Avoiding Case Hardening by Monitoring the Drying of Timber. Also Collapse & Staining of Timber

Avoiding Case Hardening by Monitoring the Drying 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) - this is to be expected since as wood dries the outer areas will be put into tension (and the inner regions into compression):-

Image showing how a prong is cut from a board to test for case hardening - prongs have bent inwards — *Reverse case hardened timber in cross section*

The prong is then left for 24 hours…

Hopefully the arms or the prong 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:-

Image showing prongs of test piece have bent inwards showing case hardening — *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:-

Image showing prong arms bent outwards indicating reverse case hardening — *Prongs of test piece bend outwards indicating reverse case hardening*

… 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.

Collapse & Staining of Timber

Collapse

This defect only arises if an attempt is made to dry wood that is at it’s maximum moisture content. As the air-water menisci recede into cells full of water they must pass through the very fine pores in the “pits” inter-connecting the cells, and this generates high tensile stresses in the water which causes the cells to implode. (the same mechanism is responsible for the “aspiration” – closing – of bordered pits).

The cells are especially prone to collapse if they are at high temperatures (more than 70C) when plasticisation of the cell wall material can occur. Collapse can lead to another defect known as:-

Wash-boarding

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-season), since once the cells start to lose water collapse cannot occur. Sometimes it is possible to reform the imploded cells by steaming.

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 timber drying defects

Other seasoning defects can be caused by fungal or chemical stains:-

Fungal staining

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.
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 - but the value might well be!

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