Temperature and relative humidity are essential elements of collections care. Get the conditions wrong and you could find your items ruined. Mould, pests, deterioration and warping are just a few of the problems that can happen if these elements are not stable and controlled.
This guide will:
- Assess the affects of temperature and humidity on items
- Examine how they affect one another
- Provide tips for creating a stable and controlled museum environment
Plants and animals contain a high proportion of water, so it's unsurprising that their products also retain moisture. When materials absorb and retain moisture, they are described as hygroscopic. Such materials can and will absorb or give off moisture until they reach a state of equilibrium with the air that surrounds them.
These materials include:
When the surrounding air is very dry, organic materials will give off some of their moisture. They become brittle and may shrink, warp, split or crack.
When the surrounding air is damp, the materials will absorb some of the moisture from the air. They may swell, cockle, warp, change shape or lose strength. Dampness can also cause mould and fungal growth on organic materials.
Inorganic materials such as glass, ceramics, metals and minerals are also affected by high or low humidity.
Materials that have a natural salt content may suffer from efflorescence when the air is dry. The salts in deteriorated glass, porous ceramics and some geological material are carried to the surface by moisture (which may have entered the pores during a period of higher humidity). The moisture evaporates and the salts crystallise on the surface.
Other effects on inorganic material include:
- Corrosion in metals
- Faded dyes and pigments
- Pyrite decay in geological materials
If the humidity of the air changes frequently, hygroscopic materials will swell and shrink repeatedly. This causes internal stress and damage.
This is particularly a problem in composite objects where the different materials have different rates of shrinkage. The expansion of one material may force changes in the dimensions of another, causing considerable tension and eventually damage. Look out for such damage in items such as skins on drums and paintings on wooden panels
Moisture can also start or speed up the damaging effect of air pollutants and other harmful substances on many museum item.
Objects themselves are rarely directly affected by temperature. Fluctuating heat, however, can damage or compromise items in indirect ways.
Uncontrolled temperatures can:
- Cause changes in humidity, damaging sensitive objects through RH fluctuations. This is the main reason for controlling temperature.
- Speed up chemical processes and biological activity.
- Make certain materials expand and contract. This is particularly damaging for composite materials where parts expand at different rates.
- Affect the comfort of people working with or visiting collection items.
How temperature and humidity affect each other
The humidity of the air depends on the temperature. Where one cubic metre of air holds 10g of water at 10°C, the same volume can hold over 30g when the air is heated up to 30°C.
Absolute and relative humidity
Measuring the amount of moisture in grammes, or absolute humidity, doesn't mean much when monitoring a museum's environment. 10g of water feels damp at 10°C but will seem dry at 30°C.
Instead, the environment in museums is measured in relative humidity (RH). This is expressed as a percentage of the maximum amount of water the air can hold at that temperature.
- At 10°C, 10g water is the maximum amount the air can hold, so the RH is 100%.
- At 30°C, 10g is about one-third of the maximum amount, therefore the RH is approximately 33%.
When the temperature changes, for instance after the sun has set, so too does the RH. This principle is one of the most important factors in environmental control in a museum.
The recommended temperature for museum items is 16 to 20°C.
Moderately fluctuating temperatures between 10 and 20°C are unlikely to adversely affect museum items. Rooms below 16°C becomes too uncomfortable for visitors, while anything below 10°C can cause condensation and affect RH. Temperatures above 20°C will be too hot for guests and can accelerate degradation in museum objects.
Museum stores can be cooler than 16°C as they are not frequented by visitors. Bear in mind that items will need to acclimatise gradually between storage and display.
Relative humidity should not drop below 40% or rise above 70%.
Relative humidity below 40% can cause sensitive items to become dry and brittle. The maximum level is determined by the point that fungal growth begins, which is at an RH of at least 70%. Reducing the fluctuation of RH is also important.
Remember, these are broad recommendations. Some items and materials require more specifically controlled levels of relative humidity. For more information, read our guide to items and their ideal conditions.
Monitoring the environment
Scotland is a problematic country for controlling temperature and humidity as the seasons here change so drastically. When the temperature drops in winter to freezing point, the heaters come on and relative humidity drops to below 30%. In summer, the days are warmer and the external humidity can fluctuate between 65 and 95%.
In a naturally ventilated building in sound condition, indoor conditions will respond to outdoor conditions. There are, however, mitigating factors that can affect the temperature and humidity inside a building, room or display case.
Factors affecting the environment
- Water penetration or rising damp can cause high humidity levels if the building is in poor condition or badly maintained.
- Sudden weather changes can cause dramatic fluctuations if the building is not insulated.
- Direct sunshine on metal roofs and glass skylights can cause considerable heat gain and day-night variations.
- Poor air circulation or ventilation can create micro-climates that differ from ambient conditions.
- Heating systems run during the day for comfort can cause day-night fluctuations.
- Visitors produce moisture, especially on rainy days.
- In-case and spot lighting can create localised pockets of high temperature and low RH.
- Radiators or heating units placed beneath items can cause severe drops in RH for that item.
- Display cases can be designed to create favorable micro-climates for an item. Choose your case carefully to ensure that the opposite does not happen.
As so many factors can affect the temperature and humidity in a museum, the environment must be regularly monitored to keep track of fluctuations. After 12 months of monitoring you will have developed a good idea of changing environmental conditions in your museum. Use the information gained from monitoring to work out where and how to display sensitive items from your collections, investing in control equipment if necessary.
When continually monitoring your museum, pay particular attention to regular, frequent fluctuations, which can cause significant damage to museum items.
The changes take place slowly enough for the objects to adjust to them, but fast enough to cause frequent movement, stress and fatigue in the material. Occasional very rapid fluctuations, within one or two hours, will have a less damaging effect on items. Very gradual fluctuations will give the items enough time to acclimatise slowly.
Some types of items are more affected by fluctuations than others. When transporting items to an area with different environmental conditions, keep items well packed and wrapped in acid-free tissue and a box or blanket. This will ensure that acclimatisation to the new conditions can take place gradually.
To reduce the adverse effects of external conditions on the internal environment, museums can:
- Undertake comprehensive building and energy efficiency surveys to ensure that the building is in good condition and well insulated.
- Maintain the building to provide a barrier against external conditions.
- Keep windows closed at all times.
- Keep entrance doors closed as much as possible, or install double doors, revolving doors or a vestibule.
- Apply solar control film on windows and skylights to reduce the effect of direct sunlight.
- Install heaters, humidifiers or dehumidifiers where necessary to create and maintain a stable environment. Ensure that control equipment complements, not negates each other.
As a stable relative humidity is the main aim when controlling temperature and humidity, the temperature can be allowed to fluctuate moderately in favour of the stability of the relative humidity. Humidity levels can be used to control the heating with a humidistat rather than a thermostat. In situations with large fluctuations in humidity the use of humidifiers or dehumidifiers can be more cost-effective.
The golden rule
Stability is key.
When controlling or creating a museum environment, aim for a stable relative humidity. Preference should be given to maintaining a stable level approximating the desired level all of the time, rather than maintaining the exact desired level only part of the time.
In some situations it can be difficult or not financially viable to control the environment of an entire building. In such cases museums can create localised environments, or micro-climates, surrounding specific, sensitive items.
This can be achieved by:
- Choosing a suitable display case
- Using silica gel, humidifiers or dehumidifiers in cases where appropriate
- Monitoring the environments in sealed cases using humidity-indicator strips and hygrometers
- Fitting large cases with thermohygrographs or electronic sensors for electronic data logging
When items are in storage, keep doors closed at all times and use museum-standard enclosures. These can act as physical buffers to adverse conditions and create stable micro-climates for stored items.
Use the following items and materials for appropriate storage and packing:
- Acid-free tissue
- Melinex sleeves
- Cotton covers
- Trays with lids
- Cupboards, wardrobes and chests of drawers
For more guidance on storage, read our advice on storage and display materials.
Keeping your museum green
Maintaining stable humidity and temperature in your museum can use up a lot of energy. This costs a lot of money and can also increase your building's negative environmental impact.
Although maintaining stability is essential to the care of museums, some factors can be put in place to increase energy efficiency. Take a slightly more flexible approach to environmental control and look for ways to reduce artificial means of altering temperature and humidity. This may mean allowing a greater range of desired RH and temperature but can reduce your carbon footprint.
Always keep your building within the recommended guidelines outlined elsewhere in this guide.
A guide to items and their ideal conditions
Although all museums should meet the recommended temperature and relative humidity as previously outlined in this guide, some items require special considerations.
Read below for a list of item materials and how they should be treated, based on advice from Conservation and exhibitions: packing, transport, storage, and environmental considerations (Stolow, N. London, Butterworths, 1987).
Specific requirements at normal temperatures
Once archaeological items have been removed from the site and placed in the museum they can become fragile and subject to quick degradation. For humidity-sensitive items such as wood, leather and fibrous components, aim for RH between 40 and 60%.
Stone, ceramic and other inert materials should aim for the same, unless salts are active. In this case aim for 20-30%. Keep iron very dry, below 15% RH. Check the stability of other metals before deciding on RH, remembering that low RH reduces the rate of corrosion if the metal is unstable.
Arms, armour and metals
Read our guide on caring for your metal collections for extensive guidance. Aim for 40-60% RH, according to the condition of metal and oxide formation or the presence of organic components. Unstable and corroding metals may need dryer conditions, as low as 10%.
Dried plants, seeds and other botanical objects are stable between 40 and 60% RH.
Ceramics, tiles and stone
Decide an RH, between 20 and 60%, based on the presence of embedded salts. These items are susceptible to freeze-thaw cycles of damage if exposed outdoors.
Coins and other numismatic collections
Corrosion products, oxides and patina formations affect the desired RH of coins. Assess their degree of stability and aim for an environment between 40 and 60% RH, although dryer for unstable corroding objects.
Costumes, textiles, rugs and tapestries
Our guide on caring for textiles gives full guidance on looking after such items in your museum. RH between 30 and 50% will protect most textiles. Silk and wool are more sensitive to moisture damage than cotton or linen.
Painted textiles are most sensitive to RH changes. Synthetic fabrics are less reactive but exhibit electrostatic properties at low RH values and will readily accumulate dust.
Dried and mounted insects should be kept in RH levels between 40 and 60%.
Ethnographic bark, cloth, basketry, manilla, sisal, masks, feathers and leather garments all have specific reactivity. Aim for RH values between 40 and 60%.
Furniture and marquetry
40-60%, depending essentially on wood content, grain, joining, and condition of surface or barrier coatings. Furniture is especially affected by seasonal RH fluctuations and cycles. Some woods are less sensitive than others, owing to resin content or construction.
Minerals, rock and fossil material containing pyrite have a lower RH maximum of 50%, ideally 30%. Other geological items are stable between 45 and 60%.
40-60%. Crizzled, or 'weeping', glass needs a narrower band of controlled RH around 40% to prevent advance of this condition. RH is not too critical for other kinds of glass.
Ivories and bone carvings
50-60%. These require more control than other anatomical collections. Responses are sped up when the ivory or bone is in thin sheets, such as in miniatures.
Aim for RH values between 50 and 60%, although Japanese authorities recommend higher levels to 70%.
Leather, skins and bindings
The stability of leather is variable, dependent on how it was tanned. RH values should be between 45 and 60%.
Paintings on canvas
Paintings and frames require extensive considerations for environmental care. Read our guide for detailed information.
In general, aim for 40-60% RH. Unlined paintings and paintings lined with hygroscopic adhesives are more reactive than those lined with wax or synthetic materials.
Paintings on wood and polychrome sculptures
The RH required here depends on the thickness, wood grain, ground and method of joining section, in a 40-60% range. Some panel paintings need narrow RH levels to minimize warping. Massive wood sculptures particularly susceptible to seasonal drifts.
Paper and paper products
Read our guide on paper collections for further advice. General guidelines are as follows:
- Parchment and vellum require narrow controls between 55 and 60% because of great hygroscopicity.
- Stretched paper, such as screens and stretched frames, need narrow RH control, with 45-55% range.
- Other paper requires a 40-50% range, although some authorities recommend less.
Photographs and films
30-45%. The gelatin in photography is reactive, as is support paper. Plastic film components are less responsive. Our guide on photographic collections has further advice on caring for such items.
30-50%. In general, plastic materials such as acrylic supports, sculptures and castings have slight humidity responses. However, they do warp when in thin sheets and exposed to varying conditions. Electrostatic properties at low RH levels with dust accumulations.
Musical instruments, models, turned wood, treen and decorative objects that are painted or coated require RH values of 40-60%.
Read more of the advice guides from Museums Galleries Scotland for expert tips on environmental monitoring and control. Contact us with any further questions.
Elsewhere, you can find useful advice from the Collections Trust and the Museum Association, who give guidance on keeping your museum green.