Temperature

Introduction to the temperature recommendations

This standard allows considerable variation in temperature over the year. This saves energy and does not imperil the artifacts, with very rare exceptions noted below. A cooler winter temperature simplifies and reduces the energy used for RH control and greatly reduces damage to the building through condensation within outer walls. A higher summer temperature limit permits collections not to be air conditioned.

For exhibitions, the temperature range is limited to the zone of human comfort, so exhibits have to endure an environment far from ideal. In storage the temperature range can be set lower for energy economy and to slow the deterioration of artifacts by retarding chemical reactions. Some chemically unstable artifacts need cold storage. The temperature requirements are listed separately for exhibition, general storage, cold storage and transport.

Exhibition

Annual temperature range

The allowable range over the year is 16°C to 30°C. This must be restricted where particular items listed in appendix T are on display.

Aim for a low temperature within this range, to reduce the rate of chemical degradation processes. The upper temperature limit is set high to avoid forcing installation of air conditioning just to cope with the rare occasions of extreme outdoor temperature. High temperature will accelerate chemical degradation reactions but low temperature in winter will compensate by reducing reaction rates.
[Padfield, T. 'The preservation index', www.conservationphysics.org/twpi/twpi_01.php]

Temperature variation rate

The daily temperature range is not specified

The demand for temperature uniformity in space indirectly limits the variation in time, making a specification unnecessary.

Temperature uniformity

The temperature may not vary more than 5°C within any continuous space within the exhibition. The target variation is less than 3°C.

This requirement is intended to minimise variation in RH within a room. Cold corners and spaces behind furniture and paintings set against outside walls will have a local RH much higher than that in the main space of the room. As a rule of thumb, the RH will increase by 3% for each 1°C fall in temperature. Therefore a room at 20°C and 55% RH at its mid point will have 70% RH at a cold wall surface at 15°C. This is a frequent cause of mould growth. The usual advice is to ventilate but the root of the problem is uneven temperature. The reverse effect, a low RH against a warm surface, is characteristic of showcases in sunlight.

It is an important design objective that an exhibition space should have a naturally uniform temperature, so that air conditioning is not needed simply to maintain uniformity in the interior climate.

The temperature in a showcase will typically increase by 2°C when the lights are turned on, so uniformity better than this would be meaningless.

Storage

Annual temperature variation

The temperature in existing storage may vary on an annual cycle between 30°C and 4°C but new storage design must aim for an annual cycle between 8°C and 16°C.

This can be achieved in a zero energy way by putting an insulated building with low air exchange rate on an uninsulated floor, which acts as a year round temperature buffer.[Ryhl-Svendsen, icom-cc-lisbon, in press] See also the note on RH buffering in storage.

The lower limit at 4°C is a practical precaution to prevent freezing of water in pipes, sprinklers, drains and items stored in formaldehyde solution. Items which are in equilibrium with a moderate RH do not suffer ice crystal damage until the temperature drops below about -30°C. [www.conservationphysics.org/cool/suprcool.php]

Temperature variation rate

The daily temperature range is not specified.

Temperature uniformity

The temperature uniformity in a storage space shall be within a 3°C range.

The layout of shelving and clear space close to floor, walls and ceiling are important in achieving this temperature uniformity.

Cold storage

Temperature in cold storage

The temperature can be a constant value set anywhere between -5°C and -20°C.

Cold storage is used for materials which deteriorate fast, by museum expectations of durability. The published rates of deterioration have all been derived by extrapolation from reaction rates measured above +30°C, so they can only be relied on as general guidance to keep the items cool. -20°C is commonly used for foodstuffs so the technology is reliable and relatively cheap. However, the energy consumption is proportional to the distance from the annual average temperature outdoors, so -20°C may be considered too great an expense for unproven preservation duration.

Temperature variation rate

The temperature range shall be less than 5°C over any period.

Since mechanical cooling is unavoidable, it will be more reliable if the temperature is held constant, regardless of outside conditions. The temperature cycling of cooling equipment depends on details of manufacture and adjustment. It is often several degrees, to reduce wear on moving parts.''

Special precautions are necessary during removal and replacement of items in cold storage. Condensation can occur on and within the individual container around an item as it passes through a temperature gradient.

Temperature uniformity

The temperature variation within the contiguous space shall be less than 5°C.

This is to keep the RH within reasonable bounds. For energy economy, the room must be well insulated but because of the large temperature gradient to the outside there is bound to be variation in temperature, moderated by convective air movement, which the layout must allow, particularly at the containing walls.

Special precautions are necessary during removal and replacement of items in cold storage. Condensation can occur on and within the individual container for an item as it passes through a temperature gradient. [Padfield, T.'Condensation in film containers during cooling and warming', Proceedings of the conference 'Preserve, then Show', The Danish Film Institute, Copenhagen, December 2001, published September 2002
www.conservationphysics.org/coolfilm/coolingfilm.pdf]

Transport

Temperature range

The temperature range is not defined but outside temperature variation shall be moderated by thermal insulation of maximum U-value 0.05 W/(K•m²) and by a thermal buffer within the insulated space.

Thermal buffers are heavy and therefore expensive to transport. The amount needed must be evaluated for each object, since some objects have considerable heat capacity themselves.

Temperature variation rate

The temperature variation rate is not defined.

Temperature uniformity

The container must be made so that instantaneous temperature difference between any two points within the airspace enclosing the object can never exceed 7°C. The target value should be less than 3°C variation.

This limit is to avoid condensation in cold places of water vapour evaporated from a warm part of the item, or its container or built in humidity buffer.

Notes: Temperature sensitive materials and assemblies

Introduction

Temperature sensitivity comes in three variants.

Nearly all organic materials suffer slow decomposition, mainly through oxidation, hydrolysis and diffusion of plasticisers. These reactions accelerate exponentially with temperature. That does not mean, however, that occasional exposure to a temperature at the upper end of human tolerance will be rapidly destructive, because the reaction rate may still be very slow.

As the temperature declines, materials become more brittle, particularly plastics which go through a transition to a glassy state which can be orders of magnitude stiffer. This brittleness is not necessarily destructive but raises the risk of handling damage and cracking of laminated structures.

A third type of temperature sensitivity can destroy items very quickly. This is phase change. For these rare materials there are temperatures which the item must not pass through, even briefly.

In the following diagram these sensitivities are plotted together with the limits for human comfort and the three environments: cold storage, cool storage and exhibition.

[Here is the svg file. Download to disk as a text file, then change the file ending to .svg. ]

Chemically unstable materials requiring cold storage to prolong usability:

Colour film and colour prints. Acetate film showing evidence of autocatalytic hydrolysis (vinegar syndrome [definition]).

Materials requiring cool storage to ensure durability for at least a century

Cellulose acetate and nitrocellulose rolled film with monochrome images. (Note that nitrocellulose in roll form also requires separation from other items because of its flammability. Note also that many nitrocellulose items of jewellery and everyday use are displayed in museums, often unidentified as nitrocellulose. If in doubt, store cool, but cold storage is not necessary.

Mercury-tin amalgam mirrors lose mercury by evaporation, at a rate which increases exponentially with temperature. This process can only be slowed, amalgam mirrors are doomed. On display, all amalgam mirrors now show evidence of deterioration. [Hadsund, Per, The tin-mercury mirror: its manufacturing technique and deterioration processes.Studies in conservation (1993) 38 pp 3-16]

Plasticised polymers such as pvc will lose slightly volatile plasticisers such as dibutylphthalate and become stiff. The plasticiser may migrate to attached, but different, plastics, causing further havoc. Plastics are best in cool storage but if they are on display - expect unpredictable, occasionally rapid decay compared with pre-20th c. items.

Rubber goods

Paper manufactured from 1850 to the present which is either mechanical wood pulp or rosin-alum treated paper.

Materials liable to sudden destruction by phase change within or near the default temperature range

Many minerals are surprisingly sensitive to their environment [Waller, R. (1992). Temperature and Humidity Sensitive Mineralogical and Petrological Specimens. pp. 25-50 In Howie, F. (ed.) Care and Conservation of Geological Material: Minerals, Meteorites and Lunar Finds. Butterworth-Heinemann, London.] having well defined phase change temperatures. For example, sodium sulphate decahydrate melts at 32.4°C. This is not as trivial an example as you may think - it is a common contaminant of porous stone and ceramic.

Tin: 13°C phase change. A rare but important temperature limitation. The process is autocatalytic, so it doesn't always happen. Alloying prevents the change, so the identification of the element tin in an item does not define its sensitivity.

Gallium melts at 29.8°C (there are several other low melting metal alloys). However, pure gallium is not a common museum item.

Waxes melt at various temperatures. There is a wide variety of composition within this generic description. However, few melt below 60°C. Therefore, avoid exposure to sunlight.

The glass transition temperature has been highlighted as a significant boundary of stability, but passing through this zone does not cause damage. In any case, the glass transition temperature of hygroscopic polymers varies with water content and the glass transition temperature of plasticised polymers varies over a huge range according the the nature and amount of plasticiser.

Materials physically damaged by cycling temperature

Marble (due to anisotropic expansion of calcite crystals). This has only been observed outdoors, on building facades.

Susceptibility to mould and insect damage

Textiles, paper, parchment and wood are susceptible to attack. RH above 50% plays a role but low temperature reduces mould growth, with significant slowing of the rate below 20°C. However, growth at high RH still happens at 0°C but requires around 90% RH. See also the RH section.

Insect infestation is unlikely below 12°C. Again, RH plays an important role though many insects generate water by digesting carbohydrates. Cool storage is the best defence.

Interactions with other environmental influences

Temperature and RH

In a non-absorbent environment, such as an exhibition hall with painted walls and objects in showcases, the RH will rise as the temperature falls. In an absorbent enclosure, such as an archive with abundant paper documents exposed on shelves, the RH will fall slightly as the temperature falls.

Temperature and Light

Light energy will warm objects. Even a moderately illuminated showcase will rise 2 ℃ over the ambient temperature. This in turn lowers the RH about 6% in the boundary layer next to the object surface, thus causing moisture movement. Direct sunlight, even through a glass window, can raise the temperature of a dark object over 60 ℃, with consequent very low RH.

Temperature and biological damage

Many biological processes achieve maximum rate at around 30 ℃. Below 12 ℃ insects become sluggish and mould growth slows considerably.

Temperature and pollution

Internal pollution increases with temperature because of the increased rate of decay of materials. Reaction rates increase generally, as do diffusive processes, but advection of pollutant gases may be the rate limiting step.

Temperature uniformity

There is no requirement 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. 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 temperature uniformity requirement is not directly for the thermal benefit of the artifact; its influence on the RH is the reason for applying it.

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 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 in this standard should not automatically be realised by forced air movement.