Heat and thermal shock
Updated
Less discussed than humidity and it accounts for a share of the damage that gets blamed on handling.
Two different problems
Thermal shock: a fast temperature change, which cracks.
Sustained warmth: which accelerates every chemical process in the collection.
Thermal shock
Minerals expand unevenly, and a crystal with cleavage or an inclusion has a plane ready to go.
The classic case is a specimen brought in from a freezing car and washed in warm water, which cracks audibly.
Quartz with fluid inclusions is particularly vulnerable: the inclusion expands faster than the host.
The rule is simply to let material reach room temperature before doing anything to it. Hours, not minutes, for a large piece.
Sustained heat
Every reaction in this section runs faster warm: pyrite oxidation, efflorescence, dehydration, fading.
A warm room is a collection aging faster, and an attic or a conservatory is a substantially worse environment than a cool interior room.
Heat also drives humidity swings, because warm air holds more water and cools overnight.
Where heat comes from
Display lighting, particularly halogen in an enclosed case.
Direct sun on a shelf or through a case front.
Radiators, and the top of anything with a motor in it.
Vehicles, which are the worst environment a specimen will meet: a car in summer cycles daily through temperatures no interior reaches.
Which is worth remembering on the way home from a show, where a boot full of new material sits in the sun for an afternoon.