Climates and Microclimates:
A New Attitude to the Storage of Archival Materials

Dale Peters
Paper Conservator
Campbell Collections of the University of Natal
May 1996

This was originally published locally in South Africa in AMLIB Newsletter (Association of Archivists and Manuscript Librarians), No.60 May 1996, 8-12.


Environmental control is a well established strategy in preventive conservation, and has become accepted as a management tool in collections maintenance programmes, yet for many institutions, the means of achieving the recommended values for environmental control are remote. The concept of archival storage under less-than-ideal conditions seems to question the very ethic of conservation, but the mechanical vagaries of full air-conditioning does not always provide a more stable environment. A new attitude to environmental control is required to achieve stable conditions for archival storage.

Recommended storage life values

Environmental control provides the single most effective tool in preventive conservation. There is a direct relationship between the effect of storage temperature and relative humidity on the permanence of paper : the lower the values of temperature and humidity--the longer the life expectancy.

The values of 20°C and 50%RH, and a proliferation of "recommended" variations, have been accepted without question, and form the basis of environmental monitoring by means of thermohygrograph, or sling psychrometer, in many institutions. However, with an increased awareness of the importance of environmental control within South African repositories, our reliance on mechanical air-conditioning plants may have given rise to the neurosis, that without air-conditioning, our museum, gallery and archive collections will rapidly deteriorate. This is not the case.

Recent research indicates that it is more important to have a stable environment than specific values of temperature and humidity, and this can be achieved by careful building design. More vulnerable are those collections where it is not possible to maintain constant 24 hour air-conditioning--housed in non-specific buildings, where there is no control--or for economic reasons. The cycling effect under these circumstances is devastating to paper-based materials, and far more detrimental than no air-conditioning at all.1

The point of consideration is whether the recommended values of 20°C and 50%RH, have any relevance. Stefan Michalski reported to the ICOM Committee for Conservation in Washington in 1993 that, in tracing the derivation of these generally accepted values, there seemed little justification for their recommendation other than mechanical feasibility in the temperate climate zones of Europe and North America, rather than any research designed to determine the values that minimised damage to collections.2 Their unquestioned acceptance under the African sun speaks more of our colonial heritage than our perception of the response of archival materials to environmental conditions.

The objective of any Conservation environment is therefore to contain fluctuations in relative humidity. The achievement of a particular relative humidity is dependent on the temperature maintained in the building. Maintaining a lower temperature increases the relative humidity and inversely, higher temperatures decrease the relative humidity along the natural path of psychrometric relations. Damage is measured in terms of internal stress factors and greater chemical reactivity caused by a fluctuating moisture content within composite structures, such as book bindings or mounted and framed artefacts. The sorption and desorption of moisture is clearly dependent on relative humidity, causing the paper to age at a faster rate.

The relative humidity level should rather be set within the individual parameters of each institution--including the nature of the collections, the ambient climate, and the limitations of the existing building, including its orientation, insulation, natural ventilation and glazing, factors which contribute to its thermal mass.

An interesting finding of this study of fluctuating relative humidity, conducted by the Library of Congress, is that bound records were found to be generally weaker and have a higher acid content than loose papers in boxes, even those stored in highly acid enclosures.3 A possible explanation may be the stable microclimate formed by the enclosure, that seems to present a barrier to atmospheric pollutants and to fluctuating environmental conditions.

Link between climate and microclimate

Recent conservation research has measured psychrometric behaviour in microclimates with startling results. The Municipal Archives of Amsterdam commissioned an investigation into the cause of browning of mounted paper artefacts stored vertically in boxes. The limited volume of air within a microclimate is unable to respond in the expected manner and the reverse phenomenon of fluctuations in relative humidity following the rise and fall temperature was experimentally measured. The degree of browning was found to increase with the frequency of these humidity fluctuations. The implications of these conclusions are significant to the storage of archival collections. They indicate that while we might adjust the temperature in the repository as a means of controlling the relative humidity, the effect thereof is reversed in the microclimate of enclosed spaces.4

The George Mervyn Lawson collection of prints and drawings, at the William Humphreys Art Gallery in Kimberley, are a case in point.5 It is unlikely that prints stored in the semi arid climate of Kimberley for the last century would be susceptible to biodeterioration, despite evidence of spotted discolouration, which--for want of a better definition--must be termed foxing. Anecdotal evidence of provenance provides an explanation related to environmental conditions.

The possession of George Mervyn Lawson (c1865-1945), the Archdeacon of Kuruman and Mission Priest of Papkuil for over 46 years, the collection bespeaks a man of discerning tastes, called to a modest existence in the harsh dry climate of Griqualand West. The drawings, dating from the 15th to the early 10th century, were attached by means of gummed glassine tape into albums, and carefully stored in his coffin under the bed!

The strong diurnal and seasonal ranges of temperature in the Northern Cape region would have driven a repeated cyclical pattern of humidity rise and fall, with following sorption and desorption curves of the paper inside the coffin as the outer microclimate. A pertinent point of interest is the evidence of discolouration on the prints on porous paper, which would respond more readily to humidity fluctuations, while the albums appear relatively unscathed. Collections exposed to strong diurnal ranges in temperature, as experienced in some of the driest regions of this country, are vulnerable to similar dewpoint cycling, as a result of the influence of temperature variation on the humidity in microclimates.

Appropriate storage materials

The implications of these findings are cause to reconsider the concept of archival storage under less-than-ideal conditions, and the alternatives presented in passive environmental control by boxed storage and enclosed display cases as microclimates.

Archivists and librarians have become increasingly aware of the threat of acid migration between adjacent materials, and have been alerted to the importance of "archival" quality material in the construction of storage enclosures. The term, often used loosely, is intended to indicate materials of an enhanced level of permanence, as prescribed by international standards. (6)

A study of the environmental factors in the mechanism of acid migration has revealed the risk of autocatalytic degradation in microclimates. High relative humidity was found to contribute to the transport and deposition of degradative volatiles present in the chambers. The alarming reality is that this is not necessarily a contact based phenomenon. Neutral pH paper decreased in pH to the same degree, whether it was in physical contact with an acidic newsprint or just exposed to the chamber environment where volatiles from the newsprint were present.

The prior deacidification of items intended for polyester encapsulation is emphasised. Of particular relevance is the related finding that Mylar (polyester/ polyethylene terepthalate) is a an effective barrier for volatile degradation products. Although it will keep the external volatiles from penetrating to attack a vulnerable paper object, it will also prevent any volatiles within, generated when deacidification is neglected, from escaping and thereby accelerate pH decline. (7)

Microclimates as environmental buffering systems:
Showcases, picture frames, boxed enclosures

With the knowledge that unstable environmental conditions are reflected in microclimates, we need to reconsider the role of the environment in the preservation of material displayed in showcases and in picture frames, or stored in boxed enclosures.

Unless a container is hermetically sealed--such as the elaborate case designed for the display of the Declaration of Independence and the Constitution of the United States in the entrance hall of the National Archives and Records Administration Office in Washington DC--atmospheric pollutants and water vapour in the interior will be exchanged with the exterior, the macroclimate. This exchange takes place in three ways, diffusion through the walls, diffusion through gaps and openings , and pressure- and temperature-driven flows as indicated. As the temperature falls, atmosphere is drawn into the container, and as the temperature increases, atmosphere comes out, drawing with it moisture from the paper substrate.(8) This supports the temperature-driven increase in RH measured in the Amsterdam Municipal Archive.

Coupled with the hydrolytic effect of pollutant concentration on outflow and dilution on influx, the box provides no protection at all unless hermetically sealed and impermeable to atmospheric pollutants. Sealed, impermeable containers prevent any transfer from the macroclimate to the microclimate but if the enclosed materials are themselves reactive, or have a high moisture content, or the storage materials are of an inferior quality, such as corrugated cardboard, they will give off degradation products that induce autocatalytic degradation within the enclosure. It would be preferable to construct enclosures with strategic openings, providing physical protection while allowing atmospheric interchange.

The primary agents of paper deterioration have been identified in environmental conditions, and in the by-products of the paper itself as it is aged. In response, Conservation Resources Ltd. have developed a range of active archival storage materials, known as MicroChamber. Comprising a laminate of alkaline buffered board, molecular sieves with activated carbon and vapour barriers, MicroChamber lining papers and box boards buffer against environmental fluctuations and protect materials from atmospheric pollutants, and absorb accumulated by-products of deterioration within the enclosure. It is necessary to point out that as with all acid-free materials, if unfavourable conditions persist, sorbent saturation loading could be reached, and the material may have to be replaced periodically. The contents will however, not only be protected, they will be preserved.


Repeated fluctuations of relative humidity, whether resulting from ambient conditions in the macroclimate, or driven by temperature changes in the microclimate, are a hazard to paper-based collections. The appropriate response therefore must be aimed primarily at a stable relative humidity, rather than a specific temperature, based on human considerations rather than the preservation of materials. Of greater importance than the specific level chosen, is the steady maintenance of the chosen value, as low as possible, and by whatever technique available: mechanical air-conditioning, dense building construction or natural ventilation to control the macroclimate of the repository.

The use of microclimates to buffer unstable conditions is an effective passive climatic control technique, though it demands careful selection of appropriate materials and a design for atmospheric interchange. Furthermore, it seems likely that this form of passive environmental control is the best possible direction to take for the long-term preservation in storage of the volumes of archival materials that are best described in linear metres and by tons of weight. Such collections are unlikely to receive Conservation treatment until mass deacidification techniques become feasible.

These perspectives on environmental control redefine accepted theory, they reflect important research into this complex topic, and they are consequential--they demand a re-evaluation of current Conservation practice.

References and Recommended Reading

1. KING, S. & PEARSON, C. (1992). Environmental control for cultural institutions--appropriate design and the use of alternative technologies. In La Conservation Préventive, Paris: ARAAFU, 63-74.

2. ERHARDT, D. & MECKLENBURG, M. (1994). Relative humidity re-examined. In Roy, A. & Smith, P. (Eds.), Preventive conservation: practice, theory and research. Preprints of the contributions to the Ottawa Congress, 12-16 September 1994. London: International Instutite for Conservation of Historic and Artistic Works, 32-38.

3. SHAHANI, C. J., HENGEMIHLE, F. H. & WEBERG, N. (1989). The effect of variations in relative humidity on the accelerated ageing of paper. In Zeronian, S. H. & Needles, H. L. (Eds.), Historic textile and paper materials II: conservation and characterization. Washington D C: American Chemical Society, 63-80.

4. HOFENK DE GRAAFF, J. (1994). Research into the cause of browning of paper mounted in mats. In Verschoor, H. & Mosk, J. (Eds.), Contributions of the Central Research Laboratory to the field of conservation and restoration. Amsterdam: Central Research Laboratory for Objects of Art and Science, 21-42.

5. - (1994). George Mervyn Lawson c1865-1945. Kimberley: William Humphreys Art Gallery.

6. - (1993). International standard for permanent paper. Restaurator, 14 (4), 253-254.

7. SLAVIN, J. & HANLAN, J. (1992). An investigation of some environmental factors affecting migration-induced degradation in paper. Restaurator, 13 (2), 78-94.

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