Stuffed specimen and hygrometry

Gerard Cobut <gerard.cobut [at] naturalsciences__be> writes

>In the Natural Sciences Museum (Brussels) I am currently facing a
>conservation issue. Relative humidity is decreasing steadily in one
>showcase; the conditions in it are now really too dry.
>...
>The showcase has 2 Art Sorb cassettes in its right wall; the holes
>through the wall (between the cassette container and the showcase)
>were made according top the Art Sorb instructions manual.
>...
>... Mean hygrometry is
>steadily decreasing (+/- 45% relative humidity on November 18, less
>than 35% on December 4, 2008).
>
>Has anyone experienced something similar? Is the instrument we use
>known to be reliable? What is the Art Sorb's behavior? Is it
>possible that it would absorb "avidly"? Can the stuffed specimen be
>highly hygroscopic (absorbing humidity from the atmosphere)?

My short answer is: Yes!, the stuffed horse may be absorbing the
moisture from the showcase microclimate. Consider the mass and
surface area of hygroscopic material in the horse's pelt alone,
without regard for the 1871 materials in the interior of the body.
Compare this to the mass and size of the two cassettes of Art Sorb
passive buffering. No contest! Horse 1, Buffering 0.

I do have some questions that might help prove the point, or solve
the mystery:

    What humidity was the stuffed horse acclimatized to before being
    put on exhibition? If the answer is "Less than 35%" you can stop
    here. If the answer is not "Less than 35% RH", then you might
    continue asking:

    What is the current moisture level of the passive buffering?
    (Place it in a bag with a hygrometer.)

    What was the moisture level of the passive buffer when you
    installed it?

    What is the ambient relative humidity outside the showcase?

    Is there any relationship seen in the showcase environmental
    data between case interior and exterior ambient humidity?

    Do you know the case leakage rate? (Can you be certain that the
    showcase is well sealed?)

Assuming that your guess (and mine) is correct, and we can assume
that the horse is acclimatizing and still taking in moisture,
there's a positive outcome: Once the horse has reached its new
equilibrium, you will have little need for the passive buffering
cassettes. As demonstrated, the stuffed horse should be able to hold
its own, with only occasional need of a boost from the buffering.

Jerry Shiner
Keepsafe Microclimate Systems
416-703-4696 ext 701


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                  Conservation DistList Instance 22:36
                 Distributed: Monday, December 15, 2008
                       Message Id: cdl-22-36-002
                                  ***