Re: Are my glass Christmas ornaments shattering from cold or heat?

I don't think i can settle your bet.  A web search on storage of 
Christmas  decorations finds several sites on people who sell storage 
boxes and give advice  http://homearts.com/cl/collect/c8clho11.htm is one.
In effect they say a cool dry place preferably in the center of the house 
avoiding attics, cellars, garages, and temperature-humidity extremes.

A careful examination of the broken bulbs might show the place of initial 
breakage.  If in the bulb proper the difference in the expansion rates of 
the coating and the glass is a likely culprit.  If so heat and cold would 
be equally suspect with cold more likely because the temperature change is 
greater in the cold direction.  Test one by poppin it in the freezer, and 
in water warmed to 140F and see.  If they are breaking at the neck at the 
small metal sleeve or collar the problem is the that the expansion of the 
glass is usually greater than that of metal and as they warm the expanding 
glass is constricted by the metal collar and cracks.

A third possibility is a rapid uneven temperature change similar to 
putting hot water quickly in a glass. Most brittle materials[quartz being 
a common exception] respond poorly to very rapid Temperature changes.  

Examine the evidence carefully and be sure to carefully pack and store 
heirloom ornaments.  

JimG