When a specimen is flipped over one has to think in reverse regarding where the specimens various parts are then located. Boney processes that pointed one way, now point in the reverse. Bone processes that pointed up are now pointing down. So do I cut or grind away the plaster jacket here? Maybe not, there was a rib or limb bone there. Usually one just guesstimates where everything is but as the incredible Gorgosaurus skull is close to the original field jacket I did not want to do this. So what to do? I tried something different and have never tried before.
First I took a red marker pen and drew a line around the skull, several centimeters away, thereby marking a safe buffer zone. Then using that line as a guide, I drilled a series of spaced holes with a power drill and a long bit. The holes were drilled all the way through the jacket. In curved areas the holes were drilled closer together. When the block is flipped over, these holes will appear and the holes can be reconnected with a red felt pen and thus accurately outline the skull. As the jacket is then pulled and cut apart, the holes will always be there until the very end to remind me where the skull is.
I also spent a couple hours photographing the specimen under various light levels, angles and distances. These pictures will serve as a valuable reference when I prepare the other side and will be useful for future researchers.
The last of the Gorgosaurus preparation (on this side) was finished February 10th. In readiness for molding, holes, cracks and undercuts have to be filled and this phase is rapidly reaching the end, too. The plan to mold the edge of the field jacket as well means the edges between the cut part of the plaster jacket and the rock itself needs special treatment.
The plaster jacket was of course made in the field with no advance knowledge that it would be molded in part later on. So these edges need to be fixed. They are rough, full of undercuts and often form vertical faces that are harder to fix to make look like rock. Most of these problematic edge areas are simply glued and crushed sand and silt thrown against the wet glue. Once dry, excess sand/silt is vacuumed off and the process repeated up to three times until the white plaster is hidden by simulated rock. Undercut areas on the edge of the jacket are fixed by using the glue/sand/silt mix which is roughly pressed into the undercut or hole, then sprinkled with dry sand/silt which is then firmly pressed (with the heel of the hand) into the damp glue/sand/silt mix underneath. This is done for any other gentle depressions or undercuts: a series of pictures are given here. Once dry the effect is quite realistic. Also, any remaining cracks are being heavily glued. I try to do heavy gluing jobs on a Friday afternoon- this way it has all weekend to dry without any disturbances from me. Come Monday morning everything has firmly set and the process begins anew until everything is done. Vertebrate paleontologist Philip J. Currie is to visit our museum (and see the Gorgosaurus) for the first time on February 18th so it is important that I be done by then.
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I must apologize to readers about the image quality. The specimen, when seen in person, is really quite spectacular- the bones have a beautiful chestnut-brown to orangey-brown (more heavy on the orange) and all with a deep, rich lustre. These color qualities, which really make the specimen all that more amazing I have found very difficult to convey photographically. The overhead lighting near the Gorgosaurus has been changed this week which also has affected image quality.
All photos here and in the series are owned by Darren Tanke and the Royal Tyrrell Museum.
Late edit: Matt van Rooijen has done a colour edit on that last image to try and perk it up a bit:
