Requirements by material
Wood wants stable humidity, metal wants it low, photographs want it cold. What each material needs and how to handle a mixed store where those needs conflict.
Updated 14 August 2026
Environmental targets are usually written for a room, but damage happens to a material. A store held at 55 percent relative humidity is comfortable for wooden furniture, marginal for iron and actively bad for a collection of cellulose acetate negatives. This section approaches the problem from the object’s end rather than the building’s.
The three axes that separate materials
Response to humidity. Hygroscopic organic material, meaning wood, textile, paper, leather, ivory and horn, exchanges moisture with the air and changes dimension as it does. For these, stability matters more than the absolute figure. Metals, by contrast, care about the absolute level: corrosion rates rise steeply above certain thresholds and archaeological iron with chloride contamination needs conditions far drier than any organic material tolerates.
Response to light. Fading is a property of dyes, pigments and some polymers. Stone, glass, ceramic and most metals are effectively unaffected. Between those extremes sits nearly everything else, and the difference between an oil painting and a watercolour is a factor of several in acceptable illuminance.
Response to temperature. Chemical degradation roughly follows the usual rule of thumb that reaction rates rise sharply with temperature, which is why cold storage extends the life of unstable film and colour photographic material dramatically. For most other material, temperature matters mainly through its effect on relative humidity.
Quick comparison
| Material | Dominant risk | Humidity | Light |
|---|---|---|---|
| Wood and furniture | Dimensional movement, joint failure | Stability above all | Low sensitivity, but surfaces fade |
| Textiles | Fading, insect attack | Moderate, stable | Highly sensitive |
| Paper and books | Acidity, fading, mould | Moderate, avoid damp | Sensitive |
| Metals | Corrosion, tarnish | Low is better | Not sensitive |
| Photographs | Chemical decay, fading | Low, cold if possible | Very sensitive for many processes |
| Plastics | Autocatalytic degradation | Varies by polymer | Sensitive for some |
The table is a starting point for triage, not a specification. Each linked page below goes into the failure modes and where the standard advice is contested.
The pages
Wood covers dimensional response, why panel paintings and veneered furniture fail, and why the stability argument matters more here than anywhere else.
Textiles covers light sensitivity, support and mounting, and the fact that textiles are the primary target of the most damaging collection pests.
Paper and books covers acidic paper, iron gall ink, enclosure choice and the deacidification question.
Metals covers tarnish, active corrosion, the humidity thresholds that matter and why storage furniture is often the culprit.
Photographs covers process identification, cold storage, vinegar syndrome and why buffered enclosures are wrong for some processes.
Plastics covers the polymers that degrade, how to recognise them and why they need isolating from the rest of the collection.
When requirements conflict
The practical resolution is almost always the same. Set the room to a defensible compromise, then treat the exceptions individually with enclosures. A sealed box with conditioned silica gel will hold a small volume at a different humidity from the room around it for months, and it costs a fraction of what a second conditioned space would.
The exception is cold storage for unstable film. That genuinely needs its own space, and the reason is not stubbornness about the specification but that the material degrades in a way that accelerates itself and damages what sits next to it.
Frequently asked questions
- What do I do when materials in one room need different conditions?
- Set the room to suit the majority or the most vulnerable, then give the outliers their own microclimate. A sealed case or a well-made box with conditioned silica gel is far cheaper than a second climate-controlled room.
- Which material is most at risk in a typical mixed collection?
- Usually the one nobody thinks about: cellulose nitrate and acetate film, which degrade autocatalytically, damage neighbouring material and in the case of nitrate present a genuine fire risk.
- Does a composite object follow the strictest requirement?
- Generally yes, but with judgement. A painted wooden panel with metal fittings is dominated by the wood and its response to humidity, because that is what moves and tears the rest apart.
In this section
Wood
Wood moves across the grain and barely along it. Almost every failure in wooden objects follows from that one fact.
Textiles
The most light-sensitive material in most collections and the first thing clothes moths find. Two risks that dominate everything else.
Paper and books
Much of the paper in any collection is destroying itself chemically. What that means for storage, enclosure and handling.
Metals
The one material group where the absolute humidity level matters more than its stability, and where the storage furniture is often the culprit.
Photographs
The material group most likely to be quietly destroying itself, and the one where cold storage changes the answer most.
Plastics
The newest material in most collections and frequently the first to fail. Four polymers cause most of the trouble.
Related reading
Target humidity ranges
The fixed 50 percent rule is gone. What the current frameworks say, where they disagree, and how to pick a defensible number for your own building.
Preventive conservation
The whole field in one page: the ten agents of deterioration, which of them you can actually measure, and a defensible order to work through them.
Climate monitoring
Choosing loggers, placing sensors where they tell the truth, keeping them calibrated, and reading a year of data without fooling yourself.
Humidity control
The range to aim for, the equipment that holds it, and the case for treating one showcase instead of one building.
Light and UV
Fading is spent permanently. How to measure it, how to filter it, and how to budget a year of display rather than dimming by feel.
Air quality
The damage that comes out of the storage furniture rather than in through the window, and how to find it before the object does.