A percentage with a temperature attached
Relative humidity, usually abbreviated RH, expresses water-vapour pressure as a percentage of the saturation vapour pressure at the same temperature. It is not a direct measure of how much water an object contains. The relationship matters because warming air without adding water vapour generally lowers its RH, while cooling it raises RH. Two traces that move in opposite directions may therefore describe a temperature-driven cycle rather than repeated moisture entry.
Read temperature and RH together before diagnosing a source. Dew point can help distinguish changes in water-vapour content from changes driven mainly by temperature, although calculated values inherit uncertainty from both sensor channels. Near a cold surface, local conditions may differ markedly from those at the logger. A stable room average cannot rule out a damp pocket behind a cabinet.
Objects do not respond instantaneously
Hygroscopic materials, including wood, paper, textiles and many adhesives, exchange moisture with the surrounding air. Their rate of response depends on thickness, coatings, construction and access to air. A thin sheet and a thick wooden support do not follow the same timetable. Layers bonded together may swell or shrink differently, creating stresses that a simple material label fails to describe.
Past treatment and environmental history also matter. Existing cracks, restrained joints, degraded binders and previous repairs change the significance of a fluctuation. An object may have adapted to a particular pattern without making that pattern suitable for every other object. This is why a generic target percentage cannot replace a condition assessment or the advice of a conservator familiar with the collection.
| Collection material | Possible moisture-related concern | Useful contextual evidence |
|---|---|---|
| Painted wood | Dimensional movement and stress between layers | Construction, restraint, cracks and fluctuation duration |
| Paper and textiles | Dimensional change; mould risk under favourable sustained conditions | Air movement, enclosures, condition and exposure history |
| Salt-contaminated ceramic | Salt dissolution and crystallisation | Salt identity, contamination and repeated threshold crossings |
| Susceptible metals | Corrosion influenced by moisture and pollutants | Alloy, surface condition and local contaminants |
Read the shape, not just the average
A daily mean can conceal repeated peaks. A monthly mean can conceal a prolonged damp episode. Examine duration, frequency, rate of change and seasonal movement, using a time scale appropriate to the material and question. Mark case openings, cleaning, weather events and building changes on the record. An unexplained line becomes more useful when it can be connected with something that actually happened.
For illustration, a case might show 48% RH before opening and 56% afterward. These are invented readings, not recommended bounds. Their meaning depends on sensor uncertainty, equilibration time, the duration of the change and the vulnerability of the contents. A short disturbance in a robust collection and sustained moisture around a sensitive composite object are not equivalent situations.
Trust depends on the measuring element
Capacitive humidity sensors are common because they are compact and can be integrated into low-power loggers. Their performance can be affected by ageing, contaminants and exposure outside their intended range. Condensation may cause delayed recovery or damage. A protective filter helps in some settings but can alter response time. The complete assembly, not just the bare sensing element, determines practical behaviour.
Use a documented verification and calibration programme suited to the consequences of error. A side-by-side comparison helps find a drifting logger, but two instruments can share a similar bias. Avoid improvised humidity checks beside collections: reference methods involving salts require controlled procedures, suitable containment and technical understanding. Follow the instrument manufacturer's instructions and obtain competent calibration support when needed.
Respond at the smallest sensible scale
After confirming the evidence, consider causes and options. A leaking case, an unconditioned buffering material, a cold wall or altered cleaning practice may deserve attention before a building-wide set-point change. Passive buffering can moderate fluctuations in a suitably sealed enclosure, but its capacity is finite and depends on preparation, air exchange and moisture load. It is not a maintenance-free cure.
Document the intended outcome, the person responsible and how success will be checked. Avoid abrupt corrective changes made only to make a chart look tidy. Collections care often involves balancing material needs, practical access, building limitations and energy use. Monitoring supports that discussion by making local behaviour visible; it cannot settle the conservation priorities by itself.
Reading notes: historical climate and material sensitivity
EN 15757 addresses specifications for temperature and relative humidity to limit climate-induced mechanical damage in organic hygroscopic materials. Its scope is not a universal prescription for every deterioration mechanism. The Canadian Conservation Institute and the Getty Conservation Institute publish wider preventive conservation research. The American Institute for Conservation and the International Institute for Conservation also provide professional resources on environmental decision-making.
Getty Conservation Institute · American Institute for Conservation · International Institute for Conservation