The Advantages and Disadvantages of Kiln Drying Timber compared to Air Seasoning of Timber

• A major benefit of kiln dried timber production is that kilns can dry to lower moisture contents than can be obtained by air seasoning – in fact we must use kilns in countries having cool, moist weather conditions (such as in the U.K.) for most indoor (heated) applications. In the 9iUK the lowest moisture content achievable by air seasoning alone is about 18%.
• Kilns dry timber faster than air seasoning does - so “Rush” orders can be met, and regular supplies can be ensured regardless of the season of the year.

It is often the case that timber is first air seasoned (down to the minimum that can be obtained in this way) and then, if required, kiln dried (the following relates to drying 1 inch (2.54 cm) stock):-

Species	Time (days)
Oak	240 – 360
Beech	150 – 200
Spruces	45 – 90

... and then Kiln Dried (20 - 6%):-

Species	Time (days)
Oak	6 - 16
Beech	5 - 8
Spruces	3 - 5

This can be compared to Kiln Drying only (green to 6%):-

Species	Time (days)
Oak	20-30
Beech	12-15
Spruces	4-7

Table comparing the time taken (in days) to dry 1 inch (2.54 cm) stock. Note, it is usual to air season to 20% and then kiln dry down to 6%.

• Air seasoning is at the mercy of the elements. Correctly controlled, the drying conditions produced and maintained in a kiln reduces losses of timber due to degradation (i.e. drying “degrade” (defects) is reduced). BUT mistakes can be made very rapidly when kilning and can be very expensive!
• Another benefit of kiln drying timber is that kilns can provide sterilization of insect or fungal infected timber. The more rapid drying also reduces the propensity to bio-degrade
• The temperature used in kilns sets or hardens gums and resins which might otherwise cause processing or finishing problems.

However:-

• Kilns are very expensive (high initial investment). They also have high energy costs. Consequently they must be fully utilized – running 24 hours a day, 7 days a week.

However, air seasoning also has it’s costs:-

• Capital is tied up in the necessary large stocks of slowly drying timber.
• The large stocks held must be insured.
• The large yard will incur a purchase or rental cost.
• A lot of expensive stickers are required.

But with regard to kiln drying of timber:-

• Desirable (or undesirable) colour changes may occur using kilns.

However it has been claimed steaming improves the colour of e.g. beech (as well as the other benefits of kiln drying timber such as sterilizing, reducing drying degrade, giving easier machining and better stability).

To realise the full benefits of kiln drying timber it is vital that the kilns are carefully controlled otherwise expensive financial losses can be incurred and the timber might be rendered valueless.

Systems used to Control Timber Drying Kilns

Entirely manual

Here an operator periodically checks the moisture content of the wood and changes to the next schedule stage as necessary. A good, reliable operator is vital!

Semi-automatic

Here the change in drying environment is made at pre-set times (there is no automatic measurement of the moisture content of the wood).

Automatically self correcting systems

Automatically self-correcting systems. Here the moisture content of the wood is measured electrically and the equipment automatically selects the next stage when required.

There is increasing computerisation in industry, and (fortunately) increasing importance being given to quality control.

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Find out How Kilns are Used to Dry Timber and how to avoid common, costly drying defects such as Case Hardening and other value reducing defects.