Cause of High Longitudinal Shrinkage in Wood

Samples were chosen randomly from breast height discs cut from two trees. The samples measured approximately 4 centimeters longitudinally, 1 centimeter tangentially, and 0.15 centimeter radially. Thirty specimens were cut from the earliest formed earlywood and only 14 specimens from latewood. All the samples were cut in the air-dry state with their long axes parallel to the grain. One flat tangential face of each sample was smoothed with fine abrasive paper and four small crosses were inscribed on it to act as reference marks for the measurements. One pair of crosses was placed in the line of the grain and the other pair in a line at right angles to the grain. The distances between the marks were measured using an. S.I.P. “Trioptic” universal measuring machine. The samples were measured first after being soaked in water under vacuum for several hours. They were then slowly dried to the air-dry condition and measured again after being dried under vacuum over phosphorus pentoxide. Microfibril angle was determined from the spread of the (002) arcs on x-ray diffraction diagrams. Longitudinal shrinkage is negligible when the microfibril angle is less than 25? to 30? but as the angle increases above this region there is a very rapid increase in longitudinal shrinkage and a corresponding decrease in tangential shrinkage. At very large microfibril angles such as can occur in the corewood of some conifers, the longitudinal shrinkage is likely to be very high and may even exceed the tangential shrinkage.

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