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Relative Humidity as an Agent of Deterioration: A Critical Annotated Bibliography of Raven’s ‘Agents of Deterioration’ Video Nr.9. Patricia Nauta - 16372727
1. Introduction Relative humidity (RH) is widely recognised as one of the most important environmental factors for the long-term preservation of cultural heritage. Fluctuations or sustained deviations in RH can induce mechanical stress in hygroscopic materials, promote microbial growth, and accelerate chemical degradation. The severity of these processes depends on interactions between temperature, humidity sources, environmental conditions and the response of materials. A structured framework for managing incorrect indoor climate is presented in Risk Management for Collections , written by Senior Scientists Agnes Brokerhof, Bart Ankersmit and Frank Ligterink of the Cultural Heritage Agency of the Netherlands (RCE).^1 Their publication applies a risk-management method developed together with the Canadian Conservation Institute (CCI).^2 These concepts are also introduced to broader audiences in the YouTube series The Agents of Deterioration , where archaeologist Raven Todd DaSilva presents key preventive-conservation mechanisms. Episode 9 of the series is devoted to relative humidity.^3 In the introductory video Raven describes the CCI framework as “the pillars of preventive conservation,” which she aims to explain to conservators or those who “want to be a conservator”.^4 However, most of Raven’s 75,000 subscribers are likely private viewers who may use this information to care for their own cherished objects. Given the potential influence of public media on these custodians—who collectively maintain a vast share of cultural heritage—it is worthwhile to examine how accurately these concepts are presented. The central research question of this annotated bibliography is therefore: To what extent does the Raven video provide an accurate and conceptually adequate introduction to relative humidity as an agent of deterioration when assessed against the professional preventive-conservation literature? To address this question, seven key sources cited in the RCE publication are analysed and annotated. The discussion follows the risk-chain structure used in the RCE handbook, progressing from conceptual foundations and moisture sources through exposure and material effects to risk-reduction strategies and interpretive cautions. Considering Raven’s scope, the critique is structured (^1) Agnes Brokerhof, Bart Ankersmit, and Frank Ligterink, Risk Management for Collections (Amersfoort: Cultural Heritage Agency of the Netherlands, 2017), Chapter “Incorrect Indoor Climate,” 165–175. (^2) Brokerhof, Ankersmit and Ligterink, Risk Management for Collections , colophon. (^3) Raven. “9 – Relative Humidity – The Agents of Deterioration.” YouTube video, 10:09. Posted by Art Conservator , accessed March 1, 2026. https://www.youtube.com/watch?v=w5qLxX7Dmnk (^4) Ibid. , 9: 16 – 9:18; Raven, “1 – Physical Forces – The Agents of Deterioration,” YouTube video, 0: 07 – 0:52. Posted by Art Conservator , accessed March 1, 2026. https://www.youtube.com/watch?v=kFkNZNENAeE.
hierarchically: first identifying omissions or inaccuracies that may affect the preservation of common household objects, and subsequently discussing limitations primarily relevant for professional museum collections.
2. Annotated Bibliography 2.1 Conceptual Foundations: Relative Humidity and Temperature Brokerhof et al. explain that relative humidity (RH) must be understood within the broader dynamics of indoor climate, where temperature, moisture sources, and material responses jointly determine preservation risk.^5 In this framework, RH is not an isolated parameter but part of an integrated thermodynamic system. In Relative Humidity , Raven provides an accessible introductory definition for a general audience, describing RH as “a measure of the amount of water vapour in the air” and noting that it is “very dependent on the temperature.”^6 These relationships are presented qualitatively, however, and do not indicate how strongly preservation outcomes depend on the interaction of temperature and humidity. Stefan Michalski, Senior Conservation Scientist at the CCI, provides a quantitative perspective. Drawing on empirical research into material degradation, he summarises preservation effects through simple rules: “double the life for each 5 °C drop” in temperature and, for humidity, “half the RH, more than double the life.”^7 His lifetime- multiplier curves show that the impact of cooling is generally larger than that of humidity reduction.^8 Raven’s explanation therefore omits a practical implication. Emphasising RH reduction without considering temperature may lead viewers to assume that heating a damp space is beneficial because it lowers RH. Michalski cautions that this reasoning is misleading: heating should be used “only to prevent damp,” that is, conditions where moisture condenses on surfaces.^9 (^5) Brokerhof, Ankersmit, and Ligterink, Risk Management for Collections , 168. (^6) Raven, “Relative Humidity,” 0:47–0:52. (^7) Stefan Michalski, Guidelines for Humidity and Temperature for Canadian Archives , CCI Technical Bulletin No. 23 (Ottawa: Canadian Conservation Institute, 2000), 2. (^8) Ibid. , Fig. 1. (^9) Ibid. , 7.
2.3 Local Exposure and Microclimates According to Brokerhof et al. , humidity risk is not just determined by room conditions but “at a local level, there can be great differences in temperature…[and a]s a result […] in relative humidity.” 15 In particular “the relative humidity at a surface … is the product of the absolute humidity of the air and the surface temperature.”^16 In the end, local condensation and moisture-related damage are governed by what happens at the surface. To analyse these relationships quantitatively, the Image Permanence Institute developed the Dew Point Calculator, a web-based tool that models how temperature and relative humidity jointly determine condensation risk.^17 The calculator demonstrates that condensation occurs when a surface becomes colder than the dew point of the surrounding air. It also shows effects on natural aging, mould growth and metal corrosion. Raven likewise notes that damp may occur when warm air cools and moisture condenses on colder surfaces.^18 She identifies typical microclimates near exterior walls, above cold floors, or around poorly insulated windows. The dew-point model, however, clarifies that condensation depends not on the absolute temperature of such surfaces, but on their temperatures relative to the humidity conditions of the surrounding air. Consequently, raising room temperature and RH may increase the condensation risk if the temperature of an object lags behind that of the surrounding air. (^15) Brokerhof, Ankersmit, and Ligterink, Risk Management for Collections , 166. (^16) Ibid. (^17) Image Permanence Institute, Dew Point Calculator (Rochester, NY: Rochester Institute of Technology, 2016), web application. http://www.dpcalc.org/. (^18) Raven, “Relative Humidity,” 1:04–1:10.
2.4 Material Effects and Conflicting RH Requirements Brokerhof et al. explain that the effects of incorrect relative humidity depend on how different materials respond to moisture.^19 They show, for example, that hydrolysis- sensitive materials such as paper and photographs are chemically most stable below roughly 30 % RH, whereas hygroscopic materials like wood or canvas become more brittle in dry conditions.^20 Because materials absorb and release moisture at different rates, indoor climate specifications inevitably represent a compromise between competing requirements. Raven acknowledges that it is difficult “to find the perfect relative humidity for all objects” and recommends keeping RH “stable and at a decent range for everything in a collection.”^21 In the same passage she suggests that this range is “a lot wider than we previously thought” and that RH control need not be “such an exact and panicky science.” However, detailed material studies challenge this conclusion. The ASHRAE Handbook notes, for example, that bronze has several critical RH values due to its corrosion chemistry, meaning that no single RH level is universally safe; the only generalisation is that “anything over 75 % RH is dangerous.”^22 While Raven mentions several practical measures to stabilise RH, she does not explain which material combinations are most likely to create conflicting humidity requirements that viewers could otherwise anticipate and avoid.^23 (^19) Brokerhof, Ankersmit, and Ligterink, Risk Management for Collections , 172, tab. 44. (^20) Ankersmit and Stappers, Managing Indoor Climate Risks , 181. (^21) Raven, “Relative Humidity,” 1:59–2:18. (^22) American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), ASHRAE Handbook—HVAC Applications , SI ed., chap. 21, “Museums, Galleries, Archives, and Libraries” (Atlanta: ASHRAE, 2007), 21.2-21.4. (^23) Op. cit. , see note 12.
2.6 Proofed Fluctuations and Interpretation of Damage While the previous sections explain how RH-related mechanisms can damage objects, conservation practice must also interpret existing deterioration. Visible cracks or losses are often treated as evidence of a poor indoor climate. Brokerhof et al. caution, however, that an object’s condition reflects the cumulative environmental conditions it has experienced rather than only the present environment. They therefore introduce the concept of “proofed relative humidity fluctuations”. 32 This means that future RH variations pose little risk if they remain smaller and fluctuate around roughly the same average RH range as before.^33 Raven does not address this interpretive problem. Her video notes that materials have a “critical value” for RH at which damage may occur, but offers little guidance on how such limits can be assessed.^34 Mechanical modelling by conservation scientists David Erhardt, Charles Tumosa and Marion Mecklenburg of the Smithsonian Institution shows that RH-induced deformation generates stress–strain responses in materials: when strain remains within the elastic limit, deformation is reversible and no permanent damage occurs.^35 However, the British Standards Institution (BSI) notes that at high humidity the relation between RH change and material deformation becomes exponential.^36 These observations explain the practical logic of proofed RH fluctuations: past environmental exposure effectively tests whether objects can tolerate a certain range of humidity variation and thus provides a pragmatic indicator of climate suitability. (^32) Brokerhof, Ankersmit, and Ligterink, Risk Management for Collections , 174. (^33) Ibid. , 174. (^34) Raven, “Relative Humidity,” 2:31–2: (^35) David Erhardt, Charles S. Tumosa, and Marion F. Mecklenburg, “Applying Science to the Question of Museum Climate,” in Museum Microclimates: Contributions to the Copenhagen Conference, 19– 23 November 2007 , ed. Tim Padfield and Karen Borchersen (Copenhagen: National Museum of Denmark, 2007), 11– 17 , 15, fig. 2. (^36) British Standards Institution, PAS 198:2012: Specification for Managing Environmental Conditions for Cultural Collections (London: British Standards Institution, 2012), 9: 5.2 Upper Limit for Relative Humidity, Note 5: “At 75% RH and above, the dimensional change due to each 5% rise in RH increases exponentially.”
2.7 Interactions between RH and Other Agents of Deterioration Brokerhof et al. conclude their chapter by emphasising that relative humidity interacts with other agents of deterioration.^37 These interactions often occur at the system level within museum environments. As discussed in §2.4, different materials respond differently to humidity, and measures intended to reduce one risk may influence others positively or negatively. 38 Protective measures such as microclimate frames may therefore mitigate several risks simultaneously by limiting exposure to fluctuating humidity as well as hazards such as fire, molest or theft.^39 Brokerhof et al. also note that humidity interacts directly with chemical deterioration processes. Incorrect RH accelerates several degradation mechanisms, including fungal growth, hydrolysis reactions, oxidation and metal corrosion.^40 In particular, higher RH combined with elevated temperature increases the rate of water-mediated reactions in materials such as paper, leather and limestone.^41 Raven’s video does not explicitly address these broader system interactions. Instead, she focuses on individual material processes occurring at elevated humidity, such as metal corrosion or acid hydrolysis, or salt crystalization mention in §2.4.^42 BSI’s Publicly Available Specification PAS 198 similarly states that environmental risks should not be assessed independently. It therefore recommends systematically evaluating “the effects of known interactions between one or more agents of deterioration”. 43 PAS 198 further notes that humidity promotes pollutant-driven deterioration, while higher temperatures reduces RH but increase chemical reaction rates.^44 (^37) Brokerhof, Ankersmit, and Ligterink, Risk Management for Collections , 174. (^38) Ankersmit and Stappers, Managing Indoor Climate Risks , 181. (^39) Brokerhof, Ankersmit, and Ligterink, Risk Management for Collections , 174. (^40) Ibid. , 169, Table 42. ‘Effects caused by incorrect relative humidity and incorrect temperature.’ (^41) Ibid. , 171. (^42) Raven, “Relative Humidity,” 2:40–2:45; 3: 33 – 4:01. (^43) British Standards Institution, PAS 198:2012: Specification for Managing Environmental Conditions for Cultural Collections (London: British Standards Institution, 2012), 4-5: 3.4 Assessing the Risks, Note 5. (^44) Ibid. , 9: 5.2 Upper Limit for Relative Humidity, Note 4.
4. References Ankersmit, Bart, and Marc H. L. Stappers. Managing Indoor Climate Risks in Museums. Cham: Springer International Publishing, 2017. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). ASHRAE Handbook—HVAC Applications. SI ed. Atlanta: ASHRAE, 2007. Chapter 21: “Museums, Galleries, Archives, and Libraries.” British Standards Institution. PAS 198:2012. Specification for Managing Environmental Conditions for Cultural Collections. London: BSI, 2012. Brokerhof, Agnes, Bart Ankersmit, and Frank Ligterink. Risk Management for Collections. Amersfoort: Cultural Heritage Agency of the Netherlands (RCE), 2017. Chapter “Incorrect Indoor Climate,” 165–175. Erhardt, David, Charles S. Tumosa, and Marion F. Mecklenburg. “Applying Science to the Question of Museum Climate.” In Museum Microclimates: Contributions to the Copenhagen Conference, 19–23 November 2007 , edited by Tim Padfield and Karen Borchersen, 11–17. Copenhagen: National Museum of Denmark, 2007. Guild, Shirley, and Maureen MacDonald. Mould Prevention and Collection Recovery: Guidelines for Heritage Collections. CCI Technical Bulletin No. 26. Ottawa: Canadian Conservation Institute, 2004. https://www.canada.ca/en/conservation- institute/services/conservation-preservation-publications/technical-bulletins/mould-prevention- collection-recovery.html Image Permanence Institute (IPI). Dew Point Calculator. Rochester, NY: Image Permanence Institute, Rochester Institute of Technology, 2016. Web application. Michalski, Stefan. Guidelines for Humidity and Temperature for Canadian Archives. CCI Technical Bulletin No. 23. Ottawa: Canadian Conservation Institute, 2000. Michalski, Stefan. “Agent of Deterioration: Incorrect Relative Humidity.” Canadian Conservation Institute, 2016. https://www.canada.ca/en/conservation-institute/services/agents- deterioration/humidity.html#sources Raven. “1 – Physical Forces – The Agents of Deterioration.” YouTube video, 8:33. Posted by Art Conservator. Accessed March 1, 2026. https://www.youtube.com/watch?v=kFkNZNENAeE. Raven. “9 – Relative Humidity – The Agents of Deterioration.” YouTube video, 10:09. Posted by Art Conservator , accessed March 1 , 2026. https://www.youtube.com/watch?v=w5qLxX7Dmnk
