SeasoningSeasoning
Seasoned timber has an average moisture content of around 15% or less. (It is permissible to have occasional moisture readings as high as 18%, but the bulk of any package of seasoned timber will have a moisture content of less than 15%. Most appearance products are seasoned and may have special requirements for moisture contents significantly less than 15%. Many structural products are now specified as seasoned.
Seasoned timber tends to have superior dimensional stability than unseasoned timber and is much less prone to warping and splitting in service. In higher grades of timber, particularly hardwoods, the process of seasoning can enhance the basic characteristic properties of timber, increasing stiffness, bending strength and compression strength.
Seasoning is the process of drying timber to remove the bound moisture contained in walls of the wood cells to produce seasoned timber. Seasoning can be achieved in a number of ways, but the aim is to remove water at a uniform rate through the piece to prevent damage to the wood during drying (seasoning degrade).
Water is stored in wood in two main forms:
- As free water in the vessels and/or cells, used to move nutrients within the tree.
- As cell (or bound) water, which is an integral part of the cell walls.
The process of seasoning removes all of the free water and most of the bound water. In the removal of the bound water, the wood cells change in size and shape, so this part of the process must be carried out with careful control over drying rate.
When the timber is first cut, the initial reduction in moisture content is a result of free water loss. This usually occurs without any significant dimensional changes to the timber as the loss of moisture represents the drainage of pores in the timber. If the environmental conditions are favourable, the moisture loss continues until all the free water is released to the atmosphere. This point is known as the fibre saturation point (fsp). The fibre saturation point varies a little with each piece of timber, but it is generally taken to be at a moisture content of between 25% and 30%. The loss of free water will occur relatively quickly in small cross-sections of timber, even if the timber is exposed to rain.
After all of the free water has been lost, the timber is still classed as unseasoned as its moisture content is above 25%. Further drying is required to bring the moisture content below 15% so that it can be classed as seasoned. This additional reduction in moisture content, below fibre saturation point, requires a reduction in the amount of bound water in the wood's cell wall, which in turn requires more energy and occurs more slowly than the loss of free water. It also results in a reduction in the size of the cell walls, which causes the timber to shrink in size.]
A number of commercial processes for seasoning of timber are available, the most common of which is kiln-drying. Kiln seasoning accelerates the process of seasoning by using external energy to drive the moisture out. The timber is stacked in much the same way as it is for air drying, and is placed inside a chamber in which the conditions can be varied to give best seasoning results. Air is circulated around the charge (stacked timber) and the temperature and humidity can be varied to give optimum drying. Each species has different cell characteristics and therefore requires different drying schedules. Typically the timber may be in the kiln for a period of between two days to one week.
Generally, it is not feasible to kiln-dry structural timber in thicknesses greater than 45 mm, although there are limited amounts of 70 mm thick kiln-dried softwood members in the market place. All untreated structural pine and some commercial hardwoods are seasoned, mostly using kilns that are often heated by sawmill by-products or gas.
Plenty of research is continually being done to determine kiln drying schedules for different species. An optimum schedule will remove the moisture as quickly as possible, with the minimum quantity of introduced energy, and giving minimum damage to the dried timber.
After kiln seasoning, there is often some damage to cells near the surface of the wood. (All of the moisture passes through those cells.) They have in fact collapsed, but can easily be ‘pumped back up’ in a reconditioning chamber. This chamber introduces steam for a period and puts some moisture back into the outer cells and removes the effect of seasoning collapse.
Whilst kiln-seasoning of softwoods such as pine species is generally a fairly quick process, seasoning of hardwoods tends to be a much longer process. This is mainly due to the different (closed) cell structure of hardwoods.
Seasoning of plantation softwoods in Australia mainly utilises “high temperature” drying, where the kiln temperature is significantly higher than boiling point. This higher temperature permits softening of lignin in the timber, which begins to occur at about 90°C. The softened lignin allows the timber fibres to realign themselves while the stack of timber is being held straight by weights. The tendency to twist and distort during the drying process can be avoided, producing timber, which on cooling under restraint, will remain straight. A typical drying process for plantation softwoods using this principle involves pre-steaming for 4 hours, then drying at temperatures of about 115°C until the moisture content falls below 10%, which will typically take about 3 ‑ 5 days. The timber is then humidified with steam for about 4 hours and cooled under restraint.
Conventional kiln-drying for hardwoods uses temperatures well below the boiling point of water. This avoids damage to the wood fibres which, if the seasoning is not properly controlled, can experience loss of strength. In seasoning hardwoods, the process is slow and can be economically undertaken by first of all air-drying or partially seasoned for a period which can be anywhere from 3 to 9 months, depending upon the thickness of material, the ambient temperature and humidity and the species. The partial seasoning process can be accelerated for some hardwoods by using fan-forced pre-drying in a controlled (but not kiln) environment, to reduce the lead time for kiln-drying to 20 to 30 days.
Once the sawn hardwood material reaches fibre saturation point or slightly below (at a moisture content of about 20% to 25%), it is then placed in kilns usually for up to 10 ‑ 14 days (depending upon the thickness of the sawn timber) in order to bring the moisture content down to between 10% and 15%. This drying process must be strictly controlled and monitored in order to avoid drying degrade.
The traditional method of seasoning timber was to stack it in air and let the heat of the atmosphere and the natural air movement around the stacked timber remove the moisture. The process has undergone a number of refinements over the years that have made it more efficient and reduced the quantity of wood that was damaged by drying too quickly near the ends in air seasoning.
The basic principle is to stack the timber so that plenty of air can circulate around each piece. The timber is stacked with wide spaces between each piece horizontally, and with strips of wood between each layer ensuring that there is a vertical separation too. Air can then circulate around and through the stack, to slowly remove moisture. In some cases, weights can be placed on top of the stacks to prevent warping of the timber as it dries.
Moisture loss from the side of the wood is at about the right rate not to cause collapse of the cells, but near the ends of the wood, the moisture loss can prove to be too fast. Often the ends are wrapped or painted to slow the moisture loss from the end grain.
While little additional energy needs to be supplied for this type of seasoning, the stacks of timber require a lot of land, represent a potential fire hazard, and the product is not able to be sold for a considerable time. The interest costs on holding stock for long periods can prove significant.
Air-drying of timber is really a more controlled facilitation of what happens to unseasoned sawn, timber, once it is placed into its “work” environment. The amount of drying that can occur is very much governed by the relative humidity of the drying environment and will often vary within individual boards as well as within the stack itself. The time taken for air-drying is a function of the thickness of the timber.
Air-drying is necessarily a slow process, particularly for hardwoods, typically taking 6 to 9 months to reach a moisture content in the range 20% to 25%.
Solar drying
Solar seasoning offers a compromise between the low energy requirement of air drying and the speed of kiln drying. A number of different configurations have been tried, including a double skinned inflatable kiln. Because the energy input is variable, the kilns often have effective insulation to hold the heat inside at night time. Some kilns developed overseas can generate temperatures of over 40C inside even though the temperature outside is – 20C.
Solar-drying, which can take nearly twice the time required for kiln-drying, is well suited to high-grade applications such as furniture. Solar-drying is really a slower and gentler form of kiln drying. Some seasoned hardwood producers use air-drying down to fibre saturation point (FSP) prior to stacking the timber in the solar kiln.
Microwave seasoning, is a process still in the investigative stages in Australia, but is an established technology in Canada and is now used for drying some commercial sizes of North American softwood timber.
Pulsed microwave energy is directed into layers of timber in a manner that will drive the moisture out of the timber at rates that will not cause seasoning degrade. The process has the ability to deliver energy that can be varied from second to second to suit the moisture content of the timber at the time, and the ambient conditions.
In some cases, microwave energy delivery can be combined with low ambient pressure to give optimal seasoning speeds.
Chemical seasoning
Chemical seasoning, which is sometimes used for very high value applications such as rifle butts, golf club heads and carvings, where the timber is required to be completely free of surface checks. The process involves soaking the green timber (as soon as it is cut) in a hydroscopic chemical for about a day, which slows down the rate of moisture loss and minimises the risk of inducing the internal stresses which cause checking to occur during the subsequent air-drying.
Seasoning degrade
All producers aim to minimise the extent of damage that occurs as the timber is being seasoned. They remove any pieces that have degraded significantly during the grading process.
Normally users of timber do not have to worry about seasoning degrade because all affected pieces have been rejected during production.
Seasoning degrade can produce timber that has reduced utility due to the following factors:
- Twisting and other deformations such as bow and spring. One of the most commonly observed forms of degrade in wide pieces of timber (e.g. boards) is cupping of the member. The board becomes concave on the face further from the corewood (or centre of the log) and convex on the other face.
- Checking ‑ cracks that do not extend right through the timber. They often present as a small surface crack that doesn't extend deep into the timber, but will reduce the appearance value of the timber and make it more difficult to coat with a surface finish
- Internal checking ‑ also known as honeycombing. These checks are not visible on the surface, but are exposed when the piece of timber is cut.
- Splitting ‑ cracks that extend right through the timber (most prevalent at ends).
- Collapse ‑ some cells collapse and the surface of the timber will have a corrugated or washboard appearance. The cross-section may be irregular rather than the rectangular shape it started with. Collapse can be recovered in some species, by controlled re-humidification in a steaming chamber.
- Black heart ‑ a number of species are prone to severe checking and collapse, in the zone of dark-coloured wood in the core material of logs. This can cause problems where this otherwise decorative material is exposed on the face of a piece of timber.
The term “seasoning degrade” can also be used to refer to the population of timber that has been downgraded because of the degradation that occurs during drying.
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