Tempered glass

A few weeks ago, an incident occurred at our museum and I am hoping
that recipients of the DistList can help explain it in order to
avoid similar occurrences in the future.   As our staff Christmas
party was progressing in an open courtyard one floor below, one
large glass pane from one of our exhibit cases suddenly shattered of
its own accord.  The only person nearby (about 30 feet or 10 m away)
was one of our security guards, who saw it happen while the whole
staff downstairs suddenly went very quiet from the tremendous sound
the glass made as it cracked and then hit the floor.

The pane that broke was made of tempered glass, 3/8 inch (1 cm)
think. The pane measured 48 X 72 inches (122 X 183 cm) and was
installed on one side of a four-sided open-top display case.  It was
retained to the other panes of glass by padded locks at both upper
corners.  The lower end of the glass pane slid in an 8 inches (20
cm) deep groove along the case platform.   As the glass broke, it
shattered in thousands of small pieces and spread on an area ten
feet (3 m) in diameter.  The locks remained in place, holding on to
a few fragments of broken glass.

Within a few minutes after it happened, we were on the scene,
recuperating the objects from the case and cleaning up the area.
The case contained a small number of 20th century objects related to
hockey (skates, sweater, gloves, hockey cards).  Five cards were
slightly damaged (four with small indentations from the broken
pieces of glass; one with a crease from the weight of the broken
glass), but all other objects were found intact, after all the
broken glass had been carefully removed.

One identical glass pane already had broken at our museum, but it
happened as it was being moved and a corner accidentally hit a wall.
I also know of one personal instance when a drinking cup, also made
of tempered glass, shattered of its own in our cupboard shortly
after it was removed from a dishwasher.   As this latter incident
indicated the cause as a sudden temperature fluctuation, I carefully
checked our environmental records for that day and found no
significant temperature or relative humidity fluctuation for several
days before the incident.  Local heating caused by lighting is also
not a factor as one single 60 watt incandescent track light shines
through the pane, from 8 feet (250 cm) away.

This incident has raised concerns at our museum for both the safety
of people (visitors and staff) and the safety of the artifacts
within our exhibit cases, most of which are built with tempered
glass. Besides the very real possibility of injury, a person
standing by could even be blamed for causing the break, while it may
have occurred on its own.

Already our staff was using proper protective equipment when
handling the panes and for the immediate future, we plan to install
clear acrylic shatterproof resistant safety films (manufactured by
Madico Inc. in Massachusetts) on all our tempered glass panes.
Before we go ahead with the project, I need to find out if something
similar ever happened in other institutions and if so, what
solutions were adopted.  I also believe that it is crucial that we
all become aware of this hazard, especially if it is in fact more
than a "freak" occurrence.

Bruno Pouliot
Conservator of Three-Dimensional Objects
McCord Museum of Canadian History
690 Sherbrooke West
Montreal, QC H3A 1E9
Canada

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                  Conservation DistList Instance 10:65
                 Distributed: Friday, January 17, 1997
                       Message Id: cdl-10-65-008
                                  ***