Observation of the "washboard effect" on the surface of quarter-sawn Pinus radiata boards under kiln-drying conditions

Authors: A. Deyev, R. B. Keey
Publication: New Zealand Journal of Forestry, Volume N.Z.J.For. 2001, Issue N.Z.J.For. 46(2) 2001, pp 26-33, Aug 2001
Publisher: New Zealand Institute of Forestry

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Abstract: This paper examines the onset of the "washboard effect" or waviness of the surface of sap wood Pinus radiata boards on kiln-drying at moisture contents above fibre saturation. Green quarter-sawn boards were dried in an air tunnel at 4—4.5 m/s under high-temperature conditions, with the top surface viewed with a video camera. When washboarding appeared, the drying was interrupted to measure the specimen with a high-resolution digital camera. The pressure was measured to produce a depression on the surface of the earlywood of similar size to that produced by the washboarding. The depth of most depressions was approximately 0.8mm, with the average moisture content ranging from 97 to 122%, or about 80% of the initial moisture content in the earlywood rings 7 to 10mm wide. The depressions, however, did not reach the sides of the specimen. The waviness of the surface slowly recovered after further drying.

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