What goes wrong

The seal failed

On this page10

The desiccant exhausted The liner never sealed Dust or a chip on the rim Opened too often Sealed on a damp day Temperature cycling Check once a year Tighten properly, once Keep spare liners References

A microclimate is only as good as the worst hour it has.

The desiccant exhausted

Silica gel has a capacity. Once it is saturated the jar returns to room humidity and nothing announces it. Indicating gel or a humidity card is the only cheap warning.

The liner never sealed

27 of the closures on this shelf carry no liner at all, and a cap without one is a dust cover. It is also indistinguishable from a sealing cap at a glance.

Dust or a chip on the rim

A liner seals against a clean, undamaged rim. A grain of grit or a small chip is a slow leak that no amount of tightening fixes.

Opened too often

Every opening replaces the dry air with the room. A jar that gets opened monthly is not a microclimate, which is a strong argument for clear containers or good labels for anything consulted regularly.

Sealed on a damp day

The jar preserves whatever went in. Sealing in humid conditions traps humid air against metal, and the desiccant spends its capacity on that before it does anything useful.

Temperature cycling

A loft swings widely, which drives moisture in and out of materials and stresses the seal. Stable and moderate beats cold and variable, even at a slightly higher humidity.

Check once a year

Read the indicator, change the desiccant, look at the contents, note the date on the label. Fifteen minutes a year is the entire maintenance burden of this approach.

Tighten properly, once

A compression liner needs real torque against the rim to seal, and hand tight on a large diameter is often less than it feels. Over-tightening distorts the liner and seals worse, so consistent beats hard.

Keep spare liners

A liner that has been compressed, released and recompressed several times loses its recovery. For a jar opened annually, replacing the liner at the same time as the desiccant is a sensible habit.

References

National Park Service, Conserve O Gram, on the storage of metal objects: metals corrode in the presence of moisture and pollutants and should be housed below 35 percent relative humidity, stabilised in a dry microclimate using a desiccant such as silica gel. Deteriorating plastics emit acidic vapours that corrode metal, which is why a metal coupon is used to detect an off-gassing plastic. nps.gov

The Davis Pack Supply catalog, measured 18 October 2026. The numismatic and philatelic shelf is 391 rows: 146 containers and 245 closures. 81 PET, 64 PP and 1 glass, from 6 to 250 mL with a median of 118. Zero PVC. By colour 39 white, 35 clear, 23 black, 15 amber, 15 natural and 8 blue. Necks 70-400 at 113, 89-400 at 93 and 58-400 at 66.

Written from Library of Congress and NARA preservation guidance and from National Park Service Conserve O Gram material, and from the catalog as it stood on 18 October 2026. Conservation advice for a specific object is a job for a conservator; nothing here is a substitute for one, and a valuable or unusual item deserves that conversation rather than a jar.