hello Almut:
I have some slight experience of repairing shellac discs over
the years, with rather variable results. Methods I have tried
for producing a physically less precarious object include --
with varying levels of success in varyingly desperate cases:
1 'superglue' -- no gap-filling, which may be essential;
it can be applied externally to cracks as it finds its own
way in; used successfully to stabilise small edge or internal
hair cracks
2 epoxy resin -- in strategically-drilled holes near the edge
and/or near the label (to avoid the unwanted gap-filling
and bulking effect); only really feasible to stabilise
large edge/internal cracks
3 self-adhesive tape -- feasible on single-sided discs
4 judicious application of a soldering-iron [!!] -- ditto [??]
I have a couple of nasty-looking but otherwise completely
successful examples of two half-discs re-assembled in this
way many years ago...
However, there are very different situations to be covered,
ranging from discs with more or less safe (and closed) edge
cracks that may be stabilised very successfully, via lamination
cracks (impossible to deal with properly), 'open' cracks
(stabilisation possible, but clicks remain) to badly
warped/cracked discs and complete breakages (two clean halves
really being the limit).
Sometimes these methods have worked better than expected, with
nicely lined-up grooves and no obvious ticking sound when played
(at least in mono). On other occasions even apparently well
aligned grooves remain very 'clicky': this is especially
true for laminated discs where almost invisible 'surface'
shifts occur.
The above methods generally suffer from being (to all intents)
irreversible -- not exactly an archival/restoration ideal!
A big problem is that, as Will Prentice implies, the result
of breaking 'brittle' material is variable. Generally, the
elastic and plastic limits do not coincide, so that even a very
hard and brittle disc will deform slightly before breaking,
depending on how the force is applied (ranging from slow
bending pressure to a sharp blow). This means that the pieces
(either separated or only across a crack) are very likely not
to join up properly, even if dealt with immediately -- and
that's not including possible further deformation due to
stored stress (as Will explains) from either original pressing
or from subsequent poor storage that caused the breakage.
As Will points out, the only workable solution is not to have
any breakages in the first place. Successful moving requires
sensible (firm!) handling and storage -- and preferably
some experience of what the discs can withstand. I once
received an overloaded parcel of 45+ [!] 78rpm discs from the
USA that arrived with (almost literally) a spherical shape: but
the discs were so firmly strapped together that there was not
one single breakage.
best wishes,
Peter
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