IAQ in Museum Display Cases: History
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Subjects: Environmental Sciences
Contributor:
Oscar Chiantore

The control of air quality in museum showcases is a growing issue for the conservation of the displayed artefacts. Inside an airtight showcase, volatile substances may rapidly concentrate and favor or directly cause the degradation or other unwanted phenomena on the objects. The role of materials used in the construction of museum display cases as a source of pollutants and volatile compounds dangerous for the cultural heritage integrity is here reviewed with an illustration of consequences and critical damages. Ways of assessing the suitability of materials used either in the construction or in use of the display cases are also discussed altogether with an overview of the possible choices for monitoring the air quality and limiting the concentration of volatile compounds in their interior.

  • indoor air quality
  • display cases
  • volatile organic compounds
  • pollutants
  • emissions
  • preventive conservation
  • heritage science

1. Introduction

The quality of air in the museum environment is of primary importance in determining conditions appropriate for the collections conservation. It is widely acknowledged that in museum, galleries, and archives, the building materials, furnishings, and technical equipment can act as important sources of chemical emissions capable of interacting with the works of art inside [1][2]. Materials, either traditional or modern, may act as a source of indoor pollutants, and this fact with all related consequences has been recognized for a while in the museum environment [3].

The interactions between chemical compounds conveyed in air and the artwork materials may take place in different ways, more generally through reactions eventually fostered by the temperature and relative humidity conditions, together with the contribution of dust and pollutants interactions finally show up in the form of disfiguring effects on the surfaces and of materials degradation.

In museums, specific and more demanding situations are represented by containers such as storage boxes, cabinets, drawers, and display cases, where the emission of volatile chemical compounds from constituent materials may seriously harm the items therein sheltered. In all of the above enclosures, the air exchange rate is more or less severely restricted with the consequence that compounds that originated from the local emissions will easily accumulate in the close environment until reaching concentrations capable of inducing damages to the objects. Current museum guidelines consider that well-designed display cases are not only intended to provide an object’s visibility and physical protection, but they are indeed the tools for the primary environmental control of the objects [4]. High-performance display cases must exclude the ingress of dust, pollutants, and insects and form a buffer to ambient temperature and relative humidity changes. They are classified according to their air exchange rates: low rates of exchange allow for high relative humidity stability against reasonable temperature changes and therefore reduced amounts of humidity buffer and pollutant adsorbents become necessary, making in such a way the maintenance more affordable [5][6].

Museum display cases are labeled conservation grade when they guarantee 0.1, or less, air change per day. With such air tightness values, gases and pollutants originated by the construction materials of the display case may become a problem when their concentration builds up and reaches values dangerous for the exposed objects. When items to exhibit are less demanding, i.e., more chemically inert or for temporary exhibitions, display cases with air exchange rates of 0.25 or 1 day−1 are also employed. One advantage of showcases without absolute air tightness is that volatile products emitted can disperse on the outside [5]. From the preventive conservation point, whether it is more advisable to have museum display cases completely sealed or leaky is still a question of debate among conservators, scientists, and curators—without even considering what can be the contribution of products offgassed by the objects themselves.

2. Effects of Volatile Emissions in Display Cases and in Other Museum Enclosures

A variety of detrimental effects may occur on objects, from the corrosion of metals to stone deterioration to the degradation of organic materials, depending on the type of VOC and the susceptibility of material types; in some cases, a merely aesthetical damage is produced that is not directly harmful for the object but nevertheless making it necessary to open the show case and undertake a cleaning operation. The most common visible appearances on museum objects and surfaces are efflorescences, salt crystallizations, and hazes.

Crystalline efflorescences on shells in museum collections were found to be mixed acetate/formate salts. The acetic and formic acid vapors derived from wooden storage cabinets with hardwoods, particularly oak, emit the highest concentrations [7]. The production of formic acid from wood is generally much lower than that of acetic acid, and its source is less well understood. It might come from the hydrolysis of formyl groups in hemicelluloses, although a more important route is attributed to the oxidation of wooden originated formaldehyde. Formaldehyde in wood is formed to different extents from cellulose, hemicelluloses, and lignin components depending on the moisture conditions, temperature treatments, and processing procedures [8].

The role of acetic acid as a corrosion agent against many different types of materials in museums has been broadly investigated. Typical examples are acetate efflorescences found on bronze antiquities, lead artifacts, and copper alloys stored in wooden cabinets [9][10][11][12]. The acetic acid emissions could be attributed not only to the wood but also to the adhesives and varnishes used to fabricate the cabinets.

Several different acetate salts have been frequently observed as efflorescences on calcareous objects stored in museum cabinets and display cases: not only pure calcium acetate, but mixed salts where the acetate forms phases with calcium chloride and nitrate in different hydrated forms [13][14]. Similar crystals of hydrated calcium acetate–chloride were identified on Greek ceramics stored in cherrywood display cases [15]. On porous calcareous objects, the chloride presence is due to the archaeological burial environments from which the objects originate.

In a museum collection, dramatic efflorescences and damages were observed on lead and tin glazed ceramic tiles of various origins and dates [16]. Analysis of the salts revealed the presence of a complex calcium acetate–chloride–nitrate hydrated structure formed on the surface of the tiles after reaction with acetic acid vapors from the storage environment. The concentrations of gaseous pollutants were measured in several display cases and halls of the museum, and in the cases, they were fairly high. From the efflorescence compositions, it could be concluded that they were formed as result of the interaction between the acetic acid vapor from the wooden storage and the salt contamination, chlorides, and nitrates, which are usually present in the tiles [17][18].

Other types of efflorescence salts on calcareous historic objects stored in wood cabinets have been also described. White crystals were collected from four different objects in three museums under different environmental conditions (relative humidity, temperature, furniture, etc.) [19]. On ancient archaeological ceramics and on a copper alloy bowl, formate mixed salts were present in addition to acetates. On objects stored under different conditions, a white needle-like salt was identified as hydrated calcium acetate–formate–nitrate. Although hard- and softwoods emit 3 to 10 times more acetic acid than formic acid, the crystal backbone structure was found to be based on formate ions. It was concluded that aggression by formic acid happens to a much larger extent than by acetic acid as a consequence of the higher strength of formic acid [20].

Corrosion effects produced by wood acidic emissions in museum collections have been well documented also on metal objects [21]. The active corrosion of metals, in particular lead and lead-containing alloys, takes place in the indoor environment of museums with acetic acid as prevalent pollutant. Wood is not the only source because adhesives, coatings, and sealants can be acetic acid emitters. Acetate solvents are most commonly employed in paints, and if they undergo hydrolytic degradation, acetic acid is formed. The corrosion potential of acetic acid against metals is strongly enhanced by high relative humidity values, but even under the controlled conditions of museum rooms and display cases, the corrosion of copper and lead-based metals is seen to proceed steadily [22].

The effects of museum environment and particularly of storage and display materials on metal coins have been also described [23]. Shocking disfigurations were detected in coins confined in the close spaces of showcases and cabinet drawers without ventilation. The parts emitting harmful gases were not only wood used in the construction of cases and furniture but also textiles, paints, adhesives, carboard varnishes, and different lining or flooring materials. In addition to the acidic emissions from wood and paints or adhesives, an important contribution to the coin damages was given by sulfur, which is known to be emitted in the form of hydrogen sulfide by wool and other proteinaceous materials. In its reduced form of sulfides, sulfur is one of the most problematic pollutants to attack metals in museum and collection environments. Minute amounts of sulfur can cause significant visual and physical effects on metal surfaces, and particularly on silver. Hydrogen sulfide reacts with silver to form a surface film of silver sulfide, and the same will happen on copper. Sources of sulfur in museums reside in wool fabrics from historical collections and in finishing or decoration parts made with vulcanized rubber such as floor coverings or carpet backing. Moreover, with the development and diffusion of plastic materials and synthetic dyes, the presence of chemical compounds containing sulfur has increased substantially in the recent years because of their use in dye structures and additives for polymer stabilization [24].

The formation of dark films on silver objects when they are exposed to polluted atmosphere is said to tarnish, and the principal component of a tarnish film is silver sulfide [25]. Tarnish is aesthetically displeasing, and the thin corrosion layers can be removed in different ways, but in the sealed environment of display cases, the accumulation of high levels of aggressive gases produces on silver objects more severe irrecoverable damages. Lacquered silver Tibetan artifacts after being stored for more than 20 years in well-closed display cases were found to be affected by two types of severe corrosion: the predominant one was in the form of thick layers of black silver sulfide crystals growing as whiskers perpendicular to the surface; a second type of corrosion was in the form of black fine filaments growing parallel to the surface [26]. Evaluation of the environment in the display cases revealed fairly high concentrations of tarnishing gases, which were much higher than the levels in the open spaces of the museum. In the display cases, polyisoprene rubber carpet backing and carpet wood fibers were the source of hydrogen sulfide causing the corrosion. The two different crystallization forms of the corrosion were attributed to the fact that the objects were lacquered and the silver was exposed to the tarnishing gases only where lacquer was missing, with the formation of whiskers, or where breaks occurred in the film coating with the filiform corrosion taking place underneath the coating.

Glass objects may be also affected by gaseous emissions in the museum environment. The conservation of glass collections strongly depends on climate, being particularly sensitive to large fluctuations in relative humidity (RH). According to the different compositions of glass, deterioration appears as droplets or moisture films or efflorescences on the surfaces [27]. Ions sampled on the surface of glass vessels from different museums revealed, among others, the presence of acetate and formate anions, which are attributed to the emission of formic acid, acetic acid, and formaldehyde from the wood or wood composite materials used in storage or display [28].

As a consequence of all the collected evidences, conservation guidelines for museums and exhibitions are nowadays fully recognizing that careful control of the atmosphere in the storage and exhibition room is essential for the conservation of different types of historic objects [3][4]. Modern guidelines of preventive conservation ban the use of wood for the permanent storage and display of objects in order to avoid efflorescence salts crystallization, corrosion on surfaces, and other damages to the artefacts. As a result of the aesthetic qualities of wood, the above caveat is not always fulfilled, particularly when in museums, the final decision for presentation of objects in galleries and display cases is made without the professional conservators approval. When wooden parts in storage boxes or display cases cannot be avoided, it has been suggested that to prevent the release of volatiles, the wood surfaces should be sealed with a coating. The main problem here is the choice of wood sealant, which not only must provide a barrier to the volatiles but must itself be free of dangerous emissions. A study on wood coatings for display and storage cases showed that most of the coatings are not able to make the wood completely safe, and maximum attention must be paid to the coating emissions and the safety time necessary for drying or curing [29].

There is worldwide recognition in museums and among the professional conservators that display cases must be built with chemically stable materials, which primarily means avoiding the use of wood and wood composites [4][6][30]. The structure of nowadays conservation grade display cases is essentially made of glass panels and metal sheets, but for the whole construction, many organic materials are involved as well under the form of paints, adhesives, gaskets, or insulators, and in recent years, it became clear that new sources and new forms of degradation and of disturbing effects on art objects were caused by the modern constructions.

The phenomena of extensive visual disturbance were detected on glasses of display cases that are fairly new in construction in different museum environments [31]. These appearances are named haze or fog from the type of pattern and form they have. It was natural at first to think that pollutants from the case materials or from the objects on display could be the cause of the glass fogging, but it was as well considered that glass is not completely chemically inert, and the type or treatment of glass may have effects. A detailed study of the fogging on glass display cases was performed on new galleries of Royal Ontario Museum, in Toronto, when more than 2000 glass panels from display cases showed some haze [32]. Fogging residues were fully characterized in order to determine their source and propose an effective cleaning protocol. The residues showed a surprising abundance of compounds, from sodium salts of inorganic and organic acids to n-alkane hydrocarbons, ketones (both long-chain and alkyl phenyl), long-chain nitriles, silicones, and phthalates. Synthetic polymers and polymer additives were also identified. Some of the fogging components were deposited from the air in the museum, whereas the sodium salts of formic, lactic, and fatty acids were attributed to reactions of available sodium with the volatile acids condensed on glass. Most of the glass residues contained n-alkane hydrocarbons, which, being more or less abundant in different glass batches, were attributed to manufacturing lubricants or release agents from production and finishing machinery. In the display cases, laminated glass is generally employed, which is formed by different glass layers heated and pressed for adhering to polyvinyl butyral or ethylene-vinyl acetate film. During the production process, the glass sheets come into contact with vacuum suction cup glass lifters, conveyor belts, and shipping supports, all of them using lubricants. It was concluded that part of the lubricants remained from manufacturing processes and were not removed by previous cleaning operations. The problem of effective cleaning then came to the fore, with the possibility that some residue on the panels (glycols, acids, surfactant components, antifoaming, etc.) could originate from glass cleaners and polishes. Thorough cleaning of the glass appeared necessary to remove all greasy residues, and a new cleaning protocol was developed and validated for museum applications.

The formation of hazy films on glass surfaces has long been known in the conservation field, particularly with framed oil paintings, which occasionally show so-called ghost images on the inside of the protective glasses. A scientific study of ghost images revealed that they were generated by mixtures of free fatty acids evaporating from the oil binders and condensing on the glass interiors [33]. Palmitic and stearic acids were the main evaporating components, and the hazing intensity could be attributed to the binder nature with slow-drying paint formulations, which are richer in free fatty acids, producing more intense ghost images. Haziness from free fatty acid deposits in museums has been seen not only on protective glasses but also on the surfaces of many types of artworks dating from the late 19th through 20th centuries, going from paintings to sculptures or works on paper [34]. With the evolution of artistic techniques, the introduction of new materials and the countless formulation changes experimented by the artists, the sources of fatty acids in artworks are greater than could be expected. In addition to the variety of oil binders having different compositions, other relevant sources are alkyd paints, beeswax coating formulations, pigment extenders, and surface-active coatings such as stearate salts. As fogging patterns and ghost images are immediate visible evidence of fatty acids deposition on the glass enclosures of paintings, it is obvious to expect that in the presence of emitters, the same acids can deposit to some extent on glass surfaces within display cases. Measurements done on samples wiped from the glass interior of an enclosure containing wooden and metal objects at the Viking ship museum in Oslo indeed confirmed the presence of fatty acids, together with formic and acetic acid [35]. Interestingly enough is the fact than in the deposited films, more than 30 volatile organic compounds were present.

In recent years, other types of fogging on glass and efflorescences on different exposed surfaces were discovered in the interior of modern and newly built cases [36]. As no wooden material was involved, and in many cases, neither was there interaction with objects on display, it became necessary to consider an unexpected contribution from some of the construction materials, all of them previously tested and approved for not giving acidic emissions. Checks on the materials and analysis performed by museum and conservation labs led to the conclusion that the efflorescences were derived from the reaction of a volatile organic amine, a substitute piperidinol compound, with acidic molecules present in the closed environment [36][37]. In particular, in empty display cases or in cases containing objects without interacting materials, crystalline efflorescences were seen to grow on gaskets as a result of the reaction of the above-mentioned amine with a dichlorobenzoic acid present as a by-product of the curing process of peroxide-cured silicone elastomers. Efflorescence on the objects, on the other hand, was the result of the volatile amine deposition and subsequent reaction with some acid component on the surfaces. The organic amine was identified as a molecular fragment coming from an hindered amine light stabilizer (HALS) added to the adhesive used to bond the glass and metal of the display cases. Its high reactivity against acidic compounds was confirmed with laboratory experiments where different types of crystal salts could be obtained in close environments simulating the display case condition [37].

The above findings are bringing into the conservation community much more understanding of the problems that are to be faced with the introduction in galleries and museums of new materials whose production generally involves complex and undetermined formulations containing additives, stabilizers, pigments, and fillers. Experience has shown that not only the corrosion effects from acidic compounds must be considered, but also damaging reactions from unusual organic bases. The standard tests normally used by museums for the approval of materials are not effective, as they do not cover all the phenomena that can possibly occur. This becomes especially true with the widespread recommended use of airtight enclosures where the concentration of emissions can increase up to critical threshold reaction values.

3. Removal and Control of Emissions

Any construction material of organic origin introduced in a museum environment is a potential carrier of volatile and semi-volatile compounds that are capable of harming heritage artifacts. As far as modern display cases are considered, construction characteristics and materials involved are such that emission-free display cases are not possible, and indeed, we must refer to low-emission showcases. Constructors and the conservation community in museums are aware that with new installations, the necessity arises of an adequate maturing of the enclosures, which should be kept completely open in well-ventilated spaces to allow for the outgassing of VOCs and SVOCs. Time constraints most often make it unfeasible to properly season the enclosures, or else the time for complete outgassing of the low molecular weight compounds contained in the materials is extremely long (months) to fulfill. Moreover, in the showcases, the contribution of volatiles emitted by the exhibited objects must be considered. This implies that in airtight modern display cases, VOCs and SVOCs originating either from objects therein contained or from some construction material can accumulate in the indoor air until they eventually reach troublesome concentrations. Therefore, the way for air cleaning in museum enclosures is the use of suitable adsorbents.

Adsorbents for pollutants reduction have been used for a long time, and interested readers may refer to the existing specific literature for the general aspects of production, physical characteristics, and working mechanisms. What is worth mentioning here is that adsorbents in cultural heritage environments must be highly efficient, commercially accessible, and affordable. An excellent report on the types, structures and general properties of adsorbents for pollution reduction in cultural heritage collections was produced at the Swedish National Heritage Board [38]. The adsorbents most used are activated carbons, activated alumina, silica gel, zeolites and, more recently, polymer-based substrates. Their performance against target molecules to adsorb depends on the overall physical properties of the porous substrate and the working environment. Within display cases, the adsorbents efficiency for the reduction of pollutants is strongly dependent on whether the system works in passive or active mode. In passive mode, there is no forced air circulation in the enclosure, and adsorption rates are obviously slower than in active mode, where air is forced through the adsorbent material. The air flow in the display case can be operated continuously or at selected intervals for fixed times, and this indoor air recirculation constitutes a multi-pass filtration.

The behavior of different types of adsorbents for the removal of air pollutants from museum display cases has been the subject of specific investigations. Most of the comparisons are between activated carbons and activated alumina, and the principal target molecules are formic and acetic acids and formaldehyde with, in some cases, the inclusion of major atmospheric pollutants [39]. Laboratory results in general point out that activated carbon materials perform better than other substrates for the adsorption capacity against a wide range of adsorbates, with some zeolites occasionally competing with carbon materials for interaction with acetic acid [40][41]. The efficacy of a set of carbon materials for reducing the concentration of formic and acetic acids has been investigated under real museum conditions by installing the adsorbents on different showcases containing heritage objects [42]. The reduction of acid concentration could be obtained in all the showcases but at different extents depending on the type of carbon material, pure or impregnated granulate, carbon-coated foam, or activated carbon cloth. Carbon cloth and carbon-coated foam were more effective than granulate. For the different adsorbents, the efficiency in acid reduction was also strongly influenced by the air exchange rate of the showcase, and under favorable conditions, some adsorbents could reduce the acids concentration below the recommended target level.

The efficiency for acetic acid adsorption of different types of commercial materials (activated carbon, activated alumina, zeolite, two bentonites) has been also studied by measuring the corrosion rates on lead probes [43]. All the sorbents were considered applicable for the sorption of low concentrations of acetic acid vapors in museum depositary environments. The physical characteristics of the porous adsorbents were also determined to find correlations with the protection efficiency. The most effective adsorbents of acetic acid vapors turned out to be the activated alumina and the activated carbon, which were the materials with the largest specific surface areas.

Acetic and formic acids are again the target molecules in a study about the capacity of carbon adsorbents to efficiently remove excess organic acids in museum storage rooms [44]. The interest was about measurements made in situ in two different real spaces: a storage of historical books and a storage containing mixed materials, from archaeological waterlogged wood to modern plastics. Two cartridge filters with carbon adsorbents were tested, having different air cleaning characteristics. The removal efficiency of the two filters was found to be strongly influenced by air flow conditions, and the benefits from active air filtration in rooms could not be confirmed for the operating conditions.

In a recent systematic study dedicated to adsorbents for the removal of museum pollutants, the filtration efficiency of 37 different adsorbent media has been assessed under conditions simulating active and passive (with and without forced air exchange) display cases [45]. Adsorbents comprised activated carbons with and without impregnation, activated carbon cloths and carbon-coated foams, natural and synthetic zeolites, molecular sieves, silica gels, archival cardboards, polymer-impregnated matrixes, and others. Efficiency of the adsorbents was determined from the achieved concentration reductions of selected target substances under the active or passive mode of operation. Target substances were the usual main harmful compounds in the museum environment: formaldehyde, formic, and acetic acid to which toluene and alpha-pinene were added as representative of the many VOCs detected in museum enclosures. From the overall extensive testing, a confirmation comes that pure and impregnated activated carbons have the best adsorption efficiency against the selected target substances. One surprising result was that in the passive mode, nearly all adsorbent media showed performances better than those under active conditions. The possible explanation is that under the test experimental conditions, in the passive mode, the residence time of pollutants with the adsorbents is longer than in the active mode. With high air exchange rates, the reduction of pollutants turned out to be lower than that occurring at lower air rates. Apparently, this consideration seems to favor passive adsorbent installations in display cases, but it must be considered that the active mode results have been obtained with only one pass through the filter unit, whereas in museum display cases, filtration devices will recirculate the air many times during operation.

An interesting finding was also that commercial materials specially designed for museum applications were not well performing or even performed badly.

In the conservation community, there is growing agreement about considering carbon-based materials the best choice for the control of pollutants in the museum environment. The critical point can eventually be what type of carbon sorbent should be selected between the many commercial options. The results above described are in some way comforting, as they point to the general good performance of different types of carbon sorbents. Therefore, primary choice criteria can become the cost of the material and handling facility.

While research on indoor air quality and on the role of modern materials are strongly growing, it cannot be disregarded that new types of adsorbents are constantly under development, and new suggestions may come for the heritage wellness. Examples of new materials for the removal of indoor air pollutants in museums concern optimized mesoporous silica [46] and metal–organic framework (MOF) structures tailored for the adsorption of low concentrations of acetic acid [47].

It appears evident that in modern museum display cases, the ventilation with air flowing through active sorbent material will be often necessary to maintain acceptable concentration levels of the gaseous products emitted by construction materials and by the exhibited artifacts. A comprehensive preventive conservation scheme implying active air circulation within the showcase and through functional sorbent materials has been proposed for the complete suppression of emitted compounds [48]. For monitoring the filtration efficacy in the display case, an affordable system by direct SPME sampling in the air flow has been developed. Moreover, a continuous monitoring of the air quality within the showcase was done by the insertion of a photoionization detector that is capable of measuring VOCs in the air. The system is not selective, as it gives the total VOC concentration, with ppb sensitivity. In addition, it includes temperature and humidity sensors, making the whole a useful instrument for the display case environment control.

This entry is adapted from the peer-reviewed paper 10.3390/atmos12030364

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