Remy Dreyfuss-Deseigne described research related to mending methods for transparent materials using nanocellulose films. His research has been carried out with several institutional partners, at the National Library of France (BnF, Paris, France), Research Center for Conservation (CRC, Paris, France), French Museum of Cinema, and during his 2015-2016 NEA fellowship in paper conservation at the Conservation Center for Art & Historic Artifacts (CCAHA, Philadelphia, PA).
Remy opened with some images of difficult structural problems: torn gelatin windows, animation cells, and architectural drawings on tracing paper. He then introduced nanocellulose, explaining how it is made, what its properties are, and its potential for use in conservation.
His work focuses on one kind of nanocellulose, microfibrillated cellulose (abbreviated MFC). Nanocellulose materials are produced for a variety of uses in electronics and biotech, and are being researched and manufactured by several universities including in Grenoble, France and at the University of Maine.
Nanocellulose is produced by mechanically shearing wood to rip apart the fibers until they are nano in scale. Cotton, spruce and birch can all be used as sources for nanocellulose. The amorphous parts of the remaining cellulose structure are treated with acid in order to dissolve them, leaving highly crystalline fibrils. There is a lot of ongoing research into the production of nanocellulose in the nanotechnology, renewable materials, and sustainable engineering fields.
Nanocellulose fibrils vs. crystals, image from: https://3dprint.com/69012/american-process-and-ornl-say-nanocellulose-will-rival-carbon-fiber-for-3d-printing/
For conservation applications, Remy compared the properties of nanocellulose films to lightweight Japanese papers like gampi and kozo used to mend tears on translucent artworks. Nanocellulose is supplied as a gel that can be cast out by pouring into a petri dish and evaporating out the water, creating films that vary proportionally in thickness related to concentration. Remy’s research investigates its properties in combination with different adhesives, and its response to artificial aging tests (light, temperature and humidity) as well as mechanical strength tests.
He found that the nanocellulose films were thinner than papers but quite strong (nearly as strong as Gampi), and mostly behaved like cellulose, a good thing for their use as a paper conservation material. Most importantly, mends made with the thin films are practically invisible in regular and transmitted light. These mends were demonstrated on translucent slides with tears from the collection of the French Museum of Cinema (impressive work!). Ongoing testing will include further analysis of the material, e.g. pH and mechanical strength measurements and fungal resistance tests.
While this was the first time I had heard about nanocellulose it has many potential uses, and not just for mending translucent materials. As a biomaterial derived from renewable forestry resources, nanocellulose has gotten a lot of attention over the past five years for its potential in industrial applications. Given its high ratio of strength to weight it has great potential for use in fill materials of all types, and has already found applications in industrial 3D printing as a substitute for carbon fibers in composites. Since it is compatible with many adhesives, it may find wide-ranging applications in conservation. I am looking forward to hearing more about Remy’s ongoing research and thank him for the excellent introduction to an interesting material. You can learn more about Remy’s work at his website.
In January 2017, Sarah Nunberg (Conservator in private practice, Stockman Foundation Fellow at Pratt Institute) and Dr. Cindie Kehlet (Professor, Department of Math and Science, Pratt Institute) organized a four-day workshop on Nanotechnologies in Conservation at the Pratt Institute in Brooklyn, NY. Dr. Piero Baglioni and Dr. Rodorico Giorgi of the University of Florence Research Center for Colloids and Surface Science (CSGI) lectured on two EU-funded projects, NANOFORART and NANORESTART. The workshop focused on three of the technologies CSGI developed for conservation of cultural heritage: gels, microemulsions, and nanoparticle solutions. Conservators specializing in varied disciplines from institutions and private practices on the east coast attended the workshop.
CSGI has developed materials to address consolidation, cleaning and deacidification problems identified by colleagues in conservation, and continues to consult and collaborate internationally on a wide range of conservation projects. They have developed conservation products for cleaning and consolidation using nanotechnologies.[1]
Gels
Conservators in all specialities have adopted the use of gels and thickeners for controlled cleaning of artworks. In the United States, over the past decades Richard Wolbers and Chris Stavroudis (among others) have introduced and popularized viscosity modifiers made of natural or synthetic polymers (e.g. Xanthan gum, Pemulen, Klucel, Carbopol, Velvesil plus). These materials allow conservators more options for tailoring cleaning solutions and restricting penetration into porous substrates. [2] Viscous solutions can limit solvent volatility, increase contact time, and combine immiscible solvents to form stabilized emulsions. More recently, physical gels made of polysaccharides (e.g. agar, gellan) have been used as “containers” for aqueous and some solvent cleaning applications.
Testing nanogels with water for removing Wolbers’ artificial dirt applied to a painted surface mockup “Pollock.” image: Jessica Walthew
While a variety of gels and thickeners have been readily adopted for use in treatment, concerns remain about controlling their application (e.g. conforming to different surfaces), specifying pore size, controlling solvent release and eliminating residues. Gels made of crosslinked polymers in semi-interpenetrating networks – such as those developed and introduced by CSGI – offer new options in the conservator’s toolkit. Imaging and analysis conducted by Baglioni and others has worked to identify and localize residues, and confirmed that the hydrogels and organogels do not leave a residue due to their structural and physiochemical properties.[3]
At the workshop we worked with two types of CSGI hydrogels: chemical gels made of poly(2-hydroxyethyl methacrylate) (pHEMA)/ polyvinylpyrrolidone (PVP) formed by covalent bonds and physical gels made of polyvinyl alcohol (PVA)/ polyvinylpyrrolidone (PVP) or PVA/PVA formed with secondary bonds (dispersion forces or hydrogen bonds).[4] Hydrogels are compatible with aqueous cleaning solutions, some polar solvents, and microemulsions (i.e. oil-in-water). CSGI’s chemical hydrogels are rigid, clear sheets that are cut to the size of the area treated. The physical gels are similar to Jello in consistency and texture, but clear and colorless. Another class of gels being developed by CSGI, organogels, open up a wider variety of solvent options. [5] Depending on the desired characteristics, these gels can be engineered to have different properties (e.g. elasticity, solvent retention, solvent compatibility) based on the polymers used, synthesis procedures, and degree of crosslinking.
Microemulsions
Conservators often remove natural and synthetic adhesives and coatings from fine art surfaces by solubilizing the unwanted material. Solubilization risks incomplete removal, penetration into the substrate (especially if the substrate is porous), leaching out original materials, redeposition, and tidelines. Moreover, solubilization and removal of material must often be accompanied by mechanical action, which can damage sensitive underlying surfaces. Additionally, solubilization of aged coatings with neat volatile organic solvents often requires the use of polar, aromatic, or otherwise aggressive solvents that pose risks to the environment and human health.
Three of the Nanorestore microemulsion products available from CSGI. image: Jessica Walthew
Microemulsions are made from a micellar solution (a dispersion of surfactants formed when the concentration of surfactant exceeds a threshold value called the critical micellar concentration), in which a surfactant is used to contain a dispersed phase in a continuous phase (either water-in-oil or oil-in-water). Stable microemulsions can be extremely effective at cleaning because of the exponential increase in interphase surface area, where the cleaning activity occurs. As a result, smaller amounts of solvents are needed for highly effective cleaning solutions. Other microemulsion formulations have already been used in conservation for cleaning acrylic painted surfaces and plastics, as developed by Dow Chemical/Getty Conservation Institute/Tate. [6]
Several of the nanostructured solutions developed by CSGI can also work through dewetting (the opposite of surface wetting) instead of solubilization. In dewetting, the microemulsion activates and swells the polymer coating, forming a discrete layer that can be removed. Minimal mechanical action with a dry swab rolls off the swelled, dewetted polymer.
Acrylic overpaint removed from a test painting with a microemulsion demonstrating the dewetting mechanism. image: Cindie Kehlet
The CSGI microemulsions were initially developed for conservators working on wall paintings in Italy and Mexico that were deteriorating owing to aged acrylic and polyvinyl acetate coatings. The microemulsions can be used for removing synthetic polymers such as coatings or graffiti, but must be tested carefully for each application. Careful formulation is crucial: therefore it is important for conservators to work closely with CSGI to understand the product components and devise the best systems for their treatments.
As for the gels and viscosity modifiers described above, residues left behind from cleaning agents have been a major concern for those considering using emulsions and microemulsions for cleaning painted surfaces and plastics. For emulsions, high proportions of surfactants are sometimes needed to stabilize mixtures, and surfactant residues may attract dirt to the surface, or change surface gloss. Going forward CSGI and their research partners aim to identify self-degrading surfactants (that decompose without leaving a residue) for use in future conservation products. CSGI is also working to limit the amount of toxic solvents (such as methyl ethyl ketone) used in their microemulsions.
Dr. Giorgi showing a painting with test cleanings performed with gels and areas to test removal of acrylic overpaint. image: Jessica Walthew
Nanoparticles
Nanoparticles (0.1-0.2 um in size) in suspension can be used for a variety of applications where penetration into surfaces is desirable. The main applications of nanoparticle solutions in conservation are for consolidating carbonate materials (i.e. stones or frescoes) and deacidifying paper and canvas. Nanoparticles can be dispersed in alcohol to form stable solutions, called nanoconsolidants. These can be applied to stabilize surfaces in preparation for subsequent treatment steps. As an example, they can be used to pre-consolidate friable wall paintings before salt removal, as they do not interfere with subsequent treatments or mobilize soluble salts. These treatments reproduce the original physiochemical properties of the artwork by undergoing the lime-cycle carbonation process, allowing crystals to bridge gaps as they form, effectively reconstituting the same binder as the original paint (inorganic CaCO3). Another application is to counteract and prevent acid hydrolysis in cellulosic materials: spraying Ca(OH)2 particles onto paper or canvas can effectively adjust and neutralize the material’s pH. [7]
Workshop participants experimenting with nanomaterials. Pictured: Cathy Silverman, Fellow in Objects Conservation at the Metropolitan Museum of Art. image: Jessica Walthew
After learning about the chemistry and principles behind these materials through Dr.Baglioni and Dr. Giorgi’s lectures, workshop participants experimented using a variety of mock-ups and artworks. We tested aqueous cleaning of paper and painted surfaces with the highly retentive chemical gels, microemulsions applied with cotton poultices and/or hydrogels for the removal of coatings on terracotta and fresco surfaces, and removal of acrylic paint covering oil paint, as well as applying solutions of Ca(OH)2 nanoparticles for fresco consolidation. As always, each conservator needs to develop a sense of the working properties of any tool or material to see how they will be useful. Dr Baglioni and Dr. Giorgi admirably contextualized the need for these materials, the underlying chemistry and physics, and the particular benefits these nanotechnologies provide.
The final day of the workshop focused on a project investigating the materials and restoration history of a Louise Nevelson painted wooden sculptural installation at St. Peter’s Church in Manhattan. A variety of treatment options were considered and tested, including chemical gels. Sarah Nunberg will present on this project at AIC:“Treatment of a White Louise Nevelson Installation” in the General Session, You Can’t Go It Alone.[8]
Sarah Nunberg discussing the cleaning treatment of a sculptural environment by Louise Nevelson in the Chapel of the Good Shepherd at St. Peter’s Church. images: Jessica Walthew
Overall, the workshop was an extremely well-organized and exciting introduction to the vast applications of nanotechnologies in conservation. Dr. Baglioni, Dr. Giorgi, and their colleagues have published widely on their research, see the CSGI site for more information and selected references.
Thanks very much to Dr. Baglioni, Dr. Giorgi, the organizers, Pratt Institute, and the other participants for a wonderful and stimulating few days. Keep an eye out for the upcoming WAAC newsletter, which will feature a discussion of the practical use of these technologies.
[1] These materials are available for low cost directly from the University of Florence, per the arrangements of the EU funding source supporting this research: see the CSGI website (www.csgi.unifi.it). Reducing the environmental and human health impact is an important goal of CSGI’s projects. For an overview, see Giorgi, Rodorico, Michele Baglioni, Debora Berti, and Piero Baglioni. “New Methodologies for the Conservation of Cultural Heritage: Micellar Solutions, Microemulsions, and Hydroxide Nanoparticles.” Accounts of Chemical Research 43, no. 6 (June 15, 2010): 695–704. doi:10.1021/ar900193h.
[3] 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. See Domingues, Joana A. L., Nicole Bonelli, Rodorico Giorgi, Emiliano Fratini, Florence Gorel, and Piero Baglioni. “Innovative Hydrogels Based on Semi-Interpenetrating p(HEMA)/PVP Networks for the Cleaning of Water-Sensitive Cultural Heritage Artifacts.” Langmuir 29, no. 8 (February 26, 2013): 2746–55. doi:10.1021/la3048664.
[4] 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.
[5] Organogels are described in greater detail in Baglioni, P. et al. 2015. Organogel formulations for the cleaning of easel paintings. Applied Physics A 121 (3): 857–868. doi:10.1007/s00339-015-9364-0 and Piero Baglioni, David Chelazzi, and Rodorico Giorgi. Nanotechnologies in the Conservation of Cultural Heritage: A compendium of materials and techniques. Springer Netherlands. 2015. DOI 10.1007/978-94-017-9303-2
[6] See “Mineral Spirits-Based Microemulsions: A Novel Cleaning System for Painted Surfaces” Bronwyn Ormsby, Melinda Keefe, Alan Phenix, Eleanor von Aderkas, Tom Learner, Christopher Tucker, and Christopher Kozak, Journal Of The American Institute For Conservation Vol. 55 , Iss. 1, 2016. Note that for all microemulsions, the phase diagrams describing stable formulations can be complex, and it is difficult to formulate these in most museum labs.
[7] See Giorgi et al. 2010 (full citation in reference 1)
[8] This work will also be presented at the 2017 Gels in Conservation Conference in London. (Nunberg, S. C. Kehlet, S. Alcala, C. Tomkiewicz, C. McGlinchy, M. Henry, J. Dittmer. “Conservation of a White Louise Nevelson Installation: Gel Systems Explored”) and has been submitted for review for the 18th Triennial ICOM-CC Conference in Copenhagen: Nunberg, S., C. Kehlet, S. Alcala, C. Tomkiewicz, C. McGlinchy, M. Henry, J. Dittmer. 2017. Conservation of a White Louise Nevelson Installation: Treatment Choices Based on Ethical Discussions and Analytical Studies.
Need to subtly drop some hints for holiday gifts? ECPN has compiled a list of great gifts for conservators to make it easy for you. We posted our suggestions on our Facebook page last week and got some great additions from ECPN members, which we’ve added below. image: calobeedoodles
Lucy-Anne Skinner spoke about the conservation of human remains at the site of Abydos, Egypt. Working for the Institute of Fine Arts, New York University project, Lucy had to engineer some novel solutions when two unusual burials needed to be block lifted. Lucy explained that block lifting an entire burial is not common at Abydos, as most graves are fully excavated in the field. Two graves excavated in the 2012 season in the North Cemetery presented unusually complete assemblages of grave goods, with one wearing a beaded headdress over well-preserved hair, and these factors prompted the conservators to block lift rather than to try to treat the components in situ.
The team of workmen working with the conservator helped build a wooden frame and slide metal sheets underneath, so that the entire burial could be lifted. Foil was used as a barrier layer, and expandable spray foam was used to lock components into place so that the burial could be transported back to the lab and eventually (in another field season) flipped over to work from the bottom to stabilize the lower portion of the coffin before addressing the human remains and other components. Lucy showed many details of the extraordinary finds including beautiful hippo ivory clappers. Though this project began in 2012, conservation of the burials was only completed a few days before this talk was presented (really!). Political instability in Egypt and the extraordinary logistical challenges surrounding excavating in a country undergoing political and social upheaval complicated the timeline and created extra challenges for the conservators.
Transporting a block-lifted burial to the depot at Abydos, Egypt (image courtesy Abydos excavations, Institute of Fine Arts, New York University)
For me, the big takeaways were that conservation in the field requires a great deal of planning and then a lot of on-the-fly creativity. Many digs lack a field conservator, but clearly the planning and execution of this complicated project really benefited from having Lucy on site over the course of multiple seasons. The project took several years to complete, so communication and planning in the off-season was needed for a successful outcome. Particularly challenging issues of working within Egypt while in the middle of a period of crisis were dealt with admirably by Lucy and her colleagues.
I was excited to see the most recent update on VIL imaging as it is an accessible imaging technique that can be used to localize pigments with specific characteristics. It is useful for anyone interested in painted surfaces, and can be used in conjunction with other multispectral imaging, or as a standalone technique.
The basic idea is that you need a light source to produce visible light, a camera with its infrared filter removed, and a bandpass filter to limit the type of light that gets to the camera sensor, along with some standards to help process the images. The pigment particles on the object are excited in the visible range, and emit infrared radiation which is detected by the modified camera. This technique can be used to detect trace remains of pigments that are all but undetectable to the naked eye. The technique was developed by scientists from the British Museum and the Courtauld Institute (see Verri et al., 2009) [1].
In the case studies shown in Dawn and Anna’s presentation the focus was on Egyptian blue, which produces luminescence in the infrared (~910nm) when exposed to visible light. Optimizing the capture and processing protocols will mean better results and hopefully, a means of standardizing and sharing information between conservators working in different labs. While VIL is gaining popularity as more museums add it to their workflow (for example. as part of the APPEAR project spearheaded by the Getty), the technique is still being developed, with much more progress on the horizon. Dawn and Anna reported on results of a survey of VIL users to show where progress has been made and where we can still expect some improvements in the technique.
Click for larger image and to view transition! A gif showing VIL and normal illumination images of Brooklyn Museum’s portrait, Noblewoman, ca. 150 C.E. Encaustic on wood. Brooklyn Museum, Gift of the Ernest Erickson Foundation, Inc., 86.226.1, image courtesy Brooklyn Museum
Capture: varying light sources
There are many options for lights used for excitation, so choosing a light source that is targeted to your research question is critical. As an example, the authors described work by my classmate Brian Castriota showing that red LEDs with an output centered at 630 nm caused greater luminescence of Egyptian blue compared to white LEDs. More research on the luminescence characteristics of other pigments will help users optimize their light sources to target specific pigments.
Processing: calibration, standards, and protocols
While many conservators using VIL use the CHARISMA protocols (developed by the British Museum), others are using Photoshop to process the images. Egyptian blue VIL images are usually shown in monochrome, but as the technique is expanded different overlays or crossfades will help communicate the results by registering the images with other photographs, as Dawn and Anna did for the images shown in their presentation. This is one of the greatest advantages of VIL: it’s very easy to understand the images that are generated and easy to communicate the results to the public. However, capturing good metadata and using appropriate standards are critical for the intercomparability of these data in the future. It will be crucial to develop a luminescence scale or target in order to compare images from institutions who may not be using the exact same capture or processing parameters.
What do we have to look forward to?
While its initial development as a tool for identifying Egyptian blue has led to its popularity among archaeological conservators, it seems like the technique is ripe for more widespread adoption for research into modern pigments, some of which also have unique luminescence properties.
Conservators can use a variety of wavelengths using targeted or tunable light sources (e.g. the CrimeScope, adapted from the forensics field) to survey visible-induced luminescent pigments (other examples of which include dragon’s blood, Indian yellow, Han blue, cadmium red and yellow). Dawn and Anna showed an example of imaging surveying cadmium pigments used in Stuart Davis’s Mellow Pad carried out by their Brooklyn colleague Jessica Ford. For more on the work from the team at the Brooklyn Museum, see their recent blog post here.
References.
[1] Verri, Giovanni, et al. “Assyrian Colours: Pigments on a Neo-Assyrian Relief of a Parade Horse.” The British Museum Technical Research Bulletin 3 (2009): 57–62.
