Consolidants and Adhesives in Fossil Preparation
Consolidants used in the field in waterlogged environments are different to those used in the lab during preparation.
Consolidation of a fossil bone with Paraloid B-72, applied with a bottle and brush precision applicator to prevent reverse migration.
Paraloid B-72, an acrylic resin that can be used as both consolidant and adhesive in its pellet form. Linked here.
What is conservation grade?
Permanent (not conservation grade)
Paraloid B-72 (formerly called Acryloid B-72) is an extremely widely used acrylic resin that can be an adhesive, consolidant or varnish depending on the concentration of the solution. Thick and viscous Paraloid solutions can be used as a conservation grade adhesive. We almost exclusively use B-72 as an adhesive in fossil prep. Paraloid comes in pellet form, which you dissolve to your desired concentration in a solvent, or you can purchase it pre-mixed in solvent.
Cyanoacrylates are widely used reaction adhesives (they set in contact with air). Good quality, medium to very high viscosities make for fantastic and strong adhesives. Medium viscosities should be used as a general purpose, but higher viscosities like gels can also be used for pore filling and consolidating rough, loose surfaces whilst bonding. In fossil preparation, aim to use only good quality superglues. We wouldn't put superglue from the pound or dollar store anywhere near our collection as it will 'bloom', go brittle and quite probably yellow over time.
Epoxy Adhesives (two-part)
Two part epoxy adhesives (also known as two component epoxy) are a popular choice when joining large, heavy pieces together due to their very high bond strength and excellent resistance to shear. They typically have very good chemical, water and temperature resistance. They are stored as two separate components, a resin and a hardener, which need to be mixed in specific proportions to trigger polymerisation which leads to curing. The chemical reaction that occurs is exothermic (heat-releasing) and so it is worth first establishing whether your fossil can be subjected to a sharp change in temperature.
They offer a permanent solution, but caution must be taken. Epoxies, although sold clear, are famous for yellowing over time. Some yellow more than others. This is fine if used internally on a surface that will never be seen again, but less desirable on pale coloured fossils or where the joins are visible. Two-part epoxies can cure anytime from 5 minutes to many hours, often depending on temperature. They can be modified (within the working time) with pigments and fillers although this will change viscosity, bond strength and flexibility so experimenting first is recommended.
Polyester resins can be used as permanent adhesives, but are more commonly used as combined fillers/adhesives or simply fillers. They are popular in fossil preparation as many are provided in colours used by stonemasons to match the colours of various stones and minerals, and so are popular when restoring or joining minerals like yellow/honey-coloured calcite. This makes polyester resins a good candidate for the adhesion of broken calcitic ammonites, also providing gap-filling properties (they can be very high viscosity and semi-manipulatable).
Like epoxy resins, polyester resins must also be mixed with a hardener to cure in a specific ratio. Polyester resins provide excellent chemical and moisture resistance and some types are particularly popular as an adhesive prior to chemical preparation as they resist acid etching well. However, most are less resistant to solvents like acetone. The downside is that the bond strength is less strong than epoxies, and it is more likely that there is some retraction within a joint upon curing. They are however usually more UV stable but can delaminate over time.
Paraloid B-72 (formerly called Acryloid B-72) is an extremely widely used acrylic resin that can be an adhesive, consolidant or varnish depending on the concentration of the solution. It sets when the solvent evaporates. Paraloid comes in pellet form, which you dissolve to your desired concentration in a solvent, or you can purchase it pre-mixed in solvent (for UK Shipping only).
Very low concentration solutions from 2-5% and occasionally 10% solutions are used for consolidation. Paraloid B-72 is probably the most popular consolidant in fossil preparation. Learn more about how to use, mix and store Paraloid here.
Very low viscosity cyanoacrylates (superglues) are widely used as consolidants, particularly in commercial fossil preparation. A low viscosity superglue should behave almost like water in order to penetrate deeply. It provides strong and permanent consolidation and uses wicking or capillary action to seep into the rock/fossil. In fossil preparation, aim to use only good quality superglues. We wouldn't put superglue from the pound or dollar store anywhere near our collection as it will 'bloom', go brittle and quite probably yellow over time.
Learn more about how to use cyanoacrylates in fossil preparation here.
Butvar B-76 is a polyvinyl butyral resin. It was used more so historically as the default solution adhesive and so you may read about in older books. It was used widely as a consolidant, but rarely as an adhesive as layers of higher concentration can just be peeled away.
Butvar B-76 is supplied in powder form with an unlimited shelf life, which dissolves (albeit slowly) in a solution of either acetone or ethanol. It sets when the solvent evaporates. It is often best used as a field consolidant or mid-mechanical prep consolidant as it is easier to remove than Paraloid B-72. The matrix isn't consolidated quite as firmly and adhesion with the specimen isn't as strong as with Paraloid B-72. In most applications, Paraloid B-72 would be preferable.
However, if using as a varnish, Butvar B-76 has a more matte appearance than Paraloid B-72 which is often desirable. In this sense, it is similar to Mowithal B30H. It also has a higher glass transition temperature (62-72C) than Paraloid B-72, making it better suited to fieldwork in hotter climates.
Carbowax is essentially a water-soluble 'wax' produced by Dow Chemical Company. It is polyethylene glycol (PEG), and is available in various different molecular weights that have different hardnesses (e.g. PEG 4000 is usually a good bet for fossil preparation). It behaves very much like wax, but is water soluble. This makes it useful as a consolidant in many different respects. It can either be used to make a temporary support, e.g. prepare one side of a fossil, set it in carbowax and then prepare the other side. It can also be used to temporarily fill a void if it is at risk during mechanical preparation. It also offers a solution for temporary protection of fragile fossils or features during shipping (e.g. can be applied to the spines of an ammonite to prevent them from breaking off). To remove, the fossil and it simply need to be soaked in warm water.
Primal® (Rhoplex) WS 24
Primal® (Rhoplex) WS 24 is an acrylic colloidal dispersion in water with a solids content of 36%. It is supplied as a liquid that requires dilution with water. It is a conservation grade alternative to PVA or Elmer's glue which is less susceptible to cracking, yellowing and becoming brittle over time.
Primal (Rhoplex) WS 24 is used particularly for preservation of mammoth tusks/teeth and ice age vertebrate remains that require consolidation. It is also used as a consolidant in the excavation and recovery of fossils in damp, wet or waterlogged environments. It can be used as an alternative to Paraloid B-72 (see our other listings) in wet conditions on most materials (although not recommended for metals).
It is a consolidant with a very small particle size (0.03 Microns) which can penetrate deeply into fine grained materials. Used for consolidation of bones (both fossil and modern and a particular favourite with archaeological finds and Ice Age bones), plaster, wall paintings and rocks/fossils/archaeological material in damp, wet or waterlogged environments.
Butvar B-98 is a polyvinyl butyral resin that is used (although infrequently) as a consolidant. It is supplied in powder form with an unlimited shelf life and only dissolves in ethanol. It has the capacity for deeper penetration than Butvar B-76 or Paraloid B-72 if the surface to be consolidated is first wetted with ethanol. It is suitable for consolidating very loose sandstones.
There are other consolidants and adhesives used in fossil preparation, but these are perhaps the most widely used that would be recommended for their specific applications. We do not recommend the use of PVA, Elmer's Glue, Shellac, low quality Cyanoacrylates or low-quality Epoxy Resins, as these will not stand the test of time and may be difficult/impossible to remove for future conservation efforts. Different levels of additives, or simply inappropriate formulations will likely yellow, crack and go brittle over time. Try to avoid making a conservation nightmare for your future self and use consolidants and adhesives that are documented as suitable for fossil preparation.
If you would like more information on various consolidants and adhesives, it's well worth having a look at the following references. Amy Davidson is a hero of ours, and a phenomenal preparator with a wealth of experience and knowledge.
Davidson, A. and G. Brown, 2012. Paraloid B-72: Practical Tips for the Vertebrate Fossil Preparator. 2012 Collection Forum 26 (1-2): 99-119.
'Understanding and Use of Adhesives' by Amy Davidson, (AMNH), series of videos by Association for Materials & Methods in Paleontology
Davidson, A. and S. Alderson. 2009. An introduction to solution and reaction adhesives for fossil preparation. In: Methods In Fossil Preparation: Proceedings of the First Annual Fossil Preparation and Collections Symposium, pp 53-62. Brown, M.A., Kane, J.F., and Parker, W.G. Eds.
Cavigelli, J-P., 2009. A primer to Polyethelene Glycol use in Palaeontological Preparation or How I learned to stop worrying and love prepping thin and delicate bones. 15th Annual Tate Conference, The Annual Fossil Preparation and Collections Symposium, pp.24-33.