**1. Introduction**

As solid woods are natural porous media, many of their properties depend on the size of the pores, the way they are interconnected or isolated from the neighboring pores, and even the quality of the surface of the pores [1–4]. Many factors influence the formation of wood and its structure and porous system thereof—factors such as initial spacing, intercropping with different plants, drying procedures [5–7], growing season, extractive content, moisture content, and hygroscopicity of wood [8,9]. Therefore, researchers constantly look for new methods to modify wood for better mechanical properties because few species offer radial and axial uniformity in their produced wood [10].

Thermal modification is generally considered the most commercially-used wood modification method [11]. It has been recognized as a method to improve the dimensional stabilization of wood and increase its decay resistance [11–13]. Thermal modification at high temperatures has decreasing effects on some mechanical properties of wood. However, there are some ways to mitigate the decreasing results [14]. Thermal modification is mostly carried out between the temperatures of 160 and 260 ◦C. Temperatures that are lower than 140 ◦C usually result in very little changes in the mechanical properties, but higher temperatures result in so much degradation that the mechanical properties are usually unacceptable. Thermal modification processes that are currently used at an industrial scale are not higher than 260 ◦C; in practice, temperatures between 150 and 230 ◦C are more accepted [11,15]. The reduction of swelling in wood specimens caused by increase in temperature and duration of heat treatment was often attributed to the destruction in hemicellulose compounds [15]. However, structural modifications and chemical changes of lignin were suggested to also be involved in the process [15]. Moreover, Borrega and Kärenlampi [8] revealed that a reduction in hygroscopicity can not only be attributed to mass loss, but another mechanism that was also active in the process. They suggested that the active mechanism might be an irreversible hydrogen bonding that occurs during the process of water movement within the porous system of wood structures. This bonding was reported by other researchers to change physical and mechanical properties, as well as fluid flow in the solid woods.

High thermal conductivity coefficients of metal nanoparticles [16–20] were used in improving some of the properties in solid woods and wood-composite materials [21]. Impregnation with nanosilver suspension as well as heat treatment were also reported to alter the porous structure of solid woods, significantly altering the gas and liquid permeability [22], and possibly the penetration of paints in to the porous structure, eventually changing the adhesion strength of paints. However, authors found no or little literature on the effects of impregnation with metal nanosuspension and thermal treatment on the correlation between the paint pull-off adhesion strength with permeability in solid woods. Therefore, the present study was carried out to firstly find out the effects of nanosilver-impregnation and heat treatment on the gas permeability of beech wood, as a commercial wood species. Thereafter, and as to the nondestructive nature of permeability measurement process, pull-off strength was measured in the same specimens, providing the possibility to find out the effects of nanosilver-impregnation and heat treatment on this property, too. With regard to the fact that both of these properties, permeability and pull-off strength, depend in some way on the porous structure of the substrate, correlation between them was calculated. Moreover, as to the low thermal conductivity coefficient of wood, a separate set of specimens was impregnated with nanosilver suspension to increase thermal conductivity in the specimens and decrease the heat treatment gradient in them. This can also accelerate thermal treatment.

#### **2. Materials and Methods**
