Palaeo-conservation – An emerging field

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.

paraloid

Paraloid B-72 beads, and a tube containing 50% w/w Paraloid B-72 in acetone.

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.

Paleobond PB100, a cyanoacrylate formulated for fossil preparation, and Devcon 2-Ton epoxy, a two-part epoxy resin.

Paleobond PB100, a cyanoacrylate formulated for fossil preparation, and Devcon 2-Ton epoxy, a two-part epoxy resin.

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.

Limestone tensile test samples after testing (From Fossil Friday on the Museum’s Facebook page) https://www.facebook.com/photo.php?fbid=10150591947295172&set=a.494557285171.266761.37946265171&type=3&theater

Limestone tensile test samples after testing (From Fossil Friday on the Museum’s Facebook page) https://www.facebook.com/photo.php?fbid=10150591947295172&set=a.494557285171.266761.37946265171&type=3&theater

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 rhian.russell@gov.ab.ca

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

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4 comments
  1. Jeffrey said:

    Thank you for a very informative article about Paraloid B-72 versus Epoxy Resin. I am an artist working on a project where I need to adhere light pieces of metal to paper. I came across your article when searching the web on the subject of the tensile strength of B-72 versus epoxy resin (2 part slow set 60 minute). Do you happen to have a quantitative number as to how much stronger the epoxy resin was as compared to the B-72 when pulling apart the aluminum from the limestone? I have not yet purchased B-72 to test myself, so I hope you can help.

    Best,
    Jeffrey

    • Hi Jeffrey! The bonds we tested were limestone to limestone, the aluminum tabs were attached with epoxy to the sides of the blocks, but their function was as an attachment for the universal testing machine. The epoxy resin samples ranged from about 700 – 800lbs/in2 tensile force before the join failed, and the Paraloid B-72 samples failed at around 500lbs/in2 once completely set, although they were much weaker to start with. When we tested the Paraloid samples 3 days after they were joined, the average was 275lbs/in2.

      Also, when we made our first samples, we glued the smooth aluminum tabs to the limestone and they didn’t adhere very well at all – I could break them off with my hands. We had to sand them and score them thoroughly so that the epoxy would hold. I would definitely recommend doing something similar if you need a strong bond!

      Hope that helps,
      Rhian

  2. Jeffrey said:

    Hi Rhian,

    Thank you for getting back to me so quickly with the tensile force information.

    Do you happen to remember the working time of B-72? I use epoxies which have a working time of 1 minute, 5 minutes and 60 minutes. where I can reposition work.

    Lastly, as far as B-72 being an “archival” product, do you know if it is PH neutral or acid free? I am wondering if it will damage watercolor paper over time. I use PVA glue as an adhesive and have one that is acid free which is critical for museums that collect my work.

    Thanks again for any thoughts or advice you can offer.

    Kind regards,
    Jeffrey

    • Hey Jeffrey,

      Sorry for the delay! The working time of Paraloid B-72 depends on the solvent used to dissolve it, and also the concentration of the solution. I usually use a 50% w/w solution of B-72 in acetone, and I can work with it for 15 minutes or so. Ethanol gives a much longer working time. The great thing about it is that you can just add more solvent if you need to rework anything, and this is discussed in a helpful paper by Davidson and Brown (2012) in SPNHC’s Collection Forum. It’s called ‘Paraloid B-72: Practical tips for the vertebrate fossil preparator’ and you can access it on the SPNHC website.

      The pH of Paraloid B-72 is reported as 6.41 by Down et al. (1996) in ‘Adhesive Testing at the Canadian Conservation Institute: An Evaluation of Selected Poly(Vinyl Acetate) and Acrylic Adhesives’ from Studies in Conservation, Vol. 41, No.1., pp.19-44. I am not sure how B-72 interacts with watercolour paper, but I know it has been used as a picture varnish with good results. I’ve seen some spray formulations but these often use more toxic solvents such as toluene and xylene.

      Hope that helps a little…if you can, maybe experiment with it a bit and see if it works for you.

      Cheers!

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