Continued from Case Hardening of Timber
The development of stress as timber is dried can be monitored by cutting a “U” shaped
prong from the end of one of the boards in the kiln (below, left). Soon the arms
of this prong will open out (below right) -
Cutting a test prong from a board
The prong is then left for 24 hours…
Hopefully the arms will have straightened back out because the stresses in the material were not permanent. Kilning can continue.
But if the prongs are found to have bent inwards:
The result of case hardening on a test prong
Unfortunately the timber has been case hardened! Kilning must be slowed down! We might even have to steam the timber.
..and if:-
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The result of reverse case hardening on a test prong
… the timber is reverse case hardened!
Severe case hardening can cause another serious defect known as Honeycombing. This defect is only visible after resawing when checks are seen in the interior of the boards. Oak, beech and eucalyptus are susceptible to this defect.
This defect only arises if an attempt is made to dry wood that is at it’s maximum
moisture content. As the air-
The cells are especially prone to collapse if they are at high temperatures (>70°C) when plasticisation of the cell wall material can occur. Collapse can lead to another defect known as:
Here evidence of collapse is seen as a corrugation or rippling of the surface of the wood. Oaks, Eucalyptus, Western Red Cedar are prone to collapse, especially if they are grown on very wet sites leading to thin walled, weak cells.
To prevent collapse, the wood must be initially dried at cool temperatures (or air-
Currently, research is being carried out which aims to generate gas bubbles in the water in the cells so that these bubbles can expand rather than the cells collapsing. This might be achieved by diffusing chemicals (urea) into the wood or causing air bubbles using low pressures or ultrasonics.
Other seasoning defects can be caused by fungal or chemical stains:
This will occur if suitable conditions arise for sufficient time:
· Blue stain: softwood sapwood.
· Yellow stain: oak, sweet chestnut
Interaction of chemicals
e.g.
· “Ink Spots”: caused by rain dripping off iron roofs onto timber.
· “Water stain” caused by contaminated rain or snow falling onto wood.
Also sodium bicarbonate, used to protect timber from fungi, can itself cause a yellow stain.
In these cases the strength of the timber might not be affected -
Reasons to Dry Timber: An Introduction to Timber Drying
Timber Drying -
Factors controlling the Drying of Wood
The Structures of Softwoods and Hardwoods and their effect on Wood Drying
An Introduction to the Air Seasoning of Timber
Layout of a Timber Drying Yard
Design of Stacks in the Timber Drying Yard
Types of Kiln Drying Equipment
Benefits of Kiln Dried Timber production compared to Air Seasoning Timber
Using a Dehumidifier to Dry Wood
High Temperature Timber Drying
Avoiding Case Hardening by Monitoring the Drying of Timber. Also Collapse & Staining of Timber