Most books have some mold by the time they're ~100 years old, it's just not enough to cause a serious problem. Sealed enclosures (wrappings, bags, tubs) are a nightmare situation. Even packing books too tighly on shelves accelerates mold growth to problematic levels.
Archivists recommend standard ambient conditions or a little drier for long term storage. As you've said, too dry and the pages fall apart; permanent damage.
I suppose the fancy silica gels that maintain specific humidities would work in bags.
> I would think this totally dries the pages out and then they just turn to dust, from experience.
They used to sell Boveda two-way humidity control packs that would keep a bag at a constant low-ish 32% humidity, but last I looked those were discontinued (it looks like they've pivoted pretty heavily to marijuana storage and higher-humidity products).
Who is we?
> then store the pages in a sealed plastic bag which goes in the archives; they're stored there indefinitely
Is that the best thing for archival storage? Like could things like chemical breakdown increase the humidity in the sealed bag or concentrate corrosive chemical vapors? I was under the impression the best environment was an actively climate-controlled environment.