Impact of juvenile wood on hemlock timber drying characteristics.
Large volumes of relatively small-diameter logs are currently harvested from sustainable sources such as the second-growth coastal forests of British Columbia. The percentage of juvenile wood in these trees is higher compared to limited old-growth wood supplies. Hence, this investigation evaluated the drying quality of green 115 mm square timbers from second-growth mixed coastal forests of hemlock baby-squares, under the influence of juvenile wood presence shown with a pith location at least on one of their end-surfaces, the tree harvesting season and drying target MC. Timber specimens were classified into four groups depending on the presence and location of the tree pith. They were dried in a laboratory conventional kiln in four drying runs to 15 percent and 20 percent target MCs, with that were cut either in the summer or fall season. The results revealed that specimens with the pith shown in the center exhibited high propensity for bow, twist and surface checks, due to variable shrinkage coupling of juvenile and mature wood areas. Specimens with the pith shown close to one of the sides in the cross section had a lower, but acceptable quality according to commercial grading rules, opposite to specimens with the centrally located pith.
In the era of increased degradation of the natural environment, the old-growth forested areas as a natural source of wood, are obtaining protected status. At the same time, growth in total wood consumption closely follows human population growth (Hammett and Youngs 2002). For that reason, in recent years, wood manufacturers have been trying to find rational solutions for the problems of processing smalldiameter wood from second-growth and ecologically suitable plantation forests (LeVan-Green and Livingston 2001). This type of tree is harvested at a younger age, and because the growth is generally greatest during the formation of juvenile wood, its core may represent a larger proportion of a smaller diameter of modern log supplies compared with old-growth trees (Kretschmann et al. 1993). Juvenile wood typically has characteristics that negatively impact a number of wood properties. The greatest concerns regarding the presence of juvenile wood in solid products are focused on its behavior under changing MC, mainly due to its inconsistent density and higher longitudinal shrinkage as a result of greater fibril angle in the [S.sub.2] layer of cell wall (Cave 1976). In spite of all their differences, it is almost impossible to separate juvenile from mature wood by visual inspection. The only significant visual indicator of the presence of juvenile wood in a piece of wood is the presence of the pith as the center of the log.
The modern wood-drying concept should be a strong component in current issues regarding the rational use of natural resources and the reduction of material losses. Unfortunately, the current kiln-drying approach to new wood supplies is basically the same as the strategy used for old-growth wood. The constant shift toward the use of smaller diameter logs requires different strategies from those used and acceptable during the era of large old-growth supplies (Smith and Briggs 1985).
Canada accounts for approximately 48 percent of all international trade in softwood lumber products (Council of Forest Industries 2000), and British Columbia is the source of more than 50 percent of that production (Morton and Greenwood 2004). The Ministry of Finance and Corporate Relations (2001) reported that approximately 95 percent of BC coastal sawmills' remanufacturing production is exported to Japan. Using the commercial name "baby-square", they manufacture Pacific Coast Hemlock (PCH) to the standard 105 by 105 mm cross section.
In this context, the objective of our study is to evaluate the drying quality of green baby-squares from second-growth mixed coastal forests of hemlock, under the influence of juvenile wood presence shown with a pith location at least on one of their end-surfaces, the tree harvesting season and dry target MC. This kind of study on PCH baby-squares, one of BC's most profitable wood products, is an important contribution to the knowledge required for the utilization of natural resources.
Materials and methods
Green timbers of mixed western hemlock (Tsuga heterophylla) and amabilis fir (Abies amabilis), commercially known as PCH, obtained from a local sawmill were used in all four test drying runs. Specimens for the first two runs were acquired during the hot and dry summer period (S), and for the last two runs during the rainy autumn period (F) of 2004. Runs one and four were randomly selected to be dried to target MC ([M.sub.t]) of 15 percent (15S or 15F) whereas runs two and three to 20 percent (20S or 20F) shown in Table 1. The four-run sample size was 640 pieces, and for each run 160 specimens were randomly selected. In every run four different pith locations classes (PL) were determined to visually represent the presence of juvenile wood. The classification of specimens regarding the presence and location of the tree pith within the square area of the end cross section is shown in Figure 1.
[FIGURE 1 OMITTED]
Rough green specimens were 114 by 114 mm in cross section and slightly over 3 m in length. Before drying, all specimens were trimmed to 2.44 m long (Fig. 2). One 25.4 mm thick slab (cookie) was cut from each end, next to the trimmed end (Fig. 2), for determination of basic density and initial MC ([M.sub.i]), weight was immediately measured with a digital balance (resolution 0.01 g) and the green volume by the water displacement method. The cookies were then ovendried at 103 [+ or -] 2[degrees]C for 24 hours and ovendry weights were immediately measured with the electronic balance. [M.sub.i] was calculated as dry-basis, and basic density was calculated as the ratio of the ovendry weight to the green volume. An average value of two cookies from each specimen was considered as the average [M.sub.i] value of the particular kiln specimen.
[FIGURE 2 OMITTED]
The kiln specimens were loaded into the steam-heated laboratory kiln with their individual location completely randomized within each charge with location and orientation noted on digital photographs. The drying schedule used for this study is shown in Table 2. Upon completion of each run, the wood was left to cool down in the closed kiln for 12 hours, followed by after-drying evaluation and planing in a local custom planer mill to 105 by 105 mm in cross section. Thereafter, each specimen was graded by a professional lumber grader according to the CFLA E120 visual grading procedure.
Each specimen was weighed before and after drying. Based on [M.sub.i] and the weight difference the final MC ([M.sub.f]) was calculated. To calculate shrinkage, the width and thickness of each specimen was measured before and after drying, at the same midlength point by using a pair of electronic digital calipers (0.01 mm resolution). Shape distortions (twist, bow, and diamonding) and the total length of checks on each side of a specimen were measured before and after drying, and after planing. Deformations were measured on a shop-built aluminum table calibrated for straightness and flatness, using the methodology explained by Hao and Avramidis (2004), and checks using the Starrett C1-8M8 measuring tape.
Results and discussion
The values for basic densities, [M.sub.i] and [M.sub.f] are listed in Table 3. Basic density was similar to results obtained by other researchers (Zhang et al. 1996, Avramidis and Hao 2004) and the analysis of variance (Table 4) did not show significant difference between drying runs or PL. Initial MC of freshly cut PCH timbers was previously reported to range from 26 percent to 60 percent (Oliveira and Wallace 2001) and 58 percent to 72 percent (Avramidis and Hao 2004), and the similar relatively wide range was noticed in our results, namely, 41.6 percent to 79.2 percent (Table 3). The analysis in Table 4 shows that [M.sub.i] of S specimens was lower compared to F specimens. Timbers with the pith shown on their end cross sections are often considered to be low-quality timbers, so they are stored in an open area where they are not protected from environmental conditions. However, with proper storage procedures, this variability could be lower. Also, the Bonferroni multiple comparison test was able to detect significantly higher [M.sub.i] of the PL4 compared with the other three classes. A possible explanation could be that because of their location (close to the center of a log), the first three classes of specimens are sapwood free. In the mature class, there is a probability for sapwood presence, which is expected to have much higher green MC than heartwood. Nielson et al. (1985) reported that an average [M.sub.i] of western hemlock heartwood could be three times lower compared with sapwood.
Focusing on individual runs, the well-known lack of uniformity in [M.sub.f] of PCH timber (Kozlik and Hamlin 1972, Zhang et al. 1996) could be observed through the SDs shown in Table 3. Kozlik and Hamlin (1972) indicated that this variability is attributed directly to wet pockets. The analysis of variance listed in Table 4 shows significant differences in [M.sub.f] between different [M.sub.t].
The average volumetric shrinkage for each pith class per drying run is plotted in Figure 3. In drying runs with an [M.sub.t] of 15 percent, the average volumetric shrinkage was 5.06 percent with a SD of 1.91 percent, and for drying runs with an [M.sub.t] of 20 percent it was 4.00 percent and 1.58 percent, respectively. These shrinkages are similar to values previously reported by Avramidis and Hao (2004) and Zhang et al. (1996). The analysis of variance listed in Table 4 shows a significant interaction between the cutting season and [M.sub.t] on the volumetric shrinkage and the Bonferroni's multiple comparison test shows that run 15S had significantly greater volumetric shrinkage from the run 20S and that run 15F had significantly greater shrinkage from all other runs.
[FIGURE 3 OMITTED]
The trend of [M.sub.i] in the interaction was unexpected because of the fact that shrinkage starts below the fiber saturation point (FSP), and in this study both cutting seasons had an average [M.sub.i] above FSP. Nevertheless, it should be kept in mind that in green lumber, because of the surface air-drying, a certain volume is below FSP and shrinkage could begin before kiln-drying, even when average MC is above FSP (Simpson and TenWolde 1999). With a lower [M.sub.i], this prekiln-drying shrinkage will be higher and it will not appear in the kiln-drying timber evaluation. Consequently, the last run as the run with the largest difference between [M.sub.i] and [M.sub.t] showed a significantly greater volumetric shrinkage compared to all other runs.
For shape distortions, the influence of controlled factors was tested on values that were developed during kiln-drying. These values for bow are plotted in Figure 4. Every PL and drying run shows positive and negative changes in shape distortions. Negative changes come from specimens where the weight of the load was able to reduce the initial deformation during the kiln-drying. As seen in the analysis of variance listed in Table 5, the study reveals a significant influence of PL on an average bow developed during the kiln-drying. The Bonferroni multiple comparison test shows that PL1 has a significantly larger bow difference from PL3 and PL4; and PL2 has a significantly larger bow difference from PL4. These results confirmed the fact that juvenile wood has a higher propensity for bow (Danborg 1994). The reason for this is the nature of gradual change in properties within juvenile wood. These changes include normalization of the fibril angle in the $2 layer of wood cells from the pith toward mature wood. This angle is the main reason for the higher longitudinal shrinkage of juvenile wood that cause bow. Consequently, juvenile wood in PL3 will have smaller bow when compared to juvenile wood in PL1. The kiln-drying process was insignificant for the development of bow in PL4.
[FIGURE 4 OMITTED]
Together with bow, twist was also a significant defect that impacted drying quality. Differences between green and dried rough twist are plotted in Figure 5. Similar to bow, some specimens randomly exhibited negative changes during the kiln-drying. The analysis of variance (Table 5) shows significant evidence for the interaction between the cutting season and PL, as well as the interaction between [M.sub.t] and PL. The Bonferroni multiple comparison test shows that twist differences in PL1 from the summer cutting season are significantly larger compared to PL2 from the summer cutting season; and in PL2 from the fall cutting season compared to PL2 from the summer cutting season and PL3 from the fall cutting season. Also, the test detected significant larger twist differences in PL1 dried to 15 percent compared to PL4 dried to 15 percent and all pith location classes dried to 20 percent; in PL2 dried to 15 percent compared to all PL classes dried to 20 percent; and in PL3 dried to 15 percent compared to PL2 dried to 20 percent and PL3 dried to 20 percent. Appearance of PL as a factor in two significant interactions could indicate that specimens with juvenile wood have a higher tendency for twist. One reason could be their higher slope of grain, but another reason could be the fact that specimens near the pith have larger annual ring curvature when compared with mature wood. Because of the high tangential shrinkage, this curvature could be a factor for the higher twist.
[FIGURE 5 OMITTED]
There is also a possibility that an asymmetric appearance of juvenile wood inside of a specimen, with its larger longitudinal shrinkage, could be one of the reasons. Its symmetrical position will more likely have a result in bow rather than twist, but its diagonal position could also deform a thick specimen into an asymmetric form such as a twist. Nevertheless, it must be said that the presence of PL as one of the factors in two significant interactions indicates that it is incorrect to treat it as a separate main effect, but it cannot be ignored that these interactions are probably related with influences of these factors on the shrinkage of juvenile wood. Summer cutting season will have a lower [M.sub.i] compared with the fall cutting season, so smaller shrinkage will occur during the kiln-drying period because a certain amount of wood mass could shrink even before the kiln-drying, and therefore this value will not be reported in a kiln-drying evaluation. For that reason, S specimens will result in a smaller kiln-drying twist compared to F specimens. In the same manner, a higher [M.sub.t] will develop a smaller twist. Both cases are influenced by the amount of juvenile wood on the cross section, or the pith location as its indicator. All this could be seen in Figure 4 by observing PL1, which is the class with the maximum amount of juvenile wood. PL2 and PL3 are not so representative because they have juvenile wood located on one of the specimen's sides, and this position will more likely develop bow instead of twist. This study indicates that twist is obviously a complex issue and more variables should be controlled in order to provide a clear picture about its behavior during the kiln-drying of PCH post-and-beam construction timbers. An option for scanning the internal configuration of specimens prior to kiln-drying could improve chances for research results.
Diamonding is usually the shape distortion with the smallest amplitude of all PCH kiln-drying distortions, what could be seen in our results as well (Fig. 6). The analysis of variance in Table 5 indicates a lack of evidence to claim a significant influence of any of the controlled factors on the development of diamonding during the kiln-drying process.
[FIGURE 6 OMITTED]
The analysis of surface checks was based on the sum of check lengths from all four sides of a specimen. Figure 7 represent the differences between green values and values recorded after the kiln-drying. The analysis in Table 5 did not detect significant influence of the cutting season on checking development, but it shows a significantly larger development of checks in the drying runs dried to the [M.sub.t] of 15 percent, compared with runs dried to the [M.sub.t] of 20 percent. The test also detects a significant individual main effect of the pith location. The Bonferroni multiple comparison test shows significant longer checking in PL1 compared to other classes; and in PL2 compared to classes 3 and 4. This reveals that the surface checks clearly increased when a specimen was located more centrally in the log. The reasons could be the fact that specimens near the pith have larger annual ring curvatures and a higher tangential shrinkage when comparing it to mature wood. Additional problems include faster drying and the weaker structure of juvenile wood, because of its lower density. Moreover, it is expected that the initial stages of the drying process are the most critical period since shrinkage of the surface layers of the specimen is restrained by the wetter core material. High superficial tension stresses may then lead to surface checking, if the strength of the specimen perpendicular to the grain is exceeded. For that reason, less aggressive drying in the initial stages could lead to a reduction of surface checking.
[FIGURE 7 OMITTED]
The planing process was able to reduce rough shape distortions and surface checks to a certain level. Bow is on average reduced by 11 percent, twist by 8 percent, diamonding by 48 percent and surface checks by 22 percent. A paired t-test (Table 6) is used to estimate the significance of these changes, and it shows that the reductions are significant only in the case of diamonding and surface checks, so it means that this operation was not a solution for our two most important drying problems.
The CFLA's E120 grading classification is a very important indicator that shows the market value of PCH structural products. Based on the grading results of an official CFLA grader, the contingency table was formed of PL classes, with the number of specimens which fit, or do not fit the level of drying quality for the E120 standard (Table 7). PL1 had the most problems in terms of meeting the export requirements. Chi-square test confirmed the conclusion that this class significantly decreases grading results. In this class, 28.75 percent of specimens were not acceptable for the E120 standard. This percentage was twice as large when compared with other classes, where it ranged from 13.75 to 14.38 percent. This conclusion about PL1 is synchronized with the previous results in this study. It was evident that for the downgrading of specimens, bow was the most prevalent attribute of the drying quality; however, many specimens were down-graded from the combined effect of two or more forms of shape distortions or surface checks. Although the level of degradation may be higher for a lower [M.sub.t], there is a relatively small difference (1.87%) between the runs dried to [M.sub.t] of 15 percent and the runs dried to [M.sub.t] of 20 percent.
The following conclusions can be drawn in the light of this investigation:
1. The results indicate that PCH baby-squares with variable pith presence and location on at least one of the end cross sections have similar basic density.
2. A comprehensive analysis of the MC indicated that average initial MC was affected by the tree harvesting season and pith location. Specimens from summer harvesting showed lower initial MC when compared with the fall harvested ones. Specimens located less than 30 mm from the tree pith had lower average initial MC than other specimens.
3. Final MC was affected by the target MC, but it was not affected by the cutting season or pith location.
4. If both the initial MC and final MC were significantly different between drying runs, the volumetric shrinkage was attributed to the interaction of the cutting season and target MC. Their wider range resulted in a higher shrinkage. The pith location did not necessarily have an influence on the volumetric shrinkage.
5. Pith location affects bow. Specimens with the pith shown centrally on at least one of the end cross sections, demonstrated a significantly higher propensity to bow, compared with specimens without the pith showing. Specimens with the pith shown near one of the sides on at least one of the end cross sections, demonstrated a higher bow from the mature wood specimens.
6. Kiln-dried twist could be attributed to two interactions: the interaction between the cutting season and pith location, and the target MC and pith location. Any attempt to interpret any of the controlled factors separately is not usually statistically valid; however, this study shows that this method can be an invaluable way to interpret the data.
7. There is a lack of evidence to claim an effect of the pith location, cutting season or target MC on kiln-dried diamonding.
8. Analysis of surface checking developed during drying demonstrates that the target MC and pith location are significant main effects. The lower target MC increased surface checks. This study provides the evidence that a baby-square with the pith in the center of at least one of the end cross sections develops the longest surface checks. Moreover, it shows that the surface checks are shorter if the pith is shown on a cross section near one of its sides and even shorter if the pith is outside of the cross sections.
In general, PCH timbers with the pith shown in the center on at least one of the end cross sections should not be kiln-dried because of a high propensity to drying defect formation. Their presence in a kiln load will result in an unnecessary waste of energy and a decrease in drying capacity. On the other hand, timbers with the pith shown close to one of the sides of at least one of the end cross sections will have a lower, but generally acceptable quality. Treating them in the same manner as previous types of timbers and sorting them as "out-of-class timber" before kiln-drying results in a loss of profit.
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Slobodan Bradic Stavros Avramidis *
The authors are, respectively, MS Graduate Student and Professor, Dept. of Wood Sci., The Univ. of British Columbia, Vancouver, BC, Canada (email@example.com). This paper was received for publication in February 2006. Article No. 10163.
* Forest Products Society Member.
Table 1.--Details of the four experimental drying runs. Run Cut [M.sub.t] (%) 15S July, 2004 15 20S August, 2004 20 20F November, 2004 20 15F November, 2004 15 Table 2.--Drying schedule used in the four kiln-drying runs. Step Time [T.sub.db] (a) [T.sub.wb] (a) (hrs) ([degrees]C) 1 6 49.1 49.0 2 24 52.0 51.1 3 24 55.0 52.8 4 24 58.1 54.9 5 24 62.0 57.0 6 24 66.0 59.0 7 24 70.2 60.6 8 24 74.0 62.9 9 Last until M = 15% 78.0 65.1 10 12 72.0 69.0 Air Fan Step [M.sub.EMC] (c) velocity reversal (%) (m/s) (hrs) 1 25.0 3 3 2 21.5 3 12 3 17.5 3 12 4 15.6 3 12 5 12.6 3 12 6 10.6 3 12 7 8.6 3 12 8 7.8 3 12 9 7.0 3 12 10 15.1 3 6 (a) Dry-bulb temp. (b) Wet-bulb temp. (c) Equilibrium MC. Table 3.--Basic density, initial and final MC of different pith classes and drying runs. Basic density [M.sub.i] [M.sub.f] Mean SD Mean SD Mean SD (kg/ [m.sup.3]) (%) 15-S Class 1 404 44 41.62 18.17 14.87 3.78 Class 2 411 39 42.38 17.64 15.10 3.59 Class 3 405 43 41.86 16.66 14.79 3.77 Class 4 405 46 53.25 22.88 15.81 4.40 Total run 406 43 44.78 19.44 15.14 3.88 15-F Class 1 399 59 69.43 16.69 15.46 4.12 Class 2 387 52 75.54 19.17 15.81 5.14 Class 3 401 43 73.20 16.56 15.28 4.04 Class 4 398 49 79.19 20.27 16.32 4.53 Total run 396 51 74.34 18.42 15.72 4.45 20-S Class 1 401 47 54.11 19.77 19.85 6.00 Class 2 408 46 50.57 17.25 18.87 4.37 Class 3 399 49 54.83 23.32 21.37 7.44 Class 4 404 58 58.26 25.90 20.21 6.88 Total run 403 50 54.44 21.18 20.08 6.29 20-F Class 1 406 36 64.45 17.80 20.30 5.23 Class 2 395 53 64.73 22.24 20.33 3.82 Class 3 399 46 69.51 23.96 20.71 4.92 Class 4 404 52 77.41 28.18 21.27 4.49 Total run 401 47 69.02 23.71 20.65 4.62 Table 4.--Analysis of variance for basic density, initial MC, final MC and volumetric shrinkage. Source of variation DF Sum of squares Mean square Basic density Drying run 3 8,294 2,765 PL 3 608 203 Drying run by PL 9 10,454 1162 Experimental error 624 1,440,167 2,308 Initial MC Cutting season 1 77,943 77,943 PL 3 9,170 3,057 Cutting season by PL 3 503 168 Experimental error 632 278,912 441 Final MC Cutting season 1 53.29 53.29 [M.sub.t] 1 3,895.30 3,895.29 PL 3 77.87 25.95 Cutting season by [M.sub.t] 1 0.00 0.00 Cutting season by PL 3 30.09 10.03 [M.sub.t] by PL 3 73.39 24.46 Cutting season by [M.sub.t] by PL 3 22.21 7.40 Exp. Error 624 15,018 24.07 Volumetric shrinkage Cutting season 1 91.92 91.92 [M.sub.t] 1 180.93 180.93 PL 3 8.93 2.98 Cutting season by [M.sub.t] 1 50.04 50.04 Cutting season by PL 3 5.20 1.73 [M.sub.t] by PL 3 6.64 2.21 Cutting season by [M.sub.t] by PL 3 12.37 4.12 Exp. error 624 1,794.40 2.88 Source of variation F calculated F critical Drying run 1.20 2.63 PL 0.09 2.63 Drying run by PL 0.50 1.91 Experimental error Initial MC Cutting season 176.62 (a) 3.87 PL 6.93 (a) 2.63 Cutting season by PL 0.38 2.63 Experimental error Final MC Cutting season 2.21 3.87 [M.sub.t] 161.85 (a) 3.87 PL 1.08 2.63 Cutting season by [M.sub.t] 0.00 3.87 Cutting season by PL 0.42 2.63 [M.sub.t] by PL 1.02 2.63 Cutting season by [M.sub.t] by PL 0.31 2.63 Exp. Error Volumetric shrinkage Cutting season 31.96 (a) 3.87 [M.sub.t] 62.92 (a) 3.87 PL 1.04 2.63 Cutting season by [M.sub.t] 17.40 (a) 3.87 Cutting season by PL 0.60 2.63 [M.sub.t] by PL 0.77 2.63 Cutting season by [M.sub.t] by PL 1.43 2.64 Exp. error (a) Term significant at [alpha] = 0.05 Table 5.--Analysis of variance for shape distortions and surface checks created during kiln-drying. Sum of Source of variation DF squares Mean square Bow differences created during kiln--drying --Cutting season 1 5.66 5.66 --[M.sub.t] 1 8.27 8.27 --PL 3 119.37 39.79 --Cutting season by [M.sub.t] 1 2.91 2.91 --Cutting season by PL 3 2.09 0.70 --[M.sub.t] x PL 3 19.53 6.51 --Cutting season by [M.sub.t] by PL 3 13.62 4.54 --Exp. Error 624 2,953.39 4.73 Twist differences created during kiln-drying --Cutting season 1 2.24 2.24 --[M.sub.t] 1 126.80 126.80 --PL 3 19.69 6.56 --Cutting season by [M.sub.t] 1 11.89 11.89 --Cutting season by PL 3 54.18 18.06 --[M.sub.t] by PL 3 33.59 11.20 --Cutting season by [M.sub.t] by PL 3 21.04 7.01 --Exp. error 624 2,059.56 3.30 Diamonding differences created during kiln-drying --Cutting season 1 0.12 0.12 --[M.sub.t] 1 0.00 0.00 --PL 3 3.19 1.06 --Cutting season by [M.sub.t] 1 0.00 0.00 --Cutting season by PL 3 0.45 0.15 --[M.sub.t] by PL 3 1.34 0.45 --Cutting season by [M.sub.t] by PL 3 5.20 1.73 --Exp. error 624 411.14 0.66 Surface check differences created during kiln-drying --Cutting season 1 35,120.44 35,120.44 --[M.sub.t] 1 111,540.00 111,540.00 --PL 3 2,359,368.73 786,456.24 --Cutting season by [M.sub.t] 1 28,050.26 28,050.26 --Cutting season by PL 3 28,550.99 9,517.00 --[M.sub.t] x PL 3 32,552.23 10,850.74 --Cutting season by [M.sub.t] by PL 3 32,095.62 10,698.54 --Exp. error 624 7,666,840.13 12,286.60 Source of variation F calculated F critical Bow differences created during kiln--drying --Cutting season 1.20 3.87 --[M.sub.t] 1.75 3.87 --PL 8.41 (a) 2.63 --Cutting season by [M.sub.t] 0.61 3.87 --Cutting season by PL 0.15 2.63 --[M.sub.t] x PL 1.38 2.63 --Cutting season by [M.sub.t] by PL 0.96 2.63 --Exp. Error Twist differences created during kiln-drying --Cutting season 0.68 3.87 --[M.sub.t] 38.42 (a) 3.87 --PL 1.99 2.63 --Cutting season by [M.sub.t] 3.60 3.87 --Cutting season by PL 5.47 (a) 2.63 --[M.sub.t] by PL 3.39 (a) 2.63 --Cutting season by [M.sub.t] by PL 2.12 2.63 --Exp. error Diamonding differences created during kiln-drying --Cutting season 0.19 3.87 --[M.sub.t] 0.00 3.87 --PL 1.61 2.63 --Cutting season by [M.sub.t] 0.00 3.87 --Cutting season by PL 0.23 2.63 --[M.sub.t] by PL 0.68 2.63 --Cutting season by [M.sub.t] by PL 2.63 2.63 --Exp. error Surface check differences created during kiln-drying --Cutting season 2.86 3.87 --[M.sub.t] 9.08 (a) 3.87 --PL 64.01 (a) 2.63 --Cutting season by [M.sub.t] 2.28 3.87 --Cutting season by PL 0.78 2.63 --[M.sub.t] x PL 0.88 2.63 --Cutting season by [M.sub.t] by PL 0.87 2.63 --Exp. error (a) Term significant at [alpha] = 0.05. Table 6.--Paired-t test for differences between kiln-dried rough and planed specimens. Rough - Planed > 0 t-value Probability level Bow 0.498 0.309 Twist 0.417 0.338 Diamonding 16.262 (a) 0.000 Surface checks 11.105 (a) 0.000 (a) Term significant at [alpha] = 0.05 Table 7.--Contingency table of pith location classes and acceptability for export products. Pith Pith Pith Pith Count % class 1 class 2 class 3 class 1 Total Accepted 114 138 137 137 526 71% 86% 83% 83% 82% Not accepted 46 22 23 23 114 29% 14% 17% 17% 18%
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|Author:||Bradic, Slobodan; Avramidis, Stavros|
|Publication:||Forest Products Journal|
|Date:||Jan 1, 2007|
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