Brown stain makes the appearance of ponderosa pine shop lumber look dark, streaky, and splotchy, which often results in lumber downgrading. When brown stain occurs in a piece of shop lumber, it is often sorted separately and ultimately sold for a discount. Manufacturers of ponderosa pine shop lumber have adopted low-temperature dry kiln schedules to mitigate brown stain effects. The low-temperature schedules are lengthy, often taking more than 100 hours of drying time. Preliminary high-temperature (>212°F [100°C]) drying schedules have reduced drying time while minimizing brown stain effects. The purpose of this study was to further explore drying schedules with high temperatures early in the schedules followed by conventional drying settings. Results showed that 10 hours of initial high-temperature drying can be used without brown stain affecting the visual grades of ponderosa pine shop lumber while reducing drying time up to 17 percent.ABSTRACT
Ponderosa pine (Pinus ponderosa) is widely distributed from southern British Columbia to the southern Rocky Mountains in Arizona. Ponderosa pine is an important commercial species that is often sawn into shop- and appearance-grade lumber for finish, furniture, and other secondary forest product industries. Brown stain in pine is usually caused by the migration and subsequent concentration of sugars, proteins, and phenols at or near the lumber surface reacting with nitrogen, wood, and heat during the kiln processes (Kreber et al. 1998, McDonald et al. 2000).
Ponderosa pine kiln dried with high-temperature schedules has been shown to be highly susceptible to brown staining. Kiln operators have developed antistain schedules to minimize brown stain. Antistain schedules are relatively low-temperature schedules, typically less than 160°F (71°C) and are time-consuming and costly. For 6/4 ponderosa pine shop lumber, an antistain schedule can typically result in drying times in excess of 100 hours.
In a review of the literature, Kreber et al. (1999) cited 47 articles that dealt with nonmicrobial stains during kiln drying. They found that brown stain had been reported in many softwood and hardwood species of lumber around the world. Species included oaks (Quercus sp.), pines (Pinus sp.), hemlock (Tsuga sp.), red alder (Alnus rubra), and Douglas-fir (Pseudotsuga menziesii). Much of the work involved radiata pine (Pinus radiata). However, none of the studies they reviewed included ponderosa pine as a test species.
One study showed that an accumulation of sugars occurred on the surface of air-dried Scotch pine (Pinus sylvestris) lumber (Long 1978). Glucose and xylose concentrations at surfaces were 10 times higher than at the center of boards. This finding indicated that sugars likely migrated to lumber surfaces during drying. In another study on Scotch pine lumber, drying schedule was found to have a significant effect on the redistribution of low-molecular-weight sugars (Terziev et al. 1993). With fast drying schedules, significantly higher enrichments of sugars were obtained at wood surfaces than with slower schedules.
In research on radiata pine, it was found that brown stain occurred just under (0.02 to 0.08 in. [0.5 to 2 mm]) the wood surface (Kreber et al. 1998). This was because the surfaces dried quickly and the drying front moved into the wood, not allowing the migration and deposition of sugars and nitrogen-containing compounds to reach the surface. Other research on radiata pine showed that only light to moderate brown stain occurred above 80 percent moisture content (MC) in boards dried at 113°F/95°F (45°C/35°C) dry-bulb/wet-bulb temperatures (Kreber and Haslett 1997). However, brown stain increased as drying temperature increased, and MC decreased from 74 to 12 percent. At 194°F/140°F (90°C/60°C) dry-bulb/wet-bulb temperatures, half the boards had severe brown stain when dried to 58 percent MC.
In previous work on brown stain, we compared a conventional antistain schedule (<170°F [77°C]) to a straight high-temperature schedule (>212°F ([100°C]), a conventional antistain schedule starting and ending in high temperature, and finally a schedule starting with high temperature and ending in a conventional antistain schedule. Our results showed that the initial high-temperature schedule ending with a conventional antistain schedule showed promise to decrease drying time while keeping brown stain at acceptable levels (Wagner et al. 2008).
The objective of the current investigation was to further develop a kiln schedule with an initial high-temperature component that could reduce drying time while not increasing brown stain effects that influence grade, value, color, and lightness to the point of the pine shop lumber being downgraded due to the brown stain.
Materials and Methods
Three kiln schedules were used to dry 6/4 (38-mm) ponderosa pine shop lumber with two replicates per schedule for a total of six runs. Approximately 40 boards 8 feet (2.4 m) long of freshly sawn green lumber were dried in each run. Because the boards had random width, the number of boards per run could not be held constant. The kiln schedules included conventional antistain (CV antistain), 6 hours of initial high temperature changing (within 1 h) to conventional temperature settings (6-h HTCV), and 10 hours initial high temperature changing (within 1 h) to conventional temperature settings (10-h HTCV; see Table 1). Note that for all high-temperature drying, no venting and no steam spray were used. All kiln-drying runs were followed by 8 hours of conditioning. All boards were dried to an average MC of 12 percent. MC was measured with a Wagner L612 moisture meter equipped with a stack probe.
Boards were then planed, graded by a certified mill grader, and examined for color and brightness at Bennett Lumber Products Inc. (Princeton, Idaho). The surface color and lightness of dry fleshly planed boards were determined in accordance with the ASTM D2244‐11 (ASTM International 2013) using a StellarNet EPP2000 UV-VIS spectrometer (190 to 850 nm) krypton light source (SL1; Stellar Net) with a diffuse reflectance fiber-optic probe. The spectrometer was calibrated with a RS50 white standard (>97% reflectance). The spectrometer transformed spectral data into lightness data (L) and chromaticity coordinates (redness [a] and yellowness [b]) on the basis of a D65 light source (CIE 1986). A higher L meant that the sample was lighter, and a lower L meant that the sample was darker. Black would have a reading of 0 percent, and white would have a reading of 100 percent. Eight measurements were taken on each board at locations clear of defects, such as knots and pitch streaks. The occurrence of brown stain was not considered when locating positions for measurements. Analysis of variance (SPSS v17 software) was used to test for differences between mean lightness (L), redness (a), and yellowness (b) of all boards, from duplicate runs, in each schedule.
Results and Discussion
Average dry kiln schedules times were 72, 64, and 60 hours for CV antistain, 6 hours HTCV, and 10 hours HTCV, respectively. The initial high-temperature schedules (6-h HTCV and 10-h HTCV) decreased the average drying times up to 17 percent, which will improve mill productivity.
The certified Bennett Lumber Products Inc. grader determined that the occurrence of brown stain resulting in downgraded dry finished boards was limited to one board in the CV antistain schedule and one board in the 10-hour HTCV schedule, respectively. Brown stain was observed, after scraping the surface, as a discrete band about 1 mm below the surface of the lumber. This observation was consistent with the literature (Kreber et al. 1998, McDonald et al. 2000, Wagner et al. 2008).
Due to the secondary manufacture nature of pine shop lumber, we felt it was important to further explore the color and lightness of the shop-grade boards after they were finished planed and graded by a certified pine shop grader. As shown in Figure, 1 the dried planed mean absolute lightnesses for the three kiln schedules were compared and analyzed. The resulting data showed that the mean absolute lightness of the 6-hour HTCV schedule boards was about 2 percent darker than the CV antistain schedule boards and that 10-hour HTCV schedule boards were about 5 percent darker than the CV schedule boards. These results support our previous findings and that of the literature that darkening of the wood occurs at higher kiln temperatures and extended drying times due to thermal degradation (Kreber et al. 1999). An analysis of variance (α = 0.05) indicated that the CV antistain (L = 68.8, σ = 2.3) schedule for dried planed mean absolute lightness was significantly lighter than the 6-hour HTCV (L = 67.6, σ = 2.0) and 10-hour HTCV (L = 65.4, σ = 2.5) schedules. Analysis of variance (α = 0.05) also showed a statistical difference between the 6-hour HTCV and 10-hour HTCV schedules for dried planed mean absolute lightness.
Citation: Forest Products Journal 63, 1-2; 10.13073/FPJ-D-12-00054
To further investigate color change brought on by the duration of high-temperature drying, we measured the redness and yellowness of the pine boards. As Figure 2 shows, the a redness color element for the CV antistain (a = 3.7, σ = 1.2) and 6-hour HTCV schedules were approximately 17 percent less than the 10-hour HTCV (a = 4.4, σ = 1.4) schedule. Analysis of variance (α = 0.05) showed the CV antistain schedule for the a redness color element was significantly different from the 10-hour HTCV but not different from the 6-hour HTCV. Figure 3 shows an increase in the b yellowness color element for CV antistain (b = 32.0, σ = 2.7) to 10-hour HTCV (b = 32.5, σ = 2.9), but the analysis of variance (α = 0.05) showed that there were no significant differences seen for the b yellowness color element of dried planed CV antistain, 6-hour HTCV, or 10-hour HTCV schedules.
Citation: Forest Products Journal 63, 1-2; 10.13073/FPJ-D-12-00054
Citation: Forest Products Journal 63, 1-2; 10.13073/FPJ-D-12-00054
Conclusions
This research supports our hypothesis that high temperatures (above the boiling point) can be used early in shop-grade ponderosa pine drying schedules without substantially impacting lightness, color, value, or grade of the shop lumber to the degree a certified mill grader would downgrade the pine shop wood. However, this study also indicated that color changes and a darkening of the pine shop-grade wood were possible and likely with the addition of high temperatures at the beginning of a ponderosa pine shop-grade kiln drying schedule. Considering the results of the certified grader and spectrometer, we conclude that high temperatures above the boiling point (up to 240°F) can be used up to 10 hours at the start of the kiln drying process for ponderosa pine shop lumber to reduce drying times, achieving higher throughput during the lumber manufacturing process.
Lightness (L) of kiln-dried planed boards using different schedules. CV = conventional antistain; 6 h = 6 hours of initial high temperature changing (within 1 h) to conventional temperature settings; 10 h = 10 hours of initial high temperature changing (within 1 h) to conventional temperature settings.
Redness (a) of kiln-dried planed boards using different schedules. CV = conventional antistain; 6 h = 6 hours of initial high temperature changing (within 1 h) to conventional temperature settings; 10 h = 10 hours of initial high temperature changing (within 1 h) to conventional temperature settings.
Yellowness (b) of kiln-dried planed boards using different schedules. CV = conventional antistain; 6 h = 6 hours of initial high temperature changing (within 1 h) to conventional temperature settings; 10 h = 10 hours of initial high temperature changing (within 1 h) to conventional temperature settings.
Contributor Notes
The authors are, respectively, Research Technician, Professor Emeritus, and Professor, Renewable Materials Program, Dept. of Forest, Rangeland and Fire Sci., Univ. of Idaho, Moscow (mott8849@yahoo.com, fwagner@uidaho.edu, armandm@uidaho.edu [corresponding author]). This paper was received for publication in May 2012. Article no. 12-00054.