Adhesive for glass

In DistList Instance 12:13, Robert K. MacDowell
<macdowell_r [at] mediasoft__net> writes

>Occasionally we are faced with important items of glassware having
>cracks that do not result in actual breakage of the object. We would
>like a good approach to wicking an adhesive into the cracks and
>having it cure slowly (to allow the wicking process time to
>complete).  I am not enthusiastic about the use of epoxy because of
>the risk of yellowing.

I have worked with this problem and with the adhesives mentioned as
well as others and would add the following notes (in no particular
order) to what's been said by Koob, Deller, and Sloan:

    1)  All of the epoxies will yellow to lesser or greater extent
        eventually (see Jane Downs, CCI in Studies in Conservation
        29, 1984), but most of the color change I've observed
        phenomenon occurs at the surface, so a thicker cross section
        may hold its appearance better than a thin one.

    2)  The match of refractive index, resin to glass, is a more
        important factor than yellowing (see Norman Tennent in
        Adhesives and Consolidants, IIC 1984).

    3)  The index of glass varies widely with its chemistry, the
        index of epoxy resins fit into a narrower band and cannot be
        manipulated much.

    4)  Hxtal NYL epoxy is more viscous than many of the others
        making it not my first choice for penetration in tight
        places.

    5)  There is no reason to believe that UV curing monomers are
        much more color stable than epoxies.

    6)  The use of B72, or any other solvent-based application
        system will yield a cosmetic result that is likely to be
        unsatisfactory because as the solvent evaporates the
        adhesive film left behind becomes incoherent.

    7)  The difference between the thermal expansion characteristics
        of resins and glass may lead to a failure of adhesion inside
        the crack, leaving you a little worse off than where you
        started.

Kory Berrett
Berrett Conservation Studio
3054 Reisler Road, Oxford PA 19363
610-932-2425

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                  Conservation DistList Instance 12:15
                 Distributed: Wednesday, August 5, 1998
                       Message Id: cdl-12-15-001
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