Consolidation of sepiolite-rich armorial panel

In Conservation DistList Instance: 29:19 Sunday, September 27, 2015
I was looking for "a reversible adhesive mixed with a solvent that
does not cause sepiolite to swell and which can be mixed with
micro-balloons or similar inert material to form a filler, allowing
us to re-adhere the surface in its correct shape".  An earlier
consolidation attempt of an armorial panel with Paraloid B72 in
acetone had caused surface lifting and fragmentation.

XRD analysis of samples identified near surface sepiolite pockets.
It appears that sepiolite is a rare but detrimental phenomenon that
can occur in both sand and limestones.   Although not found to be an
expansive clay, sepiolite nonetheless swells in reaction to polar
liquids e.g. water and hydration forces while being brittle in
itself.   Acetone has a polarity index (PI) of 5.  1 and during a
simple visual test, was found to cause a volumetric increase in
powdered sepiolite.

Subsequently, lower polarity solvents were tested that could still
dissolve the reversible and stable adhesive resin Paraloid B72.
Xylene has a PI of 2.5 and reacted similar to acetone but toluene
with an only slightly lower PI of 2.4 did not appear to cause such
changes in powdered sepiolite.  Further tests on exposed sepiolite
inclusions on the uncarved back of the armorial panel with solutions
of Paraloid B72 in toluene did not cause any visible surface
changes.

Subsequently, the consolidation treatment and restoration of the
surface of the armorial panel was successfully carried out with
toluene substituted for acetone.   This increased the setting time
of the resin and required adjustments in Health and Safety measures.
Improved environmental control inside the display space at Elgin
Cathedral is likely to be necessary to prevent further surface decay
to the panel.

I hope that these findings may be of use to stone conservators
coming across sepiolite inclusions in the future.

Christa Gerdwilker BA MSc ACR
Stone Conservator
Historic Environment Scotland


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                  Conservation DistList Instance 29:43
                  Distributed: Tuesday, March 29, 2016
                       Message Id: cdl-29-43-001
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