## 2 Materials and Methods  
The residual fibre mass, composed of the species Norway spruce (_Picea abies_ Karst.) and Scots pine (_Pinus silvestris_ L.) and oven-dried to 12% moisture content, was provided by the pulp and paper factory Mondi Stambolyiski EAD. The bulk density of the factory waste fibres was 44.86 kg-m\({}^{-3}\). The waste fibres had lengths from 500 to 1000 \(\upmu\)m and 7% reduced lignin content. Magnesium lignocellulosic at 15% gluing content (based on the dry weight of the fibres) was used as a binder. The lignocellulosic additive had the following characteristics: total solids content--51.2%, magnesium content--6%, reduced sugars--7% and sulfate content--2%. European beech (_Fagus sylavtica_ L.) veneers, made using the centric peeling process with an average thickness of 1.13 mm and a moisture content after drying and conditioning of approximately 7%, were supplied by the factory Welde Bulgaria AD, and used for veneering the laboratory-produced fibreboards. Under laboratory conditions, the fibreboards were produced with a 16 mm thickness and a 720 kg-m\({}^{-3}\) target density. Residual fibres were mixed with the magnesium lignocellulosic in a high-speed laboratory adhesive mixer at 850 min\({}^{-1}\). The hot pressing process was carried out using a single opening hydraulic press (PMC ST 100, Italy). The press temperature was 210 \({}^{\circ}\)C. The following four-stage pressing regime was used: in the first stage, the pressure applied was 4.5 MPa for 1 min; in the second stage, the pressure was steadily decreased to 2.23 MPa for 3 min; in the third stage, the pressure was decreased to 0.74 MPa for 10 min. The fourth pressing stage was performed at a pressure of 1.78 MPa for 2 min. The fabricated composites, after pressing, were conditioned for 10 days at \(20\pm 2\)\({}^{\circ}\)C and 60-70% relative humidity. The veneer sheets with dimensions of \(400\times 400\) mm were cut. After that, the fibreboards were veneered using magnesium lignocellulosic at the content of 80 g-m\({}^{-2}\) as an adhesive. A hand roller was used to apply the binder to the veneers in order to achieve a uniform adhesive layer. The veneering press factor used was 1 min-mm\({}^{-1}\) of board thickness; the pressure applied was 0.6 MPa at 200 \({}^{\circ}\)C.  
The physical and mechanical properties of the fabricated panels (Figure 1) were tested according to the European Standards EN 310, EN 317, EN 322 and EN 323 [73; 74; 75; 76]. A precision laboratory balance Kern (Kern & Sohn GmbH, Balingen, Germany) with an accuracy of 0.01 g was used to determine the mass of the test specimens. The dimensions of the test pieces were measured using digital callipers with an accuracy of 0.01 mm. The physical properties (water absorption and thickness swelling) were measured after 24 h of immersion in water. The thickness swelling was assessed using the differences between the initial and final panel thicknesses, and the water absorption was determined using the difference in weight. The mechanical properties of the panels were determined using a universal testing machine Zwick/Roell Z010 (Zwick/Roell GmbH, Ulm, Germany). The formaldehyde emission of the laboratory-produced panels was tested in the laboratory of Kronospan Bulgaria EOOD (Veliko Tarnovo, Bulgaria) on four test specimens in accordance with the standard perforator method [77]. Figure 1: Eco-friendly panels from industrial waste fibres bonded with magnesium lignocellulosic and veneered with beech veneers; 720 kg-m\({}^{-3}\) target density, 18 mm thickness and 15% lignocellulosic content.