## 2 Materials and Methods  
### Wood, Inorganic Preservatives, Wood Treatment with Preservatives  
The spruce wood (_Pica abies_ Karst. L.) without sap-zone was used in the experiment. Specimens with the dimensions of 80 mm \(\times\) 20 mm \(\times\) 5 mm (longitudinal \(\times\) radial \(\times\) tangential), with no knots and other abnormalities or defects, were sawn from one naturally dried spruce board with dimensions of 2000 mm \(\times\) 300 mm \(\times\) 30 mm. The board was prepared from a 96-year-old spruce tree trunk obtained from the National Forest Centrum in Zvolen. For the shear strength test, a total of 500 specimens were used (25 set types \(\times\) 10 replicates in one set type \(\times\) two specimens for preparing one set; see Section 2.3). For the wettability test, a total of 15 specimens were used (five types \(\times\) three replicates; see Section 2.2). For the microscopy analysis of \"wood--glue\" interphases, a total of 10 specimens were used; see Section 2.4). During the preparation of specimens, the surfaces of the spruce board were gradually milled in a milling machine (D630-EL, Rolband, Bruges, Belgium) in order to use smooth spruce wood surfaces for testing the wettability (Section 2.2) and for preparing the glued joints by EN 205 [49] (Section 2.3). Specimens before other tests were conditioned to the moisture content of 8 \(\pm\) 1%.  
The milled surfaces of spruce wood were treated with four types of inorganic wood preservatives--copper sulfate CuSO\({}_{4}\cdot\)5H\({}_{2}\)O, zinc chloride ZnCl\({}_{2}\), boric acid H\({}_{3}\)BO\({}_{3}\), ammonium sulfate (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\))--used in an aqueous solution with concentrations of 0.5, 1, and 2%, i.e., similarly as were they applied in practice in the past.  
Copper sulfate--blue vitriol, was as \"Holtz-Balsam\" used already in 1718, and later in the 19 th and 20 th centuries for the treatment of green wood by Boucherie technology, as well as of wood products for interiors. Today, it is not used as a stand-alone for wood protection, because of its high corrosive effect on iron and steel, discoloration of wood, and poor resistance to leaching. However, copper (II) is still applied in combinations with other biocidal and fixing compounds [50].  
Zinc chloride was a fungicide recommended for wood protection in 1815. In 1956 it was used also as an insecticide for wooden arts, however, with low or no efficiency. Today, zinc (II) fungicide cations are applied to wood and wood composites in the form of nano-zinc oxide [50].  
Boric acid was successfully used as a fire retardant from 1851, and as a biocide for wood protection from 1877 [50]. Later, it was combined in commercial biocides with chromium and copper compounds with the aim of its fixation against leaching. Today, fixation of trivalent boron in wood against water leaching is secured by colloid silicones, animal proteins, or zinc sulfate creating insoluble zinc metaborate.  
Ammonium sulfate is still one of the most effective fire retarders used for the protection of wood structures and various cellulose materials, which can be fixed with wood components and used in a humid environment [51].  
The double coating process at the atmospheric pressure and a temperature of 20 \({}^{\circ}\)C \(\pm\) 2 \({}^{\circ}\)C was performed on the milled surfaces of spruce wood specimens (80 mm \(\times\) 20 mm), with the application rate of the preservative solution 2 \(\times\) 120 \(\pm\) 10 g/m\({}^{2}\). The coating was conducted using pipettes and brushes--i.e., first, the exact volume of drops of a preservative was applied on the milled surface using a pipette, secondly, drops were equally spread on the milled surface of the specimen. The second coating was carried out after 24-h of air conditioning of the specimen.