Pit aspiration

Another chemical method of increasing the permeability of sapwood is to steam the green wood. This technique is frequently used in processing southern pine where it aids in the drying step as well as increasing the permeability. It has been shown that the probable mechanism involved in the change in permeability is acid hydrolysis which reduces the effectiveness of pit aspiration significantly (26). 

The thick cell walls of latewood [mainly caused by a thicker S2 layer in the cell wall (IS)] result in less pit aspiration on drying (ISIS). The main flow of liquids in softwoods is through the lumens of tracheids by way of bordered pit-pairs. 

Radial permeability is not as well understood as longitudinal or tangential permeability and indeed the tree has no path for - or interest in - radial flow except within ray tissue. However, after drying (and pit aspiration) the minimal radial permeability in some species can exceed tangential permeability. The greater radial air-dry permeability for Pinus sylvestris compared to either Picea sitchensis or Pseudotsuga menziesii is attributed, in part, to the pine having many more ray tracheids. 

Pines such as P. radiata are permeable and ean be dried quiekly (Table 8.4a). However their pits aspirate and if subsequently pressure impregnated with an aqueous preservative such as copper-chrome-arsenate using the full eell proeess they eannot be redried nearly as rapidly. If sueh resaturated, preservative treated timber were to be dried using the schedule in Table 8.4a steep moisture gradients would develop leading to severe checking. A milder schedule is required. Treated pine takes approximately twiee as long to dry. 

Thomas RJ and Kringstad KP (1971) The role of hydrogen bonding in pit aspiration. 

This subtle mechanism is vital for trees, but causes some difficulties in wood drying. Indeed, pit aspiration occurs as soon as water is removed from the wood, sometimes even during the heartwood formation. In particular, it is impossible to avoid pit aspiration when drying softwoods under normal conditions. 

FIGURE 40.13 The mechanism of pit aspiration a clever strategy to limit the damage caused by any gas invasion due to injury or cavitation of the sap column. (Adapted from Siau, J.F., Transport Processes in Wood, Springer, BerUn, Germany, 1984.)... 

The strategy of simulating the differences between heart-wood and sapwood lies in only two sets of parameters the permeability and the initial moisture content (for these experiments, 180% for sapwood and 70% for heartwood). The values of permeability used to differentiate sapwood from heartwood (Table 40.8) are based on the considerations concerning pit aspiration. 

Comstock, G.L. and Cote, W.E., 1968. Factors affecting permeability and pit aspiration in coniferous sapwood. Wood Sci. Technol, 2 279-291. 

Meyer, R.W., 1971. Influence of pit aspiration on earlywood permeability of Douglas-Fir, Wood Fiber, 2 328-339. 

Heartwood of Scots pine is more durable than sapwood because it generally contains more extractives and absorbs less water. Heartwood is nearly always much less permeable to water than sapwood due to pit aspiration, obstruction by extractives, and vessel tyloses (outgrowth from an adjacent ray or axial parenchyma cell through a pit cavity in a vessel wall, partially or completely blocking the vessel lumen) [9]. 

Only freeze-drying, or changing the liquid phase for a solvent with low surface tension before drying, allows air-dried samples without pit aspiration to be obtained (Comstock and Cote, 1968 Meyer, 1971 Bolton and Petty, 1978 Fumoto et ak, 1984). The permeability values depend on the species and on the author, but all these data show that air-dried samples, with aspirated pits, have permeability values considerably smaller than unaspirated samples (typically ranging from 1 to 10%). Due to thicker cell walls, smaller pit radii, and more rigid structures, the percentage of aspirated pits is much less in the latewood part of samples (Siau, 1984). 

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