Specimen jackets
Posted 4/1/2011 12:04 AM by Logan Ivy |
CommentsFossil specimens are found on the surface or at a relatively
shallow depth in the ground or rock. In such a situation the
fossils are often cracked or broken. This can be due to
weathering if on the surface, water infiltration if at a shallow
depth, root penetration, or expansion and shifting of the enclosing
rock as overlying rock is eroded away. To keep the fossil
together, the standard procedure is to encase it in a plaster
jacket, like a cast around a broken arm.
You first expose enough of the fossil's top to tell how big it
is and what bone it could be. Then a few inches outside the
fossil you start digging down. When you have excavated down
to below the fossil, based on your previous guess at which the bone
it is, you start digging under the bone a little. What you
then have is a pedestal of the fossil and some surrounding
rock. At this point you start making the jacket.
You make up a mixture of plaster and water in a bucket or tub
and then soak strips of burlap in the plaster. This will form
your jacket. Since plaster sticks to fossil bone fairly
tightly, you need to put a layer of something else over the
specimen as a separator. In the past, collectors have
used newspapers, paper towels, plastic wrap, garbage bags, aluminum
foil, or even toilet paper to act as a separator. Once the
separator is down, then the plaster soaked burlap strips are laid
out over the specimen, covering it tightly and completely.
How thick you make the plaster depends on the size of the
specimen. The main thing is you want the plaster thick enough
so it doesn't bend when you pick up the finished jacket.
Erring on the side of too thick is preferred. The burlap and
plaster strips are tucked under the pedestal as far as possible and
when it is all thick enough you let it dry. This might be
over night.
Once the jacket is dry and hard it must be turned over.
You carefully remove the jacket and its enclosed fossil and rock
from its pedestal base and carefully turn it over. This is
called flipping the jacket. If all is well, you have an
upside down specimen with a thick plaster and burlap shell below
and a rock or dirt surface above, with the fossil inside. The
next step is to place a plaster burlap cap on the exposed rock so
that the specimen is completely encased in plaster and
burlap. Once this is dry, the jacket may be hauled away
without damaging the fossil inside.
The plaster and burlap can weigh a fair amount, so along with
the fossil and rock the jacket can be heavy. Not to worry,
however. A well made jacket can protect the fossil from quite
a bit of rough handling as it is picked up and placed in a
truck. You probably don't want to drop it, however. The
jacket is taken to the museum where it is carefully opened and the
fossil inside is cleaned and exposed to the world for the first
time since it was buried.
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