## 4 Conclusions  
Infrared thermography was used to study a colorless leaded-glass window which was subjected to a conservation-restoration treatment. The window looks visually homogeneous and only contains historical glasses, but these glass panes are supposedly from different time periods and therefore are likely to have a different composition and thermal properties. Our goal was to assess to what extent IRT in combination with a clustering approach can differentiate the different glass types. The window panel was measured both in reflection and transmission heating mode. The thermograms of both measurement modes showed similar results, but the transmission mode measurement was subjected to more interference from corrosion deposition on the glass surface. The lead comes that hold the glass panes appear to have a significant impact on the thermal behavior of the surrounding glass, which can be problematic in case of small glass panes (typically smallest dimension < 40 mm) because their entire surface is affected. There was also some non-uniformity in the heating of the glass panes at the edges of the window, caused by the wooden frame. By inspecting the thermograms and applying k-means and hierarchical clustering to the thermal data, we were able to differentiate between different groups of glass panes. Although clustering approaches did not allow to distinguish more types as compared to a careful manual inspection of the individual thermograms, we feel that performing clustering on the thermal data cube provides us with clearer and more reliable results. This also automates part of the inspection process, which reduces the amount of man hours required and delivers faster results. Both clustering methods arrive at the same groups of glasses and clearly separate them. The strength of IRT lies in its compact set-up, fast measurement time, ability to measure a large area, and ease of use. However, it cannot be ruled out that glasses with different chemical compositions are not distinguished by IRT due to similar thermal properties. This might explain the fact that some visual color differences between glass panes, could not be distinguished by means of IRT. Differences in thermal behaviour are also not always caused by a different chemical composition. A systematic experiment on glasses with known chemical composition, representative for the established historical groups, would be necessary to elucidate this matter. Nevertheless, IRT shows potential as a swift and easy to use method for documenting the material intervention history of leaded-glass windows during or in preparation of conservation treatments. The potential weaknesses of IRT could be covered by making use of a  
Figure 8: **Left:** Image of the leaded-glass window where the glass pieces are color-coded according to the group they belong to, based on our interpretation of the results. **Right**: Average temperature difference plots over time for the numbered glass pieces. The temperature differences are normalized over the entire data cube. The temperature data originates from the upper area measurement in reflection mode. multimodal (imaging) system that makes use of both IRT and chemical imaging methods. In this way, IRT could be used for a first complete scan, while a more specific chemical technique is used to inspect the areas where IRT does not give conclusive results. Conceptualization, S.L., K.J., G.V.d.S. and G.S. ; formal analysis, M.H. and Y.D. ; investigation, M.H. and Y.D. ; resources, J.C. and G.S. ; writing--original draft preparation, M.H. ; writing--review and editing, S.L., K.J., G.V.d.S., J.C. and G.S. ; supervision, G.S. and G.V.d.S. ; All authors have read and agreed to the published version of the manuscript. This research has been funded by the University of Antwerp (BOF/IOF) and Research Foundation Flanders (FWO). UAntwerp BOF DOCPRO project: Depth-selective chemical imaging of Cultural Heritage Objects (DICHO) with project ID 39928.