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Conservation Notes: Get to know a hygrothermograph

Most days this summer the air outside has been hot and humid, but inside the Gallery it's been cool and relatively dry. In fact, it's been cool and relatively dry to exact specifications, in order to protect the art within the AGO's walls. How do we do it? And why, exactly?

Drawing on the expertise of the AGO's deputy director of collections management and conservation, Margaret Haupt, let us introduce you to a helpful little device, invented in the late 18th century, that allows staff to keep a close eye on conditions in the Gallery's art spaces.

A hygrothermograph in the David Milne Centre.

Hygro (from the Greek hugros, meaning "moisture/humidity") + thermo (a Greek root meaning "heat") + graph (from the Greek root grapho, meaning "write")

A hygrothermograph is an instrument that measures and records both temperature and relative humidity, simultaneously, onto a single chart. We use them at the AGO to record environmental conditions in art spaces — whether display or storage environments.

How does it work?
The temperature is recorded by the upper pen, which is controlled by the movement of a bimetallic strip. The humidity-sensing element is comprised of small bundles of human hair. The hair lengthens or shortens depending on the humidity of the surrounding air. This motion is transferred to the lower pen by means of a lever mechanism. The chart is installed on a drum which is rotated by a battery-powered clockwork mechanism. The mechanism has three output speeds, so that one-day, seven-day, and 31-day charts can all be accommodated on the one instrument.

How do we use them?
At the AGO, the charts are changed monthly and the instruments are calibrated when the charts are changed. The trends in any given gallery usually follow a consistent pattern, making it very simple and quick to identify unusual changes. These changes are reported by the Protection Service Officers patrolling the galleries, which allows us to respond very quickly to new conditions.

Hygrothermographs are particularly valued for producing an immediately visible record that allows us to assess the pattern of temperature and humidity changes that occur in a space over a period of time. AGO staff sometimes pen horizontal lines onto the charts to mark the target range for relative humidity, making it easier to spot conditions that fall outside that range. Once removed from the machines, we review the charts in order to identify galleries that are not performing to our expectations, so we can devise strategies to improve them.

The hygrothermographs provide a backup system to the electronic Building Automation System and are used by Plant Operations staff to diagnose system problems. It’s not uncommon, for example, for the charts to help staff identify sensor failure in the automated system. We also use the charts to demonstrate to lenders that the AGO is able to provide appropriate conditions for works that we would like to borrow for exhibitions.

A hygrothermograph installed in one of our Canadian galleries.

Temperature and why we control it
For mixed fine-art collections, any temperature between 16 C and 25 C is considered acceptable. At the lower limit, acrylic paint films become brittle, but otherwise the cooler the ambient environment is kept, the slower the inevitable chemical reactions that are associated with the aging of materials.

At the AGO, as at most museums, the temperature is set at a normal room temperature so that our visitors (as well as staff and volunteers) will be comfortable (20 C or 21 C; seasonally adjusted). Most materials actually do decompose at room temperature, but the time frame for complete disintegration is measured in millennia.

Humidity and its effects on the artwork
The focus for creating a preservation environment at the AGO is relative humidity (RH) rather than temperature. The RH set-point for most AGO galleries is 45 per cent. Other collections, especially those from Europe, are sometimes conditioned to other humidity levels — usually 50 per cent. When these works are exhibited at the AGO, we adjust our system to provide the right environment. Any humidity at all will cause a gradual discolouration and disintegration of organic materials — especially of materials that are chemically unstable, such as poor-quality papers.

Research undertaken in the 1990s demonstrated that cycling of RH in the mid-range does not damage most objects — the key exceptions being new materials or recently conserved works, which are conditioned to our own very stable environment. New mats will warp when subjected to relatively small shifts in relative humidity of about 15 per cent. Outside of that acceptable mid-range, or in especially fragile works of art, fluctuation of humidity can cause significant damage.

Extremely high or low RH will lead to direct damage of objects and materials. Relative humidity above 75 per cent supports mould growth, which stains and weakens both organic and inorganic materials. In addition to bad hair days, high humidity also causes the corrosion of metals and the shrinkage of tightly woven textiles — as sometimes occurs in 19th-century canvases. Low humidity has caused wood and ivory to crack.

Now that you know what it does, join us in commending the humble hygrothermograph for playing its role in a larger system of checks and balances. There are a more than a few paintings, drawings, prints and other artworks that would likely say "thank you" too.

Curious about Conservation?
If you have a burning question about Conservation, leave it in the comments below. We'll do our best to give you an answer in an upcoming Conservation Notes post!


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