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4 and 5 shows general similarity in the responses.
However there are some differences. Eq. 4 and 5 show
dependence on RH, but not Eq. 1 and 2. This may be
due to larger variation in environmental parameters,
including RH, during the 8 years natural weathering
experiment from which Eq. 4 and 5 were derived [4] ,
as compared to the indoors experimental exposures of
the PPO/EWO. Eq. 5 also shows dependence on SO 2
which was not found for the PPO/EWO, except at high
doses and humidity in the laboratory. Figure 3 shows
correlation of calculated effects on EWO with that on
PUR (Eq. 4) and PES (Eq. 5) using input from the 80
measurements of the environmental parameters in 10
European Museums in the EU project MASTER [3] .
Measured values for UV were not used in the equation
due to the difficulty of recalculating the UV values
measured in units of mW/m 2 into the global irradiance
units used in Eq. 4 and 5. As the UV values measured
in the indoors locations were 0 or very low this should
not give a large error.
Even if the correlations between EWO/PUR and
EWO/PES seen in Figure 3 are not very good they
do suggest degradation mechanisms that depend on
the same environmental parameters, as is also seen
from the Eq. 1, 2, 4 and 5. The numerical change in
the damage parameter for the PES compared with
the EWO is somewhat higher than that for PUR.
However, to compare EWO levels with damage of
these materials it would be necessary to establish
levels of “real” perceived damage dependant on
environmental levels as in Table 3 and 4. For modern
synthetic materials that in some cases may be more
vulnerable to degradation than most other organic
materials, lower tolerable levels and more stringent
environmental control may be needed.
The humidity dependence in Eq. 4 and 5 explain
94 % (PUR) and 74 % (PES) of the effect on PUR
and PES not explained by EWO (the constant in the
correlations) in Figure 3. The SO 2 dependence in
Eq. 5 only explains 0.6 % of the effect on PES not
explained by EWO in Figure 3. Thus, the EWO can
represent PUR and PES degradation effects except
the unfortunate lack of sensitivity of the EWO to
humidity. EWO values representing determined
threshold levels for the degradation of PUR and
PES (Eq. 4 and 5) can be calculated from the
correlation equations for PUR and PES in Figure
3. The constants in the correlation equations would
then mainly represent average humidity effects
expected on PUR and PES indoors in museums.
When directly comparing degradation of dosimeter
and object material, the EWO response cannot be
indirectly adjusted for RH effects using the RH-T
interdependence as was the case above when using
environmental threshold values. From Eq. 4 and 5 it
can be seen that the RH effect is linear at constant
temperature and duration of exposure and relatively
large compared to that of the pollutants. Thus, to
reduce the rates of these degradation effects it is
important to keep the RH as low as possible. It
would be recommended to perform independent
RH measurements. This should be reflected in
guidelines accompanying use of the EWO.
Conclusion
Dosimetry is a useful method to assess the quality
of museum environments to assure good preventive
conservation of museum objects. The main advantage
of dosimetry, compared to measurements of single
environmental parameters, is that dosimeters measure
generic effects comparable to those on objects, of
several environmental parameters usually including
both pollutants and climate. A main challenge in
using dosimeters is to relate the measured effects on
dosimeters to the real effects observed on objects.
This can be accomplished by relating the measured
effects to tolerable levels of the single environmental
parameters or by direct comparison with the objects
to protect. In both cases the dosimeters need to be
sensitive to the major degrading influences on the
real object. If this is not the case combined use
of several dosimeter types, e.g. one sensitive to
oxidising another to acidic pollutant gases, is an
option. EWO-dosimetry can most easily be used to
assure good environments for collections including
modern synthetic materials by applying general
threshold levels set for organic museum objects, or
possibly levels modified to fit effects on modern
synthetics. To measure degradation risk for particular
modern synthetics one needs to compare known
dose-response data for those materials with the EWO
dose-response equation, to evaluate applicability and
determine threshold levels.
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