Barrier materials used in framing

Emily Gilbert <emily_gilbert [at] hotmail__com>

>I am currently researching materials used as a barrier between the
>back of a mount and the backboard of a frame (usually oil tempered
>hardboard). I am currently looking at the effects of using melinex,
>aluminium foil, 'marvelseal' and 'moistop'

At the Netherland Institute for Cultural Heritage (ICN) we are
studying since some years the effect of backboard protections on the
conditions of a canvas. In particular we have focussed our attention
on the effect of backboard protection on the microclimate behind the
canvas. This analysis can be applied to mountings of paper objects
as well.

We have seen that the materials used can be divided in three
category:

    *   hygroscopic and  permeable materials, like simple
        cardboard,

    *   impermeable and hygroscopic materials, like kapa-line
        (polyurethane foam between sheets of paper) or lexan sheets
        (polycarbonate) with a sheet of cardboard added between the
        canvas and the lexan.

    *   impermeable, not hygroscopic materials like simple lexan,
        perspex, melinex.

They can also be attached in different ways, they can be nailed to
the stretcher or taped. Our conclusion is that in constant
temperature conditions  backboard protections have an effect in
reducing RH fluctuations on the back of the object and that the
extent of this reduction depends on:

    1.  the leakages present in the system (materials permeability,
        eventual corners cut-off in the backboard, other leakages).
        The higher the leakages, the smaller the RH fluctuations
        reduction;

    2.  the total amount of hygroscopic materials present in the
        system (the object itself and the backboard material, if
        hygroscopic, etc). The higher the amount of hygroscopic
        material, the higher the RH reduction.

    3.  the speed of the hygroscopic material to respond to RH
        changes.Thin open materials like cardboard are very fast and
        do buffer RH fluctuations, while thick materials like wood
        are very slow and have only a small effect on RH buffering
        in comparison with their mass. In the case of a temperature
        gradient between the back of the object and the front (due
        for example to a cold wall), the situation is more
        complicated.

We have performed some measurements on the effect of a cold wall on
the internal RH for the National Trust. If the object is hanged on
the cold wall (say at 10 deg. C)  with a certain distance, the
temperature gradient between the room (kept at 20 deg. C) and the
backboard is of the order of 5-8 C. In this case if the backboard is
hygroscopic, it will buffer the back of the object to a higher RH
than the room value, say about 3% higher. This value was
experimentally measured and it can be predicted looking at the
temperature dependence of the sorption isotherm of hygroscopic
materials.

The consequences of more dramatic temperature gradients are nicely
described by Tim Padfield in his "Himalayan legend"
<URL:http://www.natmus.dk/ixgb.htm>.

We plan to study in the future the temperature effects of backboards
and therefore to take into account the use of thermo insulating
materials. If you would like to know more about our research you
could read our paper on the last number of Studies in Conservation
(vol 44 number 4 (1999)) and the follow up published on the
proceeding of the 12th Triennial meeting of ICOM-CC in Lyon. For the
use of aluminium or metallic backboard you could also consult the
final thesis of Manuela Frankestein and the following research at
the Fachhochschule fuer Konservierung in Koln.

As far as I understand from the geometry you describe, you use an
impermeable material between the back of the object and the
backboard and you are basically screening off all the RH buffering
effect of the hygroscopic backboard. Did you think about applying
the impermeable material on the back of your backboard?

Giovanna Di Pietro (1-2) and Frank Ligterink (2)

(1) Abt. fuer Wissenschaftliche Photographie, University of Basel
(2) Netherlands Institute for Cultural Heritage, Amsterdam


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                  Conservation DistList Instance 13:36
               Distributed: Wednesday, December 22, 1999
                       Message Id: cdl-13-36-001
                                  ***