Our knowledge of the past is odourless. Yet, smells play an important role in our daily lives: they affect us emotionally, psychologically and physically, and influence the way we engage with history.
In this work, we propose that smells are part of our cultural heritage, and that a structured approach to researching them is required. Several aspects allow us to explore the connection between olfaction and heritage. We will define heritage smells and argue their importance, by focusing on the following: (1) a theoretical review of olfaction and odours in heritage, including (a) the consideration for smells in heritage documents and guidelines, leading to the identification of smell as part of cultural significance of a place or object and (b) the use of smell in a heritage context as a means to engage and communicate with the audience; and (2) techniques for identifying, analysing and archiving smells and therefore enabling their characterization and preservation. These techniques can be approached from two complementary angles: firstly, the chemical analysis of the source of sensation, in our case chemical analysis of the compounds that lead to perception of the smell. Secondly, sensory characterization of that smell in terms of human perception. In the case of historic smells, this dual approach can contribute to a holistic understanding of what the odour represents in terms of the nature, history and state of the object.
Since this is the first comprehensive scientific treatise of the subject, the introduction deals with the issues of significance and use, as well as characterisation, in the following three subsections.
Olfaction and heritage
The significance of olfaction in the context of cultural heritage, evidencing that smells can be fundamental in shaping who we are, where we belong and how we experience encounters with different cultures, has been recently examined in several case studies. They show that odour can be part of the local identity through history ; that a central place for olfactory experiences in a culture results in a much wider vocabulary to discuss smells  and that travel and tourism offer an opportunity to approach the world with our noses . However, the role of smells in our perception of and engagement with the past has not been systematically explored.
Odours are powerful triggers for emotions via the limbic system of the brain, which deals with emotions and memory [4, 5]. They are an effective way to evoke recollections; certain aromas can even act as part of the common memory of a generation. For example, people born before 1930 tend to display positive association with nature scents, and the fragrance of Playdough triggers nostalgia in those born after 1960 . Scents can also influence behaviour: in shops, a pleasant scent positively impacts customers’ attitude towards the store, the evaluation of products and intention to revisit the place . A British company claims that treating customers with the smell of male sweat made them 17% more likely to pay their bills than a control group . Mood and cognitive function are also affected: although olfaction is one of the least considered senses in pedagogy, aromas can improve learning through their connections with memory, mood and productivity .
In the heritage context, experiencing what the world smelled like in the past enriches our knowledge of it, and, because of the unique relation between odours and memories, allows us to engage with our history in a more emotional way . Odours are also powerful cues to remember an exhibition, as demonstrated by Aggleton and Waskett  in their work at the Jorvik Viking museum in York, England. In the case of a gallery, the presence of point-of-scent components heightens the enjoyment of the public, in comparison to experiencing the same displays without smells .
Smell as part of cultural significance
There is currently no strategy in the UK for the protection or preservation of smells. In heritage guidelines, odours are often recognized as a value associated with a place, or with certain practices. Currently, smells are viewed as an aspect of cultural significance, an overall measure of the value of a particular place to the public, as introduced by the widely adopted Burra Charter . Aesthetic value contributes to cultural significance and includes ‘aspects of sensory perception for which criteria can and should be stated’, and, as an example, ‘the smells and sounds associated with the place and its use’. In this sense, smell can be considered as an intangible property of tangible heritage and inextricably linked to it.
However, unlike some food and culinary practices, smells are not recognized in the definition of intangible cultural heritage by UNESCO. In spite of sharing a relation to other aspects of intangible cultural heritage, such as language, industries, and tourism , the olfactory world is hardly discussed or documented.
According to the guidelines published by the Historic Buildings and Monuments Commission for England (formerly English Heritage), the smells of a place are considered of value because they affect our experience of it. For this reason, they should be taken into account when defining the character of a historic area .
The connections between odours, locality, and identity are also a component of tourists’ experience. With globalization, we see that a location and the authentic experience of it are not always linked, because it is possible to have an experience that is perceived as authentic far from the original location. For this type of tourist, authenticity lies with the experience (existential authenticity), not with the place . Likewise, it could be said that reproducing historic or heritage smells without their original physical context would be looking to provide an existentially authentic experience.
The link with a place and its identity is not sufficient to encompass all kinds of smells. Odours on their own, such as historic perfumes, are not formally classified as heritage, in the way buildings are by international standards of conservation such as the Venice Charter . This was recognized by Henshaw , who studied the way smells defined the urban environment, by conducting guided ‘smellwalks’ around different European cities. She stated: “In the UK, we have something called listed building status, so that beautiful buildings are protected and we’re not allowed to redevelop them. But there aren’t those equivalents for beautiful sounds or beautiful smells”. The temporary nature of odours, as well as the absence of a standardized vocabulary to talk about them, might be contributing factors to this state of affairs.
Specific smells can also be related to cultural practices, expressions and knowledge. As an example, the art of Asian perfumery is threatened by industrialization and may be in need of protection. The smells carry the information about how practices have evolved throughout history, the materials associated with them and the conditions in which smells were experienced . In this case, smells are associated with intangible practices, although they still emanate from a tangible source, as knowledge has no smell.
An example of a community-led selection of aromas recognized as heritage can be found in the Japanese Ministry for the Environment ‘100 most fragrant’ list, which was established in 2001 after a nationwide consultation where 5600 candidate smells were submitted by local groups. The aromas included ancient woods, sea breeze, sake distilleries and a street lined with bookshops. The 100 chosen aromas and their sources are now protected and carry a seal that reads ‘scents to be handed down to our children’. In addition to the recognition of significance as a cultural legacy, these smells are also an important element of regional promotion .
This year (2017), UNESCO is considering the application to have the skills, knowledge and practices associated with perfume making in the region of Grasse (France) recognized as intangible heritage . Given that there is no mention of smells among the 38 elements inscribed on the organization’s list between 2009 and 2014, the recognition of the olfactory heritage of Grasse would create an important precedent.
As the place of smell in heritage has begun to be discussed, so has the observation that the dynamic nature of olfactory ‘objects’ does not fit well within the current definition of intangible heritage [1, 19]. This presents a specific set of challenges in current museum practice when smells are used as part of collection interpretation.
However, not every historic smell is a suitable candidate for analysis and preservation, because not all historic smells have heritage value. Therefore, the first step in the proposed framework involves the identification of heritage smells.
Some concepts from the current evaluation policies can be helpful to illustrate how the cultural value of a smell can begin to be considered for its designation as olfactory heritage. Associative characteristics, an important aspect the determination of cultural significance of Scottish historic monuments , are considered more subjective than intrinsic or contextual ones. They include ‘significance in the national consciousness or to people who use or have used the monument, or descendants of such people’ and ‘the associations the monument has with historical, traditional or artistic characters or events’. In the context of this work, the associative aspect reflects the relevance of the provenance of a certain smell. It also encapsulates the importance of understanding the role of that smell in the public’s memory or collective imagination.
Out of the four sets of values identified by Historic England, there are two that can be relevant to assess the significance of a smell: historic value, both in its illustrative aspect, which ‘has the power to aid interpretation of the past through making connections with, and providing insights into, past communities and their activities through shared experience of a place’ and its associative aspect: ‘association with a notable family, person, event, or movement gives historical value a particular resonance’. Communal value is also an assessment category that can be used to consider the cultural value of a smell. It derives ‘from the meanings of a place for the people who relate to it, or for whom it figures in their collective experience or memory’ . Finally, interpretive criteria, defined by the Heritage Collections Council of Australia as ‘the value or utility the object has for a museum as a focus for interpretive and educational programs […]’ may also be significant for its links to particular collection themes, histories, or ways of seeing the collection .
Smell in the museum
While museums were once spaces where handling objects as a way of exploring them was encouraged, these practices changed in the nineteenth century with the increase of the number of visitors (and potential for damage to collections), and more sophisticated display techniques that allowed seeing objects well without touching them .
Visual communication is still dominant in the museum of today. However, all experience of the world is multisensory, whether or not it has been designed as such . The benefits of a multisensory approach to the examination of historic objects and practices have been argued [27, 28] and since the turn of the century many heritage institutions have been staging multisensory exhibits. The inclusion of smell in museums can be related to attracting more visitors, adding a ‘dose of reality’ to the displays, exploring the connections between olfaction and other senses and even claiming a space for perfume as an art form.
In contrast with the mentioned Jorvik museum, where smells illustrate Viking life 1000 years ago, an exhibition in the Reg Vardy gallery in Sunderland in 2008 presented smells without a visible source, just in a white, clean room. Robert Blackson, curator of the show, said the scents were “inspired by absence. Their forms are drawn by the disparate stories throughout history for which few, if any, objects remains” . Among the 13 smells were a bouquet of extinct flowers, communism and the scent of Cleopatra’s hair.
In addition to engaging the visitors to rethink the past as an odorous place, smells in museums can be a way of relating to the world of the ‘other’. In their work on presenting the country’s history in the National Museum of Australia, Wehner and Sear  sought to display works to encourage visitors to engage with subjective experiences. For example, one of their sensory stations included ‘the pungent smell of dried sea cucumber’ alongside cooking tools of Trepang fishermen.
Scenting a heritage space poses several curatorial challenges. Drobnick  argues that there is a risk that audiences might feel manipulated when the identity of the smell is not clear. The use of synthesized scents in this context, as opposed to ‘authentic’ (as in related to a unique material source) is also questioned by this author. This is partly because, whenever no olfactory artist is signing the work, museums tend to rely on commercial fragrance providers to scent exhibitions. Their catalogues are rich in aromas related to food and daily activities (‘banana’, ‘aftershave’), and interpretation/conceptual ones: ‘burning witch’ is offered as “history lesson in a smell, turns out witches smell a bit like bacon!” (Dale Air, Sensory Scent, 2015).
The issue of authenticity of smells used in heritage spaces leads us to consider the relationship between the olfactory properties of a source and the perception of its smell. A comparison with colour, another intangible property of tangible objects, might be useful. Colour can be described as an attribute of an object, and different theories consider it objective (i.e. depending on the object), subjective (i.e. depending on the viewer), or relational, where colour is a property that involves both physical objects and those who experience them . Smells can be treated in a similar way: as an attribute of the object, independent of the nose which smells it, a perception completely dependent on the smeller, or a communication between source and receptor, where meaning is created. The perception of a smell as authentic is, then, the result of an interpretation process, which we will explore on the basis of a case study.
Characterisation of smells
Since most odours are composed of volatiles organic compounds (VOCs) [33, 34], the analytical methods frequently involve headspace solid phase microextraction (HS-SPME) and gas chromatography-mass spectrometry (GC–MS). However, it needs to be stressed that not all compounds that could trigger the sense of smell can be determined using this technique. For inorganic compounds, and some organic compounds that are difficult to sample using SPME, other sampling and analytical techniques may be more useful, from direct detection to various types of separation techniques .
SPME was developed in the 1990s by Pawliszyn et al. [36, 37] and is used for extracting volatiles in the headspace over liquid and solid samples. It has been used successfully to extract and analyse VOCs from historic materials , including paper [39, 40]. GC–MS techniques are routinely used to analyse perfumes and cosmetic preparations .
SPME–GC–MS as used in this paper has been specifically optimized for analysis of the headspace of objects made of organic materials, such as book and paper , leather and parchment  and has been successfully used to sample air in libraries . Recent research shows that the profiles of volatile organic compounds found in historic libraries can be directly linked to the emissions from decaying books and wood furnishing , which makes it reasonable to assume that sampling VOCs both from books and from library environments can be done using the same technique, i.e. SPME-GC–MS.
The vocabulary we use to describe smells is important and it is essential that a methodology to describe odours for archival purposes includes a sensory description, in addition to the chemical one. In some industries, the human nose is the main tool to characterize odours due to its accuracy and sensitivity . Human olfactory experience depends on several factors, including genetic profile, ethnic background, gender, age , cultural background , and overall physical environment. The person’s mood at the time of sampling can also have an impact on description of the hedonic tone (pleasant/unpleasant) of the smell . Information on the evaluator and the evaluation circumstances can therefore be valuable metadata on the heritage smell.
The terminology to describe heritage smells is not standardized, in line with the general poverty of the olfactory vocabulary [4, 47]. However, this is independent of our ability to perceive and identify different smells, and respond to them . Many attempts have been made to unify the way to describe odours pertaining to flavour, fragrances, or malodours [47, 49, 50]. Working with reports of odour nuisance, Curren  developed an odour wheel based on descriptions by the complainants and cross-referenced it with potential odour-causing compounds. Recently, a bilingual (English–Spanish) dictionary for urban smells was created, using information from literature and urban smellwalks, and relating the selected terms to social media tagging . This latest experiment evidenced that, despite challenges posed by the ephemerality and invisibility of smells, techniques such as the ‘nose-led’ walks and crowdsourcing make the documenting of odours possible and even accessible.
All of the aspects considered in the Introduction could serve as a general framework to identify and document smells with historic value: (i) Significance Assessment; (ii) Chemical Analysis; (iii) Sensory Analysis; (iv) Archiving. Along with studies of the human experience of the odour, these aspects benefit the conservation, management and interpretation of cultural heritage, and are thus within the domain of heritage science as set out by the UK’s Institute for Conservation (ICON) in 2006 .
This opens up a new field of documentation and archiving (as well as conservation) of historic smells with heritage value, the foundations of which we explore in the frame of a case study based on the well-known and appreciated historic library smell, where we propose the Historic Book Odour Wheel as a key documentation tool.