# Drying Behaviour of 50 mm Thick Turkey Oak Lumber
## 1 Introduction  
The drying of _Quercus_ wood species represents a challenge for the industry due to the high density, the heterogenous structure, the sensitivity to temperatures higher than 60-70 \({}^{\circ}\)C, and the tendency to crack, especially along the rays. The reference literature [1, 2, 3, 4, 5, 6] recommends for European oak wood (_Quercus robur_ and _Quercus petaea_) drying schedules with low temperatures (30-max.40 \({}^{\circ}\)C) and high relative air humidity (85-95%) during the first stage of actual drying (free water removal) and low drying gradients (1.5-2.0) during the second stage (drying below the fiber saturation point) in order to prevent cracking, but these conditions lead to very long drying times. Therefore, it is tempting to apply air-drying at least until the fiber saturation point is reached, in order to save energy. However, this approach is not always an appropriate one, because the lumber is exposed to biological attack and case-hardening [4] during this period. This often affects the wood quality, and sometimes it slows down the subsequent kiln-drying. Turkey oak (_Quercus cerris_) belongs to the group of red oaks [7]. It has a high proportion of sapwood (50%) and significantly higher density (\(\rho_{0}\) = 728 kg/m\({}^{3}\)) than sessile oak (\(\rho_{0}\) = 667 kg/m\({}^{3}\)), and its fiber saturation moisture content is lower (23-24%), according to determinations performed by the authors, compared with sessile oak (26-27%). These properties make it more difficult to dry than European oak lumber. The difficult drying, combined with the scarcity of the reference sources, results in the reticence of the industry to valorize it [8]. Steaming was found as a solution to reduce wood hygroscopicity and also to attenuate the color differences within lumber boards that contain both sapwood and heartwood [9, 10]. According to [9], the best results in terms of the equilibrium moisture content (EMC) reduction and color uniformization were obtained at 120 \({}^{\circ}\)C and 18 h. It is also known that steaming has a favorable effect upon the drying of wood, especially in the case of hardwoods. This is because it temporarily plastifies wood and thus allows a more rapid water removal, with fewer cracks. However, the steaming of Turkey oak wood is a difficulttask. Poorly chosen steaming parameters (temperature and duration) may cause internal surface cracks, collapse, or warp. A combined steaming-air-drying-thermo-vacuum treatment was applied by [11] to Turkey oak wood, in order to obtain a wooden material with improved processing properties. The freshly harvested logs were steamed at 110 \({}^{\circ}\)C and 140 kPa for 24 h, then cooled for 5 days and then cut into boards. The boards were air-seasoned for 5 months, until they reached a moisture content of 20-25%, then half of them were introduced in a press-vacuum plant (PVP) [12] and the other half in a thermo-vacuum cylinder (TVP) (both kilns being manufactured by WDE Maspell Italy). The PVP treatment avoided the deformation of the boards. The TVP treatment produced a more severe darkening. The incipient steaming had a decisive effect upon the EMC reduction. It was also observed that the defect-free boards suffered no qualitative damage, but the boards containing knots, for example, displayed severe cracks along a great part of the board. According to the authors' knowledge, in Romania, there is only one plant where vacuum-press drying is applied to oak lumber. Conventional drying is by far the most common method applied. According to a survey conducted in Romanian wood-processing companies [8], 30% of them perform kiln-drying from a green state, while 60% of the companies prefer pre-drying by air-seasoning followed by conventional kiln-drying. Most of the respondents (70%) indicated as a necessary optimization direction the reduction in the drying time and of the cracking tendency, especially with thicker grades (>40 mm).