IAP 1998, Presentation 1 :


University of East Anglia


The interest in air pollution in museums has often focused on measuring the concentration of pollutants rather than addressing the damage that they cause. If we examine the damage caused then we become aware of which pollutants should be of principal concern. An interest in damage raises the issue of the "health of collections" and an analogy with medicine even though human populations and the museum collections offer some interesting distinctions.

Same species, but genetically
diverse with different health status
Different materials in different conditions
80 years
Long term for most objects
Chronic and acute
Cumulative (from air pollution)
Consider sensitive individuals
Sensitivity, uniqueness, value
Depends on toxin
Exist thermodynamically, but practically improbable
Concentration - in reality : exposure or dose
Usually concentration , but cumulative flux is best

Thresholds are required by the laws of thermodynamics for the damage of solid objects by gases, but these would set up the allowable concentration of pollutants at impossibly low values. Kinetic thresholds are also possible where reactions are exceedingly slow.

In a practical sense we should consider operational thresholds. This can be thought of in two ways :

  1. We could consider this as the concentration where the rate of damage from the pollutant becomes less significant than damage by other mechanisms.
  2. Alternatively, it could be the concentration where survival time is sufficient.
In the case of leather, we have shown that book bindings in the British Library in London have accumulated 4.8% sulfate, while those at the National Library of Wales in Aberystwyth have accumulated only 0.5 % since the 1930's. The latter bindings are serviceable, while the former have weakened to a point where they are unusable. On this basis and setting a lifetime of a century we have suggested 0.4 ppb SO2 as a standard or operational threshold for the preservation of leather bindings.

The question of standards raises the issue of units. These can be as concentrations with a specified time interval, but this could be represented more explicitly by dose concentrations. In museums, concentrations are often measured but expressing these as a dose parameter would continually remind the conservator that cumulative flux causes the damage not concentration. Dose-flux conversions require thought about deposition velocity. Where possible, gas concentration should be expressed in dimensionless units such as the mole fraction, but more commonly represented as ppb (by volume in the gas phase). Mass units lead to problems because the formula must be expressed.


Peter began by outlining the dangers of thinking of thresholds.
Population : Whereas humans can be generally regarded as genetically homogeneous, museums contain fundamentally different materials.
Damage : Humans experience chronic, acute damage effects, whereas museum items are effected by cumulative damage.
Lifetime : Humans have a finite lifetime which one can approximate to be about 80 years, in contrast, museums generally need to consider a much longer time for the preservation of collections.
Standards/Ethics : For humans, it depends on their sensitivity, for museums sensitivity, value and uniqueness must be considered. Who determines what is valuable or unique ?
Thresholds : For humans, this depends on the toxins, for museums, in the strictest sense thresholds are impossible to achieve.
Standards : For humans, need to think only of the concentration - Exposure-Dose. For museums, need to consider not only concentration but also the cumulative flux.

Types of damage
Pollution-induced damage is only one type of threat to an artefact. All must be considered when dealing with the protection of objects including;
EROSION OF OBJECTS : Polishing, dust, air and water movement all follow a dose law. With solids, erosion causes loss of surface detail, scratches etc.
BIODEGRADATION : multiple variables
FREEZING : induces physical stress
SOILING : particle deposition
CHEMICAL TRANSFORMATION STRESS : Pollutants, Dose-solid reaction with large volume changes (for example sulphation of stone).

Peter continuted to drive home the message that dose is the KEY, not concentration, as we need to consider the 'concentration multiplied by the time' parameter. Also need to consider the aerodynamic deposition velocity which is related to turbulent air processes and the surface of the objects. The product of both parameters is given the term cumulative flux. Need to integrate concentration verses time curve to get the cumulative results of the pollution deposition. We might also want to consider thermodynamic thresholds.

Finally, questions, or issues, that need to be tackled were raised :

  1. Should standards/guidelines be listed as concentration or dose ?
  2. Is concentration an instantaneous measurement ?
  3. Should spatially and temporal variables be considered ?
  4. Don't forget to consider the deposition velocity may be different for different materials.

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Index of presentations at IAP 1998 meeting

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