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Preparation
DMNS paleontologists work long hours in the field, but field season is short. When in the field, they simply try to collect as many fossils as they safely can before the weather turns. Before those fossils can be displayed or used in research, however, someone has to remove all the extra sediment. So the amount of time spent in the field is dwarfed by the amount of time invested in the DMNS fossil preparation lab. Fortunately, conditions in the lab are less spartan than they are in the field.

The DMNS fossil lab is located in the Prehistoric Journey exhibition. The lab is open whenever Prehistoric Journey is open, and visitors can look through windows to watch preparators work, or even chat with preparators working at the window seats. The lab is staffed mostly by DMNS volunteers, and supervised by chief preparator Dr. Ken Carpenter and preparator Bryan Small. To work in the fossil lab, volunteers must complete a lab preparation course. Many of those volunteers go on to complete the DMNS Certification in Paleontology Program.

Jacket removal: Click to enlarge
Lab volunteer removes plaster jacket
When starting work on a new fossil, the first thing the preparator must do is complete the required paperwork, namely a form that records where the fossil was collected. This form stays with the fossil throughout the preparation process.

If the fossil arrives in a jacket, and most of them do, jacket removal comes next. A jacket can be removed with water (to soften the plaster) and pliers or, for the more brave-hearted, a buzz saw. Either way, the advice of someone who helped jacket the fossil in the field is always appreciated as it gives the preparator a sense for what's inside. (Sometimes, a forward-thinking collector in the field will mark up the jacket with outlines of its contents.) Seldom is the entire jacket removed at once; preparators usually leave part of the jacket on the fossil and remove it a little at a time as they expose more of the fossil.

Air scribe: Click to enlarge
Using an air scribe to prepare a fossil
With the jacket out of the way, the preparator starts removing excess sediment. The trick is to remove as much sediment as possible without damaging the fossil. On many occasions, some rock must remain since it's all that's supporting thin bone. Preparators soften the matrix with acetone (nail polish remover, but without the scent). Sediment can be removed with brushes, dental tools, pin vises (straight pins or needles in special handles), or air scribes. Probably the most commonly used tool is the air scribe. It's basically a tiny jackhammer with a rotating tip to break up sediment, and a steady stream of air to blow away debris. Preparators use two different kinds of air scribes: AERO® scribes for delicate work, and Chicago® scribes for bigger jobs. Which air scribe is better — and whether an air scribe is used at all — depends on the fossil, the matrix, and the preparator's preference. Although it sounds a little counterintuitive, preparators using air scribes aim toward the bone. There is a plane of resistance along the surface of the fossil, so sediment flakes away just before the air scribe tip touches the bone, assuming the preparator is working carefully.

While in the field, DMNS paleontologists collect fossils inside chunks of sediment. Fossils of small animals may be entirely encased in rock. So the first person to actually see the whole fossil is usually the preparator. As the preparator works, he or she exposes fossils to air for the first time for thousands, millions, even hundreds of millions of years, and this can cause the bones to crack and break. To solidify bones, preparators use vinyl acetate, better known as vinac, that penetrates their porous surfaces. If bones break, preparators repair the damage with strong adhesives.

Amphibian fossil: Click to enlarge
Amphibian fossil
Needlework: Click to enlarge
Delicate preparation is done with a pin vise
Although it makes sense that the biggest fossils take the most time to prepare, often the reverse is true. Tiny fossils are frequently the most time-consuming. Preparators working on the smallest fossils use microscopes or visors with magnifying lenses, and use needles to pick away the sediment. DMNS lab volunteers usually work part time, and it's not uncommon for a lab volunteer to spend several months on a single project.

In the early days of paleontology, fossil collectors spent most of their time collecting and preparing the biggest fossils. Since those early days, a growing number of paleontologists have turned their attention to smaller fossils, such as teeth, fish scales, and tiny bits of bone. A good way to find those fossils is through a process of washing and picking. Preparators start by placing a big screen over a sink. Next, they spread sediment collected from the field on the screen and gently hose it down. Softened sediment dissolves and falls through the screen, leaving fossils on top, which are then picked away with tweezers. It's important to choose the screen mesh (how coarse or fine) wisely. Using a mesh that's too fine will catch too much sediment and take more time than necessary, but using a mesh that's too coarse will let too many fossils slip through the screen. The picking process continues under a microscope as a preparator sorts the fossils.

Cast adjustment: Click to enlarge
Chief preparator Dr. Ken Carpenter adjusts a cast of a baby stegosaur
Most vertebrate fossils are rare, and many of them are one of a kind. When the Museum gives a duplicate of a fossil to another institution, or puts a duplicate on display while keeping the original in collections, it makes a cast of the fossil. Some people are disappointed when they learn that articulations they see on display are really casts, but they shouldn't be. Casts are exact replicas of the original bones, with the added advantage of being lighter and easier to mount. What's more, they're replaceable in the event of damage, unlike the original fossil.

To make a cast, the preparator starts with molding material such as latex, and places it over the original fossil. The molding material is usually applied in two parts, one on either side. As the molding material hardens, it makes a negative image of the fossil. After the mold has hardened, the preparator peels it off the fossil, then puts the mold (minus the fossil) back together and fills it with epoxy or other plastic material. When the plastic material inside the mold hardens, the cast is done, except for painting.


References:
Carpenter, Kenneth. 1997. "It's a baby!" Museum Quarterly (Summer issue) 16-17

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