Preamble to the museum environmental standard

The environmental standards for exhibiting and storing historic artifacts have strongly emphasised constancy and purity.

It is certain that a constant environment does no further physical harm to artifacts, once they have settled down to a climate that may be unfamiliar. Once cracked, artifacts will remain stable, even if the climate is far from that in which they were made.

That is the only advantage of constancy.

If the environment is constantly warm for the sake of human comfort, 23°C for the lightly clothed, slowly moving person, the rate of chemical degradation may be seriously rapid for some organic materials, notably movie films and colour prints and some widely used plastics.

If the environment is illuminated for human study of the objects, photochemical degradation of many coloured materials is inevitable, and roughly proportional to the light intensity. Constancy is a disadvantage.

If the environment is kept clean by piping air through pollution filters there is benefit in lower degradation rate and in slower soiling with consequently less abrasion by cleaning.

These consideration lead to this standard separating exhibition climate from storage climate. A popular exhibition hall will require mechanical cooling and vigorous ventilation to control visitors' heat and carbon dioxide output. This document is also a guide for the many smaller local museums and historic house museums which are unlikely to be air conditioned. For these places we recommend minimising lighting energy to reduce summer temperature. Airtight, buffered showcases can moderate the relative humidity variation around delicate exhibits, but cannot control the temperature. They do, however, allow the temperature to be moderated by ventilation, which inevitably destabilises the RH in an open space.

It is controversial to question the tradition for air conditioning in exhibition spaces. The argument for museums saving energy to save the planet is weak, since all the world's museums running air conditioning would amount to a negligible fraction of our energy consumption. The argument that we need to show a good example is ethically doubtful if saving energy increases degradation. Museums are charged with keeping mankind's historical record for as long as possible with the technical resources we have.

In storage we can stop light damage and greatly reduce pollution damage because minimal fresh air from outside is required for the rare human visitors. In storage the temperature can be kept uncomfortably low for humans, with the advantage of lowering reaction rates.

An analysis of the engineering needed for climate control in storage spaces leads to the recommendation for cool storage at an annually cycling temperature of small amplitude around the outdoor average and an annual RH cycle of small amplitude around 50%. This can be achieved with almost zero energy use. For general museum storage there is no conflict between saving energy and saving the collection. However, particularly unstable objects, notably movie film, must be kept in cold storage below the average ambient temperature and therefore require power.

The argument for relaxing the rules for constancy of the climate in museum exhibitions must be backed up by evidence that no extra damage is caused, or that damage is even diminished. The acute danger is presumed to come from RH variation, but it has proved difficult to quantify, so we are left with a precautionary attitude. Experiments on new wood suggest that a variation of 20% around the long term mean is acceptable. However, battered ancient objects, and particularly objects contaminated with water soluble salts, will crumble at the slightest change of environment. No amount of research will quantify the infinite variation in delicacy of our museum heritage.

Standards are usually designed for quality control of well defined new materials, such as compressive strength of concrete for bridge pillars. Less well defined, natural materials worked by ancient techniques and tired by centuries of use can hardly be forced to attain given values of Young's modulus, or colour fastness.

One of the purposes of this standard is to improve record keeping so that museums acquire a better knowledge of the vulnerability of their artifacts to the environment. In a hundred years we will be better informed by the accumulation of observations of damage to real objects in all their diversity.

Museums continuously accumulate objects and host temporary exhibitions. Their climate must suit diverse objects with different histories. I argue that the standard should explicitly limit itself to new build and deep renovation events, where its advice can be implemented with legally binding specifications. For old stuff in old buildings, expertise is needed. I don't much like advice which lamely suggests "consult a qualified professional", but it really does apply to old buildings.

An Informal introduction to the formal temperature and relative humidity sections of the standard

History

A strange feature of all museum environment standards which contain numbers for required temperature and relative humidity is that they ignore the earthly seasons. Consequently, the richer museums hum with concealed machinery transmitting air and vibration through ducts into exhibition rooms. The interior is only tenuously connected with the outside world. Compared with the proven durability of surviving ancient artifacts, this is a very recent development. The museum interior as a spaceship cruising with perfect constancy through time and tempest first became a requirement in well funded museums after the 1939 war. The technology started to evolve around 1900. Thereafter, air conditioning steadily improved and spread, as described by Reyner Banham in The Architecture of the Well-Tempered Environment (1969). This standard questions the necessity for this constancy and emphasises the cost, in money and irreplaceable earthly resources, of blind subservience to a dogma for which there is surprisingly scant support in experimental science. A seasonally varying temperature uses less energy for both temperature and for RH control. It also moderates the natural RH of the outside air brought to the inside temperature, suggesting the possibility of making further savings in energy and complexity by abandoning air conditioning and using only winter heating. However, abandoning air conditioning exposes the artifacts to a varying relative humidity, for which there is ample evidence of damaging effect if the value dips below 25%.

Exhibition

There is no evidence that temperature change on an annual cycle from 28°C to 5°C causes damage to artifacts. This range exceeds the temperature range for human comfort. In museums therefore, a practical lower limit would be around 16°C, maybe higher in tropical regions. The range for museum display is therefore set between 28°C and 16°C, with an allowed annual cycle and with a plea to keep the winter temperature low, to reduce chemical decay and winter dryness in cool climates.

This wide temperature range means that in summer there should be no need to cool and dehumidify, so air conditioning is unnecessary. It won't be possible to stop the temperature rising over 28°C but the top ambient in the UK is about 30°C, which isn't instantly destructive and won't last long. There will also be energy released by lighting but opening the windows to through circulation will bring the temperature close to ambient, while letting in some air pollution, again for only a short time. A winter temperature at 16°C gives a typical RH of 40%. This is safe, but the RH will go lower in cold snaps so humidification may be required for particularly sensitive collections, such as musical instruments, lacquer and painted wood from churches. However, full air conditioning will not be necessary. Sensitive objects can alternatively be put in buffered display cases (with passive buffer needing no regeneration). The energy saving from this major change in temperature requirements for collections is large.

The daily temperature cycle and the temperature uniformity should be smaller, we suggest a 7°C amplitude maximum as a design aim, since this variation, in both time and space, will not cause dangerous excursions of RH.

In an air conditioned building the RH is controlled independently of the temperature. Without air conditioning the RH variation can be moderated on the annual cycle by setting the indoor temperature at a smoothly varying value which will make the outside air water vapour content give a moderate RH between 40% and 60% as it leaks in. The required temperature cycle for each region on earth can be predicted from published data (energy plus reference and the calculator on this website for 'conservation heating'). Some artifacts require very stable RH, unlikely to be achieved on open display without air conditioning. Their display and conservation is discussed in a later paragraph.

Storage

For objects in storage the temperature range can be centred on the annual average temperature which gives a safe RH, say 60%. In northern Europe this temperature is about 15°C and requires some energy. However, technical improvements in building construction allow such good airtightness for storage buildings that dehumidification is very cheap, even free to run if solar powered. It is therefore feasible to maintain an annual temperature cycle which is centred on the annual average temperature, typically 10°C in Europe. Theoretically, this requires no energy at all for heating and cooling. The temperature cycle amplitude can be minimised (target 7°C) by one or a combination of several methods: building underground; using a low, wide building with uninsulated floor directly on the earth; installing a heat exchanger with the ground. The RH can be controlled to near constancy by one or a combination of these processes: dehumidification; pumping in outside air when by chance it has the right water vapour content; using humidity buffering by the contents (in an archive) or by construction materials (for a locomotive store). In practice, the daily variation and the spacial variation in both temperature and RH can be made negligibly small by good design [ref Ryhl-Svendsen et al]

Low temperature generally reduces chemical decay rates. However, below 5°C some complications arise. Several common polymers (acrylic and oil paint) become brittle and require delicate handling. Another problem with cold buildings is freezing water pipes with consequent flooding. So for a general purpose museum store, 5°C is a reasonable lower limit, but it is unlikely ever to be reached in a well insulated building with ground heat buffering.

Cold storage

Some artifacts are too unstable to survive even the cool climate storage described above. These need cold storage. Confidence in the safety of cold storage is based on the long experience of movie film preservation at -20°C and the experiments of Kodak scientists [ref Calhoun] repeatedly cooling and warming movie film. Experience with more varied materials is anecdotal - based on the rarity of obvious damage to artifacts plunged rapidly to -30°C to kill bugs, and as rapidly warmed a day later.

Cold storage requires mechanical equipment. The temperature can be set anywhere from -5°C to -20°C with no confidence about what is best, because the only evidence for the preservative effect of cold is extrapolation from experiments conducted above 30°C [check this assertion]. RH control at low temperature is not expensive because there is very little water vapour in the air. Leakage will always increase the RH so low capacity dehumidification combined with good airtightness will work. The main running cost is for cooling. This can be read from calculator c2 'energy requirements for T and RH settings'.

Close control of RH

A few artifacts require unusually constant RH. These materials are mostly water soluble minerals, present either as specimens or as contaminants of porous artifacts such as stone and ceramic. For exhibition, a sealed showcase with humidity buffer will provide adequate stability. For storage, a sealed box with humidity buffer will do. The most difficult to preserve artifacts are wall paintings in ancient buildings.