Impact of Relative Humidity on Wood Sample: A Climate Chamber Experimental Simulation Monitored by Digital Holographic Speckle Pattern Interferometry

Relative humidity (RH) changes are a natural environmental effect that forces organic materials to a constant cycle of achieving equilibrium. The present work is part of an ongoing research based on the hypothesis that the inevitable deleterious effects of the RH natural cycle may be prevented or minimized if a deformation threshold is assigned to each monitored endangered object prior to exposure to structural damage. In this paper the characterization of the behavior of a softwood sample (1.0 cm thick) submitted to RH abrupt cycles has been performed, in terms of mass and rate of displacement of the surface. The exemplary study is based on the concept of recording the RH impact directly from the material surface, allowing us to identify diversity in reaction with time, which in turn could determine the onset of structural changes prior to irreversible damage. The RH impact is measured as surface deformation from interference fringes, using a custom-made real time holography system with interferometric precision termed digital holographic speckle-pattern interferometry (DHSPI). The main observations presented here are a hysteresis in the dynamic sorption isotherm and a greater rate of displacement during the drying. A long-term experiment was performed to identify signs of ageing of the sample. The evolution of the mass and the rate of displacement stayed similar, an offset with an interesting behavior was observed and highlights ageing of wood. In order to produce a future preventive model for distinct art objects it is necessary to determine a deformation threshold for each material. In this context the study was planned to continue with organic samples bearing variable density and thickness under longer-term RH cycles and monitoring until the samples show visible signs of irreversible damage.

[1]  David Erhardt,et al.  Structural response of painted wood surfaces to changes in ambient relative humidity , 1998 .

[2]  T. Vyhlídal,et al.  Diffusion-model-based risk assessment of moisture originated wood deterioration in historic buildings , 2015 .

[3]  C. Skaar.  Wood-Water Relations , 1988, Springer Series in Wood Science.

[4]  C. Skaar,et al.  Dynamic sorption and hygroexpansion of wood wafers exposed to sinusoidally varying humidity , 1983, Wood Science and Technology.

[5]  Stefan Michalski,et al.  Relative humidity: a discussion of correct/incorrect values , 1993 .

[6]  R. Van Grieken,et al.  Environmental monitoring in four European museums , 2001 .

[7]  Hl Henk Schellen,et al.  Climate risk assessment in museums , 2013 .

[8]  Eirini Bernikola,et al.  Fully-Non-Contact Masking-Based Holography Inspection on Dimensionally Responsive Artwork Materials , 2008, Sensors.

[9]  C. Wykes,et al.  Holographic and speckle interferometry: Speckle pattern interferometry , 1989 .

[10]  H. Meier.  Chemical and morphological aspects of the fine structure of wood , 1962 .

[11]  Eirini Bernikola,et al.  Surface monitoring measurements of materials on environmental change conditions , 2013, Optical Metrology.

[12]  V. Tornari,et al.  Preventive deformation measurements on cultural heritage materials based on non-contact surface response of model samples , 2015 .

[13]  Marion F. Mecklenburg,et al.  The Effects of Relative Humidity on the Structural Response of Selected Wood Samples in the Cross-Grained Direction , 1995 .

[14]  John C. F. Walker.  Primary Wood Processing: Principles and Practice , 1993 .

[15]  C. Vest.  Holographic Interferometry , 1979 .

[16]  Vivi Tornari,et al.  On development of portable digital holographic speckle pattern interferometry system for remote‐access monitoring and documentation in art conservation , 2018, Strain.

[17]  V. Tornari,et al.  Rapid initial dimensional changes in wooden panel paintings due to simulated climate-induced alterations monitored by digital coherent out-of-plane interferometry , 2009 .

[18]  Callum A. S. Hill,et al.  A critical discussion of the physics of wood–water interactions , 2012, Wood Science and Technology.

[19]  Eirini Bernikola,et al.  Measuring environmental impact by real time laser differential displacement technique in simulated climate conditions , 2015, Optical Metrology.

[20]  V. Nik,et al.  Simulations of Moisture Gradients in Wood Subjected to Changes in Relative Humidity and Temperature Due to Climate Change , 2018, Geosciences.

[21]  Peter Brimblecombe,et al.  The impact of future climate on historic interiors. , 2012, The Science of the total environment.

[22]  Vivi Tornari,et al.  Digital holographic interferometry for cultural heritage structural diagnostics: A coherent and a low‐coherence optical set‐up for the study of a marquetry sample , 2018 .

[23]  F. Becherini,et al.  A critical analysis of one standard and five methods to monitor surface wetness and time-of-wetness , 2018, Theoretical and Applied Climatology.

[24]  T. Eiju,et al.  Digital phase-shifting interferometry: a simple error-compensating phase calculation algorithm. , 1987, Applied optics.

[25]  B. Time,et al.  Studies on hygroscopic moisture transport in Norway spruce (Picea abies) Part 1: Sorption measurements of spruce exposed to cyclic step changes in relative humidity , 2002, Holz als Roh- und Werkstoff.