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In this study, the development of non-homogeneous color changes in oak timber during conventional drying and their relationship with the moisture gradient across the wood thickness were analyzed. The research was conducted on radial and tangential boards with a thickness of 38 mm, cut from two oak logs, one of sessile oak (Quercus petraea L.) and one of pedunculate oak (Quercus robur L.). A conventional drying schedule commonly used in enterprises was applied, and at specified time intervals samples were taken to determine the moisture content profile and monitor color changes on the cross sections of the samples. The findings reveal a complex relationship between the wood moisture content, the moisture content profile across the thickness, and the occurrence of color changes. Additionally, it was found that the wood of sessile oak dries slower and with less intense color changes compared to the wood of pedunculate oak. It was demonstrated that quarter-sawn boards dry slower compared to flat-sawn boards for both wood species. No differences in the appearance of non-homogeneous color between quarter-sawn and flat-sawn boards were identified.

The paper examined the strength of rotationally welded joints between untreated beech dowels and samples of thermally modified and unmodified oak wood (used as substrates). The results were compared with the control group where beech dowels were glued using PVAc (polyvinyl acetate) glue for the same types of samples. Maximum dowel pull-out force was significantly higher for the glued connection compared to the welded connection in both groups - thermally treated and untreated oak wood. Welded joint strength was higher for unmodified group, compared to thermally treated group, which was expected due to negative effects of thermal modification on mechanical properties of wood.

The subject of this paper is the analysis of the influence of higher initial temperatures during the drying of oak lamellas on the final quality and wood colour. Two different drying schedules (initial temperature: 45°C and 50°C; final temperature: 55°C) were used and then the drying quality and wood colour were determined. The colour change was expressed by parameter ΔE but also by the corrected parameter ΔE00. It was shown that the drying quality was very high in both cases, and that the colour change that occurred during drying was invisible to the naked eye. The application of higher temperatures does not pose a risk to the drying quality due to the small thickness of the wood and the short drying process. In industry, a higher initial temperature is justified in situations where sufficient quantities of (cheap) thermal energy are available. The results confirmed that the percentage of lamellas’ deformation was reduced when the load was applied while drying.

The subject of this paper is the analysis of moisture content (MC) changes of beech and ash wood during two years in room conditions (heating during winter; no air conditioning during summer). The registered MC changes are primarily the result of changes in relative humidity of the air (measured by capacitive probes). The average relative humidity of the air in the interior is lower today than in the past, as also shown in this experiment (the average relative humidity of air during two years was 44%). The lowest wood MC was reached on very cold winter days when the heating was on even during the night - between 5% and 6%, and the highest one at the beginning of June: 10.4-10.9%. As expected, the wood did not reach equilibrium moisture content - during winter, MC is by about 1% higher, and in summer it is lower by up to 2.5% than the equilibrium. The recommendation that the sawn timber from which the interior products will be made should be dried at 7-8% MC was confirmed.

This study presents preliminary results of the basic properties of the northern red oak wood as well as the relationship between the properties of the core samples obtained on the live tree and the tested properties. The core samples 5 mm in diameter were extracted from the tree and used for determining the fractometer properties: compressive strength parallel to grain, radial bending strength and braking angle. Using descriptive statistics and multiple regression, the relationship between fractometric parameters and basic physical and mechanical properties of wood determined by destructive methods (wood density, compressive strength in parallel with fibers and bending properties (bending strength and modulus of elasticity). The obtained values of the examined properties of northern red oak are within the limits of the literature data. Statistical analysis has shown that radial bending strength cannot be used as a reliable fractometric parameter in assessing wood quality. On the other hand, the obtained results suggest that the relationship between the other two parameters (compressive strength parallel to grain and fracture angle) and the properties of the northern red oak wood can be successfully described using the multiple regression models (coefficient of determination ranged from 0.945 to 0.990).

The subject of this paper is to analyse the drying process of oak lamellas, which are the solid wood top layer of engineered wood flooring. The focus of the first part of the paper is on dehumidification kilns. Drying in a dehumidification kiln is an interesting alternative to conventional drying of thin solid oak wood with the aim of reaching high drying quality in a reasonable time. Drying tests were done in an industrial dehumidification kiln, and drying parameters were compared with the drying in the conventional kiln. Simultaneously, a drying test at a higher temperature was done in the programmable climate chamber. It was demonstrated that thin oak lamellas (approx. 5 mm thick) could be successfully dried in a dehumidification kiln in a relatively short time and with high drying quality. With the applied drying schedule (initial temperature of 36°C, final temperature of 46°C), the drying cycle will last 2 to 5 days, depending on the amount of wood and the initial MC. Due to the high rate of water evaporation and the inability of the kilns to remove it fast enough, the drying of lamellas in both dehumidification and conventional kilns takes place at a higher equilibrium moisture content than the set values.

The subject of this paper is the analysis of the possibility of drying oak and ash elements with the presence of tree bark on one side of the element (bark edge). One batch was dried per species, and the process was controlled by moisture content (MC) probes in the kiln, but also determined by the oven-dry method. Drying defects and their causes were also analyzed. The results showed that conventional drying can be used for this purpose - the duration is relatively short (up to 7 days), and the variation of the final MC is relatively small. The main problem is a relatively large number of crooked elements at the end of drying, particularly those where the share of wood in relation to the bark was small. Very high axial shrinkage coefficients of the bark (4.5 times higher than wood) are the cause of these deformations. [Project of the Serbian Ministry of Education, Science and Technological Development, Grant no. TR 31041 and Grant no. TR 37008]

The subject of this paper is the analysis of the quality of natural and conventional drying of subfossil oak wood. In addition to the drying quality, the colour of this material, as well as the effect of the drying process on colour change have been tested. The subfossil oak logs that were used in this experiment originate from the Morava River in Central Serbia. After sawing, timber was air-dried to about 20% moisture content (MC) followed by kiln-drying to 9.5% MC. By examining the quality of air-drying of the subfossil oak wood, a high presence of cracks was found, as well as the inaccuracy of humidity measurement using electric moisture meters. The stack for kiln-drying consisted of 24 boards, of which 4 were used for the monitoring of MC and MC distribution across the thickness of plank during drying. Drying quality was evaluated after the kiln-drying process. Although mild drying conditions during kiln-drying were applied, the gap as a measure of case-hardening had a value greater than usual for oak of same thickness. It was found that the drying process has no significant effect on the change in the colour of subfossil wood. The colour of the subfossil oak wood was significantly darker than the usual oak wood, and the colour difference between the central and the outer parts of the trunk of the subfossil oak was determined.

The goal of this word was to examine the parameters of color of beech wood before and after thermal treatment, and to determine if there is a difference in color between sapwood and red heartwood before and after application of high temperature. Samples were treated at temperatures of 170°C, 190°C and 210°C, respectively, during 4 hours. Color coordinates (CIEL*a*b* system) were measured before and after treatment by using the following parameters: ΔL, Δa, Δb and ΔE - color change. Color was assessed at four spots on the radial and cross-section surfaces of wood and their mean values were taken for further calculations. Acquired results show that the applied heat treatment of sapwood did not significantly influence the change of color difference between cross-section and radial surfaces. The difference between cross-section and radial surfaces, in nontreated red heartwood was smaller than that measured in sapwood. The color difference of nontreated sapwood and red heartwood was more pronounced on the radial than on the cross-section surface. Application of high temperature caused a significant decrease of this difference. Temperature of 210°C caused the smallest color difference between these two parts of wood which was in this research determined on the radial surface. It was concluded that the applied thermal treatment almost equalized the colors of sapwood and red heartwood, especially on the radial surface, and from this aspect it can be concluded that these two parts of beech wood were completely equalized. It should also be noted that in the case of equalized properties, sapwood and red heartwood can be equally applied in a final product made of thermally treated beech wood. [Projekat Ministarstva nauke Republike Srbije, br. TR 37008 i br. TR 31041]

The paper shows results of analysis of influences of oscillating parameters of drying on measuring wood moisture content in the kiln, rate and quality of drying. For this analysis, we used a conventional drying cycle, a cycle with oscillating equilibrium moisture content (EMC), and a cycle with oscillating temperatures. A special software tool was created for managing the oscillations. It was shown that oscillations of EMC and temperatures result in cyclic changes in wood MC, but also in the additional inaccuracies of MC measurements in the kiln. The drying process of the cycle with oscillating EMC lasted somewhat shorter than the other two cycles. Drying quality was the same or better in the cycles with oscillations as compared to the conventionally dried cycle, and the smaller tensions in the wood confirmed the activation of the additional mechano-sorptive effect during cyclic changes of MC in surface layers. [Projekat Ministarstva nauke Republike Srbije, br. TR 31041 i br. TR 37008]

This paper presents the results of an analysis of influence of the “light” steaming process on conventional drying and colour of beech timber. A stack consisting of steamed and unsteamed boards was dried. Moisture content and moisture content profiles were recorded, and at the end of drying final moisture content, moisture content difference across thickness, case-hardening and colour were determinated. No statistically significant difference between steamed and unsteamed timber was established in any of the examined parameters (MC, ΔMC, gap). The results indicate that equal (or even) better drying quality can be expected with steamed beech timber than with unsteamed timber, given the same drying schedule. Dry steamed timber was darker and more reddish than the unsteamed one. An imprecision of measurement by probes in the kiln was noticed in measuring steamed and unsteamed beech timber having a moisture content of between 7% and 15%, and a solution for overcoming this problem was given. [Projekat Ministarstva nauke Republike Srbije, br. TR 31041]

This paper presents the results of a research of velocity of air circulation in the conventional kiln drying of sawn timber and its influence on the uniformity of final moisture content. The data showed that the air velocity (between 0.8 - 1.2 m•s -1) is significantly lower than the one that is optimal for timber drying of coniferous species (3 - 4 m•s-1). This results in a reduction in the capacity of installations for drying, increased energy (thermal and electrical) consumption and thereby increase in the cost of drying. The correlation between the speed of air circulation and the final MC of timber due to prolonged drying was not established. Recommendations that should eliminate the identified deficiencies are also given.