Obrada jasena (Fraxinus excelsiorL.) na temperaturi od 200 oC uzrokovala je značajne promjene tehničkih svojstava odnosno fizikalnih i mehaničkih svojstava, tako obrađenog drva. Gustoća jasenovine u apsolutno suhom stanju signifikantno je manja od gustoće recentne. Ista je pojava uočena i kod utezanja u radijalnom i tangencijalnom smjeru i također volumnog utezanja. Istraživana mehanička svojstava: čvrstoće na tlak u longitudinalnom smjeru, statič ke čvrstoće na savijanje, tvrdoće u longitudinalnom, radijalnom i tangencijalnom smjeru toplinski obrađene jasenovine, također su signifikantno manja od recentne. Obrada jasenovine na visokoj temperaturi polučila je drvo veće dimenzionalne stabilnosti, odnosno manjih veličina linearnih i volumnog utezanja, ali manjih mehaničkih svojstava jasenovine.The aim of this paper is to study differences in some physical and mechanical properties of heat treated and recent wood. Wood used in this research was ash (Fraxinus excelsiorL.) from the area of Otok, region of Slavonia in Croatia. The felled tree was 85 years old with the diameter at breast height of 53 centimetres. A heart board was made from the tree trunk in the north-south direction, 4 metre long and 6 centimetres thick. The heart board was dried naturally with an average water content of 12 %. The heart board was sawn in four equal parts so that four heart boards were obtained – each 1 metre long. Looking from stump to top of the tree, the first and third heart boards were not heat treated and they were used for establishing the properties of recent ash. From the above four heart boards, samples were made for determining macroscopic, physical and mechanical properties of recent and heat treated ash. Treatment of ash (Fraxinus excelsiorL.) at the temperature of 200 oC caused significant changes of technical properties relatively physical and mechanical properties of the treated wood. Maximum possible number of samples was made for each physical and mechanical property in radial direction, i.e. from heart to bark. Maximum shrinkage was determined in radial and tangential direction, as well as maximum volume shrinkage, density in absolutely dry condition, nominal density and water content after a fortnight of soaking in water of physical properties. Determination was also made of compression strength parallel to the grain, static bending strength, hardness in longitudinal, radial and tangential direction. The investigation of some physical and mechanical properties of recent ash (Fraxinus excelsiorL.) and heat treated ash at 200 oC show diametrically different values of investigated properties.
The investigation of mechanical properties of recent ash was carried out on samples with water content of 12 %, while the water content of the heat treated ash samples was 4 %. In order to be able to compare the results of mechanical properties, their values were converted to the water content of 4 % for recent ash and 12 % for heat treated ash. For increasing or decreasing mechanical properties of wood, correction factor was used in accordance with Forest Product Laboratory, Madison. The comparison of mean values of physical and mechanical properties of recent and heat treated ash was carried out in accordance with Mann-Whitney test with the help of statistical software Statistica. Density distribution in absolutely dry condition in radial direction of heat treated ash deviates from the distribution common for recent ash. The loss of mass with a relatively constant sample volume is evident in radial direction, and especially in the heart section. Mean value of density in absolutely dry condition of recent ash is higher by 8.4 % than density of heat treated ash. Shrinkage in radial and tangential direction and volume shrinkage of recent ash are higher that shrinkage in heat treated ash. Shrinkage in radial direction is higher by 70 %, in tangential direction by 62.2 % and volume shrinkage by 63.7 %. All above said physical properties of recent ash are significantly different from physical properties of heat treated ash. The investigated mechanical properties of recent ash are also significantly higher than mechanical properties of heat treated ash. Compression strength in longitudinal direction of recent ash is higher by 36.8 %, static bending strength is higher by 48 %, hardness in longitudinal direction (cross-cut) is higher by 31.3 %, hardness in radial direction (tangential cut) is higher by 22.8 % and hardness in tangential direction (radial cut) is higher by 25.9 % than the stated mechanical properties of heat treated ash at 200 oC. All investigated mechanical properties of recent ash differ significantly from these properties of heat treated ash. Dimensional stability of ash under such regime of heat treatment resulted in increased dimensional stability, and however the investigated mechanical properties are significantly lower. The above said percentages of higher values of mechanical properties of recent ash than of heat treated ash with water content of 12 % and 4 % indicate that they are similar at 12 % and 4 %. The similarity of these percentages indicates that the correction coefficients for calculating mechanical properties at different water contents is also satisfactory for heat treated ash
