The Collections Program at the Royal Tyrrell Museum is responsible for preserving the integrity of its 130,000 fossil specimens. This includes conservation of all the fossils in storage and on exhibit, which is my job—I’m Rhian Russell, the Museum’s Conservation Technician.
Conservation can be preventive in nature, such as controlling the storage environment, or interventive—stabilizing a deteriorating fossil with consolidant before it falls apart completely and cannot be saved.
We commonly use Paraloid B-72 to consolidate and adhere fossils. It’s an acrylic polymer in the form of clear beads that are dissolved in a solvent (we use acetone) to make a thin consolidant or a thick adhesive.
When applied to a fossil, the acetone slowly evaporates, and the remaining polymer holds the fossil together. Paraloid B-72 is widely used in many fields of object conservation as it has proved to be a highly reversible, stable material. These qualities are important because a fossil will only hold together as long as the glue connecting it does.
However, because Paraloid B-72 depends on the evaporation of solvent, it can take a long time to cure. This means it can be necessary to use adhesives that set faster and offer a strong bond immediately. These adhesives are usually much harder to reverse and may not be as stable as Paraloid B-72 in the long term.
Over the last couple of years, we’ve been working on a project to test the tensile strength of Paraloid B-72 compared with other adhesives that are used in fossil preparation, namely the cyanoacrylates (superglues) and epoxy resins. I want to see exactly how well B-72 performs compared to other adhesives, so that a more informed decision can be made about which adhesive to choose.
To test this, we worked with technicians at the Department of Mechanical Engineering at the University of Alberta using their Universal Testing Machine to pull apart test samples joined with our adhesives, and measured the exact amount of tensile force required to break the adhesive bonds. We made lots of test samples from limestone paving slab, attached aluminum tabs to the sides to connect them to the testing machine, and glued them together with Paraloid B-72, cyanoacrylate, or epoxy.
Paraloid B-72 was the weakest adhesive of the three, but it still took a great deal of tensile force to break the join.
The limestone blocks themselves broke under the same amount of tensile force as the epoxy did; meaning that many of the epoxy test samples broke in the limestone before the adhesive gave way. This is not something you want to happen to a fossil, as it means that the fossil will break somewhere new rather than at the existing break when subjected to stress. Therefore in this case, epoxy would be too strong for the material. Many of the fossils we work with would break under far less tensile force than the limestone blocks, so we don’t need to use such a strong adhesive to repair them. Paraloid B-72 should have sufficient strength for most repairs, and in my own experience, I’ve rarely found cases where it hasn’t been strong enough.
As you can see, there is much to consider when choosing materials for fossil conservation. We want specimens to last as long as possible so that they can be used for research, education, and exhibition for many generations to come.
I hope to post updates on my projects in the future, so stay tuned. If you have any questions or comments please feel free to email me at firstname.lastname@example.org
If you would like to know more about this project, watch my talk at http://www.youtube.com/watch?v=ru9KcmQHIrM&list=PLE5C051E20D553713&index=34
- Rhian Russell, Conservation Technician