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DISCUSSION
The Smithsonian Institution’s National Museum of Natural History (NMNH) has developed a multi-step procedure to construct two-sided, padded jackets to fully protect and hold fragile vertebrate fossils. The process is similar to that described by Dan Chaney (1992), but with some modifications. In general, hydrocal gypsum cement, surmat fiberglass cloth, and ethafoam padding are used to create two-sided jackets that bolt together to fully encase the specimens. Handles and feet on either side of the jacket enable a person to lift off a side, fully examine one side of even the most delicate specimen, recap it and then flip it over to examine the other side. This eliminates excessive handling while the jacket uniformly supports the specimen and reduces the chances for breakage. The technique has enabled NMNH to repair and jacket many vertebrate fossils in our collections, and all newly prepared specimens automatically get a jacket.
When a specimen is to be jacketed, the first step is to conserve it through cleansing, consolidation, and restoration as much as possible. Broken specimens also need to be repaired. (Figures 02, 03) Cleaning can be done just with a
Figure 2: The broken carapace of Amyda cf. Figure 03:The cleaned, repaired and aequa, a soft-shelled Eocene turtle collected consolidated carapace of Amyda cf. aequa in 1930 by George Sternberg
dry brush and compressed air or in combination with damp cotton swabs if the specimen can withstand moisture. Occasionally old plaster bases and armature may have to be removed from the fossil before it can be jacketed. Newly prepared specimens should be in a stable state before the jacketing begins.
After the conservation of the specimen is completed, it is necessary to locate a good dividing line to separate the specimen into imaginary halves. This determines the size and shape of the two sides of the jacket. One detail to look for in a dividing line is a natural separation in the specimen, such as the top and bottom of a turtle shell. It is best to keep any diagnostic elements, like teeth, fully exposed, and the size and weight distribution of the jacket also have to be considered. Once a dividing line is determined, a two to three inch wide flange must be made along this line around the specimen using a material such as sand or cardboard. The quickest and easiest way to do this is to sink the fossil half-way deep in a sandbox, and use the sand surface as the flange. (Figures 4, 5) Otherwise, it must be constructed out of cardboard or some
Figure 4: The turtle carapace on its original plaster base, sunken into a sandbox to create the flange along the dividing line.
Figure 5: Plan view of the flange
other material. If necessary, block out any overhangs or vertical walls of the specimen with newspaper or foil. There can be no “undercuts"that the rigid jacket will get hung-up on when it is removed. It’s the same principle as making a plaster mothermold.
Once the flange and any undercuts are taken care of, the entire surface is covered with foil. (Figure 6) The foil acts as a separator between the specimen and the clay layer that will be applied next.
Figure 6:
The specimen and flange covered in
Figure 7: A clay roller that is used to roll
foil and ready for the clay layer.
out clean clay to 3/16-inch to act
as a
spacer for the foam in the jacket.
Figure 8: The
specimen and the flange, layered Figure 9:
Plastic wrap is applied to the
with the rolled clay. specimen and flange as another separator.
Figure 10:
Fiberglass cloth and FGR-95 Hydrocal Figure 11:
The completion of the
Gypsum cement being applied to the
specimen. fiberglass and hydrocal application.
Sheets of clean clay
are rolled to 3/16-inch, and the entire surface of the specimen is covered with
it - including the flange. (Figures 7, 8) The clay acts as a spacer for where
the foam padding will go in the jacket. All of this is covered with plastic
wrap, which acts as another separator. (Figure 9)
The whole thing is
then layered with 10 or 15 mil fiberglass cloth and liberal amounts of FGR-95
Hydrocal Gypsum Cement. There must be at least four fiberglass / hydrocal layers
on the specimen, and five layers around the flange. More layers may have to be
applied for large specimens. (Figures 10, 11)
After the
layering is completed, it is necessary to ensure that the final product will not
rock back and forth while on a flat surface. This is done by constructing feet
on which the jacket will stand - along with handles to lift it off. The handles
are usually made from electrical conduit and built into the feet. (Figure 12) A
typical way to install the handles is to first figure out where they will best
serve their purpose, based, in part, on the contours of the jacket. The handles
should be as low profile as possible, but far away enough from the jacket to
keep it off the ground and to get fingers between the handle and the jacket.
Creative placement of the handles can help with structural support on long
jackets. Flattening the ends of a handle may make it easier for it to conform to
the shape of the jacket. Once it is determined where the handles go, measure and
cut the lengths of conduit needed and set them in place on pieces of clay of the
correct height to level them. The clay should not be in the area where the
handles will be attached to the jacket. Remember to first thoroughly wet the
attachment areas on the plaster jacket surface with water so the fresh hydrocal
is not dewatered by a dry undersurface. Pieces of fiberglass cloth are soaked
in hydrocal and folded and stacked under the conduit until they make contact
with it. Strips of fiberglass and hydrocal are then applied over the top of the
conduit and attached to the jacket. Plywood or another flat surface can be
placed over the feet while they are drying to ensure that they are level. In some cases, as
with a smaller, simple-shaped specimen, it is easier and more efficient to just
make a solid plaster pad with finger holds to lift the jacket off. (Figures 13,
14) The jacket surfaces, including the feet and handles, must be smooth for
comfortable handling. Remove the clay immediately after the hydrocal is set.
Figure 13:
Hydrocal cement will be poured Figure 14:
The finished resting pad.
into this clay dike to make a solid pad, with
finger holds, on which the jacket will rest.
Now
the plaster jacket, plastic wrap, clay and foil are removed from the specimen,
and the jacket left to dry completely. (Figure 15) The edges of the jacket are
trimmed and sanded smooth. It is also advisable to take a propane torch and burn
off any fiberglass along the edges.
The foam padding can
now be adhered to the jacket. Pieces of 1/4-inch, high-density ethafoam are
trimmed to tightly fit onto the interior surface and flange of the jacket. A
coating of contact cement, such as 3M Hi-Strength 90 Spray Adhesive or MISTY
Heavy Duty Adhesive Spray, is applied to the interior surface of the jacket and
flange and to the contact surfaces of ethafoam. It is
advisable to always work in a well ventilated area, or wear a respirator with
organic vapor filters when applying the contact cement. After waiting the
prescribed amount of time, the ethafoam is carefully pressed into the jacket and
the surfaces worked completely down. (Figure 16) If possible, the jacket can be
put back on the specimen and weighted down until the cement dries to make sure
everything stays in place.
The foam edges are then trimmed,
the contact cement allowed to completely off-gas, and the specimen is put back
into the jacket and flipped. (Figure 17)
Now, the same general
procedures are performed on this side of the specimen. It is layered with a
foil separator, then 3/16-inch of clean clay - but the clay is not applied
around the flange on this side of the jacket. Foam will not be applied to the
flange on this side either as it isn’t necessary to have both flanges with foam
on both sides of the jacket. An extra layer of foam makes it easier for the
jacket to be overcompressed if the bolts are overtightened, possibly
resulting in damage to the underlying specimen. Plastic wrap is applied as
another separator, and then surmat and hydrocal. The jacket is trimmed and
sanded, and the feet and handles are constructed for this side. (Figure 18)
Ethafoam is then adhered to the inside of this jacket - but not around the
flange.
Figure 19:
The finished, labeled jacket showing
the bolts and washers
that hold it together, and the
original catalogue labels in a plastic bag.
Holes are drilled
through the flange, and 1/4-inch bolts, washers, and wing nuts are
inserted and tightened just until they are snug to hold
the jacket
Figure 20:
The finished jacket, opened to show the (A) ventral and (B) dorsal
views of the
turtle carapace.
Figure 21:
A well-designed jacket will take up little more space on the shelf than the
specimen itself.
together.
The appropriate data to identify the specimen is written on the outside of the
jacket. (Figure 19) A photograph of the specimen may be attached to the jacket
for easier identification. Now the specimen can be studied, and any
unnecessary handling is eliminated. (Figure 20) It is also ready to be
transported and placed into the collections. (Figures 21, 22)
Modifications:
As always, there can be any number of variations on this theme. The procedures
can be, and are, continually modified for specific specimens and conditions. A
couple of common concerns are addressed below.
Non-Metal Handles:
There may be
the occasion when the specimen being jacketed will be x-rayed or CT scanned or
MRI scanned. Metal handles in a storage jacket will interfere with these
processes. If the specimen is scheduled for scanning, or if there is a chance
it will be, the handles must be made of an inert substance such as PVC pipe.
Lightweight, Delicate Specimens:
If the specimen is very light – 15 pounds or
less – one-eighth inch thick foam can be used instead of the 1/4 inch thick
material. Also, the clay must be rolled out to 1/8 inch thickness to get the
right spacing. The plaster and fiberglass making up the jacket does not have to
be as thick as for a heavier specimen, either.
Fragile
Projections:
Thinking in terms of negative space, projecting teeth, such as
those on a Eurhinodelphis rostrum, can be protected by making the clay layer
over the teeth slightly thicker. Adding a little more clay over the normal layer
will create a slightly larger air space around the teeth once the jacket is
made. After the jacket is lined with foam, the teeth should just barely come
into contact with it (or possibly just miss making contact - it's better to err
on the side of caution within reason) and eliminate the potential to break
because of too much pressure. However, because the teeth will then be, in
effect, "floating", it is essential that there are areas of bone that contact
the jacket foam to act as the load bearers for the weight of the jacket. The
palate or the rostrum or the sides of the mandibles are all areas to consider.
Make sure the normal thickness of clay for the jacket you are making still
contacts them. The larger or longer the contact area is, the better it will
disperse the weight.
Fragile processes can
be taken care of in the same way. Make the clay on those areas slightly thicker
than normal so that when the specimen sinks down into the foam it will not
create too much pressure on the processes and break them off. The weight and the
shape of the specimen must be taken into consideration as to how far it will
sink into the foam. The processes shouldn't be floating in the air, though,
because there is risk of breakage when the jacket is flipped or jostled.
Finally, if a jacket
is constructed for a fragile specimen or one that has delicate features, be sure
not to crank down too hard on the wing nuts and bolts holding the jacket
together. Too much pressure could be created on those features inside the
jacket and break them. Write "FRAGILE" on the outside of the jacket.
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