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.