Experts warn sealed time capsules risk internal chemical damage from mixed materials

Library of Congress preservation staff described a less obvious but serious long‑term hazard for sealed time capsules: chemical interactions and off‑gassing among different materials. "It's essentially stewing in its own juices," the presenter said, illustrating how aging paper emits furfural or acetic acid that can tarnish coins and damage other objects.

To manage the risk, the lab told contributors which general classes of materials were preferable (archival paper, stable metals, glass) and warned against seeds, food, many plastics and reactive metals. They operated a risk‑rating system—green for low concern, amber for items that may self‑destruct without harming others, and red for items requiring further discussion or exclusion.

The presenter gave examples of negotiated changes: Montana's beaded elk‑hide piece was remade on canvas to avoid organic hide deterioration; the Supreme Court's pocket constitution was reissued on a different paper to avoid problematic glue and faux leather; dried Rose Parade petals were advised against. Those advisory requests were passed to the project manager for final decisions.