The bead that fell apart

As you can imagine, glass deterioration greatly affects the strength of a piece of glass, but why would a weathered glass spontaneously fall apart after years of apparent stability? We recently had that happen.

The piece in question is a core-formed bead dating to 500-250 B.C. The bead is made of opaque white glass with trails and prunts of blue, yellow, turquoise, and red-brown glass, and was heavily weathered with a bubbly and pitted thick milky-white weathering crust with patches of dark enamel-like weathering over the entire bead. It has been in the collection since 1964 and had appeared stable. The bead was recently photographed and, to our surprise, an hour or so after it was photographed and safely replaced into a plastic bag, the bead was found broken into 3 larger fragments and numerous small bits.

So what happened? Direct physical causes may have contributed but do not seem to be the main reason that the bead fell apart, which points towards the glass deterioration itself and environmental factors, especially temperature and relative humidity (RH), playing a role. Blackish dendritic staining (possibly manganese) on all the break edges indicates that there were cracks in the glass which had already gone most of the way through the width of the bead. It is likely that these structural weaknesses along with (slight?) climate changes and the stress from transport and handling caused the bead to break.

Was this preventable? Maybe not, but to better understand what happened we need to determine if the bead could have experienced any changes in temperature and RH and how big those changes would need to be to affect the glass. In this case the main source of possible climate changes is the photography studio itself, specifically the lighting. We’re hoping that a repeat of the conditions, monitored with a data logger, will give us some insight into the effects lighting for photography has on the ambient temperature and humidity, especially on days when the lights are used for a long time.