Ottaviano Allegretti

Barrier Effect to Water Vapour of Early European Painting Materials on Wood Panels

Abstract This paper reports results of experimental measurements of diffusive water vapour flux through spruce wood specimens coated with 16 combinations of paint layers replicated according to materials and techniques used in the period from the late Middle Ages to the Renaissance period. Layers represented two base preparations (ground), four polychrome paint layers and presence or absence of varnish. The flux of water vapour was measured from sorption tests in unsteady-state conditions. The internal resistance to the transport of water in wood and the external resistance due to the coating layers was calculated with a numerical method. The results show that all painting layers reduce the flux and that there are significant differences among paint layer combinations. The calculated mass transfer coefficient is an important physical characteristic of the material and it allows improved models for the understanding and prediction of the influence of coatings and painting layers on the hygroscopic and deformative behaviour of wooden objects subjected to changes in environmental climatic conditions.

Chemical and ultrastructural changes in compound middle lamella (CML) regions of softwoods thermally modified by the Termovuoto process

Abstract Silver fir and Norway spruce wood have been thermally modified (TMW) for 3–4 h at 160°C, 180°C, 200°C, and 220°C by means of the thermovacuum process (Termovuoto), and the ultrastructural and chemical changes in the compound middle lamella (CML), including the middle lamella cell corner (MLcc) regions (CMLcc), were investigated. Severe anatomical and histochemical changes were prominent above treatment temperatures of 200°C; thus, woods treated at 220°C for 4 h were in focus. Immunocytochemical studies showed that noncellulosic polysaccharides, such as pectin, xyloglucan, xylan, and mannan, were significantly degraded in CMLcc regions of TMWs. After treatment, the CMLcc regions were composed almost entirely of modified lignin with increased amounts of acidic groups. With cytochemical staining for lignin, many electron dense particulates were detected in the CMLcc regions of TMWs, indicating early degradation/alteration by the Termovuoto treatment.

Chemical and ultrastructural changes of ash wood thermally modified (TMW) using the thermo-vacuum process: II. Immunocytochemical study of the distribution of noncellulosic polysaccharides

Abstract Following structural and cytochemical studies (Part I) on thermally modified ash wood (TMW) by the thermo-vacuum (Termovuoto) process, changes in the distribution of noncellulosic polysaccharides have been investigated in TMW treated for 3 h at 220°C (TMW3 h, 220°C) by means of immunogold localization methods. Pectins (homogalacturonan, rhamnogalacturonan-I) and xyloglucan were significantly degraded in compound middle lamella (CML), including the middle lamella cell corner regions (CMLcc), of all xylem cells after thermal modification. Xylan and mannan degradation were also visible in fiber cell walls. In particular, degradation of mannan was very significant and showed variation between cell wall regions even within the same cell wall. The degradation of pectins was more significant than that of hemicelluloses. In summary, results suggest that each noncellulosic polysaccharide may have a different degradation process in ash TMWs.

Monitoring of Wood Decay by near Infrared Spectroscopy

Wood as a natural resource is subject to continuous degradation by means of different environmental agents, where fungal decay is one of the main factors affecting timber structure elements. The goal of this work was to monitor and model effects of fungal growth on the chemical-physical properties of Norway spruce (Picea abies L. Karst.). The spectra of wood samples were obtained with near infrared spectrometer and subjected to chemometric analysis. It was found that the white root fungi (Trametes versicolor) affected spectral bands related to lignin and also hemicelluloses. Brown root fungi (Postia placenta, Coniphora puteana, Gleophyllum trabeum) exhibit spectra deviations mostly in the regions related to carbohydrates. However, it was evident that the degradation efficiency of Coniphora puteana was the highest among all the investigated fungi. It was possible to classify the wood samples to clusters separating the investigated fungi. Accordingly, different effects on the strength loss of the wooden members can be identified. It was shown that near infrared spectroscopy has a great potential for in-field assessment of the fungal degradation of wood.

The influence of dovetailed cross beams on the dimensional stability of a panel painting from the Middle Ages

Abstract This paper describes experiments carried out to determine the influence of dovetailed cross beams on the dimensional stability of a panel painting from the Middle Ages. Besides a serious cup (transverse distortion), the panel featured an unusual longitudinal distortion, a bow, mostly originating from bending in the proximity of the middle cross beam. Because of its localized occurrence, bending appeared to be caused by the crossbeam somehow pushing on the walls of its channel. Thanks to the cooperation offered by the curator and the restorer, some non-damaging tests were performed to measure the distortions of the panel, with and without the cross beams, after conditioning it in different climates. Processing and analysis of the measurement results demonstrated that the bow of the panel increases as the wood moisture content decreases. This led to the conclusion that the bow is actually produced by forces exerted by the cross beam along the panels length, according to a mechanism named thrust transformation. By this mechanism the cross beam partly transforms the panels tendency towards cupping, into a bow distortion.

Nonsymmetrical drying tests—Experimental and numerical results for free and constrained spruce samples

ABSTRACT The paper brings new insights into nonsymmetrical convective drying, a configuration in which stresses induce a global change of the section shape of a wooden board with five insulated faces. Two experimental devices that allow to test simultaneously “twin” wooden boards have been developed: the first board is free and its curvature is collected vs. time; the second one is constrained and the force required to keep it flat is collected vs. time. Convective drying tests have been performed on radial specimens of spruce, 5 and 15 mm thick at 45 and 65°C. The results are commented and compared with simulation runs. For this purpose, a 1-D mechanical formulation is embedded in the computational code of heat and mass transfer TransPore. The objective of the comparison between numerical and experimental results is a better understanding of the coupled phenomena and the identification of some transfer and mechanical parameters.

Chemical changes to woody polymers due to high temperature thermal treatment assessed with near infrared spectroscopy

Near infrared spectroscopy and an original approach for visualisation of spectra are proposed here for characterisation of vacuum thermally modified samples of eight wood species representing softwoods and hardwoods. Significant changes in absorption bands were observed in conjunction with an increase of treatment temperature. A novel method for visualisation of the degradation patterns (xylograms) is capable of highlighting peculiarities in chemical changes to woody polymers due to the thermal treatment. This method also allows observation of kinetics and permits comparison of thermal stability of investigated species. Such knowledge might be helpful for further optimisation of thermal treatment procedures at an industrial scale. Furthermore, xylograms as a simple and illustrative method might be suitable for visualisation of other modification processes of wood as well as of other materials.

Industrial Thermowood® and Termovuoto thermal modification of two hardwoods from Mozambique

Abstract The study aimed at treating metil (Sterculia appendiculata K. Schum) and neem (Azadirachta indica A. Juss) timber from Mozambique under industrial conditions by steam [Thermowood® (TW)] and vacuum [Termovuoto (TV)] thermal modifications (TM). Matched boards were treated identically and wood alterations in chemistry, colour, mass loss (ML), mechanical properties and durability were compared. The applied vacuum partly removed the acetic acid that causes carbohydrate degradation, i.e. heat applied under vacuum was less destructive. TM under vacuum generated a lighter colour than that caused by steam treatment. ML was significantly higher after the TW process namely, 14.1 vs. 9.9% after thermo-vacuum treatment for metil and 14.2 and 12.1% for neem. Colour and ML changes correlated with the decrease in shear strength, rupture and elasticity moduli and increase in wood decay resistance. Metil wood is more permeable and demonstrated significant differences between the treatments; the thermo-vacuum process was less destructive but led to less improvement of durability compared to TW treatment.