Sometimes, we need one-of-a-kind fossils to be two or more places at the same time. That's not possible, of course, but our ability to make precise replicas of fossils usually solves the problem. Well-made replicas retain much of the scientific information found in the originals. We often share them with other institutions that wish to display them, and we may loan replicas to scientists who can't travel here to conduct their research. We also make replicas when we wish to display fossils that are judged too fragile to withstand life on exhibit, and we retain replicas in our collections when we collect fossils in countries that require their return once our research is finished.
The fossil plant on the left, above, is the trunk of a cycadeoid that was collected in the Black Hills of South Dakota. It has been in our collections since the late 1800s and is a type specimen, meaning that its scientific description defined a new species. The replica on the right, photographed from a slightly different angle, was just made in the Department of Paleobiology's fossil preparation lab at the request of the National Park Service.
We employ several methods for making replicas, but they all share common steps, shown below. Click any photo to enlarge.
The fossil is coated with a pliable substance that conforms to every contour, nook and cranny, forming the mold. The mold needs to be thin, stretchy and strong so that it can be peeled off without tearing and without damaging delicate portions of the fossil. The mold for the cycadeoid, shown on the right, is made of several layers of latex rubber.
A hard plaster "mother mold" is built around the mold. This hard outer layer keeps the pliable mold from losing its shape when filled with casting material. The mother mold is constructed in pieces. To understand why this is important, imagine what would happen if you made a fist and stuck it into a bucket of plaster -- you wouldn't be able to pull your hand out once the plaster hardened around its contours! If a mother mold is made in enough pieces, even the most irregularly-shaped fossil can be freed from it. The photo on the left shows the latex-coated fossil and one half of the two-part mother mold. Matthew Miller, who made this replica, stands behind it.
The mother mold is removed and the fossil is freed from the mold. The photo below on the left shows Matthew peeling the mold off the upside-down cycadeoid. He's applying plaster powder to the mold to keep it from sticking to itself. The fossil was turned right side up to finish the job (right).
Next, the mold and mother mold are assembled. The photo below shows it sitting upside-down in sand, ready to be filled with casting material.
Casting material is placed in the mold. The materials used depend on the intended use of the replica. In this case, Matthew coated the inside of the mold with thick plaster to fill all the small pockets and then lined it with fiberglass cloth for strength (below, left). Then, he filled the cavity with expanding foam, a very light but rigid material (below, right).
Finally, the mother mold is removed and the mold is peeled off the replica. The photo below shows this step in progress. The replica sits right side up on a built-in base.
A replica intended for display will be painted to resemble the original. One intended for scientific study, only, will be left unpainted. The original fossil is cleaned of any residue left from the casting process and returned to collections. The mold is stored in our "mold library" in case we need to make additional replicas in the future.
Photos by Matthew Miller and Abby Telfer.
Read another post about fossil cycadeoids which includes a beautiful image of their interesting internal structures.