Assessing the structural integrity of cultural heritage objects is of great importance for their structural conservation and long-term preservation. This paper focuses on the development of a non-destructive inspection (NDI) approach using 3D shearography to evaluate the structural integrity of wax-resin lined paintings, specifically for The Night Watch (1642), a large-format 17th-century canvas painting by Rembrandt van Rijn (1606–1669) that is on display in the Rijksmuseum, Amsterdam. The Night Watch has a complex treatment history that has many old repairs of structural defects and damages (holes, tears, etc.) and three wax-resin relinings. In 2021, before a new structural intervention involving retensioning of the canvas support, it was vital to evaluate the structural integrity of the painting, specifically the condition of the treatment carried out in 1975–76 when, among other actions, several long cuts in the area of Captain Frans Banninck Cocq's breeches were repaired and an old canvas insert in the drum was replaced. To assess the structural condition, we applied 3D shearography to quantitatively analyse the in- and out-of-plane surface strains with controlled thermal loading. First, a safe loading procedure was developed by inspecting a representative wax-resin lined test painting where reference delaminations and structural repairs to canvas supports were reliably identified with 3D shearography by raising the temperature with 1-2 °C. As part of Operation Night Watch, in November 2021 an in-situ investigation was carried out in the Rijksmuseum gallery. Two areas of interest in The Night Watch, the restored slashes in the Captain's breeches (0.5 × 1 m) and the canvas insert in the drum (0.2 × 0.5 m), were inspected from the reverse of the painting. Results revealed no critical structural problems associated with the repaired slashes, nor with adhesion of the lining. For the patched canvas in the drum, it showed higher in- and out-of-plane strain variations. Overall, 3D shearography provided valuable non-destructive inspection results for assurances regarding the structural integrity of the 1975 repairs and the adhesion of the lining canvas in The Night Watch.

The assessment of the structural condition of cultural heritage objects is important for conservation interventions and their long-term preservation. This investigation concerns The Night Watch (1642), a large-format 17^th-century canvas painting by Rembrandt van Rijn that is on display in the Rijksmuseum, Amsterdam. This painting, which has a complex treatment history, has various damaged areas and has undergone three wax-resin relinings. In 1975 the canvas was slashed twelve times with a serrated dinner knife, including several long slashes in the area of Captain Frans Banninck Cocq’s breeches. In 2021, prior to a proposed new structural intervention involving retensioning of the canvas, it was important to evaluate the structural condition of the repaired slashes and of another repair, specifically an old canvas insert in the drum. For this, an in-situ inspection was carried out in the Rijksmuseum as a part of Operation Nightwatch. 3D shearography instrument with thermal loading was used to inspect these two areas of interest on the reverse of The Night Watch. The results showed that the out-of-plane strain in the breeches does not show any large deviations, which alleviated conservators’ concerns about the adhesion of the lining canvas and stability of previous repairs in this region. The patch in the drum showed higher out-of-plane strain variations. This was explained by the lower quality of the patched canvas compared to the repaired slashes in the breeches of Banninck Cocq. Overall, 3D shearography provided valuable inspection results for assurances regarding the structural integrity of the 1975 repairs and the wax-resin lining in The Night Watch, reducing the risks and providing the confidence to proceed with the planned retensioning of the canvas.