Fill materials for stone

I am writing the section on structural fills for stone in the
upcoming JAIC special edition on loss compensation, and would
appreciate any input on this or related subjects for possible
inclusion in the overview.  I would like to find out how individual
conservators have developed their own recipes for making translucent
fills with fumed silica-bulked epoxy, etc. I am also interested in
how conservators make porous structural fills on other types of
stone where water vapor transmission, liquid water transport,
overall strength, and other properties of the stone must be
considered.

My co-author, Sari Uricheck has recently completed a survey report of
marble fill techniques, and her work will be included. In light of
the recent thread of discussion on the use of fumed silica in epoxy
resin systems (see Mark Vine's query of July 27, and  Barbara
Applebaum's and Kory Berrett's responses), I feel there may be a lot
of practical experience out there which hasn't been published.  I
offer the observations below in the hope that others will share
their success or failure stories:

I have personally been using Cabosil fumed silica in Hxtal (both
Nyl-1 and Crystal Plus versions) for a long time for the main
ingredients for a recipe to match translucent white marble in an
outdoor setting (see WAAC News Vol 12, No. 2, May 1990, pp. 9-15,
where I described a recipe which utilized small amounts of
Thompson's hard fusing white glass enamel powder to give a very
white but translucent fill, and to simulate a saccharroidal
surface).  The thixotropic nature of the mixture does allow a great
deal of fumed silica to be added to a given quantity of epoxy.
Increasing the amount of fumed silica increases the tendency of the
mixture not to slump, and reduces the ultimate strength of the fill,
but it seems to be the amount of stirring which determines whether a
more opaque, doughy texture is achieved or a clear, Vaseline-like
consistency and translucency is reached.  I have mixed up to ten
times (by volume) the amount of fumed silica to epoxy and have
eventually gotten it to be a Vaseline-like translucent gel by slow
stirring.

With regard to the strength of the fill, it is often desirable to
reduce the strength for the sake of workability and for
compatibility/reversibility on deteriorated materials.  Of course,
when using abrasive techniques, especially rotary tools to shape the
fills, extreme care should be used to avoid breathing or other
contact with the dust.

Another problem associated with the use of fumed silica-bulked epoxy
is the tendency of the fill, no matter how stiff the paste, to bleed
a liquid component into the surrounding surface, causing
irreversible darkening. Even at the risk of weakening the bond
between the fill and the original surface, an isolating layer must
be applied (e.g. Acryloid B72).  I am not aware of any studies
determining the effectiveness of such a barrier to block migration
of amine hardener, etc. but from practical experience I find that
this works.  There is a great temptation not to use it, especially
where the substrate is very translucent and the resin may tend to
outline the fill and draw attention to it.  This temptation must be
resisted at all costs.

It has now been about six years since we used the above recipe
outdoors on three marble artifacts at Hearst Castle, and we are now
able to evaluate the resistance of the translucent fills to
yellowing. One of the artifacts, a Roman well font, is in direct
sunlight year round.  The large, more translucent fills on areas of
smooth, polished marble have yellowed noticeably (not yet to such a
degree that they are disfiguring).  Those fills where a sugary
appearance was required are still a very close match. This is
probably due to the white glass frit dispersing the UV at the
surface.  No other means of UV stabilization was used, but there may
be hope for the use of UV stabilizers in the mixture.

If anyone has any experiences to share or references in the
literature, please contact me directly at:  griswold [at] silcom__com, by
phone at (805) 565-3639, or by fax at (805) 565-3649.

John Griswold
Wharton and Griswold Associates, Inc.
549 Hot Springs Rd.
Santa Barbara, CA 93108

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                  Conservation DistList Instance 10:22
                Distributed: Wednesday, August 28, 1996
                       Message Id: cdl-10-22-001
                                  ***