Glass

Julia Merkel <merkeljm [at] jmu__edu> writes

>I'm sorry to ask a "Dear Heloise" type question to such highly
>trained professionals, but a colleague of mine here in the library
>has inquired about the repair of a small, broken painted glass
>lampshade.  She would like to repair it herself as it has
>sentimental rather than monetary value. Is superglue a reasonable
>suggestion?  Is there an a preferred "over the counter" epoxy for
>the lay restorer? I have no experience with glass but cautioned to
>support with sand/bean bags and try to glue the pieces as
>simultaneously as possible so that by the time she gets to the last
>piece--it is still a circular fit. ...

You are on the right track when you try to work as simultaneously as
possible. And the sandbags might be of some help as well. A bed of
sand can help to position your piece very well. Superglue is no
solution, you will see why. Be aware that glued glass can not be
compared to glued wood e.g.. The splice will always be the weak
point! Maybe you can also find an expert for UV-setting adhesive
systems. I do not know enough about that to give you advice.

The system I describe is used by the glass-specialists at the RGZM
(Roemisch Germanisches Zentral Museum) at Mayence in Germany to
repair (and  on the next level even to complete) archeological
glass-artefacts. The main clue is to glue *after* the whole artefact
has been put back together again. You do not go piece by piece, as
you can work in the field of archeological ceramics e.g.

We use an epoxy-type of very low viscosity (Araldit 2020A/B). In
general high temperatures will soften epoxy for the time of
exploration and cause yellowing and aging as well. So the lampshade
should retire after restoration if you decide to use epoxy.

The following is a quick overview rather than a manual since
restoring glass is a very complex and sophisticated field. But why
shouldn't you try it:

    1.  Clean all fragments thoroughly with acetone or some other
        solvent, focus on the breakings. You might want to wear
        gloves to keep the fragments clean. Depending on the design
        of the artefact you can work the following steps from both
        sides.

    2.  Put all the fragments together using very small strips of
        tape about every centimeter (we cut very small strips of a
        sort of brown package tape and use tweezers to attach them).
        The breaks have to be put together absolutely exact! Like in
        a computer there is only yes or no (but with glass it is
        easy). This is essential since later on we will need the
        capillary suction to bring the epoxy in. Also this is why we
        do not use superglue which is setting too fast and why piece
        by piece will not work. Check the breakings softly with your
        fingernail to see wether they are alright or not.

    3.  Use omega-shaped wire pieces (we use brass, 1mm in diameter)
        and glue them between the tapestrips like bridges over the
        breaks. Use superglue therefore and make sure that you do
        not bring it into the breaks! Hold the bridge with tweezers,
        dip the ends into a drop of glue (do use moderate amount of
        glue) and position it over the break.

    4.  Remove all the tape.

    5.  Bring the epoxy onto the breaks and you will see it running
        into the breaks. Bring drop by drop with some dentists tool
        and make sure to not extra-glue the bridges as well.

    6.  Leave it for several days, allowing the epoxy to set
        completely. Then take off the bridges. Depending on the the
        stability of the whole object you can carefully break them
        off or you have to detach them by using solvents. Before you
        use solvents give the epoxy another few days. Clean the
        surface carefully of superglue and epoxy remains, use
        cotton-wool tips with solvents for the superglue and small
        scalpel-blades for the epoxy.

If the glass does not have an even surface you have to be much more
cautious with (5) since then it is much harder to clean it
afterwards.

Sebastian Keil
Conservator
Germany


                                  ***
                  Conservation DistList Instance 19:14
               Distributed: Saturday, September 10, 2005
                       Message Id: cdl-19-14-004
                                  ***