Summerasaurus Part III: The Jacketing Process

Mark Farmer recently returned from an expedition to the badlands of southern Alberta with Dr. David Evans, Associate Curator of Vertebrate Paleontology at the ROM, in search of dinosaurs. Join us over the course of the next month as Mark and Dr. Evans put up their notes from the field, detailing discoveries, how dinosaurs are found and excavated, life in the field and more.

You’d think once the team had located a bone, it would be a simple process to remove it. However, digging around a fossil is only part of the work: making sure it doesn’t break into pieces on the way back to the trucks is just as important.

To do that, team members wrap fossils in jackets. Microfossils such as teeth don’t need this treatment, but almost anything bigger than a few inches needs a field jacket to keep it safe. Here’s how they do it:

Step 1: dig a trench around the fossil.

In this step, a team member digs around and slightly underneath the bone. This gives the whole thing almost a mushroom-like shape, with the fossil sitting on a pedestal slightly smaller than the fossil itself. This allows the field jacket to partially wrap around the underside of the fossil, protecting it and allowing the team to roll the fossil out of the ground complete and undamaged.

Step 2: apply paper towel as a protective layer.

This layer separates the fossil from the plaster used in later steps, which would otherwise penetrate the bone and harden, making it impossible to remove the jacket from the bone it’s meant to protect. All that’s required to avoid that situation is to gently apply a layer of paper towel onto the bone, using a damp paintbrush.

Step 3: use mud to fill in the gaps around the fossil.

Mud is used to fill in gaps and cracks around the fossil, to prevent the plaster used in later steps from hardening in areas that would make the jacket difficult to remove in the lab. Just like in step #1, this makes it easier to remove the jacket later in the lab.

Step 4: soak strips of burlap in water, to prepare them for the jacketing process.

Burlap is strong, light and porous, and wetting it first allows it to soak up the plaster more easily.

Step 5: mix plaster of Paris.

It’s essential that the plaster be neither too thick nor too thin. If it’s too thick, it will dry too fast to work with. If it’s too thin, it won’t form a thick enough protective layer around the fossil. To make sure the mix of plaster and water is just right, workers mix some plaster in water and then gradually add more by hand until it stops dissolving quickly. If it’s about the same consistency as a milkshake, it’s ready to use.

Step 6: soak the burlap in plaster.

This turns the plaster-infused burlap into something a lot like what’s used to make a cast when a human breaks a bone. Once the plaster hardens around the fossil, it will form a strong, tight jacket that protects the fossil, supports it, and makes sure no pieces fall off.

Step 7: layer the strips of wet burlap onto the fossil

It’s important to work quickly so the plaster-soaked burlap doesn’t get a chance to dry before it’s been shaped around the bone. As team members apply layer after layer of burlap, they have to mold it around the fossil so that it takes the fossils’s shape, in order to really protect it.

Step 8: use wood or other materials to add support as needed

Larger jackets may need reinforcing before they can be removed. Usually this is provided by pieces of wood which are embedded in the layers of burlap that make up the protective jacket. These pieces can be cut lumber such as 2X4s or even just tree branches found on-site. This wood gives the jacket the strength it needs to support the bone and resist twisting, which would damage the bone inside. The more strength a field jacket has, the better, in order to ensure the fossil doesn’t break when it’s removed.

After this last step, it’s a matter of waiting for the plaster to dry. For large jackets, that can mean letting dry overnight. Smaller jackets can dry in as little as one hour. After it’s dry, the finished jacket is flipped. The flipped side may also be jacketed, or it may only need minor stabilization. At that point it’s ready for the long trip back to the lab for study.