Gels

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Gels, Thickeners, and Viscosity Modifiers
The use of these materials in conservation spans disciplines. This is intended to be a cross-disciplinary starting bibliography for those seeking a brief overview of important conservation materials. Please see the references for further information. Members of the Emerging Conservation Professionals Network compiled and edited the first version of this bibliography, and we are grateful for our colleagues' contributions. 
Contributors: Lora Angelova, Rebecca Gridley, Amy Hughes, Laura Mina, Kari Rayner, Emma Schmitt, Samantha Skelton, Michelle Sullivan, Jessica Walthew, Miranda Dunn, Kerith Koss Schrager 
Your name could be here! Please contribute.
The Specialty Group Catalogs are a publication of the American Institute for Conservation of Historic and Artistic Works. The Specialty Group Conservation Catalogs are published as a convenience for the members of the AIC. Publication does not endorse nor recommend any treatments, methods, or techniques described herein.
==Introduction: Gels, Thickeners & Viscosity Modifiers==
Conservators in all specialities have adopted the use of thickeners, gelling agents, and rigid gels for more controlled cleaning of artworks. These substances can be formulated to display a wide range of physical properties: fluid, soft, and weakly bound, to dense, hard, and brittle. While thickening or poulticing materials have been in use in conservation for some time to contain liquids (often water) and reduce evaporation (e.g. methylcellulose, hydroxypropyl methylcellulose (HPMC), cellulose powder, blotter, Laponite or other clays, etc.), within the last decade, many new materials have been introduced which broaden the range of these applications. 
In the United States, Richard Wolbers and Chris Stavroudis (among others) first introduced and popularized viscosity modifiers made of natural or synthetic polymers (e.g. Xanthan gum, Pemulen, Carbopol, and more recently, Velvesil plus, and other polyacrylic acid derivatives). These modifiers allow conservators more options for tailoring cleaning solutions and restricting unwanted penetration of these solutions into porous substrates. Increasing the viscosity of a solution by adding a gelling agent can help to limit solvent volatility, increase contact time with the substrate, and allow immiscible solvents to form jelly-like stabilized emulsions. Emulsions are mixtures of two or more liquids that are normally immiscible (a related term is colloid). By definition, in emulsions one liquid is dispersed in the other (called the continuous phase). 
More recently, rigid physical gels made of polysaccharides (e.g. agar/agarose, gellan) have been used as delivery systems or “containers” for water or other polar solvents (see below). While a variety of gels and thickeners have been readily adopted for use in treatment, ongoing research seeks to better control their application and removal (e.g. for rigid gels, conforming to different surfaces, specifying pore size, and controlling solvent release). For all of these materials, a great concern has been limiting residues from their use. Natural polymers that form gel structures include collagen and gelatin (protein) and starch, alginate and agarose (polysaccharide). 
 
Synthetic chemical gels (i.e. those made of crosslinked polymers, such as those developed and introduced by the University of Florence’s Center for Colloid and Surface Science, CSGI) offer new options for the conservation toolkit and have the benefit of leaving no residues. This was a key concern during development of these products and was tested with a variety of analytical techniques e.g. Focal plane array FTIR. While these may seem unfamiliar to you at first glance, interpenetrating network polymeric gels are the same kind of technology used for making soft contact lenses. See also the forthcoming workshop review for “Nanotechnologies in Conservation” which occurred in January 2017 at the Pratt Institute. The key difference between chemical and physical gels is that: “Chemically cross-linked networks have permanent junctions, while physical networks have transient junctions that arise from either polymer chain entanglements or physical interactions, such as ionic interactions, hydrogen bonds, or hydrophobic interactions.” (Ahmed 2013) 
===References: Overview-Theory-Reviews-Comparisons===
Ashley-Smith, Jonathan. 2002. Science for conservators, Vol. 2: cleaning. Routledge. (many previous editions: 1983, 1987, and reprinted in 1994, 1996, 1999, 2001. overview on the principles of cleaning more generally) 
Baas, Fran. 2013. Review of “New and current materials and approaches for localized cleaning in textile conservation” by Elizabeth Shaeffer and Joy Gardiner at the 41st Annual Meeting – Textile Session, May 30. Conservators Converse: The Blog of the American Institute for Conservation. Accessed May 31, 2016. http://www.conservators-converse.org/2013/06/41st-annual-meeting-textile-session-may-30-new-and-current-materials-and-approaches-for-localized-cleaning-in-textile-conservation-by-elizabeth-shaeffer-co-authored-by-j/. 
Cremonesi, Paolo, and M. Mecklenburg. 2013. Rigid gels and enzyme cleaning. New insights into the cleaning of paintings. Washington, DC: Smithsonian Institution Scholarly Press. 179-183. 
Getty Conservation Institute. Gels cleaning research (1998–2003). http://www.getty.edu/conservation/our_projects/science/gels/ (accessed September 13, 2016). Highly recommended 
Hawkes, Richard. 2013. Review of “Gel media in aqueous cleaning methods on paper: A lecture” by Professor Richard Wolbers, University of Delaware, presented at the Wellcome Institute, London, June 19th, 2013. The Book & Paper Gathering. https://thebookandpapergathering.org/2013/11/08/gel-media-in-aqueous-cleaning-methods-on-paper-a-lecture-by-professor-richard-wolbers-university-of-delaware-presented-at-the-wellcome-institute-london-june-19th-2013/. 
Kavda, S., N. Dhopatkar, A. Angelova., E. Richardson, S. Golfomitsou, and A. Dhinojwala. 2016. Surface behavior of PMMA: Is gel cleaning the way to go? Studies in Conservation 61 (2016): 297–299. doi: 10.1080/00393630.2016.1200858. PVOH-borax, Gellan	 
Khandekar, N., 2000. A survey of the conservation literature relating to the development of aqueous gel cleaning on painted and varnished surfaces, Reviews in Conservation, 1: 10-20. 
Shaeffer, Elizabeth and Joy Gardiner. 2013. New and current materials and approaches for localized cleaning in textile conservation. Textile Specialty Group Postprints, 41st Annual Meeting of the American Institute for Conservation. Washington, DC: AIC. 23: 109-124. 
Stavroudis, Chris, Tiarna Doherty, and Richard Wolbers. 2005. A new approach to cleaning I: Using mixtures of concentrated stock solutions and a database to arrive at an optimal aqueous cleaning system. WAAC Newsletter 27 (2): 17-28. 
Warda, Jeffrey, Irene Brückle, Anikó Bezúr, and Dan Kushel. 2007. Analysis of agarose, carbopol, and laponite gel poultices in paper conservation. Journal of the American Institute for Conservation 46 (3): 263-279. 
Wolbers, Richard, and Chris Stavroudis. 2012. Aqueous methods for the cleaning of paintings. Conservation of easel paintings. Edited by Joyce Hill Stoner and Rebecca Rushfield. London and New York: Routledge, Taylor & Francis Group. 500-523.Highly recommended 
Wolbers, Richard. 2000. Cleaning painted surfaces: Aqueous methods. London: Archetype Publications. 
Wolbers, Richard, Nanette T. Sterman, Chris Stavroudis, and Getty Conservation Institute (editors). 1990. Notes for workshop on new methods in the cleaning of paintings. Marina del Rey, CA: Getty Conservation Institute. 
Wolbers, Richard and ICON Book and Paper Group. 2013. The use of gels in aqueous conservation of paper (parts 1-5). Video (digital). https://www.youtube.com/channel/UCdTol4MgefFNr-3PsvsmHIw (accessed November 25, 2013). Highly recommended
 
===2017 Gels in Conservation Conference===
Gels in Conservation Conference – London, Emmanuel Centre, 16th – 18th October 2017 
 https://academicprojects.co.uk/archive/gels-conference/'
 
==Viscosity modifiers==
Polymeric-stabilized emulsions are made with the aid of polymers with regions of differing affinity (i.e. hydrophilic vs lipophilic), which allow two liquids to form emulsions without adding surfactants. Thorough clearance is a major concern as these materials could leave undesired residues behind. These polymers can be natural or synthetic.
 
* Methylcellulose, hydroxypropyl methylcellulose (Methocel, Klucel) are synthetic cellulose derivatives dissolved in water in low concentrations (typically 0.5-3% w/v) to form clear viscous solutions. The properties of these cellulose derivatives are related to the degree of substitution of the cellulose chain (the average number of substituted hydroxyl groups per glucose). See also https://en.wikipedia.org/wiki/Methyl_cellulose and https://en.wikipedia.org/wiki/Hypromellose 
* Xanthan gum is a gluten-free additive produced from a fermented bacterial secretion that is used in cosmetics and food as a thickener or stabilizer (i.e. a rheology modifier). In art conservation, it can be used to make oil-in-water emulsions for non-water-miscible solvents. The polysaccharide polymer is made up of tangled helix chains, which will aggregate at higher concentrations to make little cages that hold water molecules. When the gel on a surface is agitated, the shear force breaks down the cage structure temporarily, allowing solvent to be activated. (see also: https://en.wikipedia.org/wiki/Xanthan_gum) 
right|250 px|Pemulen gel photo: Rebecca Gridley
*Carbopol (poly acrylic acid) gels are made with a base in water (NaOH, NH4OH, TEA) or with an amine in solvent (for solvent gel). The polymer needs to be unfurled by neutralization, causing it to open up. When it’s mixed up with amine (Ethomeen C12 and C25 are two main amines used by conservators) it will form a gel. Carbopol solvent gels were originally formulated for paintings and furniture varnish removal by Richard Wolbers for use with a variety of solvents and solvent combinations. 
**See also https://en.wikipedia.org/wiki/Polyacrylic_acid 
**Clearance: The Getty's Solvent gel residues research project found that residues can be left on surfaces if you don’t clear carefully. Some conservators apply solvent gels through a barrier layer of Japanese tissue to help mitigate this problem. Of course, careful clearance is a concern for all conservation cleaning materials. There is extensive literature on this subject: see the bibliography below for further details. 
 
*Pemulen TR2 is a copolymer of acrylate and alkyl acrylate that makes oil-in-water emulsions by thickening the continuous phase (this is a way of making the system more stable by keeping the droplets of dispersed phase separate). Pemulen has polar (PAA, like carbopol) and non-polar (fatty regions of the hydrocarbon alkyl acrylate) parts, which allows the gel to hold a non-water miscible solvent in an aqueous system, like benzyl alcohol. See the Noveon product information sheet for further description. 
*Velvesil Plus and Shin Etsu KSG make thickened gels of silicone solvents with a modified water dispersed phase (up to 30% water phase). While there is still discussion about the health effects of silicone solvents, Volatile Methyl Siloxanes (VMS) solvents are used in many cosmetics applications. The gelling agents are silicone polymers with ethoxylate/propoxylate chains or polyether chains to bridge between silicone chains. The polar groups can emulsify water into the silicone system, while the silicone side chains are compatible with silicone solvents and hydrocarbon side chains are compatible with other conventional solvents. Emulsions made with these polymers can hold acetone, acetone/alcohol mixes (e.g. acetone/isopropanol) or benzyl alcohol. Modular Cleaning Program Notes (from BAACG workshop June 2016, led by Chris Stavroudis)
 
===References: Viscosity Modifiers===
Burnstock, Aviva, and Tanya Kieslich. 1996. A study of the clearance of solvent gels used for varnish removal from paintings. 11th Triennial Meeting, Edinburgh, Scotland, 1-6 September, 1996: Preprints (ICOM Committee for Conservation). Edited by Janet Bridgland. London: Earthscan Ltd. 253–262.
Chung, Jae Youn. 2016. An investigation of methods for cleaning water-sensitive oil paint based on methods developed for surface cleaning acrylic paintings. Gerry Hedley Student Symposium Proceedings. Details tests with Velvesil Plus and Shin-Etsu KSG 210/240 
Kronkright, Dale. 2009. Review of Solvent gels for the cleaning of works of art: The residue question and Cleaning painted surfaces: Aqueous methods. Journal of the American Institute for Conservation 48 (1): 83–96.
Lennig, Heidi. 2010. Solvent gels for removing aged pressure-sensitive tape from paper. Restaurator: International Journal for the Preservation of Library and Archival Material 31: 92–105.
Stavroudis, Chris, and Tiarna Doherty. 2007. A novel approach to cleaning II: Extending the Modular Cleaning Program to solvent gels and free solvents, part 1. WAAC Newsletter 29: 9-15.
Stulik, Dusan, and Valerie Dorge. 2004. Solvent gels for the cleaning of works of art: The residue question. Marina del Rey, CA: Getty Conservation Institute.
Wolbers, Richard. 2003. Gel residue studies at the GCI: Implications for testing methodologies and future research. Materiali Tradizionali Ed Innovativi Nella Pulitura Dei Dipinti E Delle Opere Policrome Mobili: Atti Del Convegno, Piazzola Sul Brenta (Padova) 25-26 Ottobre 2002. Edited by Paolo Cremonesi. Padua: Il Prato. 25-26.
===References: Pemulen TR2===
Ravenel, Nancie. “pemulentr2 / Pemulen TR2 wiki.” Accessed October 17, 2016. Doi: http://pemulentr2.pbworks.com/w/page/15636419/Pemulen%20TR2. (best starter resource, great overview)
Kerschner, R., & Ravenel, N. 2006. Here we go 'round again: Cleaning linseed oil from carousel animals at the Shelburne Museum. Journal of the American Institute for Conservation 45 (3): 201-210. http://www.jstor.org/stable/40026691.
Ravenel, Nancie. 2010. Pemulen® TR-2: An emulsifying agent with promise. WAAC Newsletter 32 (3): 10–12.
Stavroudis, Chris. 2010. Using Pemulen with the MCP. WAAC Newsletter 32 (3): 16.
Stavroudis, Chris. 2012. Pemulen revised: pHuck the pH Meter. WAAC Newsletter 34 (2): 19.
Travers, Kirsten, Richard Wolbers, and Carolyn Tomkiewicz. 2010. Pemulen case study: Holy Innocents mural project. WAAC Newsletter 32 (3): 13–14.
Williams, Donna. 2010. Pemulen case study: A midsummer-night’s dream. WAAC Newsletter 32 (3): 14–15.
==Physical Gels==
 
Currently, two of the most commonly used physical gels in art conservation are agar/agarose and gellan gum. Both are naturally occurring polysaccharides. Agarose is derived from the cell wall of Gelidium and Gracilaria, species of red algae, or Sphaerococcus euchema, seaweed (Armisén 2000). Along with agaropectin, it is one component of agar. 
Gellan gum is the product of Sphingomonas elodea, a bacterium found on lily pads, and is available in two forms: low-acyl and high-acyl (CP Kelco 2007). When prepared, low-acyl gellan gum is typically more rigid and brittle. Most published examples of paper conservation treatments using gellan gum cite the low-acyl form. However, textile conservators have also begun employing high-acyl gellan gum, which is softer and has greater conformability (Peranteau 2013). While agarose is electronically neutral, gellan gum is anionic with negatively charged carboxyl groups, which are available to complex with cations (Sworn 2000 and Mao et al 2001). 
Agarose and gellan gum are available in powder form and are prepared by dispersing dry polymer in pure water—or modified aqueous solution—and heating until clear and boiling. As the polymer dispersions cool, both agarose and gellan gum form helical structures. These helical structures proceed to form aggregates, which define a porous network (Armisén 2000 and Valli and Miskiel 2001). 
 
These aggregates form in agarose by hydrogen bonding. In the case of gellan gum, hydrogen bonding also occurs, but the gel network is more robust when prepared with an aqueous solution that contains cations that crosslink at the negatively charged carboxyl groups. Both gels are thermo-reversible and can sustain numerous heating-cooling cycles, transitioning between liquid solution and solid gel (CP Kelco 2007). Hughes and Sullivan 2016.
===References: Agar and Agarose===
Armisén, R., et al. 2000. Agar. Handbook of hydrocolloids. Edited by G. O. Phillips and P. A. Williams. Cambridge, UK: Woodhead Publishing. 82-107.
Barkovic, Margaret. 2016. Integrative solutions for treating water stains on acrylic canvases: Case study Composition, 1963 by Justin Knowles. Gerry Hedley Student Symposium Proceedings.
Campani, Elisa, Antonella Casoli, Paolo Cremonesi, Ilaria Saccani, and Erminio Signorini. 2007. Use of agarose and agar for preparing rigid gels. Translated by D. Kunzelman. Quaderni del Cesmar 7: 31-51. 
Davis, Ellen. 2015. The unfurling of Margaret Watherston’s method: Research into the retreatability of Morris Louis’ Alpha (1960). Proceedings of the 2015 Conference of the Association of North American Graduate Programs in Conservation of Cultural Heritage http://resources.conservation-us.org/anagpic-student-papers/wp-content/uploads/sites/11/2016/01/2015ANAGPIC_Davis_Paper.pdf 
Hughes, A. and M. Sullivan. 2016. Targeted cleaning of works on paper: rigid polysaccharide gels and conductivity in aqueous solutions. The Book and Paper Group Annual 35: 30-41. Agarose and Gellan gum
Låås, T. 1975. Agar derivatives for chromatography, electrophoresis and gel-bound enzymes. Uppsala, Univ., Diss., 1975.
Iannuccelli, Simonetta, and Silvia Sotgio. 2010. Wet treatments of works of art on paper with rigid gellan gels.The Book and Paper Annual 29: 25-39.
Pouliot, Bruno, Lauren Fair, and Richard Wolbers. 2013. Rethinking the approach: Techniques explored at Winterthur for the stain reduction of ceramics. Recent Advances in Glass, Stained Glass, and Ceramics Conservation 2013: ICOM-CC Glass and Ceramics Working Group Interim Meeting and Forum of the International Scientific Committee for the Conservation of Stained Glass. Amsterdam: ICOM. 211-223. 
Roig, Pilar Bosch, Jose Luis Regidor Ros, and Rosa Montes Estellés. 2013. Biocleaning of nitrate alterations on wall paintings by Pseudomonas Stutzeri. International Biodeterioration & Biodegradation 84 (October): 266-274. doi:10.1016/j.ibiod.2012.09.009.
Sahmel, Katherine, Laura Mina, Ken Sutherland, and Nobuko Shibayama. 2012. Removing dye bleed from a sampler: New methods for an old problem. The Textile Specialty Group Postprints of Papers Delivered at the Textile Subgroup Session: American Institute for Conservation 40th Annual Meeting. Washington, DC: AIC. 22: 78-90.
Scott, Cindy Lee. 2012. The use of agar as a solvent gel in objects conservation. Objects Specialty Group Postprints. American Institute for Conservation 40th Annual Meeting. Washington, DC: AIC. 19: 71-83.
Skelton, Samantha, Corina Rogge, and Zahira Véliz Bomford. 2016. Testing the limits: The theoretical development and practical reality of a large-scale agarose treatment for a discolored Morris Louis. Studies in Conservation 61: 214-218.
Stavroudis, Chris and Getty Conservation Institute. 2013. Measuring surface pH and conductivity using water drop and agarose plug methods - YouTube. https://www.youtube.com/watch?v=bOqZEE7Kb8Y&list=PL5ZJP2yRFa3aNtJ61U-IMifrfoGzAhdAC&index=4 (accessed June 8, 2016).
Sullivan, M., et al. 2014. New approaches to cleaning works on paper and photographs. Proceedings of the 2014 Conference of the Association of North American Graduate Programs in Conservation of Cultural Heritage. http://cool.conservation-us.org/anagpic/2014pdf/anagpic2014_sullivan_etal_paper.pdf
Tuvikene, R. et al. 2008. Gel-forming structures and stages of red algal galactans of different sulfation levels. Journal of Applied Phycology 20: 527-535.
University of Delaware College of Arts and Sciences. Art conservation with innovative cleaning techniques. ARTC Spotlight/September 2012. Accessed March 3, 2017. https://www.artcons.udel.edu/Documents/artc-eblast-september-2012.original.pdf.
Van Dyke, Yana. 2004. Practical applications of protease enzymes in paper conservation. The Book and Paper Group Annual 23: 93-107.
Wolbers, Richard and ICON Book and Paper Group. 2013. The use of gels in aqueous conservation of paper (parts 1-5). Video (digital).https://www.youtube.com/channel/UCdTol4MgefFNr-3PsvsmHIw (accessed November 25, 2013).
===References: Gellan===
300 px|right|High-acyl and low-acyl gellan gum monomers (CP Kelco. 2007. KELCOGEL gellan gum book. 5th edition. www.cpkelco.com)
Botti, Lorena, Aldo Corazza, Simonetta Iannuccelli, Matteo Placido, Luciano Residori, Daniele Ruggiero, Silvia Sotgiu, Lorena Tireni, Michela Berzioli, Antonella Casoli, Clelia Isca, and Paolo Cremonesi. 2011. Evaluation of cleaning and chemical stabilization of paper treated with a rigid hydrogel of gellan gum by means of chemical and physical analyses. Preprints for ICOM-CC 16th Triennial Conference, Lisbon. ICOM Committee for Conservation. Lisbon, Portugal: Critério--Produção Grafica, Lda. 1057-1068.
Breare, Caitlin. 2015. The other woman: The nature of a copy after Paul Gauguin. Proceedings of American Institute for Conservation 43rd Annual Meeting. Describes using gellan gum to reverse a lining 
Casoli, Antonella, Clelia Isca, Sergio De Iasio, Simonetta Iannuccelli, Luciano Residori, Daniele Ruggiero, and Silvia Sotgiu. 2014. Analytical evaluation, by GC/MS, of gelatine removal from ancient papers induced by wet cleaning: A comparison between immersion treatment and application of rigid gellan gum gel. Microchemical Journal 117 (November): 61–67.
CP Kelco. 2007. KELCOGEL gellan gum book. 5th edition. www.cpkelco.com
Cremonesi, P. 2015. Surface cleaning? Yes, freshly grated agar gel, please. Studies in Conservation 61 (6): 362-367.
Iannuccelli, Simonetta, and Silvia Sotgiu. 2010. Wet treatments of works of art on paper with rigid gellan gels. The Book and Paper Group Annual 29: 25-39.
Mao, R. et al. 2001. Water holding capacity and microstructure of gellan gels. Carbohydrate Polymers 46: 365-371.
Mayheux, A. 2015. Cross-disciplinary uses for gellan gum in conservation. The Book and Paper Group Annual 34: 69-79.
Mazzuca, C. et al. 2014. Cleaning of paper artworks: development of an efficient gel-based material able to remove starch paste. ACS Applied Materials & Interfaces 6 (19): 16519–16528.
Mazzuca, C. et al. 2014. Gellan hydrogel as a powerful tool in paper cleaning process: a detailed study. Journal of Colloid and Interface Science 416: 205-211.
Mazzuca, C., L. Micheli, R. Lettieri, E. Cervelli, T. Coviello, T. Cencetti, S. Sotgiu, S. Iannuccelli, G. Palleschi, and A. Palleschi. 2016. How to tune a paper cleaning process by means of modified gellan hydrogels. Microchemical Journal 126 (May): 359-367.
Micheli, Laura, Claudia Mazzuca, Eleonora Cervelli, Antonio Palleschi. 2014. New strategy for the cleaning of paper artworks: A smart combination of gels and biosensors. Advances in Chemistry (September): 1-10. doi:10.1155/2014/385674, 10.1155/2014/385674.
Miyoshi, Emako. 2009. Our recent findings on the functional properties of gellan. Osaka University Departmental Bulletin 35: 20-50. doi: 10.18910/6362.
Möller, Lotta. 2014. Cleaning of watercolour drawings : A study of the use of gellan gum gel on water sensitive media. Goteburg University. https://gupea.ub.gu.se/handle/2077/36483.
Peranteau, Anne. 2013. Gellan gum as a material for local stain reduction. Conserving modernity: The articulation of innovation. 9th North American Textile Conservation Conference, San Francisco, CA. Philadelphia: North American Textile Conservation Conference. 72-85.
Sworn, G. 2000. Gellan gum. Handbook of hydrocolloids. Edited by G. O. Phillips and P. A. Williams. Cambridge, UK: Woodhead Publishing.117-135.
Valli, R. C. and F. J. Miskiel. 2001. Gellan gum. Handbook of dietary fiber. Edited by G. A. Spiller. Boca Raton: CRC Press. 695-720.
Yip, Vivian. The conservation of two contemporary Chinese woodblock prints using gellan gum (以結冷膠修復兩幅近代中國木刻版畫). International Institute for Conservation of Historic and Artistic Works (IIC), News in Conservation, issue 48, June 2015, pp 10-13. (PDF available here https://www.iiconservation.org/node/5731).
===References: Other Rigid Polysaccharide Gels===
Lennig, H. 2010. Solvent gels for removing aged pressure-sensitive tape from paper. Restaurator 31: 92-105. etyl-hydroxy- ethylcellulose (PolySurf 67TM CS)
==Chemical Gels==
Chemical gels can be engineered to have a wide variety of physical and chemical properties. They are made by reactions between polymers Examples of polymers include acrylamide/bisacrylamide, with polyethylene glycol (PEG) and silanes (cross linkers), polyvinyl alcohol, pHEMA, poly vinyl pyrrolidone, and methyl methacrylate, all of which have been tested for conservation applications. which create covalent chemical bonds crosslinking the polymers together, yielding more stable products which have physiochemically unique properties. Synthesis parameters can be modified to yield a variety of different features, for example, controlling the mesh size of the gel. Giorgi et al 2009 in Accounts of Chemical Research Chemical gels are structurally different from physical gels, and may be more suitable for some conservation purposes (for example, one of their main benefits is reducing the danger of leaving residues from the gel itself on the substrate being cleaned).
left|300 px|Chemical hydrogels made by the University of Florence's CSGI. Gels are supplied as flat sheets and are stored in deionized water. image: Jessica Walthew
Chemical hydrogels are chemical gels compatible with aqueous cleaning solutions, some polar solvents, and microemulsions that are mostly water (i.e. oil-in-water). Hydrophilic functional groups attached to the polymeric backbone allow hydrogels to absorb water and swell, while cross-links between network chains make them chemically and thermally resistant. ibid The University of Florence CSGI research group has recently developed semi-interpenetrated network hydrogels based on pHEMA and PVA polymers. CSGI's gels are available for conservators to purchase from their website. CSGI has made poly(2-hydroxyethyl methacrylate) (pHEMA)/ polyvinylpyrrolidone (PVP) hydrogels formed by covalent bonds as well as physical gels made of polyvinyl alcohol (PVA)/ polyvinylpyrrolidone (PVP) or PVA/PVA formed with secondary bonds (dispersion forces or hydrogen bonds). Domingues et al. 2013. "Nanorestore gels" http://www.csgi.unifi.it/products/gel.html, see also https://en.wikipedia.org/wiki/Polyhydroxyethylmethacrylate 
Organogels are compatible with organic solvents. Depending on the desired characteristics, these gels can be engineered to have different properties based on the polymers used, synthesis procedures, and degree of crosslinking. Baglioni et al. 2015 
Many applications for chemical gels have been investigated by research groups in European universities, and this research has been supported by two EU projects called Nanoforart and Nanorestart. Among the new types of chemical gels are rheoreversible gels, which “become free-flowing on application of a chemical or thermal switch,” Carretti et al 2010 which allows them to be applied and removed with ease after interacting with the surface in question. In addition, researchers have experimented with the incorporation of magnetic nanoparticles which allow precise positioning and removal of gels from surfaces using magnetic forces. 
Poly(vinyl alcohol)−borate gels (a class of gels popular on youtube in various “Slime” formulations”- see video links) Of course note that these do not follow precisely the same chemical principles as conservation applications, as they include ingredients such as glitter and shaving cream. In addition, youtube slime makers often use Elmer's glue (polyvinyl acetate) as a base rather than polyvinyl alcohol, and substitute a variety of household cleaning products (contact lens solution, detergents) for rheology modifiers used in conservation formulations. are also used in art conservation for cleaning painted surfaces. These gels accept a range of organic co-solvents, can be stiff, spreadable, non-sticky and peel-able,Carretti et al 2010 and have a consistency more like conservation materials made from cellulose derivatives (e.g. Klucel) or polyacrylic acids (Carbopol-Ethomeen). 
 
===References: Chemical gels===
Ahmed, Enas M. 2015. Hydrogel: Preparation, characterization, and applications: A review. Journal of Advanced Research 6 (2): 105-121. doi:10.1016/j.jare.2013.07.006. excellent overview of synthesis and hydrogel classification 
Baglioni, Piero, David Chelazzi, and Rodorico Giorgi. 2015. Nanotechnologies in the Conservation of Cultural Heritage: A compendium of materials and techniques. Springer Netherlands. DOI 10.1007/978-94-017-9303-2 book, excellent overview 
Baglioni, Piero, Nicole Bonelli, David Chelazzi, Aurelia Chevalier, Luigi Dei, Joana Domingues, Emiliano Fratini, Rodorico Giorgi, and Morgane Martin. 2015. Organogel formulations for the cleaning of easel paintings. Applied Physics A 121 (3): 857–868. doi:10.1007/s00339-015-9364-0. 
Baglioni, Piero, David Chelazzi, Rodorico Giorgi, and Giovanna Poggi. 2013. Colloid and materials science for the conservation of cultural heritage: Cleaning, consolidation, and deacidification. Langmuir 29 (17): 5110–5122. doi:10.1021/la304456n. 
Baglioni, Piero, Luigi Dei, Emiliano Carretti, and Rodorico Giorgi. 2009. Gels for the conservation of cultural heritage. Langmuir 25 (15): 8373–8374. doi:10.1021/la900961k. 
Bonini, Massimo, Sebastian Lenz, Rodorico Giorgi, and Piero Baglioni. 2007. Nanomagnetic sponges for the cleaning of works of art. Langmuir 23 (17): 8681–8685. doi:10.1021/la701292d. 
Carretti, Emiliano, Emiliano Fratini, Debora Berti, Luigi Dei, and Piero Baglioni. 2009. Nanoscience for art conservation: Oil-in-water microemulsions embedded in a polymeric network for the cleaning of works of art. Angewandte Chemie International Edition 48 (47): 8966-8969. doi:10.1002/anie.200904244. 
Carretti, Emiliano, Massimo Bonini, Luigi Dei, Barbara H. Berrie, Lora V. Angelova, Piero Baglioni, and Richard G. Weiss. 2010. New frontiers in materials science for art conservation: Responsive gels and beyond. Accounts of Chemical Research 43 (6): 751-760. doi:10.1021/ar900282h. excellent overview 
Domingues, Joana A. L., Nicole Bonelli, Rodorico Giorgi, Emiliano Fratini, Florence Gorel, and Piero Baglioni. 2013. Innovative hydrogels based on semi-interpenetrating p(HEMA)/PVP networks for the cleaning of water-sensitive cultural heritage artifacts. Langmuir 29 (8): 2746–2755. doi:10.1021/la3048664. 
Domingues, Joana, Nicole Bonelli, Rodorico Giorgi, and Piero Baglioni. 2014. Chemical semi-IPN hydrogels for the removal of adhesives from canvas paintings. Applied Physics A 114 (3): 705-710. doi:10.1007/s00339-013-8150-0. 
Giorgi, Rodorico, Michele Baglioni, Debora Berti, and Piero Baglioni. 2010. New methodologies for the conservation of cultural heritage: Micellar solutions, microemulsions, and hydroxide nanoparticles. Accounts of Chemical Research 43 (6): 695–704. doi:10.1021/ar900193h. 
Pizzorusso, Giacomo, Emiliano Fratini, Josef Eiblmeier, Rodorico Giorgi, David Chelazzi, Aurelia Chevalier, and Piero Baglioni. 2012. Physicochemical characterization of acrylamide/bisacrylamide hydrogels and their application for the conservation of easel paintings. Langmuir 28(8): 3952–3961. doi:10.1021/la2044619. 
===References: PVOH-Borax ===
For the basics of slimy gels, first watch this:
 Glasgow Science Centre. “How to make slime - a non-Newtonian fluid.” Accessed March 4, 2017. https://www.youtube.com/watch?v=c4hwLzMNwyk
 
(or this one: ElieOops. “HOW TO MAKE CRYSTAL CLEAR SLIME , LIQUID GLASS THINKING PUTTY” https://www.youtube.com/watch?v=3qAqtYpi2wQ or glow in the dark version: https://www.youtube.com/watch?v=uGRsS9B3Vh4)
 
or see this Chemistry Lesson plan from the Royal Society of Chemists: “PVA Polymer Slime- Learn Chemistry.” Accessed September 12, 2016. http://www.rsc.org/learn-chemistry/resource/res00000756/pva-polymer-slime?cmpid=CMP00006699.
 
Angelova, Lora V., Richard G. Weiss, and Barbara H. Berrie. 2015. Partially hydrolyzed poly(vinyl acetate)-borax based gel-like materials for conservation of art: Characterization and applications. Studies in Conservation, 60(4), 227-244. DOI: 10.1179/2047058413Y.0000000112 
Angelova, L.V., B. Ormsby, and E. Richardson. 2015. Diffusion of Water from a Range of Conservation Treatment Gels into Paint Films Studied by Unilateral NMR: Part I: Acrylic Emulsion Paint. Microchemical Journal, 124, 311-320. DOI: 10.1016/j.microc.2015.09.012 
Angelova, Lora V. 2013. Gels from Borate-Crosslinked Partially Hydrolyzed Poly(vinyl acetate)s: Characterization of Physical and Chemical Properties and Applications in Art Conservation, PhD Dissertation, Georgetown University (Available open access: https://repository.library.georgetown.edu/handle/10822/559467) 
Angelova, Lora V., Pierre Terech, Irene Natali, Luigi Dei, Emiliano Carretti, and Richard G. Weiss. 2011. Cosolvent gel-like materials from partially hydrolyzed poly(vinyl acetate)s and borax. Langmuir 27 (18): 11671–11682. doi:10.1021/la202179e. 
Carretti, Emiliano, Massimo Bonini, Luigi Dei, Barbara H. Berrie, Lora V. Angelova, Piero Baglioni, and Richard G. Weiss. 2010. New frontiers in materials science for art conservation: Responsive gels and beyond. Accounts of Chemical Research 43 (6): 751-760. doi:10.1021/ar900282h. 
Carretti, Emiliano, Luigi Dei, Azzurra Macherelli, and Richard G. Weiss. 2004. Rheoreversible polymeric organogels: The art of science for art conservation. Langmuir: 20 (20): 8414-8418. doi:10.1021/la0495175. 
Carretti, Emiliano, Scilla Grassi, Manuela Cossalter, Irene Natali, Gabriella Caminati, Richard G. Weiss, Piero Baglioni, and Luigi Dei. 2009. Poly(vinyl alcohol)−borate hydro/cosolvent gels: Viscoelastic properties, solubilizing power, and application to art conservation. Langmuir 25 (15): 8656-8662. doi:10.1021/la804306w. 
Duncan, Teresa T., Barbara H. Berrie and Richard G. Weiss. 2016. Colloidal Properties of Aqueous Poly(vinyl acetate)–Borate Dispersions with Short-Chain Glycol Ethers. ChemPhysChem 17(16): 2535-2544. 
Natali, I., E. Carretti, L.V. Angelova, P. Baglioni, R.G. Weiss, and L. Dei. 2011. Structural and Mechanical Properties of “Peelable” Organo-Aqueous Dispersions of Borate-Crosslinked 80% Hydrolyzed Poly(vinyl acetate). Applications to Cleaning Painted Surfaces. Langmuir, 13226-13235. DOI: 10.1021/la2015786 
Riedo, C., F. Caldera, T. Poli, and O. Chiantore. 2015. Poly(vinyl alcohol)-borate hydrogels with improved features for the cleaning of cultural heritage surfaces. Heritage Science 3(1): 23.
==Health & Safety==
Disclaimer: Some of the information included on this page
may be out of date, particularly with regard to toxicological data and regulatory standards. Also, because new information on safety issues is continually published, resources outside of AIC should be consulted for more specific information. 
There are many benefits of using gel systems, such as increased solvent retention on the surface, more controlled 
application and penetration, cleaning selectivity, and reduced 
solvent waste. The reduced amount of solvent required in a gel-
based application versus a free solvent application suggests that 
there may also be health and safety benefits. 
A simple experiment was conducted by Health & Safety Committee student member Miranda Dunn (Dunn 2015) to determine how much 
the gel formulation slows solvent evaporation, a property of 
the gel system that would further reduce solvent exposure for 
the conservator. The choice and amount of solvent and gelling 
medium will affect rate of evaporation, but in a simple test 
with acetone, a Carbopol-based gel significantly decreased the 
evaporation rate of the solvent. 
For this test, the time for half of 
the amount of acetone (by weight) to evaporate from an open 
container was found to be approximately three times faster than 
the same amount of acetone contained in the Carbopol gel. 
While 
more in-depth experimentation would give a broader sense of 
how gels affect evaporation rates, this experiment does provide an 
indication of how much gels can reduce solvent evaporation rate, 
resulting in less exposure over time. 
It should be noted that while using solvent gels potentially 
reduces solvent exposure, they also introduce additional mate-
rials that pose their own health hazards for the conservator. It is 
important to consider the safety issues associated with the gel 
components or pre-made gels; this information is readily available 
in the Safety Data Sheet (SDS) for a given material.
Carbopol polymers are lightly cross-linked polyacrylic acids 
with different poly alkenyl polyether bridges or cross-links that 
combine with common bases to make clear, viscous gels (Wolbers 
and Stavroudis 2012). Carbopol 934, 940, 941, and EZ-4 are 
common thickeners used to make conservation gels. As powders, 
respiratory irritation and inhalation toxicity are of concern, 
so careful and cautious handling will prevent aerosolization. 
Carbopols may form a combustible (explosive) dust air mixture, 
so be sure to handle in small amounts; do not let dust accumu-
late; minimize dust when vacuuming a dry spill or emptying a 
container; and keep storage containers tightly closed between use. Keep powders away from heat, sparks, open flames, or hot surfaces. 
Ethomeen C-25 and Ethomeen C-12 (both tertiary amine 
ethoxylates) are polymeric emulsifiers that function as weak basic 
detergents and form links with Carbopol to help form the gel 
(Stulik, et al 2004). Ethomeen C-25 is designated with a category 
4 acute oral toxicity hazard and a category 1 serious eye damage 
hazard. Ethomeen C-12 can cause severe skin burns, eye damage, 
and other health issues if not properly handled with appropriate 
personal protective equipment (PPE). 
Pemulen TR-1 and TR-2 are polymeric emulsifiers (high 
molecular weight copolymers of acrylic acid and a hydrophobic 
comonomer) that act as the gelling agents. They may be harmful 
if inhaled and cause eye irritation. Although Pemulen TR-1and 
TR-2 are chemically stable and the powder has a low dust explo-
sion risk, it would be wise to handle in small amounts and mini-
mize dust when vacuuming a dry spill or emptying a container. 
Although the experiment demonstrates that the use of gels 
slows evaporation, thus presenting less inhalation concern than 
using free solvents, gels should not replace prudent health and 
safety practices. 
Gels reduce the amount of solvent exposure in a 
given time, but the conservator will still be exposed during the 
duration of their use, therefore proper ventilation and PPE should 
still be used. 
Gelling polymers can be very specifically sensitive 
to solvent choice, and their longer contact time with a treatable 
surface requires adjustment in solvent mixture and treatment 
technique. The conservator should also consider using less toxic 
solvent alternatives, when possible. 
Always consult the Safety Data 
Sheets for any chemical that you are using.
===References: Health & Safety===
Dunn, M. 2015. Safe Gelling. AIC News 40(6): 13.
Stulik, D., Miller, D., Khanjian, H., Khandekar, N., Wolbers, R., 
Carlson, J. and Petersen, W C. 2004. Solvent Gels for the Cleaning 
of Works of Art: the Residue Question. Marina del Rey, CA: 
Getty Conservation Institute. 
Wolbers, R. and Stavroudis, C. 2012. “Aqueous methods for 
the cleaning of paintings.” In Conservation of Easel Paintings, 
Stoner, J H. and Rushfield, R (eds). London: Routledge Taylor 
and Francis Group, 501-523.
Safety Data Sheet, US. Lubrizol. Carbopol® 934 NF Polymer. 
Revised 10/14/2014
Safety Data Sheet, US. NoveonInc. PEMULEN* TR-2 
Polymeric emulsion. Revised 6/18/2004
Safety Data Sheet, US. The Personal Formulator. Pemulen 
TR-1. Revised 1/1/2014
Safety Data Sheet. Kremer Pigmente. Ethomeen® C 25. 
Revised 1/24/2012
Safety Data Sheet. Kremer Pigmente. Ethomeen® C 12. Revised 4/15/2014
Category:Health & Safety