## 2 Materials and Methods  
Scots pine (_Pinus sylvestris_ L.) was used as a wood substrate in all experiments. Kiln-dried rift-sawn boards composed solely of sapwood and measuring 3000 \(\times\) 100 \(\times\) 25 mm were sourced from a local sawmill. Seven boards were used to prepare test specimens measuring 110 \(\times\) 40 \(\times\) 10 mm (EN 152). During the specimen preparation, all the original surfaces of the boards were removed by planning to ensure that the exposed surfaces were free of extra nutrients resulting from the migration of the extractives to the surface due to kiln-drying [34; 35]. One specimen from each board was allocated to each test ensuring similar sets of wood substrate for all experiments. Consequently, 7 replica specimens were used for each test. After preparing, all specimens were conditioned in a constant temperature (20 \(\pm\) 1 \({}^{\circ}\)C) and RH (50 \(\pm\) 3%) environment. The specimens conditioned in this way without any further treatment are referred to as 'Dry' through the paper. Four experiments were carried out. In the **Experiment I**, mould development on the 'Dry' specimens depending on temperature and RH was evaluated. In the **Experiment II**, accidental wetting of wood was simulated by immersing the specimens in water for 30 min followed by blotting off the excess of water. These specimens are referred to as 'Wet'. In the Experiment I and Experiment II, the only source of mould was spores normally present on wood and in the air (\"clean' conditions). In the **Experiment III**, 'Dry' specimens were exposed to environments with and without extra source of mould contamination. The specimens exposed to environment with a contaminator are referred to as 'With contaminator'. In the **Experiment IV**, influence of the moisture content of wood at the moment, when conditions favourable for mould growth are provided, was evaluated. The specimens were conditioned until a constant weight in a container of the definite RH equipped with a vessel containing toluene for preventing mould growth. No direct contact between wood and toluene was ensured. In such a way, the specimens had reached equilibrium moisture content (EMC) for the definite RH before they were inserted into the test chamber. These specimens are referred to as 'At EMC'. The experiment IV was carried out in an environment with a source of mould contamination. Simulations of six constant environmental conditions were included in the experiments, which were formed by a combination of two RH (91% and 97%) and three temperatures (10, 20 and 30 \({}^{\circ}\)C). Saturated salt solutions were used to maintain definite RH of an environment in sealed containers during a test. The salts used were: BaCl\({}_{2}\) for RH 91% at 10 \({}^{\circ}\)C and at 20 \({}^{\circ}\)C and KNO\({}_{3}\) for 30 \({}^{\circ}\)C; K\({}_{2}\)SO\({}_{4}\) was used for RH of 97% for all tested temperatures. Desiccators were used as the containers. During the test, the containers were kept in climate chambers to maintain the required temperature (Figure 1a). To monitor the climate during tests, the containers were equipped with wireless thermo-hygrometers (TFA Dostmann GmbH & Co. KG) transmitting the results to a receiver. For evaluating the effect of presence of a contaminated wood material on development of mould (Experiment III and Experiment IV), the chambers were equipped with an elevated stage on which a mould-infected piece of wood was mounted (Figure 1b). The infected wood was obtained by exposure of pine sapwood to high RH (97%) until more than 50% of the specimen was covered by visually detectable mould (Figure 1c). The specimens were regularly withdrawn from the containers to assess the mould growth. The examination of specimens to detect germination of mould growth was carried out by microscopic inspection of surface using optical microscope ZEISS Stemi 508 fitted with a LED ring light at 40\(\times\) magnification.