# Drying Behaviour of 50 mm Thick Turkey Oak Lumber
## 2 Materials and Methods  
The wooden material used within this research consisted of 0.6 m long, 15-30 cm wide, and 50 mm thick lumber parts, cut from 14 logs existing in the log yard of a wood processing company, having average diameters of 45-50 cm and an age of 90-95 years. They were cut so that some of the boards included only heartwood, and some included both heartwood and sapwood. Half of the selected boards, originating from the same plot, were stacked under a shed, tied together (Figure 1a), and exposed for 7 months (from 30 November 2022 to 3 July 2023) to air-drying conditions. The other half were stacked and kiln-dried in a FEUTRON KPK200 climate chamber (Figure 1b), with controlled air parameters. Figure 1: Mini-stacks of 50 mm thick Turkey oak lumber prepared for air-drying (**a**) and for kiln-drying in a KPK200 climate chamber (**b**). Half of the kiln-dried lumber boards were previously steamed at 100 \({}^{\circ}\)C for 48 h in an industrial steaming chamber. In both drying tests, half of the lumber pieces placed within the same stack were end-protected. For the air-dried boards, a paraffin-based sealer was used, and for the kiln-dried boards, a mixture made with PVA adhesive and sawdust was used. Also, in both drying situations, the boards were dried under pressed conditions, in order to avoid deformations. Before stacking, specimens were sampled (Figure 2) in order to assess the inital state of the boards, namely, the average initial moisture content, the moisture gradient across the lumber thickness, and the eventual inner stresses. The visible cracks were marked with white chalk. The climatic conditions recorded during the air-drying exposure period are presented in Figure 3. The moisture content of the air-dried boards was monitored monthly by means of a resistive-type moisture meter type LG9 NG from Holzmeister (Italy). Both the core moisture content (at 20 mm depth) and that in the surface layer (at 10 mm depth) were recorded, in order to assess not only the drying rate but also the evolution of the moisture gradient across the lumber thickness. Figure 3: Climatic conditions during the air-drying. Figure 2: Samples for the evaluation of the initial state of the lumber boards: a\({}_{1}\) and a\({}_{2}\)—samples for determination of the average initial moisture content of the board; b—layer sample; c—fork sample. The drying schedule applied within the kiln-drying test is presented in Table 1. It was established based on the reference literature [6; 13; 14; 15; 16] and on the authors' experience with oak lumber drying. In this trial, the moisture content was continuously monitored by means of V2A sensors with insulated rods from Gann (Germany) (Figure 4a). The 25 mm long electrodes were used to assess the core moisture content, and short (15 mm) electrodes were placed nearby in the board (Figure 4b) to assess the moisture content in the surface layer. The drying diagram was drawn for each load, and the drying rate (w) was calculated according to Equation (1), as a function of the moisture decrease interval (\(\Delta\)MC) and the time interval (\(\Delta\)t) during which this moisture decrease occurred:  
\[\mathbf{w}=\frac{\Delta\text{MC}}{\Delta\text{t}},\ \%/\text{day} \tag{1}\]  
The final quality control included the visual examination of each board and registration of the established defects within a drying record sheet, where also observations regarding the initial state of the examined board were noted. Additionally, a case-hardening test [17] for randomly chosen boards was performed (Figure 5). Through the adopted conditions, this research envisaged establishing the following:  
* Which drying method is more suitable for Turkey oak lumber (air-drying vs. conventional kiln-drying from a green state);