viewing top, openings for the preparator's hands, and an opening for the vacuum unit.

The second workstation is portable. "We made an abrasion chamber out of PVC pipe, a clear plastic tarp, and a Plexiglas viewing top—then added arm slits in the sheeting," Williams said.

The museum uses sodium bicarbonate as the abrasive because it's softer than the bone itself but harder than the matrix being removed. Williams said the process uses about a pound of sodium bicarbonate a day.

"We were previously using acetone to soften the matrix and then working with hand picks and brushes," Williams said. "We want to open the exhibit by June 2005. We have a lot of work to do, and frankly we are on schedule only because of this machine."

The author, Joyce Laird, is a freelance writer based in Arleta, Calif.