GCI and AE monitoring

The NGV Conservation department is partnering with scientists from the Los Angeles–based Getty Conservation Institute (GCI) to undertake real-time monitoring of a Flemish carved polychrome retable from the early 16th century Carved retable of the Passion of Christ. The research project uses acoustic emission technology to detect ultrasound waves produced by microscale changes in hygroscopicThe tendency for a material to absorb moisture with increasing RH, and release moisture as RH decreases. This results in dimensional changes in response to changes in RH. materials.

Acoustic Emission Monitoring and Cultural Heritage

When a material undergoes a small structural change, such as the formation of a micro-crack, acoustic elastic waves radiate from the event and travel through the solid material. This emission of acoustic waves can be monitored by highly sensitive sensors positioned at the object surface. Acoustic emission (AE) monitoring has been used in industry to monitor critical components to identify the onset of material weakness. Examples include the use of AE monitoring to detect cracks in airplane components, high-pressure vessels and pipelines, and concrete structures such as bridges.

Wood is one of the most common materials found in cultural heritage collections. It is also hygroscopicThe tendency for a material to absorb moisture with increasing RH, and release moisture as RH decreases. This results in dimensional changes in response to changes in RH., and subject to brittle cracking events as wood cells expand and contract with changes in water content. In 2008, Slavomir Jakiela and Roman Kozlowski were the first to apply AE monitoring to a wooden artwork. Their study focused on a wooden sculpture, which was part of a medieval altarpiece in the church of Santa Maria Maddalena in Rocca Pietore, Italy. This object was subject to fluctuating temperature and relative humidity conditions caused by a heating system in the church, and the subsequent expansion and contraction of the material made it potentially vulnerable to environmentally-induced cracking.

In 2016, the Getty Conservation Institute (GCI) initiated a study of the response of a suite of museum-like wooden objects to changing environmental conditions (Łukomski et al 2017). A range of monitoring techniques were used to assess object response, including AE monitoring. While the AE results showed evidence of brittle cracking, these events were limited to RHThe amount of water vapour in the air as a percentage of the maximum amount possible at that temperature. The higher the temperature, the more water vapour the air can potentially contain. exposures below 30% suggesting that the objects (purchased in local antique shops) had been periodically exposed to extreme dry environments in southern California, and may no longer be sensitive to such conditions. In 2017, the GCI convened a meeting of experts on AE monitoring to discuss the state of the field, resulting in the 2020 GCI publication, Acoustic Emission Monitoring for Cultural Heritage.

The GCI and NGV in collaboration: AE monitoring of the Flemish retable

Motivated by an interest in reducing energy use across the gallery the NGV has adopted broader temperature and relative humidity ranges for the collection environment. This transition from previously narrow conditions to a wider environmental range aligns with recent guidance in the cultural heritage field and provides a perfect opportunity for the NGV and GCI to collaborate to implement AE monitoring of a Flemish polychrome retable.

AE sensors attached to the surface of the altarpiece will ‘listen’ for microchanges in the wood, with data observed remotely by GCI scientists and collected for the duration of the project. By monitoring the work in the different conditions over time, the degree of influence caused by variations in the Gallery’s environment can be statistically quantified to a level not possible through human observation.

NGV conservators have placed sensors at existing cracks, where the highest likelihood of change would occur. NGV mount makers produced custom mounts for the sensors that are safe for the object and can hold the sensors securely in place long term. They were also designed to be visually unobtrusive, so that visitors can still enjoy the object in all its splendour with minimal distraction.

A year of baseline AE data was collected for the object at the pre-existing environmental conditions in the gallery, with tightly controlled temperature and relative humidity. In December 2022, the gallery that houses the altarpiece was switched to wider ranges for temperature and relative humidity. AE monitoring will continue for an extended period to compare data collected under the two environmental settings. So far, there has been no adverse response from the altarpiece under the new climate controls.

This collaboration will provide important data on this specific object’s response to the environment and it will contribute a significant case study to the ongoing discussion of adaptive and sustainable environmental practice in the cultural heritage field.

Piezoelectric sensors held in contact with the retable with custom mounts