Seasoning of timber- Code of practice
1993 Edition

The IS 1141 standard defines kiln drying schedules (Schedules I to VII) for 25 mm thickness timber, specifying temperature and humidity levels that start low and gradually increase temperature while reducing humidity. Initial steaming at 55°C and 100% RH for two hours is advised to prevent mold and relieve stresses. For thicker timber (25-50 mm), relative humidity is raised by 5% at each drying phase, and for 50-100 mm thick timber, preliminary air seasoning to about 25% moisture content is required before kiln drying. For example, Schedule I for species marked 'I' begins at 52°C dry bulb temperature and 62% relative humidity at green moisture content, progressing through stages until reaching approximately 68°C and 33.5% RH for seasoned timber. These schedules ensure controlled drying minimizing defects and optimizing wood quality.

To reduce seasoning defects, timber must be stacked on a stable, level foundation using closed stacks with uniform spacing (stickers) between layers to facilitate airflow. Applying weights on top layers prevents warping. The ends of logs should be coated with moisture-resistant substances to prevent splitting. Timber stacks should be shaded or covered with adequate overhang to protect against direct sunlight and precipitation. Seasoned timber must be stored in well-ventilated, dry environments to prevent moisture absorption. Controlled drying conditions and proper stacking techniques ensure uniform moisture loss, thereby minimizing defects such as cracking, warping, and case hardening.

IS 1141, referencing IS 287:1993 and IS 11215:1991, specifies moisture content levels tailored to timber applications and climatic zones. Freshly felled timber may have moisture contents up to 100-200%. For high-quality indoor applications like cabinetry and paneling, timber should be dried to 10-12% moisture content. For rough or outdoor use, acceptable moisture content ranges from 15-25%, facilitating easier preservative treatment and durability. Proper seasoning to these moisture levels prevents shrinkage, warping, and decay, ensuring longevity and performance of timber products.

IS 1141 addresses drying defects through proper storage, stacking, and seasoning techniques. It emphasizes balanced drying rates, correct stacking to ensure adequate air circulation, and controlled kiln schedules to prevent rapid moisture loss that causes defects. Some defects inherent to timber, such as warping caused by residual growth stresses or knots, cannot be completely eliminated but can be minimized through appropriate sawing and seasoning methods. The standard advocates preseasoning treatments, careful log selection, and gradual drying procedures to reduce defects and maintain timber quality.

IS 1141 recommends solar kilns with forced air circulation combined with water spray humidification to control relative humidity, especially for drying thick or refractory timber. Solar kilns operate during daylight with forced ventilation at night, offering faster drying than air seasoning but slower than steam kilns. They require careful humidity management to prevent cracking and case hardening, with post-drying water spray to relieve stresses. While dehumidification kilns are not detailed explicitly in IS 1141, they typically provide moisture-controlled drying by extracting moisture from the air, offering energy-efficient and uniform seasoning conditions.