Tempered glass

In response to Bruno Pouliot's inquiry about the spontaneous
breakage of tempered glass:

Tempered glass can break spontaneously if there is a defect in the
edges of the sheet, like a chip or scratch.  This defect can be
minute, not something noticeable at all.  It can happen during
manufacture of the glass, handling during delivery, or installation.
I would ask your supplier to look at it--they should guarantee
against such breakage, since it's likely that a defect was caused by
manufacture or handling.

It can also be caused by a pressure point in the installation.
Check your cases to be sure there are no nails or chips of glass or
glue or old setting compound that form a bump of any sort.

Instead of using tempered glass in cases, perhaps you should
consider laminated (safety; used in automobile windshields) or
tempered laminated, which is stronger.  The laminate layer is
adhered to the two layers of glass, so if the glass breaks, it is
held in place and doesn't fly all over.  The laminate is also
difficult to penetrate, so it would protect the objects in the case
from a projectile.  There are laminates of different
strength--bullet-proof glass has a polycarbonate laminate, whereas
ordinary laminate is polyvinyl butyral.

Julie Sloan

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                  Conservation DistList Instance 10:66
                Distributed: Wednesday, January 22, 1997
                       Message Id: cdl-10-66-004
                                  ***