Bloom on display cases

In the last couple of years we have begun to notice a bloom or haze
or fogging effect on the inside of many of our display case
vitrines. It can be virtually invisible when viewed straight on, but
at an angle it shows up clearly.  It occurs on both glass and
plexiglas, in cases from 3 to 10 years old, whether or not there is
climate control within the case.  The only potential pattern is that
the vast majority of the affected cases (but not absolutely all)
have used latex paint as a finish. (All paints used in display cases
have passed the Oddy test.)

Our first thought was that the deposit was due to glycols from
inadequately dried paint, but investigation and consultation with
the manufacturer leads me to believe that this is not the case.
Lower order glycols would be expected to be hygroscopic and liquid
at room temperature, whereas the mystery substance is crystalline,
and definitely non-hygroscopic.  Further investigation and analysis
is ongoing, but in the meantime I would be glad to hear from anyone,
either on or off-list, who has experienced something similar.

Helen Coxon
Conservator
Royal Ontario Museum
Toronto
Canada

                                  ***
                  Conservation DistList Instance 11:48
               Distributed: Wednesday, November 26, 1997
                       Message Id: cdl-11-48-004
                                  ***