Marilyn,
It seems to me that this was one of those instances where one keeps a sample
of the experimental materials in the archives and for display purposes.
Knowing that the material was experimental to begin with, helps with the
decision to install a more stable, but similar in appearance product.  If it
is unrepairable and has proven to be a decisively failed experiment, it
doesn't make sense to restore L'Cs significant architectural statement with
bad stuff.
If Jefferson had tried to roof Poplar Forest with shellac and maple leaves,
would we - should we - restore with the same? and watch the building fail? or
figure out a better alternative without compromising Jefferson's intentions of
experimentation.  The public - the benefactors of the restoration, can still
experience a structure if decisions like those made at L'Cs must be taken.
Each building must speak for itself.

Candy B