Humidity, high and low
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Relative humidity is a consequential thing about a room full of minerals, and it is pulled in two directions at once.
One number, two opposite failures
Some species fail when it is high. Others fail when it is low. There is no setting that suits everything, which is why a mixed collection is a compromise and the rarities get their own boxes.
Too high
Pyrite and other sulfides oxidize, expanding and producing sulfuric acid. Above about 60% RH this proceeds; the recommendation is to stay at or below 45%.
Efflorescence on carbonates accelerates, because the reaction with acid vapor needs moisture.
Soluble species dissolve or deliquesce. Halite and several others take up water from the air and slump.
Mold and pests reach anything organic in the storage: labels, card, wooden mounts.
Too low
Hydrated minerals lose water. Borax, chalcanthite, laumontite and others dehydrate, going cloudy or white and crumbling. Laumontite is the classic case and it can happen within months.
Clays and some matrices shrink and crack.
Dehydration is usually irreversible, which makes it the more permanent of the two failures.
Where to set it
Around 45 to 50% suits the majority: low enough to hold sulfides, high enough not to dehydrate most hydrates.
Then treat the exceptions individually. Susceptible sulfides in sealed boxes with conditioned silica gel held low; known hydrates in sealed boxes held higher, or with a damp buffer.
Stability matters more than the exact number. A collection at a steady 55% is in better shape than one cycling between 35 and 65, because each swing moves water in and out of the specimens.
Which means the practical answer for many collectors is an interior room rather than a garage, a basement or an attic, and a cheap hygrometer, read occasionally, so you know what you actually have.
Finishes that will hold a seal
