ORIGINAL ARTICLE Drying of internal-check prone poplar lumber using three different conventional kiln drying schedules Soharb Rahimi • Mehdi Faezipour • Asghar Tarmian Received: 4 September 2010 / Accepted: 17 June 2011 / Published online: 17 July 2011 Ó Indian Academy of Wood Science 2011 Abstract In order to efficiently dry the internal check- prone poplar (Populus nigra) lumber, the 50 mm-thick lumber, freshly cut from Taleghan forest in Iran were dried using three different conventional kiln drying schedules (T8-F4, T8-F5 and T9-F4). The initial dry-bulb tempera- tures for the schedules were adjusted at 54, 54 and 60°C, while the final dry-bulb temperatures were 82, 82 and 71°C, respectively. After drying, the severity of surface and internal checking in the dried lumber was determined and then the statistical process control (SPC) was applied to monitor the dried lumber quality. Based on the SPC results, the quality of lumber dried by all the three drying schedules was satisfactory; however, the T9-F4 schedule resulted in lower internal checking compared to the other schedules, probably due to a greater stress relief treatment time. Keywords Drying Á Drying schedule Á Internal check Á Poplar wood Introduction Quality control in wood drying is of great importance. Drying defects, such as checks and warping reduce the grade and value of lumber. Thus, the drying of wood with as little development of defects as possible is a main objective in the drying process. One major defect in wood drying is internal checking or honeycombing. This type of defect is not visible by appearance. It happens in the inner structure of dried lumber (Perre ´ 2007). Honeycombing occurs along the wood rays, which are weak elements (Simpson 1991; Tarmian et al. 2009). Internal checking results from a too high level of stress reversal. When the stress level exceeds the modulus of rupture, checks appear as surface checking during the second drying period and internal checking at the end of drying (Perre ´ 2007). Honeycombing can result in heavy volume losses of lum- ber during cutting and machining. Thus, one of the main concerns of wood drying is to avoid this defect by suitable, improved or innovative procedures. One of the prime causes of this type of defect is use of the high dry-bulb temperature at the beginning of drying schedule or an increase in the dry-bulb temperature before the wood moisture content reaches below fiber saturation point (FSP; Simpson 1991). Therefore, honeycombing can be mini- mized by avoiding high temperatures until all the free water has been evaporated from the entire board (Simpson 1991). In addition to the drying schedule, this kind of default can be due to problems in kiln regulation (Perre ´ 2007). Zu ´n ˜iga et al. (2008) reported that drying internal checks within growth rings of Eucalyptus nitens were inversely related to the wood density and directly related to the proportion and diameter of the vessels. Harris et al. (2008) found that microwave pretreatment of Eucalyptus obliqua L’Herit backsawn boards can improve the drying quality in relation to internal and surface check formation. Poplar tree species is largely grown in plantations thought the world. Because of fast growth, a series of natural defects, such as tension wood commonly occurs in the poplar trees (Fang et al. 2007; Rahimi 2008). Furthermore, ‘‘wet wood’’ which is a critical defect in wood drying can happen in poplar trees with a high possibility. The term ‘‘wet wood’’ was introduced in the 1930s to describe the condition of discoloration and high moisture content found S. Rahimi Á M. Faezipour Á A. Tarmian (&) Department of Wood and Paper Science & Technology, Faculty of Natural Resources, University of Tehran, P.O. Box 31585-4314, Karaj, Iran e-mail: tarmian@ut.ac.ir 123 J Indian Acad Wood Sci (June 2011) 8(1):6–10 DOI 10.1007/s13196-011-0016-5