The wood from a freshly cut tree is unfit for use because it contains 50 percent or more of its dry weight in the form of water and moisture. There is a fear of the wood cracking and shrinking while using such a block of wood. This wood is weak making it difficult to paint or varnish. Hence, drying the wood or removing the water content is important before using it for any engineering purpose. This process of drying the wood is known as wood seasoning.
In this blog, Gharpedia brings a detailed guide for timber seasoning, in which you will find all the basic info about wood drying, such as seasoning of timber, its purpose, benefits, classification of timber for seasoning, timber seasoning methods, and more.
So, let’s start the journey…
What is a Seasoning of Timber?
Timber from freshly cut trees is too wet for normal use and dimensionally unsuitable. Wood seasoning reduces the moisture content in the wood to prevent its dissolving and make it more suitable for use.
Seasoning of timber means drying wood to a moisture content approximately equal to the average humidity of the surroundings where it is to be permanently fixed, as this causes matter to harden and thus increases resistance to the penetration of surge protection.
During the wood seasoning process, the moisture must be evenly removed from all wood parts at an almost uniform rate under controlled conditions. If the drying is uneven, it will cause excessive internal stresses that can overcome the same particle of the fibres, resulting in the formation of wood warps and shacks.
Wood is a hygroscopic material; the capacity of wood to absorb water vapour from the air is called the hygroscopicity of wood. Hence, the dry wood absorbs the moisture from the surrounding air. But, the humidity of the air is not constant; hence, the moisture content of the wood varies accordingly.
Wood attains equilibrium moisture content levels given the climatic conditions of temperature and relative humidity. During the seasoning process, wood is dried in such a way that the moisture content of seasoned wood matches the moisture content required in the wood for the environment in which it is to be used.
Seasoning is the first step in preparing the wood for use. It is important, especially in the country, where the climatic conditions are challenging for woodworking and commercially important wood species are hard, heavy, and prone to defects during drying and usage.
All timbers, whether teak wood or other species, naturally durable or non-durable, require proper seasoning. Therefore, seasoning should be considered an integral part of the use of timber and close attention should be paid to the wood seasoning process before usage.
The relationship between climatic conditions and moisture content in timber has been established through tests on various types of timber. Hence, protecting the seasoned timber from exposure to rain and excessively high humidity is crucial.
Purposes of Seasoning of Timber
The following are the purposes of seasoning timber:
- To make timber suitable for treatment with paints, varnishes, preservatives etc.
- To protect the timber from mould and insect attack.
- To maintain the shape and size of the component of the timber articles.
- To impart hardness, toughness, strength and better electrical resistance to wood.
- To reduce the tendency of wood to crack, warp and shape.
- To make timber easily workable and facilitate handling during conversion.
Benefits of Seasoning of Timber
Following are the advantages of seasoning timber:
- Painting and finishing wood become easier with the seasoning of timber.
- After removing the moisture content from the timber, it gains strength, and durability and is easy to transport.
- The insect and fungal attacks reduce after the seasoning of timber.
- They maintain the size and shape of timber even after leaving the hands of carpenters and joiners.
- To decrease the volume and weight of timber, thereby lowering the cost of transportation and handling.
- To reduce the tendency of timber to crack, warp, bend or shrink.
Classification of Timber for Seasoning
The three classifications of timber for seasoning, depending upon their behaviours concerning cracking, drying and splitting rate are:
01. Class A: Highly Refractory
These timbers are slow-drying and difficult to season, but they are free from cracking and splitting. Examples of highly refractory woods are heavy structural timbers, like Sal.
02. Class B: Moderately Refractory
This type of timber can be seasoned and made free from the surface and end cracking within reasonably short periods, by giving little protection against rapid drying conditions. Examples are moderately heavy furniture grade-timbers like teak.
03. Class C: Non-Refractory
Non-refractory timbers can also be rapidly seasoned and made free from surface and end cracking even when exposed to open air and sunlight. However, they develop blue stains and mould on the surface if not rapidly dried. Examples of non-refractory woods are light broad-leaved (hardwood) species for packing cases, such as salai and all coniferous species.
Precaution for the Seasoning of Timber
The following are the precautions for wood drying:
- During air seasoning, seasoned wood must be protected from exposure to high humidity and rain.
- A suitable gap between the logs will ensure an easy and uniform flow of air and water.
- Controlling environmental conditions will help remove moisture during seasoning.
- The rate of moisture extraction should be the same for all logs because of the possibility of irregular shapes due to differential dryness.
Timber Seasoning Methods
There are two types of timber seasoning methods, which have been discussed below.
1. Natural Seasoning
i. Air Seasoning
ii. Water Seasoning
2. Artificial Seasoning
i. Boiling Seasoning
ii. Chemical or Salt Seasoning
iii. Electrical Seasoning
iv. Seasoning by Steaming
v. Mc. Nell’s Seasoning
vi. Kiln seasoning
01. Natural seasoning
Wood is seasoned by exposing it to natural elements like air or water in the natural seasoning process. The followings are the natural wood seasoning process:
i. Air Seasoning
Air-drying seasoning is one of the easiest and cheapest methods of seasoning timber. This method does not require skilled labour. The technique of air seasoning mainly consists of building a good stack of sawn timber with the help of spacers on a raised foundation under shade (like a verandah) in a clean and dry place. Since atmospheric air serves as the drying agent, the rate and quality of the drying agent and the rate and quality of drying depend largely on climatic conditions. However, the damage caused by wood cracking, splitting, and decaying through a fungal attack, warping and insect attack can be brought down by paying proper attention to the method of stacking, protecting the stack against the sun, hot and dry winds, and maintaining sanitary conditions in the yard. It is always advisable to apply two coats of asphalt paint on both cross-sections of each log before cracking at the ends. This prevents waste of wood.
Air seasoning is usually a slow process. It decreases moisture content by 12 to 25 per cent. The actual time required will vary with the size and variety of timber. Depending on the climate of the place, the wood stack takes about less than 6 months to naturally season and become fit for making household furniture like windows, door frames etc. The pedestal fans and other methods of forced air drying can speed up the drying of the timber stack.
i. Water Seasoning
In this technique, the wooden log is completely submerged in a running stream of water, with its larger ends pointing upstream. Consequently, the sap, sugar and gum leach out and get replaced by water. The log is kept out for drying. While it is not a time-consuming process, it reduces the strength and elastic properties of the wood.
02. Artificial Seasoning
In artificial seasoning, artificial methods are used to speed up the wood drying process. Though natural wood seasoning gives pleasing results, it takes a long time. Hence, artificial methods have been developed. Wood can be seasoned in 4-5 days from the artificial seasoning. The followings are the artificial seasoning process:
i. Boiling Seasoning
In the boiling timber seasoning process, timber is left in boiling water for three to four hours and then allowed to dry. Because it is challenging to boil a huge quantity of wood, sometimes wooden logs are exposed to hot steam in a contained space. This method enhances the properties of wood, like strength and elasticity, and reduces shrinkage, but it’s an expensive seasoning process.
ii. Chemical or Salt Seasoning
This technique prevents splitting and cracking during the seasoning of highly refractory hardwoods and thick sections. These defects are difficult to avoid under conventional air and kiln seasoning.
First, the fresh wood is soaked in a solution of an anti-shrink chemical or anti-shrink-cum-hygroscopic chemical for a suitable period. The soaking period depends upon the timber’s type and the thickness. This salt solution has tendency to absorb water from the timber, hence the moisture content is removed. The treated timber is then kiln seasoned or air seasoned.
iii. Electrical Seasoning
In the electrical seasoning process, the ends of the logs touch the electrodes. Green timber is a conductor, while dry wood is a non-conductor of electricity. Thus, when the electricity passes through the setup, the wood opposes the flow of current, generating heat and thus drying it out. The disadvantage of this technique is that wood can crack. Furthermore, this process involves higher initial costs and maintenance costs. However, this is not a time-consuming process.
iv. Seasoning by Steaming
This technique is like boiling, but the timber is dry soon after the steaming.
v. Mc. Nell’s Seasoning
This is one of the best methods. Here, timber is stacked in a chamber with free airspace (1/3rd of its capacity) and contains the products of combustion of fuel in the fire place. This technique has a time requirement of 15 to 60 days to complete. Although there are no major adverse effects to this method, its drawback is that it is expensive.
vi. Kiln Seasoning
It is meant for the rapid seasoning of timber on a large scale. The scantlings provided here ensure the free circulation of hot air with superheated steam or moisture. The circulating air takes the necessary moisture from the wood and seasons it.
The two types of kiln seasons: progressive and compartment are in use. Kiln drying results in a slight reduction in the strength of the wood, usually less than 10 per cent, but the wood dries thoroughly and evenly, thus reducing its hygroscopicity.
Improvement of Wood Quality after Seasoning
- Weight:
An effective weight reduction happens by reducing the water content of wood by 10 to 15 per cent from its initial green moisture content value. It helps with handling and reduces the cost of transportation.
- Finishing and Painting:
Timber seasoning is necessary before painting, varnishing and polishing to get the desired result. The concentration of moisture on the surface of the unseasoned wood results in peeling, blistering or cracking of the surface layer.
- Provide Resistance to Electricity:
After the seasoning process, the wood has increased in resistance to electricity. Green timber has low resistance because it has more moisture content and is an excellent conductor of electricity, but dried wood acts as an insulator.
- Resistance to Insect and Fungal Attacks:
Insect attack is more prevalent if the timber is unseasoned. Hence, wood seasoning is necessary to protect the wood from insects and fungal attacks.
- Durability:
Timber with a moisture level of at least 24% is more susceptible to decay and staining. This further softens the wood and results in unsightly spots. Seasoning is necessary to increase its life and prevent deterioration of the wood’s appearance. Seasoned wood must be stored in a controlled environment at a moisture content of less than 24%.
- Hardness:
The seasoning process makes the timber tougher. It is advantageous, as flooring and furniture grow more resilient to damage from jerks and punches. However, it has drawbacks if you want to drive nails into seasoned hardwood.
How to Inspect Seasoned Timber?
Seasoned timber should be inspected for conformance to specifications for moisture content, residential case-hardening stress, where required, and natural as well as timber seasoning defects.
Test samples shall be measured for dimensions, quality and moisture content for the inspection of a given lot of timber concerning moisture content and residual case-hardening stress. The number of samples to be tested depends on the size of the lot, the acceptable range of moisture content depending on the end user, the composition of the lot, and the possible variations in moisture content between individual pieces.
The maximum permissible limits of moisture content conform to the requirements of moisture content and the tolerances recommended in these limits for seasoned wood for various end uses, as prescribed in ‘IS 287:1993’ (Indian Standard Code: Permissible Moisture Content For Timber Used For Different Purposes Recommendations), and following the testing procedure prescribed in ‘IS 11215:1991’ (Indian Standard Moisture Content Of Timber And Timber Products – Methods For Determination)
Inspect seasoned timber for defects (both natural and seasoning defects) by specifications laid down for sawn timber quality for various end-uses.
How to Store and Transport after Seasoned Timber?
- It is good to use seasoned wood immediately before its moisture content changes due to seasonal changes. However, when storage becomes unavoidable, make sure it is stored in closed stacks under sheds maintained under dry conditions to prevent a change in moisture content.
- Cover the stack with a polythene cover that surrounds each stack and top. Store the planks and scantlings in small lots, strapped with metal bands.
- The seasoned timbers should be routinely maintained with close tolerances of moisture content, if it is stored for large-scale production work, such as aircraft manufacturing, railway coach shutters, rifles, furniture, pattern making etc.
- Store them in climate-controlled sheds maintained under properly balanced humidity conditions.
- Transport seasoned timber in covered wagons or closed stacks under a tarpaulin cover to protect it against rain.
- Pack the seasoned and half-wrought timber for special use, in gunny bags for transport.
What is the Percent of Moisture Content Needed in Seasoned Timber?
Usually, 12-15% moisture content is what you should maintain in seasoned timber. However, the final moisture content depends upon the service requirement of the finished article and the climate conditions of the locality where it will be used. It should also conform to IS 287:1973, which gives the guidelines for the permissible limit of moisture content. The recommended tolerances and methods of determination of moisture content have been mentioned in IS 11215:1991.
Know More
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Author Bio
Bhamini Gamit – I’m Bhamini Gamit. I had completed my graduation in 2019 from S.N.P.I.T & R C, Umrakh, Surat, Gujarat. My core interest is to write blogs on civil engineering of various subjects like geotechnical, Building materials, Building construction, and Reinforcement cement concrete. For the last 6 months, I have been a guest writer on Gharpedia. You can easily find me on Instagram & LinkedIn.
