The paper shows the results obtained by drying 38-mm thick beech timber, namely one control test run under usual, conventional drying schedule and four test runs dried under the same schedule but with oscillations of equilibrium moisture content. During oscillation drying, temperature curve was identical with the temperature curve in the control test run, and the values of equilibrium moisture content oscillated according to previously set amplitudes and frequencies. Oscillation amplitudes were ±10% in the first two test runs and ±20% in the second two test runs compared to the value anticipated in the schedule. Duration of each oscillation was 3 h in the first and third test runs and 6 h in the second and fourth test runs. Special addition to the manufacturer's software was developed for the realization of these oscillations. When comparing the results, drying time (for the same intervals of initial and final timber moisture content), energy consumption and drying quality were taken into consideration. Drying quality was determined at the end of each test run (both before and after conditioning), and based on the value of final timber moisture content, distribution of moisture content across thickness and gap size. Drying time was shorter in all four test runs with oscillations, and energy consumption was lower than in the conventional test run. Slightly shorter drying period was recorded in test runs with oscillation frequency of 3 hours compared to the test runs with 6 hour oscillations. Final timber moisture content was almost even in all test runs. Profiles of moisture content across timber thickness were also similar in the test runs, but the biggest difference between core and surface at the end of drying was achieved in the test run without oscillations. Before conditioning, average gap value of over 2 mm was recorded in all test runs, but the biggest average value was recorded in the test run without oscillations.

The research results of measuring and analyzing moisture content distribution across thickness of oak and beech lumber during the conditioning phase are presented. Moisture content difference between the core and the surface layers of wood was reduced during conditioning due to the increase of moisture in the surface layers. It was shown that by means of conditioning it was possible for both species oak and beech to reach the highest drying quality conforming to the European Drying Group (EDG) Recommendation even in shorter times than usually needed for oak, 25 mm thick. The conditioning regimes, using EMC that is lower than final MC, will not give adequate results of conditioning. The drying schedule has a relevant influence on moisture content distribution prior to the conditioning phase. Therefore, the time of conditioning depends on the drying schedule and the required drying quality. Other parameters that influence drying quality (final moisture content, casehardening) should also be considered.

Owing to its anatomical structure and chemical composition, wood changes its moisture content (MC) continually, affected by environmental humidity, and consequently it changes its size and shape. The stability of wood shape and size is definitely significant for the quality of final products. One of the potential methods of increasing wood stability is to expose wood to elevated temperatures.The results of the effect of thermal treatment on some physical properties of the most important industrial species of wood in our country (beech, oak, poplar, spruce and fir) show that, with, the increase of temperature, hygroscopicity limit and total swelling/shrinking of wood decrease.In order to study the direct causes of the change of physical properties of wood under the effect of thermal treatment, comparative analysis of chemical properties of wood species under study was carried out before and after thermal treatments. It was determined that there was a correlation between the changes of the chemical nature of wood constituents and the physical properties of wood under study.

There are several factors influencing a fundamental property of a wood species, the limit of hygroscopicity of wood (LH) or fiber saturation point (FSP). The effect of temperature upon the limit of hygroscopicity has been mentioned in literature, but has not been precisely quantified. In this paper we have correlated the LHs with temperature of thermal pretreatment, taking also into account the properties of particular wood species, i.e. density and chemical composition. For our investigation we have selected the most important domestic wood species for industrial processing : oak, beech, poplar, fir and spruce. We have determined the limits of hygroscopicity by method of compression strength parallel to grain, for the following temperature ranges of thermal treatments, in dependence of wood species : for hardwoods from 20° to 80°C and for softwoods from 20° to 90°C. Our results demonstrate the general decrease of the limit of hygroscopicity with the increase of temperature of the thermal pretreatment of the examined wood species. Mathematical correlations have been evaluated on the basis of experimental data by polynomial regression statistical method. Wood density is another factor examined as influential upon the limit of hygroscopicity. We have determined the decrease of the LH with the increase of wood density, i.e. denser wood species have lower capacity for water adsorption. The differences obtained for the LHs between the examined wood species have been explained in relation to their chemical compositions. Our results indicate that cellulose is the major wood component determining its capacity for water adsorption.

In this paper we are promoting the properties related to wood anatomy and chemical composition as the factor of influence to limit of hygroscopicity of wood (LH).The wood samples of the most important domestic wood species: oak Quercus robur, beech Fagus sylvatica, fir Abies alba and spruce Picea abies were selected for this investigation.For determining LH in our investigations we have applied the method of compression strength parallel to grain.The results of experimental measurements were statistically analyzed and the empirical equation for LH dependance upon the significant (influential) factors: temperature, cellullse content and wood density in oven dry state is defined.