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About CU Conservation, Objects and Treatments

Paraloid B72 is not the only adhesive or how many adhesive puns can I use before it gets tacky?

18 November 2022
Tessa Corton

Tessa Corton

 

 

Paraloid B72. Or Acryloid B72, if you’re American, or even ethyl methacrylate and methacrylate if you’re feeling fancy (Preservation Equipment Ltd, 2021). By whatever name, you’re bound to have come across it at some point if you haven’t already. It’s very easy to perceive it to be a wonder polymer: it’s both chemically and physically stable, easy to vary the concentration and viscosity and to work with, water white, works with a wide variety of bulking materials, and (importantly in the current conservation school of thought) reversible (Koob, 1986: 7, 9, 11; Preservation Equipment Ltd, 2021; Horie, 2010: 159). It’s also soluble in a wide range of solvents, from the fast release acetone to the slow toluene and xylene and can even be mixed with ethanol to control its setting rate (Koob, 1986: 8-9). Pioneered by Stephen Koob in the 1980s for reconstructing glass (seriously, this man loves the stuff), the conservation world has stuck fast in its love for the adhesive (Zhao et al, 2018: 2422).

But is this necessarily a good thing?

It’s very easy to become stuck on all the pros of Paraloid B72, to go with the flow and stick with what you know, but B72 isn’t perfect. Its reported glass transition temperature (Tg) is 37-40°C, where It becomes soft and rubbery, attracts and traps dirt, and will eventually fail (Vincotte et al, 2019: 2; Zhao et al, 2018: 2422). For this reason, it is important to consider the final destination of an object when deciding what material to use, otherwise you end up with some interesting combinations where the logic is difficult to trace (Camurcuoglu, 2022). Even in cooler climates, the Tg is a problem during periods of high heat, like what the majority of the UK experienced during summer of 2022 (thcwordpodcast, 2022). http://

And that’s not B72’s only sticking point. Although it is recommended for consolidating and adhering metals (particularly archaeological iron), B72 does not bond readily to them (Keene, 1984: 106). Darkening is also reported on porous surfaces, especially low fired ceramics, and cellulose based materials (Gerwilker, 2016: 114). Even its reversibility is in question as it has been reported to crosslink on marble (Horie, 2010: 159).

And yet conservators seem to be stuck on B72, even when better choices are out there.

Sticking with metals, epoxies are popular alternatives. Epoxy resins are two-part adhesives where an epoxide is mixed with a hardener, with adhesion and strength provided by crosslinking (Horie, 2010: 289). This does however make them irreversible (Horie, 2010: 296). Although the low viscosity of Paraloid B72 is useful during consolidating, epoxy resins with similar properties exist which may provide higher strength. Reversibility isn’t a concern here as it would never be possible to remove all the consolidant, if that was desired, due to the corrosion’s structure. And if it’s not visible, is yellowing a problem? Also, metal has a medium-low surface energy level so it requires a strong adhesive, something with Paraloid B72 may not be able to provide but an epoxy could (Ogihara et al, 2017: 27). The same can be said with glass, which has even lower surface energy than metal (Ogihara et al, 2017: 27).  Hxtal NYL-1 is a UV resistant and water white epoxy with a refractive index close to glass which is also non-viscous, and often employed in coating, adhering, and moulding of glass (creativeglass, n.d.). Other, more viscous, epoxies have been used on non-porous ceramics, like high fired porcelains, and on metals. Famously, Hxtal was used in fixing the Portland vase in the British Museum (Painter and Whitehouse, 1990: 84). Epoxies are a big family with a range of properties and to rule them out by generalisations would do them an injustice.

The Portland Vase, which was broken in 1845, has been repaired twice – once in 1948 and again in 1989 (Painter and Whitehouse, 1990: 62, 84). The 1989 reconstruction was with Hxtal (Painter and Whitehouse, 1990: 84). Photo courtesy of the British Museum.

 

Even if epoxies aren’t your adhesive of choice, other similar acrylic resins exist. Paraloids B44, B48N, B66, B67, B82 and B99-N all have Tg’s above B72’s in the 50-80°C range that also offer similar properties  in terms of strength, viscosity, and versatility (CAMEO, 2022). Not all of them perform as well as B72 – B67 yellows and crosslinks with age (CAMEO, 2020; Horie, 2010: 164), B82 softer, and B99-N is glossier (CAMEO, 2022) – but they have their own attractive properties. B48N is a smaller molecule than B72 so has deeper penetration into porous surfaces, making it popular for use on bone and ivory, both of which are porous and often fragile material (Nethaway, 2020: 249; Snow and Weisser, 1984: 144). B44 is a key ingredient of Incralac® – a UV resistant corrosion inhibitor based on B44 and benzotriazole for copper objects – and is often used when Incralac® is unavailable (Keene, 1984: 107). Although, like B67, it has a tendency to crosslink,  the UV inhibitor helps prevent this and reversibility is less important when used as a thin coating on items such as Roman coins (Keene, 1984: 107).

 

                                 
Paraloid B44 in toluene being applied to Roman copper coins using a spray gun, June 2022. Incralac® was unavailable at the time. Photo courtesy of Emma Thomas, featuring Leonie McKenzie.

Breaking away from high strength adhesives, other polymers may be more suited to consolidation and inpainting, especially in cases where lacquers and paints are present. Aquazol® (poly(2-ethyl-2-oxaline)) is a synthetic polymer employed in art restoration which is soluble in polar and non-polar solvents, in other words it is water-soluble as well as in other solvents like acetone, ethanol, and isopropanol (Breidenstein, 2019: 190-1). It is available in four molecular weights making it a versatile material whose properties can be tailored to usage (Breidenstein, 2019: 191). Like Paraloid B72, it’s water white, reversible, and stable with no changes in appearance or mechanical strength seen after accelerated ageing via UV light, and its viscosity can be varied by percentage, solvent, and molecular weight choice (Breidenstein, 2019: 184, 191). It is less bulky than B72, sets faster, and has a higher Tg (Arslanoglu, 2004: 13; PCI, n.d). Again, it’s not perfect – it’s affected by high humidity due to its hygroscopic nature and so needs an additional protective coating, like Paraloid B72 (Arslanoglu, 2004: 13)! It’s also not recommended for weight bearing joints for this reason (Arslanoglu, 2004: 15).

 

What this blog post hoped to show is that choosing an adhesive isn’t easy – every object has different needs which are hard to match to an appropriate polymer (Leung et al, 2014: 90). It’s easy to stick with what you know and keep reaching for the B72 but take a moment and think about what about Paraloid B72 makes it appropriate for your object. Is it the viscosity, strength, reversibility, or ageing? Or maybe its solubility in a certain solvent or its ease of use? Once you’ve worked that out, consider that maybe there’s a better adhesive out there. Maybe there isn’t – B72 is so widely used because it’s a great choice for most things. But you might just miss a better one if you think of why you’re using it.

 

 

References

CAMEO. 2020, June 11. Paraloid B-67. Available at: https://cameo.mfa.org/wiki/Paraloid_B-67 [accessed 02/10/2022]

CAMEO. 2022, June 22. Paraloid. Available: https://cameo.mfa.org/wiki/Paraloid [accessed 02/10/2022]

Camurcuoglu, D., 2022, August 24. Conserving Beirut’s shattered glass. The British Museum Blog. Available at: https://blog.britishmuseum.org/conserving-beiruts-shattered-glass/ [accessed 19/09/2022]

creativeglass. N.d. Hxtal NYL-1 – A & B Kit – 110g. Creative Glass Workshop. Available at: https://www.creativeglassshop.co.uk/product/34425/hxtal-nyl-1-a-b-kit-110g.html [accessed 18/04/2022]

Gerwilker, C. 2016. Consolidation of a sepiolite-rich sandstone – learning from failure. Journal of the Institute of Conservation. 39:2. Pp110-118

Horie, C. V. 2010. Materials for Conservation. 2nd ed. London. Routledge

Keene, S. 1984. The performance of coatings and consolidants used for archaeological iron. Studies in Conservation. 29:supp1. Pp104-106

Koob, S. 1986. ‘The Use of Paraloid B-72 as an Adhesive: Its Application for Archaeological Ceramics and Other Materials.’ Studies in Conservation. 31.1. pp7-14

Nethaway, E. 2020. Conservation of a Roman Skeleton. In Studies in Archaeological Conservation. C. Caple, and V. Garlick (eds). Routledge. London. pp245-253

Ogihara, H., Xie, J., and Saji, T. 2015. Controlling surface energy of glass substrates to prepare superhydrophobic and transparent films from silica nanoparticle suspensions. Journal of Colloid and Interface Science. 437. Pp 24-27

Painter, K., & Whitehouse, D. 1990. THE VASE IN ENGLAND, 1800–1989. Journal of Glass Studies, 32, pp62–84.

Preservation Equipment Ltd. 2021.. Paraloid B-72 Adhesive Available at: https://www.preservationequipment.com/Catalogue/Conservation-Materials/Adhesives/Paraloid-B-72-Adhesive-P422-9250 [accessed 13/10/21]

Snow, C., and Weisser, T. 1984. The examination and treatment of ivory and related materials. Studies in Conservation. 29 Supplement. pp141-145

The British Museum, n.d. amphora; vessel (closed); cameo. Available at: https://www.britishmuseum.org/collection/object/G_1945-0927-1  [accessed 14/10/2022]

thecwordpodcast. 2022. Stay safe, everyone! The UK is experiencing extreme temperatures right now and while, yes, it’d be lovely to make sure everything stuck together with Paraloid B72 doesn’t fall apart this is today’s PSA: YOU ARE MORE IMPORTANT THAN THE COLLECTIONS Stay cool, stay safe. [Twitter] July 18. Available at: https://twitter.com/thecwordpodcast/status/1549011590057435136 [accessed 14/10/2022]

Vinçotte, A., Beauvoit, E., Boyard, N., and Guilminot, E. 2019. Effect of solvent on PARALOID B72 and PARALOID B44 acrylic resins used as adhesives in conservation. Heritage Science. 7:42, pp1-9

Zhao, X., Wang, L., Zhao, X., Qian, Y., Tang, and Dong, X. 2018. Synthesis, testing and application of moisture-curable polyurethane as  consolidant for fragile organic cultural objects. Journal of Adhesion Science and Technology. 32:22, pp2421-2428