How to

7 February 2017

The Importance of Temperature & Humidity Control in Museums

The world’s museums have an important job to do. Not only do they serve as viewing galleries, allowing us to marvel at the wonders of ages past; they are also charged with the preservation of these priceless objects and collections.

There are many environmental factors that can influence the structural integrity of an object over time. The major factors of deterioration are light, pollutants, pests and, of greatest interest to us, temperature and humidity. In order to get ahead of the game and ensure the longevity of their collections, museums must engage in preventative conservation, limiting the detrimental effects of external forces via careful manipulation of the environment to achieve the required conditions.

Here we will explore how museums combat the threat of incorrect temperature and humidity levels, creating environments designed to preserve their collections as best as possible.

Forms of Deterioration

The main area of concern when discussing temperature control within museums is actually relative humidity (RH), a measure of the moisture level of the air as compared to its maximum capacity at a given temperature, expressed as a percentage. Changes to relative humidity are known to cause expansion or contraction in objects; as collections often contain items comprised of more than one material, this effect can be exaggerated as various components swell or contract at different rates. However, this is far from the only detrimental effect that humidity can have on an object’s structural integrity.

The Philadelphia Museum of Art lists three forms of deterioration caused by humidity:
  • Dimensional Change - Warping, dislocation of joints, splitting, breaking of fibres, delamination, loss of surface material, cracking
  • Chemical Reaction - Corrosion of metals, fading of dyes, weeping or crizzling glass (clouding), crystallisation and movement of salts, disintegration and yellowing of paper
  • Biodeterioration - mould growth (RH 70% or above), bacteria
Museums have to take steps to combat these effects, whether items are in storage, on display, or prepped for transport.

Ideal Conditions

Most objects stored within museums require a relatively stable RH level in order to prevent damage, but the exact level can vary dramatically depending on the item in question. As a general guideline, museums are advised to maintain a RH level of 50%. However, the American Museum of Natural History stresses the importance of specially controlled micro-environments for select objects, created ‘by a combination of well-sealed storage cabinets or exhibit cases and by using buffering materials, such as acid free tissue, wooden drawers, and passive environmental measures such as silica gel’.

Of particular concern are metallic items, which often need a lower RH in order to prevent corrosion. Objects such as high-fired ceramics are less sensitive to relative humidity levels and can typically be stored without significant concern.

The American Museum of Natural History lists some basic guidelines for specific collections here.

Measuring Relative Humidity

The Philadelphia Museum of Art makes use of a device called a recording hygrothermograph (pictured) in an effort to maintain a consistent RH level in the building of around 50%, as per given guidelines. Stated to be one of the most common methods used in museums to measure RH levels, the tool records humidity and temperature using a sensor comprised of human or synthetic hair (for humidity) and a bi-metallic strip (for temperature), keeping a week-long log of recordings.

The downside to the recording hygrothermograph is its sensitive nature, which means it requires frequent calibration using another device for reference (a popular option, and the one used in Philadelphia, is the psychrometer - a relatively low-tech device much-loved for its reliability).

Other devices used for similar purposes include:
  • Dial Hygrometer - This device can be hung on a wall or shelf mounted. Measures temperature and humidity within 3 degrees.
  • Data Logger - This device can record, display and download temperature and humidity information to a computer for analysis and tracking.
  • Analog Thermohygrometer - This device is placed discreetly in exhibit cases to monitor temperature and humidity levels
  • Temp/Humidity Cards - These cards record temp/humidity levels by indicating a change in colour from blue (drier) to pink (wetter).

Controlling Temperature & Humidity

Efforts to control temperature and humidity within museums start with the obvious and make their way firmly into the less-so; let’s start with the basics.

Much like your own home, the first port-of-call when it comes to creating a stable environment is to look at simple solutions such as radiators and air-conditioning systems. There’s nothing particularly special about these, in fact they’re no different to the systems found in any other building of a similar size. Properly sealed doorways and windows are a must to minimise the impact of the weather.

As previously mentioned, many collections do require a bit of special attention, or even a specially created micro-environment, in order to best preserve them. This is where it can get a little complicated, as there is no golden rule here. Display cases can be fitted with their own climate-control systems, sealed off from the building at large, but specific conditions are needed for specific collections and those responsible for maintaining such items rely on years of study and experience in order to judge these parameters; it would be foolish of me to try and make a blanket statement here.

Sam Bonson

Sam is an aspiring novelist with a passion for fantasy and crime thrillers. He is currently working as a content writer, journalist & editor in an attempt to expand his horizons.