The Preservation Handbook Online

Table of Contents

I. Preservation
II. Assessment
III. Collection Assessment
IV. Agents of Deterioration
V. Monitoring collections
VI. Materials
VII. Storage
VIII. Conservation
IX. Disaster Planning
X. Housekeeping
XI. Resources / Suppliers
XII. Preservation Grants

Agents of Deterioration

Relative Humidity

The little known phenomenon of relative humidity is barely detectable to human skin but can be devastating for collection preservation. "Relative humidity is based on the percentage of water vapor in the air, compared to what the air can hold at 100 percent (full saturation) at a given temperature. As a consequence, when the temperature increases, the relative humidity decreases for a fixed amount of absolute humidity; conversely, a decrease in temperature causes an increase in relative humidity."xxxxiii Controlling and maintaining relative humidity is important for collections containing hygroscopic materials which are susceptible to dimensional changes during relative humidity fluctuations. The response of hygroscopic material to the ambient relative humidity will directly correlate with the equilibrium moisture content (EMC) of the object. Organic materials expand when relative humidity levels rise. The same material will shrink as the equilibrium moisture content diminishes. The lower the level of EMC the less flexible the object becomes. Composite objects such as books, paintings and photographs are highly affected by relative humidity change.

  • Fluctuating RH weakens fibers and eventually leads to cockling or fracturing.
  • Fluctuation in RH will loosen wooden joints and glued bindings.
  • High RH will encourage mold growth
  • Unstable RH can initiate chemical reactions that lead to metal corrosion, dye fading, weeping glass, and mineral deterioration.
  • Major factors that will affect the RH levels will include: building envelope; exhibit cases; climatic conditions. Stability is the most important means of minimizing negative reactions. Fluctuation in RH can accelerate rate of deterioration and cause serious damage. "Reasonable extreme limits of relative humidity in light of current understanding of it's effects on material are 50% maximum and 30% minimum."xxxxv Given the possible range of acceptable temperature and RH can leave an institution at a loss for determining the appropriate conditions for their collections. Don Sebera, a consultant to the Commission on Preservation and Access has developed the concept of isoperms which provide "...quantitative graphical measures of relative permanence."xxxxvi Although this tool has limited assessment abilities towards paper the concept is meaningful in understanding the long term affects of relative humidity and temperature. Recommended RH levels for differnt objects are discussed within the materials section. The composite nature of artifacts and the diversity of objects within collections will require some research and coordination by individual institutions.

    Controlling RH

    The HVAC system is the most effective way to ensure stable relative humidity levels. Typically dehumidification is achieved by lowering air temperture below it's dew point as it passes over chilled coils and drains off the resultant water. The degree of dehumidification which a sytem can achieve is designed into the system. Problems usuallly arise when central chillers are turned off in the winter months. Chillers should be operating year round provided there is adequate reheat available.

    Evaporative cooling systems can malfunction and are undesirable for storage control.

    Desiccant machines can be used to remove relative humidity in cool, dry storage areas.

    Increasing relative humidity can be done by steam injection into the air stream. Feed water can contain potentail contaminants and should be purified. Different size and types of "clean steam" generators are available.

    Humidifying areas using portable humidifiers can be done if: the capacity of the humidifier is appropriate; constantly maintained to remove accumulated dust and pathogens; and frequent emptying and filling of the case.


    "There are two types of silica gel. Regular silica gel is white, and the indicating type is blue. Indicating gel is especially useful because it shows when it has reached the saturation point by turning a dull pink. Indicating gel is much more expensive, but you can save money by purchasing a small amount of indicating gel and mixing it with the regular type. Once saturated, silica gel can be dried and reused by heating it in a 300 degrees F oven for three hours."xxxxvii


    XXXXIII. Weintraub, Steven. (1992) Creating and Maintaining the Right Environment. In Arthur W. Schultz (ed.), Caring for your Collections. New York: Harry N. Abrahms Inc. (18 - 29).
    XXXXIV. PaperHelp Accessed January 2003, at <>
    XXXXV. Banks, Paul N. (2000) Environment and Building Design. Preservation Issues and Planning. Chicago and London: American Library Association. (114 -144)
    XXXXVI. Sebera, D. K. (1994). Isoperms an environmental management tool. Commision on Preservation and Access.
    Accessedd February 2003, at <>
    Glaser M. T. (1999). Protecting paper and book collections during exhibition. (Technical Leaflet: the environemnet, section 2 leaflet 5). Massachusetts: Northeast Document Conservation Center.