# Sustainable Preservation of Photographs in a Hot and Humid Climate: Dry Cabinets and Metal–Organic Framework Paper Composites

Bertrand Lavédrine, Anne-Laurence Dupont, Pierre Tignol, Christian Serre, Vanessa Pimenta, Moisés Pinto, Abeer Al Mohtar

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## ► To cite this version:

Bertrand Lavédrine, Anne-Laurence Dupont, Pierre Tignol, Christian Serre, Vanessa Pimenta, et al. Sustainable Preservation of Photographs in a Hot and Humid Climate: Dry Cabinets and Metal–Organic Framework Paper Composites. Studies in Conservation, 2024, 69 (sup1), pp.190-197. 10.1080/00393630.2024.2364997. hal-04834726

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# Sustainable Preservation of Photographs in a Hot and Humid Climate: Dry Cabinets and Metal-Organic Framework Paper Composites

## Preservación Sostenible de Fotografías en un Clima Cálido y Húmedo: Gabinetes Secos y Compuestos de Papel con Estructura Organometálica

Bertrand Lavédrine¹, Anne-Laurence Dupont¹, Pierre Tignol¹,², Christian Serre², Vanessa Pimenta², Moisés L. Pinto³ and Abeer Al Mohtar³

¹Centre de Recherche sur la Conservation, Muséum national d'Histoire naturelle, Paris, France; ²Institut des Matériaux Poreux de Paris,

ESPCI Paris, Ecole Normale Supérieure, PSL University, Paris, France; ³CERENA, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal

https://doi.org/10.1080/00393630.2024.2364997

### ABSTRACT

In a project to preserve photographic archives in Southeast Asia, the authors sought to develop sustainable solutions with low environmental impact, avoiding air conditioning and using locally available materials. Dry cabinets were tested and implemented to store a glass plate negative collection that was rehoused in envelopes and boxes made locally. The project took just over a year to complete, and two years of monitoring environmental conditions have shown that relative humidity can be stabilized at low levels. However, the dry cabinets are airtight to limit the introduction of moisture and dust, preventing air exchange. In order to avoid the build-up of degradation by-products released by the stored materials, and in particular off-gassing of acetic acid from the cellulose acetate-based photographs, the use of an adsorbent is necessary. To achieve this, a cellulose-based composite with particles of a porous hybrid inorganic/ organic solid, denoted as a metal-organic framework (MOF), at a very high loading over 70% by weight and shaped as a sheet of paper, has been prepared and tested. This special paper maintains acetic acid-free air in the cabinets and also helps to reduce the energy consumption by reducing the need for cool storage. This MOF paper composite traps small carboxylic acids and was found to be an appropriate solution for maintaining clean air in the dry cabinets or any other confined space or enclosure. Additionally, it can also be designed to trap other noxious pollutants such as hydrogen sulfide or formaldehyde, by choosing other MOFs.

En un proyecto para preservar archivos fotográficos en el sudeste asiático, los autores buscaron desarrollar soluciones sostenibles con bajo impacto ambiental, evitando el aire acondicionado y utilizando materiales disponibles localmente. Se probaron e implementaron gabinetes secos para almacenar una colección de negativos en placas de vidrio que se realojó en sobres y cajas fabricados localmente. El proyecto tardó poco más de un año en completarse, y dos años de monitoreo de las condiciones ambientales han demostrado que la humedad relativa se puede estabilizar en niveles bajos. Sin embargo, los armarios secos son herméticos para limitar la introducción de humedad y polvo, impidiendo el intercambio de aire. Para evitar la acumulación de subproductos de degradación liberados por los materiales almacenados y, en particular, la liberación de gases de ácido acético de las fotografías a base de acetato de celulosa, es necesario el uso de un adsorbente. Para lograr esto, se ha desarrollado un compuesto a base de celulosa con partículas de un sólido híbrido inorgánico/orgánico poroso, denominado estructura metal-orgánica (MOF), con una carga muy alta superior al 70% en peso y con forma de hoja de papel. preparado y probado. Este papel especial mantiene el aire libre de ácido acético en los gabinetes y también ayuda a reducir el consumo de energía al reducir la necesidad de almacenamiento en frío. Este compuesto de papel MOF atrapa pequeños ácidos carboxílicos y aparece como una solución adecuada para mantener el aire limpio en gabinetes secos o cualquier otro espacio o recinto confinado. Además, también se puede diseñar para atrapar otros contaminantes nocivos como el sulfuro de hidrógeno o el formaldehído, eligiendo otros MOF.

### Introduction

Photographs are particularly sensitive to high relative humidity (RH), and storing them in a hot and humid climate leads to irreversible degradation ultimately

leading to their total destruction. Thus, the international standards and guidelines for long-term preservation recommend to store them in a dry and cool to cold environment. In a tropical climate, adopting such conservation conditions requires the implementation
of air conditioning, resulting in high cost and maintenance issues as well as a significant carbon footprint. The authors were faced with such a dilemma in a campaign for preserving a collection of silver gelatin glass plate negatives and 35 mm colour slides from the Department of Archaeology and National Museums in Yangon (Myanmar). This collection has great significance, as it holds images from important places and objects in Pagan and from across Myanmar captured during the first half of the twentieth century. Besides their artistic and aesthetic qualities, they have a high documentary value. The monuments in Pagan, dating from the eleventh to the fourteenth centuries, suffered major damage and destruction due to earthquakes in 1975 and 2016. This unique photograph collection may provide invaluable information for the restoration of the Pagan monuments. Unfortunately, these photographs have been subjected to a tropical climate for more than 70 years with the glass plate negatives exhibiting image layer degradation while the colour slides show colour fading. A preliminary survey of the glass plate negative collection indicated that the main degradation factor was the excess of moisture that promoted mould growth, and hastened the formation of silver mirroring and varnish deterioration. The priority was therefore to keep the collection in a drier environment and to rehouse it in good quality enclosures to ensure long-term preservation and provide improved access and use. The first part of this article focuses on the survey and care of the glass plate negative collection, in particular through installing commercially available dry cabinets. The second part evaluates how to extend the use of these dry cabinets to the storage of photographs on cellulose acetate supports by implementing an acetic acid adsorbent.

## Implementation of a dry environment

International standards for long-term storage of glass plate negatives recommend a temperature of 18°C and an RH of 30-40% (ISO 2000). Lowering the temperature was not easily achievable in the context of this project because it requires a heating, ventilation, and air conditioning (HVAC) system with maintenance issues and high energy consumption. On the other hand, ensuring an RH level of between 30 and 40% with a low carbon footprint seemed achievable. Silica gel is often used as a passive means to control RH by placing a certain quantity in a cassette in showcases or in cabinets but in humid climates, it saturates quickly and must be regularly regenerated, which requires constant involvement. Dry cabinets are widely used to protect photographic equipment from fungi or for storing moisture-sensitive electronic components or even for entomological collections (Eureka Dry Tech 2019). Many models produced in

India, China, Korea, or Taiwan are commercially available in Asia and beyond. These are simple and robust. In addition, they have the advantage of a low power consumption (a 60 L cabinet requires 5 W of power) and low maintenance requirements. They are made of metal with airtight glass doors and are available in different sizes (30-600 L). They include a small in-built dehumidification unit: either a Peltier cooler system, which removes moisture from the air by condensing it out on a cold surface, or an automatic regeneration desiccant unit. The desiccant, such as a zeolite, is regularly automatically reactivated with a hot air flow. The tightly closed cabinet doors ensure stable RH inside by limiting access to external moisture. This also prevents dust, dirt, insects, and rodents from entering.

## Assessing dry cabinet performances

Preliminary tests were carried out at the National Museum of Natural History in Paris to evaluate the cabinet's performance. A small dry cabinet (Sirui HC-70®, 70 L) was acquired, set to 35% RH, and filled with paper documents (books, photo albums) that had been conditioned for a few days at 90% RH at room temperature. Temperature and humidity inside the cabinet were monitored using a datalogger during the summer in Paris with the room RH over 50% and temperature fluctuating from 25°C to 30°C. When the moist documents were introduced inside the cabinet, the RH inside rose to 70%. The unit then regularly removed a small fraction of water from the air inside the cabinet. For each drying cycle, the RH decreased by less than 1% until it reached the set value (35% RH). Each step-wise drop in RH was accompanied by a brief rise in temperature due to the operation. It took eight days before the RH within the cabinet dropped from 70% to 35%. Once the RH inside the cabinet had reached the set RH level, the buffer in effect of the material inside the cabinet and the airtight insulation helped maintain the humidity level in a passive mode for a few days. When the RH inside rose over 37%RH due to air infiltration a new series of drying cycles started, which lasted a few hours until the RH dropped to the set RH level. To maintain the RH stable inside the cabinet, a total of 2 days of operation was sufficient over the following 22 days. Following these preliminary tests, six Ruggard® electronic dry cabinets (600 L) were acquired to store the 5,500 glass plate negatives. The padding made of synthetic polyurethane foam shelf was removed and the target RH set to 40% for each cabinet. The cabinets were then filled with the glass plate negatives collection over the period from January 2020 to December 2020 (Figure 1). Over the next two years of monitoring, the RH stayed very stable (Figure 2) in all six cabinets. We noticed some short-term high RH peaks, probably due to the
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Dry cabinets with the collection of glass plate negatives, after re-housing at the Department of Archaeology and National Museums of Myanmar in Yangon.</figcaption></figure>

opening of the cabinets when working on the collection. When comparing the RH inside the cabinet to the ambient RH, the improvement is impressive (Figure 2); even during the monsoon season (June to October) when the ambient RH reached over 80%, the RH inside the cabinet remained stable around from 35 to 40%, depending on the cabinet. After one year's operation, the RH set point was fine-tuned in some cabinets to get as close as possible to 40% RH (Figure 2). Considering their limited and discontinuous power consumption requirements, a further future improvement would be to power the cabinets using photovoltaic cells.

## Locally produced enclosures

### Envelopes for glass plate negatives

The original paper envelopes had induced damages on the glass plate negatives and it was necessary to

replace them with a suitable housing while keeping them separate as historical evidence. The use of synthetic polymer envelopes, either polyethylene or polyester, was excluded due to the risk of adhesion to the gelatin layer and biological degradation resulting from the moisture inside. Paper is water-vapor permeable, and depending on its thickness, it acts not only as an RH buffer but also as cushioning, mitigating uneven pressure between plates and eventually absorbing shocks. Paper used for photographs preservation has to be 'photo safe' (ISO 2013) so as not to promote further image degradation. Initially, attempts were made to find locally produced good quality paper such as that produced in Luang-Prabang (Laos) for the preservation of the Buddhist Archives (Jürgens 2015). Local mulberry paper used to make umbrellas in the northeast of Myanmar failed the photographic activity test (ISO 2007). Another issue with this paper was its coarse and uneven texture. There was a risk of abrasion of the photographic image layer and the irregular contact with the image may have generated a specific degradation pattern. In the end, paper enclosures had to be imported from abroad. Photo-safe four-flap envelopes were expensive (averaging $5US each) and not available in all formats so Chronos® paper sheets (79.2 cm × 121.9 cm) were bought in Europe instead. By optimizing the cutting, only 1300 sheets were needed to house the whole collection for a final average cost of less than $0.5US per envelope. Making a traditional four-flap envelope leads to the loss of 4/9 of the paper sheet after cutting. To avoid such waste, the four-flap envelopes were made by cutting two strips of paper and sliding the smaller one into two slits in the larger one (Figure 3(a)). The local team quickly mastered their making, how to place the plate inside image side up, and how to sequence the closing of the flaps carefully.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** RH inside dry cabinets (dark blue and red for two different cabinets) and in the room (turquoise) for 2 years, beginning in 2020. The frequent peaks in RH are due to the doors being opened. In December 2020, the set point in the cabinets was slightly increased to 40% RH.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Local production of photographic enclosures. (Left) Four-flap envelopes made with 2 paper strips. (Right) Making boxes in the background, the old storage cabinets can be seen.</figcaption></figure>

## Boxes for glass plate negatives

Originally, the plates were not housed in boxes: the smaller formats (dimensions in cm: 16.5 × 21.6, 12 × 16.5, 9.5 × 10.8) were stored vertically on the shelves while the larger plates (20.3 × 25.4 and 25.4 × 20.5) were stored flat, separately. It was decided that containers would offer extra protection and allow a safe retrieval and handling of the plates. No conservation boxes were available locally. The easiest and cheapest option was to make them from locally-available 2.8 mm thick fluted polypropylene (PP) sheets. Fluted PP boxes (or E-flute corrugated PP) (Seo, Shin, and Kim 2009) are light and mechanically resistant enough for holding heavy glass plates. They absorb shocks and are cheaper than conservation-quality cardboard boxes. Furthermore, they are not sensitive to water and hence to bio- deterioration. Two types of foldable boxes were produced for less than $1.5US each: one flat box inspired by the Metropolitan Museum of Art (New York, USA) model and one vertical box. Within a few days, the team was skilled at producing these boxes (Figure 3(b)). For some boxes, hot melt polyethylene adhesive was applied with a glue gun to secure certain parts. In that case, the upper layer of the E-flute PP was scratched to let the glue penetrate into the fluted structure and reinforce cohesion. Finally, the repackaging process could start. The three smallest plate formats were housed vertically along their long edge in the 24.5 × 19.0 × 9.5 cm boxes, while the largest plates were kept flat in the 26.0 × 30.4 × 5.0 cm boxes. For preservation it would have been easier to group the plate formats according to their format, thus limiting the risks of mechanical damage that may occur if boxes contain different plate formats (i.e. due to uneven pressure and possible movement of the plates). However, re-introducing a new numbering

system would be challenging and add complexity as well as a risk of dissociating negatives of different formats but belonging to the same series, so the order of the plates has been preserved in the boxes. These boxes therefore were filled with a limited number of plates (ca. 20) in various formats and when a mechanical risk was identified, for example with the proximity of two plates of very different formats, cushioning made of a fluted PP sheet was inserted between the two.

## Further perspective for storage of acetate films in dry cabinets

As mentioned earlier, the collection also contained 35 mm chromogenic colour slides on cellulose triacetate supports dating from the 1960s. These colour slides showed colour fading and would have benefited from storage in the dry cabinets to slow down the degradation of the dyes and also that of the cellulose acetate base (Reilly 1993). However, it is not advised to store cellulose acetate films in airtight containers since the acetic acid released by the hydrolysis of cellulose acetate builds up and catalyzes its own degradation as well as that of the surrounding photographs (Reilly 1993; Al Mohtar et al. 2021). As a result, these cellulose acetate-based films were not stored in dry cabinets but were left in their original condition for the time being, until a means of preventing the accumulation of acetic acid could be found. The use of zeolites as adsorbents proposed by Kodak (Ram et al. 1993) is less effective, as they are very hydrophilic and absorb more moisture than acetic acid. As for activated carbon, it can release adsorbed compounds. The solution emerged from collaborative work on acetic acid capture carried out as part of the European Union-funded *Nemosine* project.
## Making a metal-organic framework paper composite

We have evaluated the possibility of using metal-organic frameworks (MOFs). This new class of porous materials has been developed to selectively adsorb certain gases. They are synthesized in the laboratory by bonding metal cations to organic molecules (e.g. carboxylates), producing an ordered porous hybrid structure. The number of possible combinations of organic bonds and metal cores is very large, making it possible to obtain a wide range of structures with adjustable pore size and chemical polarity. Out of all the MOF compounds tested, the mesoporous Fe(III) tricarboxylate MOF denoted MIL-100(Fe) ('MIL' stands for 'materials of the Institute Lavoisier') was shown to be very efficient in adsorbing acetic acid at a moderate cost (Dedecker et al. 2019; Severino et al. 2023). It can be produced by a green synthesis process, at room temperature, by mixing non-toxic compounds in water (iron nitrate and trimesic acid). The MIL-100(Fe) obtained has a porous crystalline structure with a Brunauer-Emmett-Teller (BET) surface area exceeding 1500 m²g⁻¹ (Tignol et al. 2024). When exposed to acetic acid vapors in the presence of variable humidity, MIL-100(Fe) strongly adsorbs the acetic acid on its constitutive open metal sites (Lewis acid sites) with little or no desorption. The release of the trapped gas is achieved by heating the MOF at a temperature above 150°C (Severino et al. 2023). One outstanding feature of MIL100(Fe) is that water adsorption does not compete with acetic acid adsorption, the latter being thermodynamically favoured. MOFs are synthesized as powders, and shaped to form granules or beads. However, handling beads or granules is not very practical in heritage institutions, so they have to be packed in bags or cassettes, with a risk of perforation. We have therefore investigated a shaping that produces a paper-based composite that retains the mechanical properties of the paper and the sorption properties of MOF; this should be more practical for use in galleries, libraries, archives, or museums. Such shaping ensures the structural integrity of the constituents while allowing controlled airflow and efficient purification. A paper sheet is also a well-known and universally used material, easy to store, to handle and fully customizable to fit the shape of all conservation enclosures, whether envelopes, boxes, display cases or frames. The heritage conservation community is familiar with a type of adsorbent sheets made of a cellulose mixed with 20% zeolites, which was popular in the early 2000s (MicroChamber®) (Daniel et al. 1999). Such material can be more efficient at last longer if it contains a higher loading of an appropriate adsorbent. Our aim was then to load as much MOF powder as possible into a sheet of paper. However, incorporating more than 30% fillers into a cellulosic

substrate adversely hampers its mechanical properties due to the disruption of the fibre network and fibre-to-fibre bonding (Mintova and Valtchev 1996). In order to produce a sheet of paper with very high filler content while maintaining sufficient mechanical properties, bleached softwood kraft pulp fibres were mixed with microfibrillated cellulose (MFC), also called cellulose nanofibres (CNF). The CNF are made of nanometre-wide and up to several micrometres-long cellulose microfibrils (Thomas et al. 2018). In the composite, they act as a nanostructuring material. Such an entangled network of fibers and microfibrils allows satisfactory mechanical properties despite the very high MOF loading. The ultimate tensile strength (UTS), corresponding to the maximum stress that a material can endure when pulled before breaking, is around 1.5 MPa for a 75 wt % MOF paper sheet, and such a sheet can still easily be handled without breaking. The MOF paper sheets were prepared by vigorously dispersing 6.25% of softwood pulp fibers, 18.75% of NFC (Celova®, Weidmann), and 75% of MIL-100(Fe) powder in water and then filtering the suspension under vacuum on a fast filtration Whatman filter paper or a nylon fabric. Similarly, composite paper sheets loaded with as much as 90% of MIL-100(Fe) or with other porous materials other than MOFs, namely activated carbon, and zeolites, were successfully produced (Figure 4). The fibrous matt was then pressed to extract the excess of water and dried under weights and between blotting paper. Characterization and tests described below were carried out with the MOF paper loaded with 75% MIL-100(Fe). The MOF sheets exhibited a grammage of 120–140 gm⁻² and can be used as such or after activation. Activation (8 hours at 150°C under vacuum) allows all the pores to be clean before use so as to adsorb slightly higher amounts of acetic acid than the non-activated ones. The properties and morphology of the new composite were assessed using nitrogen porosimetry (Figure 5) and scanning electron microscopy (Figure 6). This MOF paper sheet exhibited equal performance to that of the native MIL-100(Fe) powder, while significantly outperforming

<figure><img src="image_6.png" /><figcaption>**Figure 4.** A 83% MIL-100 paper composite sheet. In the background, a similar composite paper loaded with activated carbon.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 5.** N₂ adsorption-desorption isotherms at 77 K for MIL-100(Fe) paper sheet.</figcaption></figure>

the more traditionally shaped products such as beads or granules, even those containing 85–95 wt % MOF. Actually, the binder used for shaping them results in blocking some pores of the MOF, but this was not observed in MOF paper (Tignol et al. 2024).

## Use of the MOF paper composite for photograph preservation

To ensure that the MIL-100(Fe) paper sheet could be safely placed in the vicinity or in contact with artefacts, the composite was submitted to the photographic activity test. Another compatibility test was carried out to verify that no residual volatile compounds emitted by it could have a deleterious effect on cellulosic artefacts in its vicinity. The test was done by measuring the degree of polymerization (DPv) of cellulose of Whatman no.1 test paper after artificial aging (21 days) in the same vial as the composite, and comparing to the DPv of a Whatman no.1 reference paper aged under the same conditions but without the composite (ISO 2020). The MOF paper successfully passed the two tests and *in situ* validation tests were final carried out to verify the efficiency of MIL-100(Fe) under real acetate film storage conditions in a fil archive (Al

Mohtar et al. 2023). In laboratory tests, MOF paper proved as effective as MOF powder in trapping acetic acid vapours produced by an acetic acid solution. To verify this in real conditions, we selected movie films suffering from vinegar syndrome. The VOCs emitted from movie films were first assessed by connecting the film container to a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) (Kammer et al. 2021). Acetic acid was the major compound detected, representing 81% of the VOCs off-gassing and being emitted at a rate of 779 ngs⁻¹. This test was repeated, but the off-gassing was passed through a filter holder containing a piece of MOF paper, acetic acid and more than 30 different VOCs emitted from cellulose acetate film were captured by the MOF until its saturation. An experiment carried out at the Valencia Film Archive (Spain) showed that when MOF was added to a movie film can, the concentration of acetic acid dropped rapidly from around 2.2 pm to 0.2 ppm. MIL-100(Fe) will avoid cross contamination between good condition cellulose acetate films and degraded films and will significantly extend their life span. A recently published theoretical model shows that using 0.1 kg of MIL-100(Fe) paper per kg of acetate film at 22°C for 10 years should have the same benefit as storing it in a cold room at 5°C (Al Mohtar et al. 2023). After 10 years, the composite may become saturated and should be replaced. The MIL-100 MOF powder or paper can be regenerated by immersion in purified water at room temperature. This simple procedure extracts the acetic acid trapped in the pores without affecting the mechanical stability of the sheet and allows it to be reused (Tignol et al. 2024).

## Conclusion

The photograph collections of the Department of Archaeology and National Museums of Myanmar was an excellent case study for implementing an

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Optical microscopy (left) and mapping with energy-dispersive X-ray spectroscopy (right) of a 75 wt % MIL-100(Fe) paper surface. On the scanning electron micrograph (right) the softwood cellulosic fibers are shown in black and the iron (MIL-100) in brown. The fine nano-fibrillated cellulose fibers are not perceptible at this magnification.</figcaption></figure>
innovative conservation strategy. As it was not possible to apply the standardized recommendations, compromises had to be made that relied on local materials and resources, collection assessment, and the training and commitment of local staff. The implementation of dry cabinets has proven to be very efficient for maintaining a stable RH level. Inside the cabinets, the RH is stabilized at an average 40% RH with low standard deviation (maximum of 5%), whereas outside the cabinets it fluctuates from about 40% to over 80% during the monsoon. This preservation solution was quick to implement and had multiple benefits. It has allowed a radical and lasting improvement in the conservation of the photograph collection, without having to resort to the traditional approach using heavy-duty solutions, such as HVAC, which would probably not have been as successful anyhow, given the political and epidemiological situation that arose shortly after the start of the project. To extend the use of such dry cabinets to the storage of the cellulose acetate films, the addition of a new type of porous sorbent was investigated and the iron carboxylate MOF denoted MIL-100(Fe) proved to be capable capturing selectively traces of acetic acid. It opens up the possibility of conserving acetate films or other materials that emit carboxylic acids all together at room temperature expanding the life expectancy of cellulose acetate films and avoiding the risk of cross contamination. Al Mohtar et al. (2023) calculated that this might also results in an average reduction of the carbon footprint related to acetate films preservation of about 50% in the current European Union (EU) energy mix policy, and by about 40% considering the 2030 EU energy mix (where a transition towards renewable energy is expected). The ability to shape adsorbents into highly charged paper sheets allows to foresee a wide range of possibilities, enabling specific pollutants to be targeted just by choosing the most appropriate adsorbent or adsorbents blend. In addition to MIL-100(Fe) for capturing carboxylic acids, other MOFs are suitable for the efficiency capture or conversion of various indoor museum pollutants. For example, Sadovnik (2021) uses the microporous Al pyrazole dicarboxylate Al-3,5 PDA to capture selectively traces of formaldehyde in the presence of humidity without any release before 50°C. Martínez-Ahumada et al. (2021) also proposed the use of different MOFs to trap or convert SO₂, H₂S, NH₃, NOx, while Sun et al. (2021) report a manganese-based MOF for high-performance catalytic ozone removal. It shall therefore be possible to produce a whole range of paper sheets loaded with a high loading of specific MOF powders to individually target specific pollutants. Then, depending on the air quality to be achieved or the sensitivity of the objects to be preserved, the collections manager might assemble different MOFs-

loaded sheets. This is expected to be highly valuable in preserving collections as well as improving air quality for human health. As mentioned, the sheets might be used as passive sorbents in envelopes, boxes, cabinets, display cases. They could also be used for air dehumidification or in cooling systems in HVAC units (Feng et al. 2018) as they require less energy (<80°C) to be dried compared to the traditional mineral porous compounds. The proposed approaches therefore represent promising solutions for effective and more sustainable heritage preservation, while making a significant contribution to reducing the carbon footprint in the cultural heritage sector.

## Materials

Celova®, Weidmann, https://weidmannfibertechnology.com/

Chronos® paper 100% pure, acid-free alpha-cellulose with an alkaline buffer, pH 8.5, and n o o p tical brighteners. It passed the photographic activity test: 90g/m2. https://secan.fr/produit/blanc-naturel-ph-85/

Ruggard® electronic dry cabinet. https://www.ruggard.com/product/11837/Ruggard-EDC_600L-Electronic-Dry-Cabinet-(Black,%20600L)

Sirui® HC-70 Dry Cabinet https://sirui.in/product-category/dry-cabinets-dehumidifiers/

## Acknowledgements

The project was carried out within the framework of an international collaboration between the UNESCO offices in Thailand and Myanmar, the Department of Archaeology of Myanmar and the Centre de Recherche sur la Conservation in France. The management team included Andrew Henderson and Misako Ito (UNESCO Thailand), Naing Naing Aye (UNESCO Myanmar), and Daw Nyo Nyo Than (Yangon Department of Archaeology). We would like to thank the following people for their support: Daw Ohnmar Aung, Director of the Department of Archaeology and National Museum, Yangon Branch Office, Ms Chaw Su Su Hlaing Department of Archaeology and National Museum, Yangon Branch Office, Ms Min Jeong Kim, Head of the UNESCO Myanmar Project Office and U. Kyaw Oo Lwin, Director-General of the Department of Archaeology and National Museums, for their support; Christine Bastard, at the National Museum of Natural History, Paris, for cutting paper strips; Martin Jürgens, at the Rijksmuseum, Amsterdam, for his work in Luang Prabang; Nora Kennedy and Georgia Southworth, at the Metropolitan Museum of Art, New York, for their model of a foldable box; Connie McCabe, at the National Gallery of Art, Washington, for our inspiring discussion; Kamonwan Petchot, at UNESCO, Bangkok, and Seng Hkawn, UNESCO, Myanmar, for the logistics; Pierre Pichard, at EFEO, for providing information on Pagan; Sabrina Paris and Léon Vilmont, at the CRC, Paris, for analysing paper fibres; Tram Vo, Los Angeles, for ensuring preservation training; and the Republic of Korea, through the National Archives of Korea, for its generous funding.
Disclosure statement

No potential conflict of interest was reported by the author(s).

Funding

The work on MOFs has received funding from the EU Horizon 2020 research and innovation program under grant agreement no. 760801\NEMOSINE.

ORCID

Bertrand Lavédrine http://orcid.org/0000-0002-9710-3895

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