Kiln drying
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WOOD WARPING NIGHTMARES...........

Improperly dried or stored wood may suffer from cracks and deformations due to shrinkage, while it may twist, bow, crook or warp caused by a combination of shrinkage and swelling. This is caused by the moisture content (MC) of the wood. The higher this content the greater the chance that one or more of these negative effects will occur. In addition wood taken from the outer rings of the log (called sapwood) is more sensitive to distortion than wood taken form the core of the log (heart wood). 

 

EFFECTS OF THE PROBLEM RESULTING FROM IMPROPER DRYING OR STORAGE.

Distortion of flat and square wood as affected by the annual rings. The white boxes indicate from which part of the log the wood was cut.  The figures A thru C show the shrinkage in drying from green to oven-dry condition.    

 

A = boards taken from the outer rings of the log (sapwood) 

B = square beam taken from the outer rings of the log (sapwood)

C = board taken from the core of the log (heart wood)

D = square beam taken from core of the log (heart wood) 

 

The moisture content is measured as the ratio of the weight of the free water in a given piece of wood to the weight of the wood when it is completely dry (oven dried, or kiln dried) and is usually expressed as a percentage.

The "green" wood of a freshly felled tree may have an MC anywhere in the range 30% to over 200%, depending on the species (hardwood (1) is far less receptive for a high MC than soft wood). Almost all of his water must be removed from the wood before it is fit to be used for its particular intended purpose (for instance; wood for furniture shall be dryer than wood for railway ties).

(1) Teak, Merbau, Iron wood, Bangkirai, Nyatoh,  Kempas, Camphor and Keruwing are the hardwoods that we use. Coconut wood is a soft wood.

Wood for structural purposes is considered dry when a moisture content lower than 20% is achieved. This implies that all of the free water has been removed and only bound water remains, starting some 10% below the fiber saturation point. (Saturation point is defined as the boundary between free water and bound water, which is around 30%). 

 

FREE WATER AND BOUND WATER IN WOOD

Cracks and warping will occur in the saturation zone (between 30% and 100%, shown in red), NOT in the bound water zone (yellow), provided the wood is properly dried in an autoclave (kiln dried), a process that may take 4 to 7 weeks, depending on the type of wood. Air drying, however, is largely preferred over kiln drying. The set back is that such will take significantly longer, a process which we often use during the progress of prefabrication when a until takes months to built.   

Furthermore, wood is a cellulose material which behaves somewhat like a sponge, so that even wood which has been kiln dried down to say 7% may in fact later reabsorb water from the atmosphere so as to reach an equilibrium state. Actually all wood is constantly gaining or losing water to or from the environment, in other words, the moisture content of wood changes as the relative humidity changes. Coats of varnish or paint can slow the process but cannot stop it.

As the moisture content of improperly dried wood (or green wood) changes, so does the wood expand or contract, potentially producing all manner of disastrous defects (wood warping and wood cracking nightmares).

Most of the wood for our bungalows and cottages is kiln dried to an MC of 12%. When arriving at destination this MC may have increased to a maximum of 20% depending on the humidity of the environment. This MC percentage of 20% is still 10% lower than saturation point, thus in the bound water (technically dry) zone and in conclusion safe against cracking and warping.

NOTE: When building in an seasonal environment interspersed with low humidity and low temperatures in winter and higher humidity and high temperatures in summer, all boards must have an MC of 10%, whereas beams and columns shall be dried up to 15 % to 20%. When this is not achieved boards will crack and warp, while beams and columns will crack and/or deform.

 

 

 

 

 

 

 

 

 

 

 

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Last modified: October 01, 2011
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