This manual is intended to serve as an educational resource and working tool for people actively involved in the drying of Spruce-Pine-Fir (SPF) lumber. The manual covers subject matter from the basic principles of drying through to the application of techniques specific to the drying of this species group. The range and depth of information presented has been selected to meet these objectives.
Disponible en français: https://library.fpinnovations.ca/en/permalink/fpipub7507
The recent interest in borate treatment for the production of termite-resistant lumber has led to the need for improved treatment processes. Pre-steaming prior to pressure treatment was known to have a number of potential benefits in terms of improved permeability, moisture distribution and effectiveness of the vacuum. This process was therefore tried on western hemlock dodai (105 mm squares) in an attempt to achieve 80% cross sectional penetration with a minimal diffusion period after pressure treatment. Western hemlock pre-steamed to a core temperature of 82 degrees C in four hours took up almost double the amount of treating solution of end-matched unsteamed samples. There was an improvement in mean heartwood penetration of 45% immediately after treatment and a 134% increase in penetration after one week storage. This was not entirely due to diffusion within the wood but to mass flow of treating solution continuing after the end of the pressure process. After one week storage 64% of samples had 80% of the cross section penetrated. Reducing the vacuum time from 30 minutes to zero had a detrimental effect on penetration. Increasing time under vacuum to 60 minutes provided no beneficial effect. Pre-steaming of hemlock dodai appears to be a very effective means of improving uptake during pressure treatment.
To retain access to European markets all green softwood lumber except cedar will have to be heat treated to a core temperature of 56°C. To reduce processing costs it may be possible to use thinner strips between rows of lumber than used in kiln drying, so that packages could be shipped with strips in place. This would eliminate unstickering and repackaging after treatment. To determine the effect of strip thickness on heating time, laboratory scale runs using 63 mm green hem-fir were compared using stickers 3, 6, 9.5 and 19 mm in thickness. Heating times increased as sticker thickness decreased. The 3 mm stickers produce too great a resistance to air flow and are not recommended. An increase in heating time of only 25 percent was found with 6 mm stickers compared to the standard 19 mm stickers. Further mill scale evaluation would determine if the costs associated with longer heating times with 6 mm stickers would be offset by the savings resulting from increased volume per kiln charge, reduced handling and repackaging costs.