Theory 08/Conservation & Heritage Science/Preventive conservation
Decay Is an Exchange Ratio
Sealed cases are assumed to protect. Without absorbents they can become chambers.
Material decay in museum enclosures is set by the ratio of pollutant ingress to air exchange rate rather than by pollutant concentration, so a well-sealed case with low exchange can concentrate pollutants and damage objects faster than a leaky room.
01The evidence
Damage has been observed in sealed enclosures despite low external pollutant levels, with the effect attributed to accumulation rather than to source strength.
02The tension
Conservation guidance uses concentration thresholds, which predict damage from the level outside and miss the accumulation that a low exchange rate produces inside.
03The mechanism
Pollutants enter an enclosure by diffusion and are removed by exchange and by absorption. At low exchange, ingress accumulates until the internal concentration exceeds the external one.
That makes the ratio of ingress to exchange the controlling variable, and predicts that sealing without absorbents can worsen conditions even as it stabilises humidity.
04What this is not
That guidance uses external levels. This uses the balance inside, which explains damage in sealed cases.
05Predictions
- P1Internal concentration should rise as exchange falls.
- P2Sealed cases without absorbents should damage faster.
- P3Absorbents should reverse the effect.
06Refutation
Decay tracking external concentration at controlled exchange would restore the threshold guidance.
07What it changes
It would change case design and monitoring practice.