Drying Problems Associated with High Pressure Preservation Treatment of Karri Cross Arms

Seventy-four green crossarms, 3-3/8 by 3-3/8 by 108 inches, of Karri (Eucalyptus diversicolor) were dried under various conditions to bring the moisture content to a level suitable for pressure treatment (about 30 percent). Karri is an impervious, check-susceptible species. Various kiln schedules, presteaming and preboring, chemical seasoning, and end coatings were tried to determine their effect on the shape-holding properties, drying rate, and degrade of the Karri crossarms. True flat-sawn material did not diamond. With bastard-sawn, material diamonding was less when using (low humidity) 110?F dry-bulb temperature and 30?F wet-bulb depression than when using 110?F dry-bulb temperature and 5?F wet-bulb depression (high humidity). A drying temperature of 200?F stopped diamonding but was unacceptable because of internal checking. Drying at 110?F dry-bulb temperature and 30?F wet-bulb depression was about 30 percent faster than drying at 110?F and 5?F wet-bulb depression. Interestingly, there were less severe moisture gradients in the material dried with the 30?F wet-bulb depression. Presteaming improved the drying rate of material dried with a 5?F wet-bulb depression but not the material dried with a 30?F wet-bulb depression. It was concluded that lower humidity masked any effect of presteaming. Preboring of insulation-pin holes reduced kiln-drying time by about 25 percent. Preboring resulted in some ovality of holes during drying and would require drilling of oversize holes or reaming after drying. End coating was not necessary for drying at 110?F and 30?F wet-bulb depression. For higher drying temperature such as 160?F and 50?F wet-bulb depression, a wax end coating prevented end degrade. Cellulose nitrate lacquer was ineffective as an end coating. Sodium chloride and urea chemical treatments were of little value in improving the drying quality. Reconditioning by steaming after kiln-drying affected some recovery in size and shape of the specimens, but reconditioning of preservative-treated material may be of more value.

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