Museum Environmental Standard

Introduction

This standard introduces the concept of the energy efficient precautionary principle: keep the environment as constant and as clean as is attainable with modest use of energy. This contrasts with previous standards which advocate using the best available technology, regardless of cost in equipment, maintenance and energy use.

The main consequence of this approach is that an annual temperature cycle is allowed. The second consequence is that storage rooms should have an annual temperature cycle centred just a little warmer than the annual outdoor average but reduced in amplitude.

The relative humidity requirement is relaxed in that the set point is defined by local conditions but the allowed variation is still restricted, because of abundant evidence that RH influences both the chemical and physical stability of many artifacts.

An unusual feature of this standard is that the customary restriction on the rate of change of the environment is replaced by a requirement for uniformity in space, rather than change through time. This is because transient temperature gradients cause significant and dangerous movement of water vapour. If temperature gradients in exhibition spaces are prevented, restriction of temperature variation in time is an automatic result which does not need to be formally defined.

Another unusual feature of this standard is the acceptance of stagnant air as usually advantageous. Other standards emphasise the need for ventilation and air movement, for which there is no scientific evidence of benefit. On the other hand dust and outdoor pollution arrive with air movement. The exception is enclosures with polluting materials within them.

The standard extends its scope to microclimates within the room. Showcases are a necessary extra protection for vulnerable artifacts against RH variation in the open gallery. Transport cases are another micro-environment which is handled by this standard.

Finally the standard introduces the novel concept that the standard must be shown to be maintained. There is a requirement that conforming institutions keep a record of the environment in a durable, standard format that is publicly accessible. There must also be regular condition surveys of the collection, otherwise we will never learn from observation how the environment influences the deterioration of artifacts.

This standard introduces architectural and air conditioning principles where they will be useful but does not specify design details.

The presumption is that nearly all classes of objects in a collection will be exposed to the same environment but particularly vulnerable materials will need special conditions. These materials are identified in an appendix. It is the curator's responsibility to identify these objects in a collection and apply the extra safety measures.

Constraints on setting a standard for the museum environment

Setting standards for the museum environment is made difficult by the paradox that the optimal environment for artifacts, and natural history specimens, hardly ever coincides with the optimal environment for people looking at them in a museum. In a store, the environment can be closer to the optimal, but one then encounters the second problem which is that different objects, or even different degradation pathways of components of a single object, demand different preservative environments.

For nearly all organic artifacts, mechanical stability is best served by a fairly high relative humidity, around 60%. Chemical durability is best served by total dryness, since many degradation reactions require water.

For nearly all organic artifacts, chemical durability is greatly enhanced by low temperature: around -5 ℃ is probably cold enough to delay damage until cleverer people think of some better way, or until uncivilised hordes destroy our monuments.

Current standards for what temperature humans can be exposed to without legal compensation demand somewhere between 25 ℃ and 18 ℃, according to country.

To these fundamental contradictions we have to add two other, practical difficulties. The first is the cost of maintaining the chosen environment, the second is the damage to the enclosing building through condensation from humidified air which may be considered necessary for preservation of the contents.

Finally we have the difficulty and expense of retrofitting existing museums and historic structures to our modern concept of what is right for the artifacts.

Looking back at previous attempts to solve the insoluble, we observe a tendency to fix the climate at conditions suitable for humans, then add constancy for the sake of the artifacts. Constancy is what engineers claim they can attain at any period in the development of climate control technology. There is a whole ecosystem of academics compiling and publishing tolerance limits for various classes of artifact. Some of these classes are not even material groups: anthropological skin objects apparently need different conditions to books with leather binding. A principle of this standard is that human sentiment on the value of an object and human conventions for grouping objects have no place in deciding the optimal environment for a material.

So what should we do?

We can start by accepting that humans like to live in air at around 22 ℃. So we can subject artifacts to around 22 ℃ but still hold a constant RH in their immediate surroundings by improving showcase design away from traditional joinery towards electron microscope containment standard. Anything regarded as humidity sensitive is awarded such a showcase.

Fortunately, hardly any objects are temperature sensitive over variation which a human would endure, so temperature instability is self correcting: artifacts cannot complain over an extreme temperature but the adjacent humans surely will.

So we are left with the relatively small number of artifacts which cannot long endure a human-friendly temperature. Colour photos for example. In a museum exhibition, we should accept copies, with assurance that the original exists, so that accredited snoops can occasionally check the accuracy of the copy.

For storage we can propose two categories: as cool as can be achieved by near passive means. This is a complicated discussion, but in the UK it means an annual average temperature around 13 ℃, which represents maybe a doubling of lifespan compared with the same object in a museum gallery.

For the few very delicate objects we can resort to a cold store. There is surely no need to go below -10 ℃. We have only extrapolation of Arrhenius plots to justify cold storage, there are no measured values for degradation at that temperature, so let us stop there until more data arrive.

Now I have proposed three and a half climate zones for artifacts: warm museum gallery containing sealed showcases for delicate items, cool storage, cold storage. All that remains is to assign each artifact in the collection to one of these zones.

A 'museum stable' object can without damage be put in the cool store and a cold store object can with only slow damage be put in the cool store.

Now we just need to try out the system with some random artifact types:

Pink Chinese porcelain: warm
Nitrate film: cool
Stinking acetate film: cold
Big old ship (Vasa): warm but RH stable. That will need a big showcase, so probably better to make the whole museum the showcase, which is how it is now.
Hawaiian oiled grass girdle: cool
Mirabilite (a deliquescent mineral) warm showcase
tin model soldier: warm but never cold, because of risk of recrystallisation.
Blu-ray DVD: probably cool but really unknown.

Which brings me to the huge 'don't know' category. This includes nearly all modern materials and nearly all modern art. It is not the duty of the standard to define the environmental sensitivity of materials, because it is not possible to do that with accuracy that is more than just defensive protection against future criticism.