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Acoustic emission (AE) technology is a promising technique for monitoring cultural monuments due to its characteristic ability to reflect status changes and perceive the development process of deterioration and damage even before their visual appearance. This study was established on the motivation of providing basic data and a methodology that can improve the signal processing, characteristics analysis and classification for the AE technique in the long-term in-situ monitoring of deterioration processes, starting from the freeze‒thaw deterioration of tuff monuments at the Chengde site. AE monitoring was carried out with an indoor freeze–thaw deterioration experiment. As a result, a set of procedures and related methodology is proposed based on the hit-based AE waveform parameters for denoising and classification of monitored AE signals by applying hierarchical cluster analysis, k-means clustering, distribution statistics, etc. The clustering results show that some signals may indicate deterioration and signals with certain characteristics are more likely to occur at a particular deterioration phase. Signals characterized by the significant absolute energy (ABE) are presumed to be related to the propagation of cracks to the outer layer. Signals characterized by a higher indirect parameter RA (Rise time divided by peak amplitude) value may connect with the opening/closing of microcracks in the earlier phase of deterioration prior to the exposure of visible surface cracks. The peak frequency (PF) is likely to decrease as the deterioration proceeds.

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Here, we investigated the wood-whitening phenomenon adjacent to metal components that are commonly observed in Japanese historic architectures. The higher detection of Cu/Zn elements and fine particles of copper/zinc oxalate hydrate in whitened wood has been verified in a previous field investigation. In order to explore the related cause of this phenomenon, this study established model experiments to evaluate some surface performance changes of wood plate samples with impregnated Cu(II)/Zn(II) contents under artificial degradation. The evaluation included the following: (1) the state of metal oxalates in the wood tissue; (2) the mechanical performance of the resistance to the external forces; and (3) the colour change of the wood surface. As the result, the accumulation of metal oxalates and their growth in particle sizes were confirmed in samples containing metal, in particular those with a higher Cu(II) content, regardless of wood species. Meanwhile, this metal-involved degradation could introduce the risk of embrittlement of wood ahead of the occurrence of visible whitening discolouration.