Peters, D.: May 1996

1. Introduction

Environmental control is a well-established preservation strategy, and has become accepted as a management tool in collections maintenance programmes. The growing trend is toward preventive conservation as a function of the full staff complement of the cultural heritage institution, in a comprehensive programme to decrease the rate of deterioration of materials. This is aimed at reducing the need for invasive treatment of individual items to ensure a more effective utilisation of personnel and often meagre financial resources.

Environmental factors which affect the deterioration of library and archival materials include temperature, humidity, light, particulate and atmospheric pollution, vibration, insect pests, etc., all of which have been adequately treated in the conservation literature. In the early 1960s recommendations for the preservation of artefacts were first promulgated citing environmental values of 20°C and 50% relative humidity (RH). A growing awareness of the different requirements of different materials has led to a proliferation of amended recommendations, unfortunately not always consistent. It is accepted that the moment at which paper will begin to deteriorate depends largely on its environment and storage conditions (Sebera 1994). This is the basis of the current debate to redefine suitable values - or limits - for the various environmental factors, some proving more straightforward than others, some hardly practicable.

UV radiation, vibration, and particulate pollutants, for example, are best excluded completely, and it is well within the realm of our technical proficiency to do so. Temperature and humidity factors cannot be eliminated, and the determination of suitable values, based on an understanding of the mechanisms of deterioration to which these factors contribute, becomes critical. This discussion is restricted to the effects of temperature and humidity on paper-based artefacts. Temperature and humidity are interdependent and their participation in various mechanisms of deterioration is more complex than we may have previously perceived.

2. The Role of Environmental Factors in Paper Deterioration

The emphasis in the environmental debate has focused on a recommended standard as a panacea for the symptoms of damage, rather than on an investigation and alleviation of the causes of deterioration. To evaluate the role of environmental conditions in paper permanence, it is important to identify the causes of deterioration and the mechanisms by which they proceed. These can be summed up in three points:

  1. the presence of sulphur dioxide and other atmospheric pollutants
  2. the presence of traces of metal in modern machine-made paper
  3. the presence of an excess of atmospheric moisture which facilitates chemical reactions

The last of these is pertinent to this discussion. It has been proven by experiment that neither sulphur dioxide in the air, nor metal inclusions are detrimental unless high humidity is also present (Pollack 1961).

While acid hydrolysis appears to be the dominant cause of paper deterioration, it is in the mechanism of oxidation that the significance of the role of environmental factors in the induction of obvious causes - dimensional stress, biodeterioration, and chemical reaction - becomes apparent (Grattan 1978). Temperature and humidity both cause and participate in oxidative reactions and also accelerate deterioration more directly in biodeterioration and in acid hydrolysis. The processes of biodeterioration, evidenced in the development of mould and the foxing phenomenon, are well documented (Florian 1994), but less well understood are the processes by which paper becomes acidic: by alum-rosin sizing on the paper machine, which leaves it a low pH, by contact with acid atmospheric pollutants, or by oxidation.

Hydroxyl groups in the cellulose macromolecule are oxidised to carbonyl and carboxyl groups. This oxidation leads to the discolouration of paper, and a decline in the physical properties indicating strength. Oxidative degradation can occur during manufacture in pulping and bleaching processes, or in the introduction of trace metals, which act as oxidation catalysts. Lignin residues, sunlight, UV light, atmospheric pollutants, heat, and humidity also facilitate the oxidation of cellulose in paper fibres.

Clearly, preservation activities in controlling environmental conditions play a vital role in diminishing or eliminating the sources of damage which lead to irreversible degradation.

2.1 Degradation Mechanisms

The identified mechanisms of deterioration, of which acid hydrolysis is the most widely documented, also include oxidation, alkaline hydrolysis, autohydrolysis, an increase in crystallinity and cross-linking, and pyrolytic chain scission (Daniels 1988). These latter mechanisms have received little investigation, and our concept of the deterioration process is restricted to the synergistic effect of moisture content, temperature, and acid content. In effect, and as a result of environmental conditions, the acid hydrolysis resulting from the combined effect of these variables is greater than the sum of the individual effects (Venter n.d.).

The critical role of environmental factors - temperature in the accelerated reaction rate, and humidity in precipitating this important oxidative mechanism of chemical deterioration - has been neglected in discussions on the causes of deterioration, and forms the subject of ongoing research.

3. Recommended Storage Life Values

We are obliged to acknowledge the obvious relationship between the effect of storage temperature and relative humidity on the permanence of paper. The generally accepted values of 20°C and 50% RH have been adopted from museum-based studies, and focus on physical stress of composite artefacts experienced under fluctuating moisture content.

Based on the diversity and composite nature of museum objects, these recommendations were spontaneously transferred to book conservation, but are misleading in their general application to library and archival collections. The problem lies in the varied material responses of composite collections, incorporating photographic holdings, particularly acetate-based, which require a very low temperature, or vellum and parchment bindings, for which close humidity control is paramount (McCormick-Goodhart & Mecklenburg 1993). The point of consideration is whether the recommended values of 20°C and 50% RH are acceptable for books and paper.

Stefan Michalski reported to the 10th Triennial Meeting of the ICOM Committee for Conservation in Washington in 1993 that, in tracing the derivation of these generally accepted values, there seemed more justification for their recommendation in their mechanical feasibility in a temperate climate zone than by any research designed to determine the values that minimised damage (Erhardt & Mecklenburg 1994). Many references to an optimum value cite Gary Thomson's excellent work, The Museum Environment, but Thomson merely chose 55% RH as midway between the upper limits of 65-70% RH to prevent mould growth, to which books are susceptible, and a lower limit of 40-45% RH for the cracking and distortion of materials such as wood and ivory, and the brittleness of paper (Thomson 1986).

A fluctuating relative humidity is critical to the development of physical stresses within composite structures and oxidative degradation of paper. The appropriate response is a stable RH - in the avoidance of damaging extremes - rather than a specific value. We have to consider whether we have not adopted too readily the values recommended for temperate climatic zones without consideration for the psychrometric implications for the conservation of paper-based collections in a variety of climatic zones. It may be more expedient to select values of relative humidity close to the ambient conditions of a given location, which fall between the lower limit of 30-40% RH and the upper limit of 70-80% RH, and which can be maintained in the narrow range of temperatures that allow for human comfort, which has frequently dictated environmental conditions.

3.1 Age Testing Techniques

The acceptance of recommended storage life values hinges on the tenuous credibility given the measurement of deterioration by age testing techniques. Accelerated ageing relies on a manipulation of temperature and humidity to predict the permanence of paper by measuring the decline in physical properties, such as folding endurance and tensile strength.

The concept of paper permanence thereby assumes a direct relationship between the chemical changes in the cellulose and the physical properties in the paper. However, this is not always the case. The property of folding endurance is highly dependent on humidity control, and will increase with greater humidity. The complexity of the degradation processes involved - primarily hydrolysis, but also oxidation and cross-linking - make this assumption questionable. The rate of change of a physical property depends on the rate at which any one of these reactions proceeds in relation to the others (Graminski, Parks, et al. 1978).

Accelerated ageing provides an indication of the life expectancy of different papers, based on environmental factors, but is a limited tool. Dry oven ageing reflects only the degradation associated with the mechanism of acid hydrolysis, while the humid oven will find both acid degradation and oxidative degradation. Moisture hysteresis, humidity excursions, and stress relaxation have a significant and permanent effect on paper properties, but are beyond the tolerances of accelerated ageing procedures. We do not have the means at present to measure the effect of these factors. Until we are able to accurately predict life expectancy, standard recommendations for environmental control have limited scientific basis.

3.2 Standard Values and Ranges of Temperature and Humidity

A growing scepticism in conservation circles reflects the concern over a reliance on standard recommendations for environmental control. A variety of research findings published in recent years have contributed significantly to our understanding of the processes of deterioration and their implications for collections. It would appear that temperature and humidity are best held as low as possible, to reduce the rate of chemical reactivity, but that the selection of appropriate values should be based on the provision of a stable environment rather than an adherence to an unfounded generalisation on environmental standards. The selection of suitable values demands cogniscence of the constraints of a particular situation - the ambient climate, mechanical feasibility, or limitations of budget - in the establishment of long term storage parameters for each institution.

The "bombshell" that threatened to completely destroy the traditional concept of environmental control in conservation management, was the announcement in August 1994 that the work of Smithsonian scientists revised guidelines for climate control in museums and archives. In dismissing the "ideal" environmental conditions of 20°C and 50% RH, they claimed to have found that museum objects can safely tolerate as much as 15% fluctuation in RH and as much as 10°C difference in temperature. This new insight, they declared, could save museums millions in construction and energy costs to maintain environmental conditions once considered essential for the preservation of artefacts (Smithsonian 1994).

While this statement begs further qualification, the current debate on environmental control is supported by the valuable ongoing research undertaken by the Smithsonian Institution (Erhardt & Mecklenburg 1994) and others.

4. Examination of Psychrometric Theory

In order to follow the debate, we need to examine the rationale and techniques used to monitor environmental conditions for cultural collections in museums, libraries, and archives.

Psychrometrics is the science involving thermodynamic properties of moist air, the definition must be broadened to include the effect of atmospheric moisture on human comfort and materials, and the method of controlling the thermal properties of moist air (Gosling 1980).

The basis for the measurement of air properties is the psychrometric chart, from which we can determine the dry bulb temperature (DB), wet bulb temperature (WB), dew point temperature (DP), specific humidity, relative humidity (RH), as well as specific volume and specific enthalpy. Any two of these independent properties locate the point of intersection on the psychrometric chart which defines the state of the air. Once this point is located, the remaining properties can be read from the chart.

The relationship between temperature and humidity is defined in terms of dry and wet bulb temperatures, read off from the hygrometric table, using a sling psychrometer. This relationship serves in the calibration of the ubiquitous thermohygrograph. While the thermohygrograph records a direct reading of temperature and humidity, and more advanced institutions may have progressed to digital thermo-hygrometer, or even to digital dataloggers with software interface for PC, these techniques tell us little of the other properties representing the state of the air, and nothing of the response of the artefacts. RH is merely an expression of that amount of water vapour as a percentage of the maximum amount which that same volume of air can hold at that temperature. Thomson simplifies the concept by explaining that "air at 50% RH, whatever the temperature, is therefore holding half the water it can" (Thomson 1986). In effect, warm air can hold more water, and cold air less. But it serves little purpose to collect thermohygrograph readings if the information is not extrapolated in a decisive strategy for the preservation of collections. The relationship between psychrometric theory and preservation practice lies in what is known as the equilibrium moisture content (EMC) of materials, which can be measured. It is the amount of water vapour held by a material when it has reached equilibrium with its environment, expressed as a percentage of its dry weight (Stolow 1966). When the RH drops, moisture is released, and when the RH increases, some moisture is re-absorbed. It is in the relationship between relative humidity and moisture content that the deterioration rate of materials can be measured.

4.1 Condensation on Unventilated Surfaces

An important consideration in the appreciation of a stable relative humidity is the concept of dewpoint, which reflects the interdependence of temperature and humidity. Familiar in the memory of childhood fingerpainting in the film of condensation on a glass surface, the dewpoint allows us to establish the maximum amount of humidity a room can hold before the air is saturated at that temperature. Below the dewpoint temperature, the air can hold less water vapour and some is condensed.

The dewpoint specifies the absolute water vapour concentration in the air. Relative humidity is the ratio of this absolute water vapour concentration to the maximum water vapour concentration possible at that temperature. This maximum increases with temperature. Air with a specific water vapour concentration (dew point) has a lower relative humidity at a higher temperature (Vitale & Erhardt 1993).

This transfer of water from the atmosphere to the liquid state as the visible condensate we used much like a slate as children, is not apparent in our library and archival repositories, as much of the moisture is taken up directly by the porous substrate of the paper in books and documents.

Acting as a natural moisture reservoir, the equilibrium moisture content of paper responds to fluctuations in RH, with dewpoint representing an extreme. Books in particular take up more moisture than single sheets, or than material protected in microclimates. The adjacent sheets of a book, exposed on three sides in the microclimate of a poorly ventilated shelf-space, provide a greater capillary force in sorption and desorption than flat paper, maps, or works of art. Ventilation plays an important role in deferring the dewpoint by mixing the air adjacent to the cool condensate surface, and it is particularly effective in conditions of high humidity. The application of the psychrometric chart to this problem should indicate likelihood and the potential risk of excursions to dewpoint.

Ongoing research will analyse the effect of the potential moisture reservoir of paper-based collections in relation to their storage environment (Brooks, Byrne, et al. 1994; Montori, Morrow, et al. 1994).

4.2 Dewpoint as a Predictor of Deterioration

In investigating the relationship of dewpoint as a predictor of paper discolouration and deterioration reactions, Timothy Vitale and David Erhardt of the Conservation Analytical Laboratory at the Smithsonian, conducted accelerated ageing tests under conditions ranging from 50-90°C and 30-80% RH at three specific dewpoints. They noted that an increased dewpoint resulted in increased colour production.

Bearing in mind the debate on the validity of accelerated ageing tests, they qualify this observation by adding that if artificial ageing conditions can be equated to room conditions, then dewpoint would be the best predictor of chemical stability in storage. The storage environment should therefore be based on the lowest feasible dewpoint rather than on the choice of temperature or specific humidity (Vitale & Erhardt 1993). The establishment of the air-conditioning set point at this chosen dewpoint should become the basis of air-conditioning systems.

The temperature can then be adjusted up or down to achieve an acceptable RH.

5. Revised Guidelines for Climate Control in Museums, Libraries, and Archives

It is useful to reconsider the guidelines for environmental control in the light of deterioration mechanisms induced and accelerated by fluctuations in relative humidity. Our traditional understanding of these guidelines, based on forces of physical stress and of biodeterioration, is developed by a consideration of the chemical stability of paper artefacts in storage. There are clearly several factors, perhaps previously neglected, that are vital to the preservation of collections:

  1. Recommendations of fixed values of temperature and humidity for the preservation of collections are not universally applicable
  2. It is preferable to maintain a consistent RH in a range as low as possible under ambient conditions, rather than a specific value
  3. The relationship between relative humidity of the air and equilibrium moisture content of materials will determine the rate of deterioration
  4. It is advisable to plot likely dewpoints under the given conditions, and to select the lowest feasible dewpoint as the air-conditioning set point

These guidelines for environmental control redefine accepted theory, and transcend the superficial treatment of the symptoms of deterioration. They reflect important research into the causes, and take cogniscence of the chemical reactions involved. Furthermore, they are consequential - they demand a re-evaluation of the current practice of environmental control for collections care.

Acknowledgements

Dr. A. N. Kaniki, Department of Information Studies and
Professor R. Osborne, Dept of Chemistry, University of Natal.


References and Bibliography

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