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Surface checking in air-drying of boxed-heart softwood in Japan.


In order to utilize air-drying effectively, the temperature and humidity levels at which surface checking in the early drying stages is still absent were examined. It was found that relative humidity affects the development of surface checking more than air temperature does. The optimum relative humidity level that eliminates surface checking and still allows drying was found to be between 80 and 90 percent when the air temperature is between 5[degrees] and 35[degrees]C. The moisture gradient near the wood surface when surface checking appears was larger than that without surface checking. If monthly-averaged relative humidity over 85 percent is assumed to be the suitable humidity condition for avoiding surface checking, the summer season in Japan is promising for extensive use of air-drying. It is possible to use air-drying with little surface checking for an average of 2.2 months per year in 19 of the 80 cities studied.


Air-drying requires no man-made energy and is low cost. The energy costs among some drying systems in Japan, conventional steam-heated kiln-drying, high-temperature drying, and multiple kiln-drying, range from $25/[m.sup.3] (U.S. dollars) to $44/[m.sup.3] when square timbers are dried from green to 20 percent moisture content (MC) (JFWA 2000). The most popular dry kiln in Japan (about 70% to 80% of all kilns) uses steam produced by burning oil as fuel. Manufacturers are re-evaluating the performance of softwood air-drying because it is an environmentally friendly and economical method. However, air-drying also has some drawbacks, that is, surface checking occurs at early stages where it is difficult to control the ambient conditions. This drawback is the main reason why air-drying has not been used extensively in Japan.

Air-drying is a fundamental drying method and there are many reports that convey how to use this process (Terazawa 1994, Edward et al. 1999). However, reports are lacking about the relation between the occurrence of surface checking and the climatic conditions for softwood squares (115 mm square green), although some authors have reported the amount of checking for boxed heart of softwood during air-drying (Ebihara et al. 2001, Shida 2002). The details of air-drying conditions when surface checking occurs are not well known. Our objective was to explore the air-drying conditions in Japanese environments that do not promote surface checking in softwood squares that contain pith.

Materials and methods

Five freshly cut green boxed-heart specimens of western hemlock that were 115 by 115 and 400 mm in length were selected for each test. Their MC range was 28 to 165 percent and their average ovendry density was 440 kg/[m.sup.3] (ranged from 320 to 680 kg/[m.sup.3]).

Polyvinyl acetate adhesive was used to coat the end surfaces of each specimen. Drying tests were carried out in a constant temperature and humidity conditioning chamber with an average air velocity of 0.6 m/sec. (Parameter Generation & Control Inc.).

The drying schedules were selected by referring to the climatic data for 80 Japanese cities. Figure 1 shows the relationship between temperature and humidity of the 80 cities (NAOJ 1999). We selected the set data of both temperature and relative humidity levels during test drying as shown in Figure 1 by the square plots.

The specimens were placed vertically in the chamber during drying. Images of one specimen surface were taken by using a web camera and the images were stored in a PC every 1 to 3 hours in a JPEG format. This was a supplemental observation method used during the night when a person could not visually observe. Just after the surface checking occurred, the specimen was taken out of the chamber to measure its MC. After the first surface checking occurred, a small block (40 mm in width, 30 mm in depth, and 30 mm in thickness) that contained a portion of the check was cut. Then, the block was cut into 15 slices along its depth by a knife (Fig. 2). An electronic balance and a digital pair of calipers were used to measure the weight and thickness of each slice, respectively. The slices were ovendried for 24 hours at 103[+ or -]2[degrees]C for the determination of their MC.



Results and discussion

The moisture gradient near the surface of the specimen is defined as follows:

[M.sub.grad] = ([M.sub.sf] - [M.sub.emc])/d (%/mm) [1]


[M.sub.grad] = apparent moisture gradient near the surface (%/mm)

[M.sub.sf] = average measured MC of the first slice contacting with ambient air (%)

[M.sub.emc] = equilibrium MC at calculated ambient air conditions

d = half thickness of the first slice that represents the position of [M.sub.sf] (mm)

Figure 3 shows the histograms of the moisture gradients of both the checked and not-checked specimens. When comparing these two distributions, it is apparent that the peak of the surface-checked specimens is higher than the peak for the not-checked specimens. Examination of the difference between these average values was done with a t-test and it was found that there was a statistical difference at the 5 percent level of significance. This difference is not strong because another wood attribute might exist to affect the development of surface checking. Knots, compression wood, and variability of wood physical and mechanical properties might affect the degree of checking. However, this result implies that the moisture gradient can be one of the parameters that causes surface checking. The moisture gradients at several depths (5, 10, and 30 mm) were also examined, but it was concluded that the moisture gradient near the surface, about 1 mm deep, might be a good parameter for predicting surface checking.

Figure 4 shows the relationship between moisture gradient near the surface ([M.sub.grad]) and the ratio without checks (the number of not-checked specimens/total numbers of specimens in a lot X 100). That is, [M.sub.grad] is here expressed by an averaged value of each lot having the same ratio without checks. Figure 4 shows the tendency that the ratio without checks is higher when [M.sub.grad] is lower. It can be also be seen that the [M.sub.grad] is under 2 percent/mm when the ratio without checks is over 80 percent.

Figure 5 shows the relationship between initial MC ([M.sub.i]) and the MC when surface checking happened ([M.sub.i]). The slope of the linear regression equation is 0.79. This shows that when the MC of the specimen during air-drying decreases to 79 percent, the danger of surface checks appearing increases. A slope value of 0.59 was reported in the case of air-drying of 120-by-120- mm boxed-heart Japanese cedar in the past (Ebihara et al. 2001). When these values were compared, hemlock developed more surface checking than Japanese cedar, probably due to its lower density. Generally, the higher density species exhibit more checking because of their low moisture diffusion and the large moisture gradient in wood. The ratio of [M.sub.f]/[M.sub.i] might be one of the important parameters that could suggest the initiation of surface checking.



Table 1 contains the results of the evaluation for the air-drying condition by using the number of specimens without surface checks divided by the number of all specimens per one test. The ratio was then divided into four rating categories. Judging from these rating categories, relative humidity seems to be affecting surface checking more than temperature. Relative humidity over 90 percent is estimated here as the proper condition to retard surface checking. At a relative humidity over 90 percent, surface checking occurred in only 20 percent of the boxed-heart squares. Finally, a range of relative humidity between 80 and 90 percent may protect the squares from surface checking during air-drying, although this requires more testing.

The possible time periods for acceptable air-drying are shown in Table 2. Air-drying can be implemented in 19 of the 80 cities tested (23.8%), with summer being the best time of the year. If the relative humidity for not causing surface checking is assumed to be over 85 percent, 39 months can be selected from Table 2. That is, the possible period for air-drying with minor surface checking averages 2.2 months per year, which is only 4.1 percent of the total number of potential drying months available for the 80 cities studied (NAOJ 1999). This result means that surface checking is a severe problem for boxed-heart softwood squares when air-drying in Japan.

For the effective use of air-drying for boxed-heart squares, a pre-treatment like a heat treatment on the surface of the squares to make a tension set before air-drying may be effective to prevent surface checking. Tests of air-drying combined with heat treatment to minimize surface checking have already been carried out on Japanese cedar squares (Ebihara et al. 2002).


In the case of western hemlock boxed-heart squares air-drying, relative humidity has a greater effect on the occurrence of surface checking than does air-drying. The relative humidity level below which surface checking might develop was found to range between 80 and 90 percent. Relative humidity over 90 percent has been established as suitable for air-drying, a condition that is quite common in Japan during the summer season. If the monthly-averaged relative humidity at each city over 85 percent is assumed to be the possible condition for air-drying to be able to minimize surface checking, the possible period for air-drying may be estimated to be only 2.2 months per year.

The moisture gradient near the surface when surface checking occurred is larger than that of not-checked specimens in this study. Furthermore, the ratio of the MC when checking occurred to the initial MC was found to be a satisfactory index that predicts when surface checking occurs in air-drying of western hemlock boxed-heart squares.

Table 1. -- Evaluation of air-drying conditions. (a)

 Relative humidity
Air temperature 60% 70% 80% 90%

 5 -- -- C (a) --
10 D B B A
20 -- D B A
30 -- C C A
35 -- -- B --

(a) A = over 80 percent of the ratio without checks; B = 50 to 79
percent; C = 20 to 49 percent; D = under 20 percent; -- = not

Table 2. -- Proper air-drying season without surface checking for
softwood. Black cells indicate the cities where air-drying can be used
(monthly average relative humidity > 85%).

Name of
City Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec

Wakkanai *
Kushiro * * *
Nemuro * * *
Suttu *
Urakawa * * *
Hakodate * * *
Miyako * *
Onahama * *
Karuizawa * * *
Mito *
Choshi * * *
Ooshima * * * *
Hathijojima * * *
Shionomisaki *
Izuhara *
Fukue *
Shimizu *
Murotomisaki * * *
Naha *

Note: * indicate the cities where air-drying can be used (monthly
average relative humidity > 85%)

Literature cited

Ebihara, Y., S. Shida, and T. Arima. 2001. Surface checking of sugi (Cryptomeria japonica) boxed-heart square sawn timber on air-drying. Kikagaku Jouhou, Japan Wood Research Soc. Kyusyu 8(3):44-45.

__________, H. Oda, and T. Sakota. 2002. Air-drying of sugi (Cryptomeria japonica) boxed-heart square sawn timber after high temperature and low humidity treatment. In: Abstracts of the 52nd Annual Meeting of the Japan Wood Research Society, Gifu, Japan. 137 pp.

Edward, C.P., W.T. Simpson, L.J. Tschernitz, and J.F. James. 1999. Air drying of lumber. Tech. Rept. FPL-GTR-117. USDA Forest Serv., Forest Prod. Lab., Madison, WI. pp. 1-62.

Japanese Federation of Wood-Industry Association (JFWA, Zenmokuren). 2000. Technical manual of dry lumber production. JFWA, Japan. 93 pp.

National Astronomical Observatory in Japan (NAOJ). 1999. Japanese science chronology. NAOJ, Mitaka, Japan.

Shida, S. 2002. Surface checking and moisture distribution in hemlock boxed-heart square lumbers during drying by the air-drying assuming conditions. No. F230915. Abstracts of the 53rd Annual Meeting of the Japan Wood Research Society, Fukuoka, Japan.

Terazawa, S. 1994. Mokuzai kansou no subete (Everything on timber drying), Kaiseisya, Japan, pp. 434-448. (in Japanese)

Satoshi Shida*

Taiga Ikeda

Stavros Avramidis*

The authors are, respectively, Associate Professor and Graduate Student, Dept. of Biomaterial Sciences, The Univ. of Tokyo, 1-1-1 yayoi, bunkyo-ku, 113-8657, Tokyo, Japan; and Professor, Dept. of Wood Science, Univ. of British Columbia, 2424 Main Mall, Vancouver, BC, Canada V6T1Z4. Funding was provided by the Japanese Society for the Promotion of Science, Scientific research category C2, No. 12660145. This paper was received for publication in September 2003. Article No. 9754.

*Forest Products Society Member.
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Author:Shida, Satoshi; Ikeda, Taiga; Avramidis, Stavros
Publication:Forest Products Journal
Geographic Code:9JAPA
Date:Jan 1, 2005
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