## 5 Wood Cultural Heritage
### Experimental Methods and Techniques  
The preservation and conservation of the world of cultural heritage demand the application of non-invasive and reversible techniques. With the development of science and technology, a wide range of techniques has been applied and adapted to the study of artwork [64]. Indeed, several techniques were originally developed for other field applications, whereby no particular attention was paid to the preservation of the analysed sample. Another limitation is represented by the application of sensor-based analyses to valuable and decorated objects. These techniques require a medium to couple sensors to the wooden surface. The coupling mediums are often highly adhesive glues with low impedance to ensure proper measurement and support sensors' weight. The use of strong adhesives is not in line with conservation principles since they are not reversible and induce further damage in wooden artefacts while being removed. For this reason, some studies have been conducted to find suitable adhesive materials [65, 66]. The selection of the adhesive method must take into account the duration of the measurements. Some binders employed are volatile, so they have been used in short-term applications [67]. The challenge that these applications bring makes it easier to use dummy samples, which mimic real artefacts. They reproduce a simplified geometry of the real artefacts, which have the same dimensions and are made of the same types of wood and layers, as in the case of a panel painting. The following sensors will be briefly addressed in terms of basic principles' pieces of knowledge and applications to cultural heritage, namely strain gauges, deformometric kit (DK), digital image correlation (DIC), photogrammetry, and the laser scanner technique. Strain gauges (Figure 9) are sensor-based techniques able to measure strain experienced by a wooden object when subjected to temperature and relative humidity fluctuations. These sensors are defined by [PERSON] et al. [64] as point measurement or point-strain measurement techniques. When the sensor, glued on the wood surface, is subjected to tensile or compressive mechanical forces, it changes its length and cross-section, i.e., width x thickness [68, 69]. According to Equation (6), these changes result in changes in electrical resistance:  
\[R=\Omega\frac{L}{A} \tag{6}\]  
where, \(\Omega\) [\(\Omega m\)] is the electrical resistivity, \(L\) [\(m\)] is the length, and A [\(m^{2}\)] is the cross-section area of the strain gauge. The use of this analytical method has been reported on wooden dummy panel paintings [45, 46], as well as complex objects such as wooden cabinets [70]. In the latter, the sensors are installed on a duplicate of the original and valuable object. Point-strain measurements also have the disadvantage of providing the strain in the gauge direction. Wood is an anisotropic material, so the strain measurements with one sensor are non-representative. For this reason, multiple sensors are used [70]. In this type of application, strain gauges are assembled in groups, assessing the strategic position on the artefact according to the aspects to be captured, e.g., the response of the joints. To obtain a whole description of the hygro-mechanical response of the object, strain gauges should be combined with other sensors that are able to detect environment parameters and moisture content. This approach allows for the performance of real-time monitoring analysis. [PERSON] et al. [46] apply this technique to study the influence of a new heating system in a church. They use a so-called 'wood sensor', a strain gauge glued on a 16 th century oak block, sacrificed from a beam of a historic house, to assess the environmental condition and the level of risk for the wooden church interior. A deformometric kit (DK) is usually used to directly monitor dimensional changes. Additionally, two parallel transducers measure the relative movement and distance of two columns fixed on the rear of a panel, i.e., the no-decorate part (Figure 10a).