What is that box with all the dials sitting in the corner of the gallery?

A hygrothermograph. Photo: Oakton.

Chances are, it’s a hygrothermograph. It’s a device that tracks temperature and relative humidity over time. Museum staff use it to understand the environment in a space, and how it is affecting the objects being displayed. So, you ask, why does this matter?

Many objects are sensitive to changes in relative humidity. Think of doors that stick in their jambs in the summer. Drawers that fit tighter when it’s humid out. Think about your hair. And that’s how hygrothermographs work. A lock of hair is attached to gauges within the device. When there is more humidity in the air, the hair becomes longer, and this relays to the pens tracking the relative humidity on the graph.

Let’s throw another word into the mix: hygroscopic. This term is applied to materials that absorb and hold water, usually resulting in some sort of physical change. Wood, ivory, textiles, and paper are examples of hygroscopic materials. Stone, ceramic, and metal are not (though I will point out that damp environments are not good for storage of metals, as this can accelerate corrosion).

Hobo datalogger. Photo: Onset.

So using hygrothermographs to determine conditions in a space, and how museum staff will display and store objects based on whether they are hygroscopic or not is a big part of preventive conservation practice. Many museums are shifting now toward the use of dataloggers to track relative humidity and temperature, so that digital graphs that may be more easily analyzed. Still, a lot of conservators like hygrothermographs for their instant visual information and ease of calibration.