Common fallacies and myths about the museum environment

Ventilation inhibits mould growth

This is an example of correlation not indicating causation. The cause of mould growth is high RH. This occurs at any point in an enclosure which is significantly cooler than the main volume. The local RH will then be high. This is often missed by the sensor, which is usually placed in the centre of the room, in a well ventilated place.

The solution is either to increase the air movement to force a uniform temperature or, much better, to design the space for natural temperature uniformity, through good insulation of the walls and rounding corners.

Ventilation to force uniformity is a bad solution, because it spreads dust and pollutants. It can also remove internally generated pollutants. This matter is discussed in the 'pollution' chapter.

Storage close to zero degrees risks frost damage

Materials pre-equilibrated to around 50% RH do not suffer ice crystal formation when the temperature drops below zero. Freezing is a property of free bulk water - it is a rearrangement of many molecules into a crystal. The individually adsorbed water molecule in an absorbent material have more affinity for their substrate than for other water molecules. The temperature must fall below about -30C before there is a risk of crystal formation.

The unfortunately widespread talk of keeping sensitive objects in a freezer is based on the technology usually being applied to foodstuffs, which are usually in equilibrium with 99%RH before being cooled.

A high building thermal mass ensures a stable temperature

Thick insulation is just as good at ensuring a stable temperature, as long as there is little internal generation of heat, as is the case in a store or archive. In exhibition areas, or even busy archives, the situation is more complicated. In this case thermal mass within a lightly insulated envelope is the way to go.

Slow change of RH is OK, rapid change is what causes damage

This is a misleading half truth. Some objects will crack however slow the RH transition - even over a year. Rapid RH change, with rapid reversion to the usual value, can be protective if water vapour does not have time to diffuse into the sensitive material. Varnished carved wood is an example. Water diffuses slowly even into bare wood. The short lived surface stress that develops will not cause distortion of the whole thickness. Short and quick heating of churches has been advocated to preserve painted wood furniture. Unfortunately it also causes a sharp decrease in RH at the surface of wall paintings, leading to recrystallisation of soluble salt contaminants.

Plunging objects into -30 ℃ or +60 ℃ is OK if the intention is to prevent insects getting into museum stores

The laws of physics and chemistry are not influenced by our moral code. It is amazing that these practices are allowed as routine treatment - in the absence of evidence for pest infestation. However, there is scant evidence of damage.

The RH of the air is...

RH is a property of water vapour in space. It is not affected by the other molecules in air, largely nitrogen, oxygen, argon and carbon dioxide. The error hardly matters at sea level, but confuses interpretation of what happens during air travel.

UV filters protect against fading

As a general rule, removing UV from daylight will halve the rate of fading of dyes. Using blue deficient light will halve the rate again. Incandescent lamp was perfect for preservation during illumination, but is now obsolete. However, LEDs can be made with similar spectra.

Specifying an acceptable integrated light dose for an object allows brighter light for a shorter period

This is not wrong - the reciprocity principle holds over a very large lux range, meaning that fading accumulates with the light dose in photons. The problem is that it is unlikely verging on impossible that sufficient records will be kept over half a century to implement this concept. Only an academic would think it feasible.

Temperature uniformity

There is no requirement in this standard for limiting the rate of change of temperature around an item. This is replaced by a requirement for temperature uniformity in the space around an item, extending into the distant recesses of the enclosure.

There is no quantitative scientific basis for establishing a limit to the rate of change of temperature. The widespread practice of subjecting items to sudden cooling through 50 °C, to freeze bugs unprepared for the sudden and premature onset of winter, has not proved to be damaging [refs]. Depending on the heat capacity and conductivity of the item, the stresses induced by uneven cooling have not been determined, even in this extreme, yet controlled situation.

Temperature uniformity in an exhibition space matters for the practical reason that the environment cannot in practice be continuously measured at every point in the space. A picture hanging on an uninsulated outer wall will experience a varying temperature, and consequently varying RH, even when the sensor in the middle of the room registers perfect constancy.

The uniformity requirement is therefore not a material property, but it is measurable, enforceable and useful.

Limits to the allowed temperature variation in space

A useful rule of thumb is that a one degree rise in temperature will cause a 3 % fall in RH. Assuming a room at 20 °C and 50% RH, the RH at the inner surface of an outside wall or window glass will be 68% if its surface temperature is 15 °C. So a 5 °C variation in temperature within a space with free air circulation and lightly regulated RH, as allowed by the RH section of the PAS, will put some items at risk of RH induced damage.

In a cold store, a constant temperature difference will appear between the wall and the centre of the room, because of the great difference in temperature across the wall.

These transient, as well as permanent, non-uniform environments are usually avoided by forced air movement to bring all surfaces to the same temperature. The generous air flow through air conditioning systems is primarily to ensure temperature uniformity. Water vapour uniformity is much easier to achieve. However, the need for ducted air propelled by fans can be avoided by careful building design - to avoid cold corners in winter, and by careful exhibition design - to hang pictures separated from outer walls.

The requirements for temperature uniformity should not automatically be realised by forced air movement.

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