The Gorgosaurus has entered its final phase of preparation on this side. All the bones are being worked on in a systematic fashion now. Glue that stabilized them mostly went inside the bones, but some is on the outside and rough in appearance or dirty with embedded sand/dust. All this “old glue” is carefully scrapped off and a final clean coat of thin glue applied to the surface where needed. Rough rock areas are being smoothed down. As said in an earlier update, the specimen will be molded in latex rubber. This will likely be done in one piece and how that is made will be covered in future updates if Dave Hone and the followers of this blog are interested [Edit: I most definitely am, it would be great to cover this too! Dave]. It is technically not fossil preparation but an important and often overlooked aspect of the technical side of vertebrate paleontology.
Part of the Gorgosaurus‘s final phase of preparation is getting it ready for latex molding. Molding a fossil is like chess- if you do something to the specimen, it will affect how the mold will be removed in the future? You always have to think ahead. For example a deep undercut under a bone can be molded, but once the latex cures how will it be removed from that deep undercut? Will it come off easy, or (more likely) will it be “stuck” and in the efforts to remove the mold, the bone above is damaged or possibly destroyed. With these thoughts in mind, the Gorgosaurus is being gone over section by section, looking for potential problem areas for molding and demolding as well as general areas of support. Bad undercuts are being fixed. Any low spots are obvious “weak” areas for support and latex will stick more firmly to them than a flat of slightly convex surface. These low areas are filled in. To do this, I first take leftover waste rock matrix from the Gorgosaurus and with a hammer break it up into sand grain consistency. The hole or depression to be fixed is glued and then enough loose sand it sprinkled into the low spot. A dry brush steers the sand in to small undercuts. Larger undercuts can be filled by using a small funnel made out of a piece of paper. The funnel’s tip is put under the undercut (usually meaning the funnel is angled somewhat) and pinches of sand dropped into the funnel until the problem area is filled up- this works great for microcracks only a sand grain wide too. Once this is all done, a very gentle puff of air administered by mouth gently blows away any brush marks in the loose sand. Then the loose sand is glued with acetone-based glue- the same used to stabilize the bones. Being acetone-based, it can be reversed at any time in the future if need be.
The final effect looks fairly good, but never adopts the same color as the untreated rock, but this can be rectified with paint later if wanted. The procedure is shown here in one small area and then a picture of an overall area after treatment is shown. The long and narrow belly ribs in the overall view on the right side are much better stabilized now. You may ask why not just leave the rock in there to begin with and that is usually the plan, but sometimes it is not there (due to a wide crack), or crumbled away into loose unstable pieces, or was deliberately removed for some reason (such as the removal of the postorbital from the antorbital fenestra in an earlier post) and needed to be refilled. We are hoping to start molding the Gorgosaurus block by mid February.
All photos here and in the series are owned by Darren Tanke and the Royal Tyrrell Museum.