Air SeasoningTo air season wood that is cut into desired lengths, stack it in layers and let it sit for 6 to 9 months. Cut the firewood in the desired lengths, and split it into smaller pieces. Stacking in layers allows the air to flow through the wood. Homesafe suggests covering the stack of wood with a tarp to keep it dry during rains. 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).Seasoning of Wood

Kiln seasoning

Kiln seasoning of wood is a commercial process that employs two processes, progressive and compartmentalization. The compartment process is conducted in a building where the lumber is stacked in a manner that allo2ws the air to circulate. A program is set to season the wood. The progressive process is conducted on a trolley system where the wood is moved through different compartments. Both processes control the humidity and air flow for the proper moisture reduction. Kiln drying allows processing of a greater amount of wood for commercial sales.Chemical SeasoningA hydroscopic chemical is used to soak the cut green wood. The chemical seasoning process reduces the risk of stressing the wood internally and slows the moisture loss, which keeps the wood in pristine shape to create rifle butts, golf club heads, carvings and other ornamental uses.Solar SeasoningSolar seasoning occurs in a specialized kiln; the wood is stacked so the air circulates while the sun heats the kiln. Occasionally wood is air dried prior to solar drying to reduce the moisture. The solar drying of wood is reserved for furniture uses, where the less moisture the better.

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.

Air seasoningThe 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. 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 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%.

Reasons for air drying wood:

There are many reasons for drying wood. Four main reasons include:

To increase dimensional stability. Wood shrinks across the grain (not along the grain) when it dries. If wood is cut to size before it is seasoned, it will shrink during drying and thus be undersized in its final form.

To reduce or eliminate attack by decay or stain. Wood that is dried below 20 percent moisture content is not susceptible to decay or sap staining.

To reduce the weight. The weight of lumber will be reduced by 35 percent or more by removing most of the water in the wood or, as we say, by "seasoning." To increase the strength. As wood dries, the stiffness, hardness and strength of the wood increases. Most species of wood increase their strength characteristics by 50 percent or more during the process of drying to 15 percent moisture content

Kiln seasoningTheory of Kiln-drying

The dry kiln has long since acquired particular appreciation at the hands of those who have witnessed its time-saving qualities, when practically applied to the drying of timber. The science of drying is itself of the simplest, the exposure to the air being, indeed, the only means needed where the matter of time is not called into question. Otherwise, where hours, even minutes, have a marked significance, then other means must be introduced to bring about the desired effect. In any event, however, the same simple and natural remedy pertains,the absorption of moisture. This moisture in green timber is known as sap, which is itself composed of a number of ingredients, most important among which are water, resin, and albumen.All dry kilns in existence use heat to season timber; that is, to drive out that portion of the sap which is volatile.

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 45mm, 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.

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.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.

Advantages of Kiln-drying over Air-drying

Some of the advantages of kiln-drying to be secured over air-drying in addition to reducing the shipping weight and lessening quantity of stock are the following:1. Less material lost.2. Better quality of product.3. Prevention of sap stain and mould.4. Fixation of gums and resins.5. Reduction of hygroscopicity.This reduction in the tendency to take up moisture means a reduction in the working of the material which, even though slight, is of importance.The problem of drying wood in the best manner divides itself into two distinct parts, one of which is entirely concerned with the behavior of the wood itself and the physical phenomena involved, while the other part has to do with the control of the drying process.

Compartmental Kiln

Progressive Kiln

Water Seasoning

On account of the time required to season timber in the natural way, various methods have been tried to effect the same purpose in a shorter time. One of the best of these is to immerse the timber in water as soon as it is cut down, and after it has remained about a fortnight in water, but not more, to take it out, and dry it in an airy situation.Evelyn directs, to "lay your boards a fortnight in water (if running the better, as at a mill-pond head) and then setting them upright in the sun and wind, so as it may pass freely through them, turning them daily; and, thus treated, even newly-sawn boards will floor far better than those of a many years dry seasoning, as they call it:" * and he adds, "I the oftener insist on this water seasoning, not only as a remedy against the worm, but for its efficacy against warping and distortions of timber, whether used within or exposed to the air.Timber that has been cut when the tree was full of sap, and particularly when that sap is of a saccharine nature, must be materially benefited by steeping in water; because it will undoubtedly remove the greater part of the fermentable matter. When timber is put in water it must be sunk so as to be completely under water, as nothing is more destructive than partial immersion. Salt water is considered best for ship-timber, but for timber to be employed in the construction of dwelling-houses fresh water is better.At the close of these several periods the boards intended for planking should be taken out and placed in store, or they might be left to season themselves naturally for two years before being worked up. As to rough timber for ribs, it should always be submitted to artificial seasoning previous to being used.

Physical Conditions governing the Drying of Wood

1. Wood is soft and plastic while hot and moist, and becomes set in whatever shape it dries. Some species are much more plastic than others.2. Wood substance begins to shrink only when it dries below the fibre-saturation point, at which it contains from 25 to 30 per cent moisture based on its dry weight. Eucalyptus and certain other species appear to be exceptions to this law.3. The shrinkage of wood is about twice as great circumferentially as in the radial direction; lengthwise, it is very slight.4. Wood shrinks most when subjected, while kept moist, to slow drying at high temperatures.5. Rapid drying produces less shrinkage than slow drying at high temperatures, but is apt to cause case-hardening and honeycombing, especially in dense woods.6. Case-hardening, honeycombing, and cupping result directly from conditions 1, 4, and 5, and chemical changes of the outer surface.7. Brittleness is caused by carrying the drying process too far, or by using too high temperatures. Safe limits of treatment vary greatly for different species.8. Wood absorbs or loses moisture in proportion to the relative humidity in the air, not according to the temperature. This property is called its hygroscopicity.9. Hygroscopicity and working are reduced but not eliminated by thorough drying.10. Moisture tends to transfuse from the hot towards the cold portion of the wood.11. Collapse of the cells may occur in some species while the wood is hot and plastic. This collapse is independent of subsequent shrinkage.

Seasoning degradeTwisting 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 finishInternal 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.Seasoning degrade can produce timber that has reduced utility due to the following factors:

Thank youSubmitted by:-Anuja SharmaAnshita AggarwalKritika GuptaAvneet Singh

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