Forest Utilisation Chapter 11: Wood Seasoning, Fibre Saturation Point, Air and Kiln Seasoning
Forest Utilisation Notes · Chapter 11
Chapter 11: Wood Seasoning
Definition of seasoning and air seasoning, free and absorbed water, fibre saturation point, moisture content, shrinkage, warping, objects of seasoning, log storage, refractoriness classes, air seasoning stacks, kilns, and salt and chemical seasoning.
1. Definitions
Air seasoning is also called air drying, open air seasoning or natural seasoning. It takes from a few months to several years.
2. Water in wood
Held by capillary action in the cell cavities (lumens). Lost first, with no shrinkage.
Held in the cell walls between the particles of wood substance. Its loss causes shrinkage.
FSP is 20 to 30% of water on the dry weight of wood. MC is found by drying a section about 1 cm thick at 100°C until two weighings agree. Air-dry wood holds 9 to 16% moisture.
Tangential shrinkage is the maximum, nearly twice the radial; longitudinal shrinkage is practically negligible. Denser woods generally shrink more.
3. Seasoning defects
| Defect | Meaning |
|---|---|
| Warping | Distortion from unequal shrinkage across the grain |
| Cupping | Curvature across the width of a board |
| Bowing | Curvature along the length of the broad face |
| Spring | Curvature along the length of the edge |
| Twist | Curvature both across the width and along the length, combining warp and cup |
| Checks and splits | Fibres separating, mostly along radii; surface cracks from faster drying of the surface than the inside |
The rate of drying is controlled by three factors: heat, humidity of the air and circulation of air. The art of seasoning is to control the rate so that the wood dries with least damage.
4. Objects of seasoning and log storage
- Reduce the risk of fungal and insect attack.
- Reduce weight.
- Avoid shakes, splits and cracks after the timber is in use.
- Secure proper penetration of preservatives.
- Make timber fit for painting and polishing.
Logs dry very little before conversion. The best way to keep logs free from cracking and attack is to store them under water (fresh, running water, fully submerged; avoid stagnant water). Sugars, gums and tannins leach out, which helps later drying. Trees may also be girdled (ring 10 to 15 cm deep and broad) to dry standing.
5. Refractoriness classes
| Class | Behaviour | Examples |
|---|---|---|
| Highly refractory | Slow and difficult to dry; crack and split | Sal, sain, dhau, mahua, tendu, jamun, sandan, aonla, bael, amaltas |
| Moderately refractory | Some tendency to crack | Teak, shisham, toon, haldu, siris, babul, walnut, neem, arjun, gamari |
| Non-refractory | Dry fast with little degrade; liable to fungi | Deodar, chir, kail, fir, spruce, semal, mango, salai, mulberry |
6. Air seasoning practice
- Foundation 30 to 45 cm high on piers about 30 cm square, 120 to 150 cm apart, with anti-termite caps.
- Stack width not more than 1.5 m; height not more than 3 m.
- Crossers (stickers) 2.5 × 4 cm or 4 × 5 cm, not more than 1.5 m apart (30 cm for 2.5 cm planks), placed vertically one above another.
- For refractory woods, stacks have their length along the prevailing wind, under sheds; non-refractory woods may be stacked in the open.
- Vertical stacking (end-racking) dries non-refractory woods fast and prevents stain.
- Sleepers: 1-in-9, close crib and open crib stacks.
7. Kiln seasoning and other methods
In a kiln, steam is the source of heat, humidity is controlled by steam or water sprays, and air is moved by flues or fans. All three drying factors are controlled.
- Process always under control
- Higher temperatures possible
- Continuous drying, not interrupted by weather
- Much shorter time
- MC needed far below air-dry
- Time matters more than cost
- Timber liable to fungus when green
- To save freight by reducing weight
- Steam-heated kilns (internal or external fans)
- Furnace kilns: direct heating (combustion gases touch the wood) and indirect heating
- Veneers in progressive dryers 15 to 35 m long
- Salt seasoning: boiling in salt and cooling 12 to 18 hours
- Chemical seasoning: calcium chloride, molasses (hygroscopic chemicals keep the surface moist)
8. Quick revision
- Seasoning = removal of moisture present in green timber.
- FSP = 20 to 30% of dry weight; shrinkage starts below FSP.
- MC on oven-dry basis, 1 cm section at 100°C. Air-dry wood 9 to 16%.
- Tangential shrinkage about 2 × radial; longitudinal negligible.
- Drying controlled by heat, humidity, air circulation.
- Best log storage: under fresh water.
- Highly refractory: sal, sain, dhau. Non-refractory: deodar, chir, semal, mango.
- Stack width up to 1.5 m, height up to 3 m; sleeper stacks 1-in-9, close crib, open crib.
9. Interactive MCQ Quiz
16 questions. Tap an option to see the answer and explanation, then go to the next question.
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