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Les je po poseku in uporabi za konstrukcije izpostavljen staranju, ki je med drugim odvisno od pogojev izpostavitve. V času življenjske dobe oz. uporabe je les izpostavljen fotodegradaciji, biodegradaciji, površinskim in notranjim strukturnim spremembam in staranju, ki se kaže v spremembi barve, kemijske sestave, higroskopnosti, dimenzijske stabilnosti ter nekaterih mehanskih lastnosti. Podajamo pregled literature, s posebnim poudarkom vpliva pogojev izpostavitve na spremembe lastnosti lesa ter pregled kemijskih in mikrostrukturnih sprememb ter sprememb barve, sorpcijskih in mehanskih lastnosti. V splošnem so spremembe zaradi staranja lahko zelo počasne in jih je težko ovrednotiti tudi zaradi velike naravne variabilnosti lesnih lastnosti. Poznavanje sprememb v naravno staranem lesu je ključno pri ohranjanju kulturne dediščine. Pri načrtovanju vzdrževanja, konzerviranja ter pri obnovi kulturne dediščine, z upoštevanjem staranja konstrukcijskega lesa bolje razumemo strukturne spremembe, vlažnostna in napetostno-deformacijska stanja lesenih konstrukcij ter interakcije z drugimi gradbenimi materiali. After harvesting, wood in constructions undergoes an ageing process that depends, among other things, on exposure conditions. During its lifetime wood is subject to photodegradation, biodegradation, surface and internal structural changes, and ageing, which is reflected in changes in colour, chemical composition, hygroscopicity, dimensional stability, and mechanical properties. We present a literature review, with particular emphasis on the influence of exposure conditions on changes in wood properties, chemical and microstructural changes, and changes in colour, sorption, and mechanical properties. In general, ageing-related changes can be very slow and difficult to detect, in part because of the wide natural variability in wood properties. Knowledge of the changes in naturally aged wood is critical for preserving cultural heritage, evaluating the safety of wood structures, and planning their conservation. When planning maintenance, conservation, and restoration of cultural heritage, we can better understand the structural changes, moisture and stress deformation states of wood structures, and interactions with other building materials if we consider the ageing of structural wood. Bibliografija: str. 53-55. Izvleček ; Abstract ; Summary.

European oak logs (Quercus robur, Quercus petraea) from the 15th auction of valuable logs in Slovenj Gradec in 2021 were included in the study. Of the 1,318 auctioned logs, 78 were randomly selected and visually graded into quality classes (A, B, C) according to the National Grading Rules (2017). In parallel, we analysed the wood growth dendrochronologically and the acoustic properties of the logs using the longitudinal vibration resonance method. The logs with the best quality had a larger diameter and, on average, wider annual rings. We confirmed the correlation between the visually determined quality and the obtained price of the logs. Longitudinal vibration velocity (v) and relative acoustic conversion efficiency (RACE) were positively correlated with log quality and price. The frequency spectra of the low-quality logs differed from the theoretical spectra. This study confirms the feasibility of non-destructive acoustic methods for evaluating oak log quality. V raziskavo smo vključili hrastovo hlodovino (Quercus robur, Quercus petraea) s 15. dražbe vrednejših sortimentov okroglega lesa v Slovenj Gradcu v letu 2021. Naključno smo izbrali 78 od 1318 licitiranih hlodov in jih skladno z veljavnim Pravilnikom o merjenju (2017) vizualno razvrstili po kakovosti (A, B, C). Vzporedno smo dendrokronološko analizirali priraščanje lesa ter z metodo frekvenčnega odziva pri vzdolžnem vzbujanju še akustične lastnosti hlodovine. Najkvalitetnejša hlodovina je bila večjih premerov, z več in povprečno širšimi branikami. Potrdili smo povezavo med vizualno določeno kakovostjo in doseženo ceno hlodovine. Hitrost vzdolžnega nihanja (v) in relativna učinkovitost akustične pretvorbe (RACE) sta bili s kakovostjo in ceno hlodovine pozitivno povezani. Frekvenčni spektri pri hlodovini nižje kakovosti odstopajo od teoretičnih. Raziskava je potrdila možnost uporabe nedestruktivnih akustičnih metod za oceno kakovosti hrastove hlodovine.

Vsebnost vode (vlažnost) je med pomembnejšimi kazalniki kakovosti drv. Za čim učinkovitejše in okolju prijazno delovanje Uredba o emisiji snovi v zrak iz malih kurilnih naprav navaja, da mora biti vsebnost vode v drveh manj kot 20 %. Za hitro preverjanje vsebnosti vode v drveh bi bila smiselna uporaba električnih uporovnih merilnikov. V pričujočem prispevku primerjamo meritve, opravljene z elektrouporovnimi merilniki različnih izvedb ter referenčno gravimetrično metodo. Rezultati kažejo, da gravimetrični metodi najbolj primerljive rezultate kažejo uporovni merilniki, ki omogočajo kompenzacijo gostote lesne vrste in temperature. V prispevku razpravljamo o uporabi različne terminologije in definicije lesne vlažnosti (t.j. relativna in absolutna vlažnost lesa, vsebnost vode) ter predlagamo postopek za čim boljšo oceno vlažnosti z enostavnimi elektro-uporovnimi merilniki. Wood moisture is one of the most important quality indicators for firewood. For the most efficient and environmentally friendly operation, the Decree on the emission of substances into the atmosphere from small combustion plants declares that firewood moisture content should be below 20 %. For fast assessment of the firewood moisture content, electric resistance meter could be used. In this paper, we compare different commercially available electrical-resistance meters with reference gravimetric method. The results show that electrical-resistance meters (which enable the compensation of wood species density and temperature) and gravimetric method provide comparable results. The paper discusses the use of different terminology and definitions of moisture content (i.e. moisture content on the dry and wet basis). Furthermore, we propose a procedure for the most reliable assessment of moisture content using simple electrical-resistance meters.

Predstavljamo zgradbo ter biološke, fizikalne in mehanske lastnosti lesa navadne bukve (Fagus sylvatica L.) v primerjavi z lastnostmi lesa smreke in izbranih listavcev. Fina tekstura, visoka homogenost, nizka anizotropija, srednja do visoka gostota, dobre mehanske lastnosti, dobra strojna obdelava, lepljenje, površinska obdelava ter možnost krivljenja omogočajo široko rabo bukovine. Danes preveč bukovine porabimo za pridobivanje energije, zato bi za njeno boljšo izrabo morali oživiti ali povečati proizvodnjo tradicionalnih in novih polizdelkov kot so žagan les, konstrukcijski in plemeniti furnir, ploščni kompoziti ter izdelki kemične predelave, za energetske potrebe pa bi morali uporabljati predvsem ostanke, ki nastanejo pri predelavi lesa. Glede na predstavljene ugodne lastnosti in uspešno reševanje problema nizke odpornosti bi se lahko usmerili v razvoj inovativnih izdelkov za večjo rabo bukovine v gradbeništvu We present the structure as well as biological, physical and mechanical properties of European beech (Fagus sylvatica L.) wood in comparison with that of Norway spruce and selected hardwoods. Fine grain, high homogeneity, low anisotropy, medium to high density, favourable mechanical properties, machining, bonding, and finishing, as well as suitability for bending, are among the main advantages which enable wide use of beechwood. Large amounts of beechwood are currently used as fuel. However, for its optimal use, we should revitalize and increase the production of traditional and new semi-finished products like sawn timber, structural and decorative veneer, composite boards and products of chemical processing. In contrast, for energy purposes we should mainly use the residues from these wood-working procedures. According to the numerous favourable properties of beechwood and fairly good possibilities of increasing its durability, we should focus on the development of innovative products to increase its use for construction purposes in buildings.

Ksilit je litotip lignita, ki ga poznamo predvsem kot kurivo. Les, iz katerega je nastal, je v obdobju skoraj dveh milijonov let doživel drastične spremembe, struktura lesa pa je delno še prepoznavna. Ksilit po videzu pogosto spominja na lesne vrste s temno jedrovino. Opravili smo karakterizacijo ksilita z vidika anatomije lesa, iz katerega je nastal, raziskali njegovo ohranjenost in določili osnovne fizikalne značilnosti. Prepoznali smo osnovno tkivo iz traheid, enoredno in homocelularno trakovno tkivo, taksodioidne piknje v križnih poljih in obilico aksialnega parenhima z vključki. To nakazuje, da je ksilit nastal iz lesa golosemenk (iglavcev) iz družine Taxodiaceae (taksodijevke) ali Cupressaceae (cipresovke). Barvnometrično je ksilit v sistemu CIELab* primerljiv z jedrovino lesa ebenovine ali wengeja. Proučeni ksilit je imel gostoto v absolutno suhem stanju 1216 kg/m3, zaradi velikega volumenskega skrčka (v povprečju 28.2 %) paje bila srednja osnovna gostota samo 873 kg/m3. Celotni linearni skrčki so veliki, kar se kot posledica kaže v neugodnih kazalnikih dimenzijske stabilnosti (qT = 0.65 %/%, qR = 0.30 %/%). Nadpovprečno visoke so tudi ravnovesne vlažnosti ksilita v procesu desorpcije, sorpcijska kapaciteta v procesu adsorpcije pa je podobna kot pri lesnih vrstah z visoko gostoto. Sorpcijski model GAB se je izkazal kot primeren za izravnavo sorpcijskih podatkov. Xylite is a lithotype of lignite, mainly used for fuel. The wood from which it was formed in a long term process (duration ca. 2 millions of years) was subjected to drastic changes. We conducted anatomical characterization of xylite, evaluated preservation of wood from which it was formed, defined its basic physical properties, and compared it with modern wood. We recognized ground tissue composed of tracheids, uniseriate and homocellular rays, taxodioid like cross-field pits, and abundant axial parenchyma with inclusions. The observed features are typical of coniferous wood species of the Taxodiaceae and Cupressaceae families. Colourimetric properties of xylite in CIELab* system are comparable to those of heartwood of ebony or wenge. The xylite had high oven dry density (1,216 kg/m3), but due to very high volumetric shrinkage (28.2 % on average), its basic density was much lower (873 kg/m3). Total linear shrinkage was considerable in all anatomical directions. Consequently it was characterized by low dimensional stability (qT= 0.65 %/%, qR = 0.30 %/%). The equilibrium moisture content (EMC) in desorption process was high, and that of adsorption was comparable to EMCs of high density wood species. GAB sorption model proved to be appropriate for fitting the data obtained in the sorption process.

Physical and mechanical properties of beech wood (Fagus sylvatica L.) from trees fallen in ice storm were determined. The properties were studied in parallel green and dried beech-wood samples, systematically collected along the radius and at different height levels of the stem. The stiffness of the timber was comparatively tested with static 4-point bending and non-destructively by measuring the speed of ultrasound, and by determining the frequency response. The bending and shear strengths were additionally determined along the grain, as well as compression strength in all anatomical directions of wood. In blown down trees with still partly preserved root system, active crown and sufficient capillary water flow, we could not detect any reduction of mechanical properties due to deterioration. The density and mechanical properties, except transverse compression strength, increased with increasing height in the tree and radially from the pith to the cambium. They were always better in dried than in green wood. The reliability of non-destructive methods was greater in the evaluation of dried wood. We recommend visual wood characterisation in addition to the use of non-destructive methods for strength assessment of wood. V raziskavi smo preučevali fizikalne in mehanske lastnosti lesa v deblih navadnih bukev (Fagus sylvatica L.), izruvanih med žledolomom. Lastnosti smo preučevali na vzporednih preizkušancih sveže in osušene bukovine, ki smo jih sistematično vzorčili po radialni in vertikalni smeri dreves. Določili smo gostoto, togost, upogibno trdnost, strižno trdnost v smeri lesnih vlaken ter tlačno trdnost lesa v vseh anatomskih smereh. Togost smo primerjalno vrednotili med statično 4-točkovnim upogibom in nedestruktivno z merjenjem hitrosti ultrazvoka ter z določanjem frekvenčnega odziva. V drevesih, ki so izruvana obležala v gozdu od februarja do konca vegetacijske dobe 2014 in so imela delno ohranjen koreninski sistem, aktivno krošnjo in zadosten kapilarni tok, nismo zaznali sprememb gostote in mehanskih lastnosti, ki bi bile posledica razkroja lesa. Tako v svežem kot osušenem stanju smo ugotovili značilni vzorec porazdelitve gostote in trdnosti. Gostota lesa je po višini dreves naraščala, v radialni smeri od stržena proti periferiji debla pa smo zaznali rahel trend zmanjševanja. Mehanske lastnosti z izjemo prečne tlačne trdnosti se povečujejo tako po višini debla kot tudi od stržena proti kambiju in so bile vedno boljše pri osušenem kot pri svežem lesu. Zanesljivost nedestruktivnih metod za določanje togosti je večja pri ocenjevanju osušenega lesa. Za oceno upogibne trdnosti z nedestruktivnimi metodami še vedno priporočamo vzporedno vizualno opredelitev značilnosti lesa.

Proučili smo fizikalne lastnosti (vlažnost in gostoto) lesa različno prizadetih izruvanih dreves navadne bukve (Fagus sylvatica L.) ob koncu prve vegetacijske dobe po velikem žledolomu v letu 2014. Raziskava je potrdila povezavo med deležem še aktivnega koreninskega sistema in stopnjo porjavelosti (izsušenosti) krošnje dreves ter vlažnostjo lesa in njenim razporedom. Pri zelo prizadetih drevesih smo v prevodnem zunanjem delu beljave potrdili zmanjšanje vlažnosti lesa pod mejo naravne biološke odpornosti (u < 60 %). Po prvi vegetacijski dobi nismo mogli potrditi zmanjšanja gostote lesa (in z njim povezanega poslabšanja mehanskih lastnosti), kar bi potrdilo okužbo in razkroj lesa z glivami. Physical properties (moisture content and density) of wood of variously damaged uprooted European beech (Fagus sylvatica L.) trees were studied at the end of the first vegetation period after the ice storm damage in Slovenian forests in 2014. The research confirmed significant correlation between the amount of active root system and necrosis of leaves and changed moisture content and its distribution in the trunks. The moisture content of wood below 60 % (indicating extinguished protection against wood destroying organisms) was found in sapwood of extremely damaged trees. At the end of the first vegetation period after damage we could not confirm any reduction of wood density (and reduced mechanical properties) which would indicate deterioration of wood.

Physical properties (moisture content and density) of wood of variously damaged uprooted European beech (Fagus sylvatica L.) trees were studied at the end of the first vegetation period after the ice storm damage in Slovenian forests in 2014. The research confirmed significant correlation between the amount of active root system and necrosis of leaves and changed moisture content and its distribution in the trunks. The moisture content of wood below 60 % (indicating extinguished protection against wood destroying organisms) was found in sapwood of extremely damaged trees. At the end of the first vegetation period after damage we could not confirm any reduction of wood density (and reduced mechanical properties) which would indicate deterioration of wood.

Determination and estimation of the moisture content in the wood is of the great importance for the proper and efficient conducting of wood production processes, and thus the achievement of desired quality of final products. Electric resistance type wood moisture meter is the most commonly used in production process for estimation of moisture content in wood, so it derived an idea to find a simple way for control of its accuracy and reliability. The precision of electrical resistance type wood moisture meter is measured by gravimetric method for the determination of moisture content. This simple method is designed, developed and tested at the Faculty of Forestry, University of Zagreb within the European IPA project Increasing competitiveness of Croatian industry of wood flooring on the EU market. The paper presents the method developed for control of accuracy of wood moisture meter, and the results of interlaboratory control of accuracy of electrical resistance wood moisture meters according to developed method.

Physical properties (moisture content and density) of wood of variously damaged uprooted European beech (Fagus sylvatica L.) trees were studied at the end of the first vegetation period after the ice storm damage in Slovenian forests in 2014. The research confirmed significant correlation between the amount of active root system and necrosis of leaves and changed moisture content and its distribution in the trunks. The moisture content of wood below 60 % (indicating extinguished protection against wood destroying organisms) was found in sapwood of extremely damaged trees. At the end of the first vegetation period after damage we could not confirm any reduction of wood density (and reduced mechanical properties) which would indicate deterioration of wood. Proučili smo fizikalne lastnosti (vlažnost in gostoto) lesa različno prizadetih izruvanih dreves navadne bukve (Fagus sylvatica L.) ob koncu prve vegetacijske dobe po velikem žledolomu v letu 2014. Raziskava je potrdila povezavo med deležem še aktivnega koreninskega sistema in stopnjo porjavelosti (izsušenosti) krošnje dreves ter vlažnostjo lesa in njenim razporedom. Pri zelo prizadetih drevesih smo v prevodnem zunanjem delu beljave potrdili zmanjšanje vlažnosti lesa pod mejo naravne biološke odpornosti (u < 60 %). Po prvi vegetacijski dobi nismo mogli potrditi zmanjšanja gostote lesa (in z njim povezanega poslabšanja mehanskih lastnosti), kar bi potrdilo okužbo in razkroj lesa z glivami.

Ksilit je litotip lignita, ki ga poznamo predvsem kot kurivo. Les, iz katerega je nastal, je v obdobju skoraj dveh milijonov let doživel drastične spremembe, struktura lesa pa je delno še prepoznavna. Ksilit po videzu pogosto spominja na lesne vrste s temno jedrovino. Opravili smo karakterizacijo ksilita z vidika anatomije lesa, iz katerega je nastal, raziskali njegovo ohranjenost in določili osnovne fizikalne značilnosti. Prepoznali smo osnovno tkivo iz traheid, enoredno in homocelularno trakovno tkivo, taksodioidne piknje v križnih poljih in obilico aksialnega parenhima z vključki. To nakazuje, da je ksilit nastal iz lesa golosemenk (iglavcev) iz družine Taxodiaceae (taksodijevke) ali Cupressaceae (cipresovke). Barvnometrično je ksilit v sistemu CIELab* primerljiv z jedrovino lesa ebenovine ali wengeja. Proučeni ksilit je imel gostoto v absolutno suhem stanju 1216 kg/m3, zaradi velikega volumenskega skrčka (v povprečju 28.2 %) paje bila srednja osnovna gostota samo 873 kg/m3. Celotni linearni skrčki so veliki, kar se kot posledica kaže v neugodnih kazalnikih dimenzijske stabilnosti (qT = 0.65 %/%, qR = 0.30 %/%). Nadpovprečno visoke so tudi ravnovesne vlažnosti ksilita v procesu desorpcije, sorpcijska kapaciteta v procesu adsorpcije pa je podobna kot pri lesnih vrstah z visoko gostoto. Sorpcijski model GAB se je izkazal kot primeren za izravnavo sorpcijskih podatkov. Xylite is a lithotype of lignite, mainly used for fuel. The wood from which it was formed in a long term process (duration ca. 2 millions of years) was subjected to drastic changes. We conducted anatomical characterization of xylite, evaluated preservation of wood from which it was formed, defined its basic physical properties, and compared it with modern wood. We recognized ground tissue composed of tracheids, uniseriate and homocellular rays, taxodioid like cross-field pits, and abundant axial parenchyma with inclusions. The observed features are typical of coniferous wood species of the Taxodiaceae and Cupressaceae families. Colourimetric properties of xylite in CIELab* system are comparable to those of heartwood of ebony or wenge. The xylite had high oven dry density (1,216 kg/m3), but due to very high volumetric shrinkage (28.2 % on average), its basic density was much lower (873 kg/m3). Total linear shrinkage was considerable in all anatomical directions. Consequently it was characterized by low dimensional stability (qT= 0.65 %/%, qR = 0.30 %/%). The equilibrium moisture content (EMC) in desorption process was high, and that of adsorption was comparable to EMCs of high density wood species. GAB sorption model proved to be appropriate for fitting the data obtained in the sorption process.

The time and energy required for wood drying and therefore drying costs are among the highest in wood products manufacturing. Analysis of the drying process costs is very complex and needs to evaluate the cost of investments, energy, labour as well as defects of lumber or added value. In the present research work, we upgraded the computer aided engineering (CAE) model of wood drying processes with the economic evaluation of different drying techniques. By analyzing and comparing the costs of different drying processes, the model is able to predict the transition wood moisture content (MCt) at which the alteration from one to another drying technique would get the best results. The model was verified on data from several air- and kiln-drying experiments on 38 mm thick beech wood (Fagus sylvatica L.) boards. Computer simulation predicted advantages of combined dying beech wood with air predrying and continued and finished with kiln drying. The MCt in the winter period was at about 40% MC and decreased to 23% in favourable climate conditions in spring and summer months. The model can be also used for optimising the combination of other drying techniques. Dolgotrajnost in energijska potratnost sušenja lesa povečuje tudi stroške, ki so med največjimi med lesnopredelovalnimi postopki. Analiza stroškov zahteva vsestransko obravnavo z vrednotenjem stroškov investicije, energije in dela kot tudi upoštevanje razvrednotenja zaradi napak ali dodane vrednosti kakovostnega osušenega lesa. V raziskavi smo nadgradili računalniški model sušilnega procesa z ekonomskim vrednotenjem različnih sušilnih tehnik. S primerjavo stroškov različnih sušilnih postopkov smo predpostaviti prehodno vlažnost lesa, pri kateri bi s kombinacijo več načinov sušenja dosegli najboljše ekonomske rezultate. Metoda je bila preverjena na sušenju bukovine debeline 38 mm s kombinacijo sušenja na prostem in komorskega sušenja. V zimskih mesecih je sušenje na prostem smotrno do vlažnosti 40 %, v najugodnejših klimatskih razmerah pa je optimalna vlažnost prehoda iz sušenja na prostem v komorsko sušenje 23 %. Predstavljeno metodo je mogoče uporabiti tudi za optimizacijo različnih tehničnih postopkov sušenja lesa.

Relevant properties of wood floorings are dimensional stability, hardness and strength, dependant on wood species, where also their workability, permeability and impregnability have the important role. Despite all the advantages, heat-treated timber has some undesirable properties comparing to the natural wood used for flooring, which depends also on the degree of thermal modification (Millitz 2002, Hill 2009, Straže et al. 2012). A lot of research on UV degradation of natural wood was done, using various surface coatings systems (Hägele 2003, Bulian et. al. 2009, Pervan 2011). There is less information about the behaviour of thermally treated wood in outdoor exposure under different weather conditions using adequate environmental- friendly surface protection.

Infrared (IR) thermography is a widely applicable non-destructive testing (NDT) method for measuring the temperature distribution of an observed object surface. Through thermographic analysis, the IR radiation intensity of the electromagnetic spectrum is detected and recorded on the obtained thermograms. IR thermography has its advantages and disadvantages but it is a promising technique, which is easily applicable in the field and has no harmful impacts on the operator or equipment. In wood science and related fields, IR thermography can be applied in production quality control, monitoring of manufacturing equipment and plants for hydrothermal wood processing, wood properties characterization, defects detection (particularly holes, rot and drying cracks), heat flux in wood, and more [1]. During the thermographic analysis, temperature is not measured directly, but indirectly. Therefore, many factors can affect the accuracy of temperature measurement during quantitative thermography analysis, and one such factor is the emissivity of the observed object. Emissivity of the material is the ability of the surface to emit radiant energy expressed as the ratio of the radiant energy emitted by a surface to that emitted by a blackbody at the same temperature and under the same conditions. Emissivity depends on wavelength, temperature, object surface (opacity, roughness, orientation), the material from which the object is made, angle of measurement. Although important, those associated parameters often are not entirely specified along with the emissivity value in the literature. Therefore the experiment for establishing thermo-modified oak wood emissivity values within spectral range 7.5 – 13 μm was conducted. Also the emissivity of innate oak wood was determined for the comparison. For the experiment, an FLIR infrared system with the following characteristics was used. Imaging and optical data were as follows : field of view 25° x 25°, minimum focus distance 0.1 m, focal length 10 mm, instantaneous field of view (spatial resolution) 2.42 mrad, F-number 1.5, NETD 100 mK, image frequency 9 Hz. Detector type: FPA, uncooled microbolometer, with spectral range 7.5–13 μm (LWIR, ISO–MIR), IRR 180 x 180 pixels. Oak wood (Quercus robur L.) specimens used in the experiment were radial texture elements clear of defects, with approximately the same three ring width. There were 60 innate oak wood specimens and 60 thermo-modified oak wood specimens with the final dimensions 270 mm (length) by 65 mm (width) by 25 mm (thickness). Innate oak wood elements were kiln dried and then four sides planed. The moisture content of the specimens was 9 %. Thermo- modified wood elements were kiln dried, then thermally treated and afterwards four sides planed. The moisture content of the specimens was 5 %. The moisture content of specimens was determined according to EN 13183-1:2002/AC:2003 Moisture content of a piece of sawn timber – Part 1: Determination by oven dry method. A polyvinyl chloride material with known emissivity in long wavelength infrared (LWIR) spectral range was applied to the surface of all specimens. The emissivity of the polyvinyl chloride material was verified and the oak wood specimens were heated to 50 °C. During the experiment, the ambient characteristics were as follows: air temperature 21 °C, relative humidity 60 %, reflected apparent temperature 20.2 °C, the distance of oak wood specimens surface from the lens was 120 mm, angle of measurement in relation to the object normal was 15°, airflow and dotted reflections were inhibited. Emissivity values were determined via FLIR software with an area box tool with following dimensions 70×30 pixels. There was a significant difference in the results for innate and thermo- modified oak wood emissivity values (two-sample t(df) = -15.6, p = 0.00).

The sorption- and chemical properties of ash- and beech wood after industrial heat treatment in water-vapour atmosphere were analysed by sorption thermodynamics, adsorbing surface fractality from sorption isotherms and by FT-NIR spectroscopy. Considerable reduction of water vapour sorption capacity after heat treatment of wood was confirmed at both wood species, more significant at beech wood in the middle hygroscopic range. Fractal dimension of internal adsorbing wood surfaces increased with amount of sorbed water and also after thermal processing. FT-NIR spectroscopy revealed most significant differences in chemical properties of hemicelluloses and revealed regions and bands, where considerable changes are appearing due to changed water sorption of innate- and heat-treated ash- and beech wood.

The FT-NIR spectroscopy showed high potential for predicting of tested physical properties of innate and heat-treated ashwood.

Xylite is a lithotype of lignite, mainly used for fuel. The wood from which it was formed in a long term process (duration ca. 2 millions of years) was subjected to drastic changes. We conducted anatomical characterization of xylite, evaluated preservation of wood from which it was formed, defined its basic physical properties, and compared it with modern wood. We recognized ground tissue composed of tracheids, uniseriate and homocellular rays, taxodioid like cross-field pits, and abundant axial parenchyma with inclusions. The observed features are typical of coniferous wood species of the Taxodiaceae and Cupressaceae families. Colourimetric properties of xylite in CIELab* system are comparable to those of heartwood of ebony or wenge. The xylite had high oven dry density (1,216 kg/m3), but due to very high volumetric shrinkage (28.2 % on average), its basic density was much lower (873 kg/m3). Total linear shrinkage was considerable in all anatomical directions. Consequently it was characterized by low dimensional stability (qT= 0.65 %/%, qR = 0.30 %/%). The equilibrium moisture content (EMC) in desorption process was high, and that of adsorption was comparable to EMCs of high density wood species. GAB sorption model proved to be appropriate for fitting the data obtained in the sorption process. Ksilit je litotip lignita, ki ga poznamo predvsem kot kurivo. Les, iz katerega je nastal, je v obdobju skoraj dveh milijonov let doživel drastične spremembe, struktura lesa pa je delno še prepoznavna. Ksilit po videzu pogosto spominja na lesne vrste s temno jedrovino. Opravili smo karakterizacijo ksilita z vidika anatomije lesa, iz katerega je nastal, raziskali njegovo ohranjenost in določili osnovne fizikalne značilnosti. Prepoznali smo osnovno tkivo iz traheid, enoredno in homocelularno trakovno tkivo, taksodioidne piknje v križnih poljih in obilico aksialnega parenhima z vključki. To nakazuje, da je ksilit nastal iz lesa golosemenk (iglavcev) iz družine Taxodiaceae (taksodijevke) ali Cupressaceae (cipresovke). Barvnometrično je ksilit v sistemu CIELab* primerljiv z jedrovino lesa ebenovine ali wengeja. Proučeni ksilit je imel gostoto v absolutno suhem stanju 1216 kg/m3, zaradi velikega volumenskega skrčka (v povprečju 28.2 %) paje bila srednja osnovna gostota samo 873 kg/m3. Celotni linearni skrčki so veliki, kar se kot posledica kaže v neugodnih kazalnikih dimenzijske stabilnosti (qT = 0.65 %/%, qR = 0.30 %/%). Nadpovprečno visoke so tudi ravnovesne vlažnosti ksilita v procesu desorpcije, sorpcijska kapaciteta v procesu adsorpcije pa je podobna kot pri lesnih vrstah z visoko gostoto. Sorpcijski model GAB se je izkazal kot primeren za izravnavo sorpcijskih podatkov.

Wood heat treatment (HT) is promising technique for improvement of dimensional stability and durability of wood, specifically needed for outdoor applications. Wood can be exposed in these cases to various loads and to continuously changing environmental and climatic conditions. Among many positive impacts the heat treatment also reduces the wood density and most of the mechanical properties, as well as changes rheological characteristics of wood (Gorišek and Straže, 2011). Characteristics of HT wood highly depend on the properties of the innate wood specie and used process parameters. Industrial heat treatment under steam atmosphere was performed on beechwood (Fagus sylvatica L.) with pre-drying, heating at 230 °C for 6 hours, followed by conditioning and cooling. Oriented micro-tensile wood specimens (n = 20), 1.5 mm (T) by 3.0 mm (R) by 50.0 mm (L), were made from innate- (C) and heat treated wood (HT). Firstly, short-term micro-tensile mechanical tests were carried out at half of the conditioned wood specimens in dry climate (T = 23 ± 0.1 °C ; RH = 33 ± 1%). Micro-tensile mechano-sorptive tests followed afterwards in specially designed apparatus (Navi et al., 2002), limited to the area of linear viscoelasticity (σ = 0.2×σmax) for 48 h, where climatic conditions were cyclically changed between relative humidities of 33 % and 75%, at temperature of 23 ± 0.1 °C. The mechano-sorptive test was combined of phase of free swelling and shrinkage (a), phase of transient moisture effect and loading on the mechanical response (b) and a phase of transient moisture effect on the strain recovery (c). Heat treatment significantly reduced density of beechwood in average for 21%, from 760 kg/m3 to 575 kg/m3. We confirmed also 15% decrease of modulus of elasticity (MOE) from 18.7 GPa to 15.7 GPa, and the loss of mean tensile strength, from 170 MPa (C) to 84 MPa (HT). The viscoelastic creep response of HT wood was slightly reduced, where the creep rate was decreased also (Fig. 1 ; Fig. 2) Differences were found out also at mechano-sorptive response between innate and HT beechwood, where the latter had almost no mechano sorptive deformation. The ratio of final mechano sorptive strain (εms) to initial elastic strain (ε0) reaches at HT beechwood less than half of the value of innate wood. The mechano-sorptive strain was the greatest in the first relative humidity cycle (Fig. 1 ; Fig. 2). Lower mechanical properties, less visco-elastic creep and mechano-sorptive deformations can be generally ascribed to chemical changes of HT wood, to reduced amount of water in the cell wall structure, to rearrangement of molecular structure as well as to less established and loosen bands between cellulosic microfibrils, hemicelluloses and matrix of lignin (Olsson et al., 2005). Micro deformations of the cell wall structure can not either be neglected.

Heat treatment (HT) of wood is a promising technique for improvement of its dimensional stability, resistivity against biological and environmental factors as well as its durability. A heat treatment, besides positive impacts, also changes cellular structure of wood, its chemical characteristics, significantly reduces the wood density and most of mechanical properties [1, 2]. HT wood has a high potential for using it in outdoor applications, where can be exposed to continuously changing climatic conditions, causing heat transfer and moisture diffusion. The HT beechwood (Fagus sylvatica L.), used in the experiments, passed standard 1-day heating procedure at saturated steam conditions, consisted of pre-drying-, heating-, conditioning- and cooling phase. Clear, oriented wood specimens, (n = 7), 20 mm (width) by 100 mm (length), having various thicknesses (5, 10, 15 and 20 mm), were made afterwards from untreated- (C) and heat treated (HT) wood. Five side surfaces sealing of specimens using epoxy resin followed thereupon and conditioning at 20 °C and 40 °C having 33% relative humidity (RH). Diffusion experiments were made in adsorption, by exposing of specimens to 75% RH. The specimens were periodically weighted in short time interval, where the thickest material was also layered into 3 mm thick slices, for gravimetrical determination of moisture content (MC) profile. Diffusion coefficients were determined by traditional unsteady state method, using fractional change in average MC and a Newman equation for separating internal and external mass resistance [3]. Diffusivity was additionally analysed numerically by finite difference method, to asses the influence of moisture content. The heat treatment, according to the traditional unsteady state method, reduced moisture diffusion for 20% at low tested temperature (DC20 = 5.8×10- 11 m2 s-1 ; DHT20 = 4.7×10-11 m2 s-1). Mean moisture diffusion was for 2.6 times higher at 40 °C (DC40 = 1.56×10-10 m2 s-1 ; DHT40 = 1.25×10-10 m2 s-1). The diffusion ratio (DHT/DC ≈ 0.8) remained similar at both tested temperatures (Fig. 1). The finite difference method also confirmed the influence of heat treatment on moisture diffusion. It gave similar values of diffusion coefficients, for 25% higher at 20 °C (DC20 = 1.0×10-1o m2 s-1 ; DHT20 = 5.9×10-11 m2 s-1), and 10% lower at 40 °C (DC40 = 1.4×10-10 m2 s-1 ; DHT40 = 6.6×10-10 m2 s- 1), whereas variability of results was higher. The greater variation is ascribed to variability of wood properties, to uneven conditions during HT process, as well as to the achieved moisture content. The innate wood maintained higher MC at established climate conditions, and reached higher equilibrium moisture content (EMC) at initial and final state (Tab. 1). Additionally, increase of diffusion coefficient with the increase of moisture content was confirmed during adsorption process, more pronounced at innate beechwood specimens. Lower diffusivity of HT beechwood can be generally ascribed to lower hygroscopicity, most probable due to removal of hemicelluloses. In coincidence, the decrease of wood density presumably has positive, but less significant influence on moisture diffusivity of HT wood. We believe therefore, that physical changes of HT wood, like possible micro deformations of the cell wall structure, can not be neglected at mass transfer experimentations on this material.

Xylite is a lithotype of lignite, mainly used for fuel. The wood from which it was formed in a long term process (duration ca. 2 millions of years) was subjected to drastic changes. We conducted anatomical characterization of xylite, evaluated preservation of wood from which it was formed, defined its basic physical properties, and compared it with modern wood. We recognized ground tissue composed of tracheids, uniseriate and homocellular rays, taxodioid like crossfield pits, and abundant axial parenchyma with inclusions. The observed features are typical of coniferous wood species of the Taxodiaceae and Cupressaceae families. Colourimetric properties of xylite in CIE Lab* system are comparable to those of heartwood of ebony or wenge. The xylite had high oven dry density (1,216 kg/m3), but due to very high volumetric shrinkage (28.2 % on average), its basic density was much lower (873 kg/m3). Total linear shrinkage was considerable in all anatomical directions. Consequently it was characterized by low dimensional stability (qT = 0.65 %/%, qR = 0.30 %/%). The equilibrium moisture content (EM C) in desorption process was high, and that of adsorption was comparable to EM Cs of high density wood species. GAB sorption model proved to be appropriate for fitting the data obtained in the sorption process.

Industrial heat treatment (HT) under steam atmosphere was performed on European beechwood (Fagus sylvatica L.), wherefrom small clear wood specimens (10 × 15 × 170 mm) were made and conditioned in standard climate. Transverse free- free vibration response and 4-point bending creep tests were carried afterwards to determine dynamic mechanical properties and creep deformation of the material. Heat treatment significantly reduced density of beechwood, equilibrium moisture content and stiffness at bending. Significantly greater and slightly faster bending creep response was confirmed at innate beechwood specimens. The heat treatment increased internal friction (tan ) of HT wood, determined by vibration response.