2.2. Extraction of tannin
White oak (Quercus alba) barks taken from Çamsan A.Ş. (Turkey) were used for tannin extraction. The extraction was carried out in a batch type digester at 85 °C, at 4 rpm of rotation velocity for 30 min. The NaOH solution (4%) was added to bark particles at a 6/1 (liquid/solid) mixture ratio. Solid tannin extract was obtained by evaporation and stored in a refrigerator before use. For each of the panels applied with tannin, a control panel without tannin solution was produced. The values of the control panels are shown in Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5 and Fig. 6. The tannin solution used was prepared at 20% based on weight. The tannin solution ratios used for the experiments are presented in Table 2.