Drying methods to evaluate the quality of Eucalyptus sawn timber

Abstract

The difficulty of obtaining dry wood with quality is one of the major obstacles when it comes to drying wood of the Eucalyptus genus, since the species of the genus have slow drying and high propensity to the appearance of defects. Due to this, it is recommended to apply methods that combine pre-drying with conventional drying as an alternative to improve the quality of this wood. The aim of this study was to compare the final quality of Eucalyptus sawn timber submitted to two different drying methods. The drying methods used were: combined drying (air predrying + drying in a conventional kiln), and only conventional drying. In each of the drying processes, moisture loss and drying speed were monitored, and the defects before and after each drying (warping, cracking and collapse) were evaluated. Statistical analysis was performed by using descriptive statistics, analysis of variance, analyzed by means of the Analysis of Variance (ANOVA) and Tukey’s test at the probability level of 95%, and regression analysis was performed by using software R. Based on the results obtained, combined drying presented a shorter time than the conventional kiln, reducing 51% of the total drying time, which is equivalent to three days less than in the dry kiln. The drying rate in the removal of hygroscopic water between conventional and combined drying methods was higher for the combined drying, i.e., higher drying speed was obtained up to the final moisture content, about twice as high as for the conventional one. Combined drying had the lowest defect rates and highest final quality of the timber, with no collapse.

Full text

Australian Journal of Basic and Applied Sciences
2020 February; 14(2): pages 22-30
DOI: 10.22587/ajbas.2020.14.2.4
Original paper AENSI Publications
Australian Journal of Basic and Applied Sciences
ISSN: 1991-8178, EISSN: 2309-8414
Journal home page: www.ajbasweb.com
Drying methods to evaluate the quality of Eucalyptus sawn timber
1Linéia Roberta Zen, 2Thiago Magalhães do Nascimento, 2Jean Henrique dos Santos, 3Ricardo Jorge Klitzke,
3Márcio Pereira da Rocha, 3Thiago Campos Monteiro
1Forest engineer, Phd student, Postgraduate Program in Forest Engeneering, Universidade Federal do Paraná - Av. Lothário Meissner, 631 – Jardim Botânico,
Campus III – 80210-170 – Curitiba, Paraná – Brazil.
2Forest engineer, Msc student, Postgraduate Program in Forest Engeneering, Universidade Federal do Paraná - Av. Lothário Meissner, 631 – Jardim Botânico,
Campus III – 80210-170 – Curitiba, Paraná – Brazil.
3Forest Engineer, Phd professor, Universidade Federal do Paraná, Departamento de Engenharia e Tecnologia Florestal, - Av. Lothário Meissner, 631 – Jardim
Botânico, Campus III – 80210-170 – Curitiba, Paraná – Brazil
Correspondence Author: Linéia Roberta Zen, Forest engineer, Phd student, Postgraduate Program in Forest Engeneering, Universidade Federal do Paraná - Av.
Lothário Meissner, 631 – Jardim Botânico, Campus III – 80210-170 – Curitiba, Paraná – Brazil
Received date: 15 December 2019, Accepted date: 24 February 2020, Online date: 3 March 2020
Copyright: © 2020 Linéia Roberta Zen et al., This is an open-access article distributed under the terms of the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Keywords: Moisture content, drying defects, drying rate, drying curve
INTRODUCTION
Currently, Eucalyptus spp. has been no longer an alternative raw material for the timber industry. It has become a reality for
the industrial sector of solid timber products (Liebl et al., 2017). Thus, the processing of Eucalyptus sawn timber needs to be
adapted and improved both in the mechanical processing and drying phases, since this genus is characterized by the rapid
development of forest stands.
However, caution on the production of sawn timber is necessary, mainly due to the manifestation of internal growth stresses,
which are responsible for the formation of checking and warping in boards during the processes of breakdown and drying (Yang
and Waugh, 2001; Murphy et al., 2005). Improving the quality of boards and optimizing the wood processing industry are
fundamental for the productive and financial success of these companies. These improvements generate subsidies, such as new
breakdown and drying techniques, especially with regards to the drying of refractory woods, as the Eucalyptus genus.
Due to its low permeability, along with its anatomical constitution, which hinders the drive of moisture out of its interior,
Eucalyptus can be considered as a species of severe drying (Oliveira and Carvalho, 2001; Rezende et al., 2018; Zen et al., 2019).
The reactions of the anatomical constitution are very expressive in drying, presenting small diameters (Oliveira and Carvalho,
2001), high cell wall fraction (Zanuncio et al., 2016), which hinders the water movement by capillarity or in liquid form in the
Abstract
The difficulty of obtaining dry wood with quality is one of the major obstacles when it comes to drying wood of the
Eucalyptus genus, since the species of the genus have slow drying and high propensity to the appearance of defects.
Due to this, it is recommended to apply methods that combine pre-drying with conventional drying as an
alternative to improve the quality of this wood. The aim of this study was to compare the final quality of Eucalyptus
sawn timber submitted to two different drying methods. The drying methods used were: combined drying (air pre-
drying + drying in a conventional kiln), and only conventional drying. In each of the drying processes, moisture
loss and drying speed were monitored, and the defects before and after each drying (warping, cracking and
collapse) were evaluated. Statistical analysis was performed by using descriptive statistics, analysis of variance,
analyzed by means of the Analysis of Variance (ANOVA) and Tukey’s test at the probability level of 95%, and
regression analysis was performed by using software R. Based on the results obtained, combined drying presented
a shorter time than the conventional kiln, reducing 51% of the total drying time, which is equivalent to three days
less than in the dry kiln. The drying rate in the removal of hygroscopic water between conventional and combined
drying methods was higher for the combined drying, i.e., higher drying speed was obtained up to the final moisture
content, about twice as high as for the conventional one. Combined drying had the lowest defect rates and
highest final quality of the timber, with no collapse.

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Citation: Linéia Roberta Zen et al., 2020. Drying methods to evaluate the quality of Eucalyptus sawn timber. Australian Journal of Basic and Applied
Sciences, 14(2): 22-30. DOI: 10.22587/ajbas.2020.14.2.4
interior of this wood, in addition to high complexity, since the same anatomical structure can help in capillary water flow and
interfere in the impregnated water flow (Monteiro et al., 2017). Another factor to hinder the process of hardwood drying is the
presence of tyloses in the vessels. Tylose is a structure found in the heartwood of Eucalyptus species and may obstruct the passage
of water (Siau, 1971; De Micco et al., 2016; Helmling et al., 2018).
Therefore, as for the drying of Eucalyptus, choosing a method influences the drying time, quality of the dry material, and
moisture content desired for particular purposes. It is possible to reduce the drying time and incidence of defects and to improve
the quality of the lumber when the process is properly conducted.
In order to minimize defects in Eucalyptus sawn timber, several studies have been conducted, such as the use of vaporization
during drying (Rezende et al., 2015) and different stacking methods (Dittmann et al., 2017). Another alternative to meet these
aspects is the use of pre-drying and the combination of air drying with subsequent conventional drying, providing cost reduction
and process optimization due to the higher efficiency of conventional drying kilns.
In addition, this alternative attenuates the moisture gradient in lumber, which is responsible for most defects during drying
(Stöhr, 1977; Campbell and Hartley, 1988; Northway, 1996; Ciniglio, 1998; Denig et al., 2000; Santos, 2003; Jankowsky, 2008).
It aims at accelerating the drying process by gradually removing the maximum amount of free water with the minimum amount of
defects as possible, then finishing the drying in a conventional kiln. Thus, it reduces the time in kiln and energy costs and initially
eliminates defects such as collapse.
When individually applied, these two drying processes (air and conventional drying) have characteristics that do not favor the
drying of Eucalyptus lumber and are often not recommended. On the one hand, air-drying depends on local atmospheric
conditions, and the environmental variables cannot be controlled. On the other, conventional drying allows for total process
control, which reduces the incidence of defects in the initial phases. However, it requires a skilled workforce, increasing operating
costs (Rosso, 2006). Thus, this study aims at evaluating the quality and drying parameters of Eucalyptus sawn timber submitted to
two different drying methods: combined drying (air pre-drying + drying in a conventional kiln), and only conventional drying.
MATERIAL AND METHODS
Material Preparation
For this study, Eucalyptus sawn timber from forest stands with an average age of 15 years, belonging to the company
MADEMAPE Madeiras (municipality of Campina Grande do Sul, state of Paraná, Brazil), was used. The logs were selected in the
yard of the sawmill and were sawn in simple vertical band saw, in the nominal dimensions of 25 mm x 110 mm x 2500 mm
(thickness, width, and length, respectively), at green state. Later, they were separated and transported for the drying processes.
The following drying methods were used: combined drying (air pre-drying + drying in a conventional kiln), and only conventional
drying. For pre-drying, the nominal dimensions of 25 mm x 110 mm x 2500 mm (thickness, width, and length, respectively) were
used. The timber was naturally exposed in the city of Curitiba (state of Paraná, Brazil), at the facilities of the Forest and Wood
Science Center (Centro de Ciências Florestais e da Madeira – CIFLOMA) of the Federal University of Paraná. A well-drained,
flat, unobstructed ventilation site was selected for good air circulation. The air-drying stacking was built on concrete load supports
with lumber blocking at the height of 30 cm from the ground. It was formed by 20 layers of boards. Seven timber battens per layer
were placed at 35 cm from each other to minimize the incidence of defects. The timber was exposed until it reached the moisture
content desired.
For drying in a conventional kiln (both combined and only conventional drying), the boards were resized at 25 mm x 110 mm
x 650 mm (thickness, width, and length, respectively). Samples of 25 mm were taken at each end of the piece to determine the
initial moisture content for both methods. Each load consisted of 60 pieces, and the wood was stacked in dry kiln trucks. The
boards were arranged in the transverse direction to the airflow of the dry kiln and were allocated and separated by stickers of
square section of 25 mm and length of 1100 mm. Airflow in the dry kiln was measured with an anemometer and had a value of
approximately 2.0 m/s.
Moisture Monitoring and Evaluations
Moisture monitoring of the lumber boards in conventional drying was performed by four pairs of pin electrodes (two long and
two short), totalling sixteen pins, which had resistance operation. Drying curves and rates were achieved based on the data
obtained by the kiln. In the air drying, moisture was monitored through control samples allocated in the stack at the entrance and
exit of air. The moisture loss in this test was monitored according to Equation 1 (Brandão, 1989):
(1)
Where: UA = moisture content of the specimen at any given time (%); MA = mass of the specimen at the same given time (g); UI
= initial moisture of the specimen (%); and MI = initial mass of the specimen (g).
Before starting the drying process, the initial moisture content of each sample was determined through the gravimetric
method, according to the recommendation NBR 7190 (Brazilian National Standards Organization – ABNT, 1997).

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Citation: Linéia Roberta Zen et al., 2020. Drying methods to evaluate the quality of Eucalyptus sawn timber. Australian Journal of Basic and Applied
Sciences, 14(2): 22-30. DOI: 10.22587/ajbas.2020.14.2.4
Based on the data obtained by the dry kiln through the pin electrodes and control samples, the drying rates for each method
were established according to Equation 2:
(2)
Where: TS = drying rate (U%.h-1); TUi = initial moisture content (%); TUf = final moisture content (%); and T = time (days).
Drying Program
Drying programs were developed under two conditions, using the same initial parameters: one was based on the greenwood,
and the other, on the pre-dry wood (air drying at 23% of moisture content). To elaborate the program, the methodology presented
by Ciniglio (1998) and Jankowsky (2009), so-called drastic drying, was used. Table 1 presents the drying program elaborated for
Eucalyptus spp, under both conditions.
Table 1: Conventional drying program for Eucalyptus spp lumber in a dry kiln.
Temperature °C
Phase
Moisture
DBT
WBT
RH %
EM %
Time (hours)
DP
Heating
-
40
40
100
3
-
1
35
40
38
87
18
-
2
2
31
44
42
82
16
-
1.9
3
28
46
42
76
14
-
1.9
4
24
48
43
73
12
-
1.9
5
20
52
43
63
10
-
1.9
6
18
56
45
58
9
-
1.9
7
15
60
46
54
8
-
1.9
8
12
63
46
40
6
-
1.9
9
9
65
47
35
5
-
1.9
Standardization
10
63
43
63
9
8
Acclimation
10
61
53
72
11
8
Cooling
10
44
37
54
9
4
Where: DBT = Dry-bulb temperature (°C); WBT = Wet-bulb temperature (°C); RH = Relative Humidity (%); EM = Equilibrium
Moisture (%); DP = Drying Potential.
Incidence of Defects and Final Quality of the Timber
Evaluations on defects were performed according to the recommendation NBR 14806 (ABNT, 2002). The defects evaluated
were crooking, bowing, cupping, and surface and top checking. Collapse checking was performed according to the
recommendation of Galvão and Jankowsky (1985) and was classified according to the recommendation of Welling (1994) (Table
2).
Table 2: Collapse degree.
Collapse Degree
Reduction in thickness (dc), representing the current level of collapse
Severe
Moderate
Light
dc ≤ 6 mm (or removed with planing)
dc ≤ 4 mm (or removed with planing)
dc ≤ 2 mm (or removed with planing)
Source: Adapted from Welling (1994).
Statistical Analysis
Statistical analysis was performed by using descriptive statistics, analysis of variance, and regression analysis. The variables
relative to the quality of the lumber were statistically analyzed by means of the Analysis of Variance (ANOVA) and Tukey’s te st
at the probability level of 95%. Bartlett’s test was applied to verify the homogeneity of the variances. Regression analysis was
performed by using software R.
RESULTS
Fig. 1 presents the results for the combined drying curve of Eucalyptus lumber according to the moisture loss over the days.
For the combined drying method, the initial moisture content in the pre-drying phase was 88%, reaching the final moisture content
of 23.1% after 65 days. Drying was finished in a conventional kiln, and the timber reached 9% of moisture in three days (72
hours).

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Citation: Linéia Roberta Zen et al., 2020. Drying methods to evaluate the quality of Eucalyptus sawn timber. Australian Journal of Basic and Applied
Sciences, 14(2): 22-30. DOI: 10.22587/ajbas.2020.14.2.4
Fig. 1: Combined drying curve for Eucalyptus sawn timber.
After this phase, the moisture content loss is reduced. At the beginning, the desorption rate is high, and as it approaches the
FSP, it is drastically reduced. When analyzing only conventional drying (Fig. 2), the average moisture content was 69%, given
172 hours (7.16 days in the dryer), up to the final moisture content of 9%.
0
5
10
15
20
25
30
35
40
010 20 30 40 50 60 70 80 90 100
Mouiture content (%)
Time (hours)
Fig. 2: Conventional drying curve for Eucalyptus sawn timber.
Drying Rate
Table 3 shows the mean drying rates in the following moisture content conditions: green up to 30% (capillary water); green
up to 9% (capillary and impregnated water); and conventional drying from 30% up to 9% and combined drying only from 23% to
9% (impregnated water). The results were presented in U%.h-1, for the pairs of pin electrodes of the dry kiln.
Table 3: Mean drying rate for Eucalyptus spp. Lumber for conventional drying and combined drying in the conventional kiln.
Drying Rate %U.h-1
Drying
Green up to 30%
(Capillary Water)
From 30 to 9% (Impregnated Water)
Green up to 9%
(Impregnated and Capillary Water)
F
Conventional
0.7468
0.17153 b*
0.2845
36.1187**
Combined
-
0.32548 a*
-
Where: * removal of impregnated water from 23% of moisture; ** significant for Tukey’s test at p < 0.01.
Conventional

26
Citation: Linéia Roberta Zen et al., 2020. Drying methods to evaluate the quality of Eucalyptus sawn timber. Australian Journal of Basic and Applied
Sciences, 14(2): 22-30. DOI: 10.22587/ajbas.2020.14.2.4
For the purpose of comparing the mean drying rates in the conventional and in the combined drying methods, F-test was
performed only for hygroscopic water range, since the combined drying results from a pre-drying in which the removal of free
water has already occurred. Thus, it only shows this moisture range.
Fig. 3: Characteristic drying curve of Eucalyptus spp. Sawn timber for conventional drying rate.
Quality of Eucalyptus Lumber for Conventional and Combined Drying Methods
Surface and Top Checking
Eucalyptus lumber showed no surface checking during conventional drying. However, during combined drying, it presented
the minimum number of checking of 1.67% (nearly insignificant). The top checking rate has reduced in both methods. On the one
hand, for conventional drying, top checking was absent at all stages of evaluation. On the other, for combined drying, top
checking rate was 3.33% after a breakdown and absented after drying.
Warping (Crooking, Bowing, and Cupping)
Table 4 presents the warping rate for Eucalyptus lumber in both methods. The average crook increased after drying for the
boards in the conventional drying. Even though crooking was low after a breakdown, the boards continued to crook during the
moisture loss. In the combined drying, the average crook decreased after drying, which can be considered as very low. After
analyzing the average crooks, according to NBR 14806 (ABNT, 2002), all pieces presented crooking inferior to 5 mm/m and were
not defective in both methods.
Table 4: Warping of Eucalyptus lumber.
Defect
Methods
Crooking (mm/m)
Conventional Drying
Combined Drying
After Breakdown
0.29
0.10
After Drying
0.87
0.05
Bowing (mm/m)
Conventional Drying
Combined Drying
After Breakdown
1.03
1.51
After Drying
1.34
1.33
Cupping (mm)
Conventional Drying
Combined Drying
After Breakdown
0.28
0.48
After Drying
1.11
0.48
Collapse
Table 5 presents the percentage of collapse, level of collapse, and mean deformation (dc) of the boards for the conventional
and combined drying methods.
Table 5: Percentage of defective parts and level of collapse of Eucalyptus lumber for conventional and combined drying methods.
Level of Collapse (%)
Drying
Pieces with Collapse (%)
Light
Moderate
Severe
Mean dc (mm)
Conventional
11.66
0.00
5
6.66
2.27
Combined
0.00
0.00
0.00
0.00
0.00
Where: dc = mean deformation of the collapsed boards (mm).

27
Citation: Linéia Roberta Zen et al., 2020. Drying methods to evaluate the quality of Eucalyptus sawn timber. Australian Journal of Basic and Applied
Sciences, 14(2): 22-30. DOI: 10.22587/ajbas.2020.14.2.4
According to Table 5, the collapse occurred in the conventional drying, presenting a high percentage of pieces (11.6%, where
5% had moderate level and 6.66%, severe level). No collapse was observed in combined drying.
DISCUSSION
Initial Moisture Content, Time, and Drying Curves
According to Hill et al. (2010), lumber sorption behaviour can be described by a model called parallel exponential kinetics
(PEK). This model is composed of two exponential terms representing the fast and slow processes with characteristic time and
moisture content associated with them (Hill et al., 2010; Hill et al., 2012).
Thus, air drying of the sawn timber was conducted up to the moisture content of 23%, since it was the moment when the control
samples presented greater moisture homogeneity for transfer to the dryer. After moving from the drying stack to the dryer, it was
possible to observe moisture gain by the heating phase and then high moisture content loss due to the conditions of the artificial
drying (Figure 1).
In addition, the moisture content increased from 23.1% to 31.7% in the heating phase for the combined drying curve, when
the lumber was in the conventional kiln. This result is normal for the dryer, as there is thermal equilibrium between air and lumber
in the heating phase. As it is undesirable for the lumber to start the drying process at this phase, high relative humidity and low
temperature are used, promoting this thermal equilibrium (Andrade, 2000).
The initial moisture content of the sawn timber is influenced by many variables, such as: age, species, site, season of the year
when it was cut, besides transport-related operational factors, log storage time, and sawdust up to drying. Despite the low initial
moisture content, the drying curve behaved as typical as the characteristic Eucalyptus curve for dryers (Jankowsky et al., 2003),
with no constant moisture loss rate, and proving its impermeability. Low permeability implies the need for slow drying, with low
drying power and temperature, especially at the beginning of the process. It is in accordance with the drying program elaborated
previously in Table 2.
According to Figures 1 and 2, a comparison between the drying times according to the moisture content for both methods
shows the reduction of 51% of the total drying time, which is equivalent to four days less in the conventional kiln, when using
only conventional drying. Therefore, it is possible to affirm that these results are satisfactory since the productive capacity of the
kiln has increased.
By reducing the days of drying, a higher rotation of loads in the kiln and significant reduction in both thermal and electrical
energy consumption, as well as low reduction of the defect rate (mainly for collapse), are possible. Several authors, such as
Northway (1996) and Ciniglio (1998), have indicated that applicable methods, such as pre-drying for Eucalyptus lumber and the
combination of air and conventional drying, are satisfactory to reduce the time in drying kilns. This combination provides cost
reduction and process optimization by higher kiln operation.
Drying Rate
According to Table 3, the drying rate for the removal of hygroscopic water between conventional and combined drying
methods differed statistically at the probability level of 1%. For the combined drying, the drying rate was higher, i.e., higher
drying speed was obtained up to the final moisture content, about twice as high as for the conventional one. The higher
temperatures used in the combined drying may explain this result, in accordance with Langrish and Walker (2006), who have
reported on the influence of temperature on the increase of the drying rate. Similar behaviour was described by Batista et al.
(2015) for the drying rate of three species of Eucalyptus. The author verified the lowest drying rate in the impregnated water
range.
In addition, according to Table 3 and Fig. 3, the drying rate for conventional drying was normal, and the removal of free
water (green up to 30%) has occurred as expected. It presented a high mean drying rate, thus providing higher drying speed, i.e.,
greater removal of free water when compared to impregnated water. This result is justified by the high moisture content of the
lumber and the short elapsed time, regardless of the drying program. In the phase of removal of hygroscopic water, the drying rate
decreased as the lumber moisture approached the final moisture content. Finally, a slight reduction in the total removal range of
both free and hygroscopic water is observed.
Quality of Eucalyptus Lumber for Conventional and Combined Drying Methods
Surface and Top Checking
The probable reasons for the low rate of surface checking found in this study may be related to the low initial temperatures
used in the drying programs, especially for conventional drying, for which this defect was absent. Besides, the surface moisture
content may have decreased, or even the drying stresses in the pieces that presented surface checking in the evaluation after
breakdown may have reversed, sealing the cracking initially verified, after drying.
Vermaas (1995) states that, above the PSF, Eucalyptus lumber presents a great tendency to check and collapse, especially at
high temperatures. To dry Eucalyptus lumber with a thickness of 25 mm or more, the temperature should not exceed 45°C during
the initial stages of the process (Campbell and Hartley, 1988; Hartley and Gough, 1990; Severo, 2000). Otherwise, this timber
becomes prone to the development of surface checking. Similar results were found by Batista et al. (2015), who has also obtained
low percentage values of surface checking when evaluating the drying of E. saligna and E. grandis.
The low top checking rates found are justified by the fact that the drying programs elaborated, both for combined and
conventional drying methods, were ideal. Low temperatures, high relative humidity, and low drying rate, along with low moisture

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Citation: Linéia Roberta Zen et al., 2020. Drying methods to evaluate the quality of Eucalyptus sawn timber. Australian Journal of Basic and Applied
Sciences, 14(2): 22-30. DOI: 10.22587/ajbas.2020.14.2.4
gradient, favored the absence of top checking after drying. According to Susin (2012), one of the biggest difficulties during
Eucalyptus lumber drying is reducing, avoiding, and controlling the incidence of top checking.
Warping (Crooking, Bowing, and Cupping)
The higher average crooks for the conventional drying, when compared to the combined one, may have resulted from the
different initial moisture contents (due to pre-drying) since the contractions in conventional drying were higher, and mainly, the
lumber still presented stresses in the boards. Rocha (2000) states that the manifestation of this defect is more associated with
stresses than with the drying process.
The average crook may have reduced in the combined drying since the dimensions of the boards were smaller. Another
explanation may be the fact that the pieces already result from pre-drying, when crooking decreases, positively contributing to
defect reduction when dried in the conventional kiln. According to Souza et al., (2012), the manifestation of crooking in pieces at
the end of the drying process is one of the most laborious defects to control, as the boards are arranged with no edge restriction
that prevents them from deforming.
After drying, the average bow reduced for the combined drying, whereas it increased for the conventional one (Table 4). This
decrease may be justified by the fact that the crooked parts were carefully positioned at the bottom of the stack (first layers) and
with the concave face down during the stacking of the boards, causing this rate to be reduced. Using additional loads on the stacks
as well as placing the bowed boards in the first layers of the stack favour the minimization of this defect at the end of the drying
process (Ciniglio, 1998; Klitzke, 2007; Susin, 2014).
Bowing increase in the conventional drying may be justified by the fact that the lumber presents higher initial moisture
content at green state in this method, unlike in the combined drying, in which it has undergone pre-drying, and the lumber water
has been gradually removed. Another factor may be the greater proportion of tangent pieces. The presence of abnormal logs and
tangentially oriented boards favour lumber bowing. Moreover, Simpson (1991) and Denig et al. (2000) report that bowing is not
one of the most problematic lumber defects, as it can be eliminated by performing proper lumber stacking for drying. According
to Klitzke, (2007), bowing is influenced more by stacking than by the drying process.
Table 4 shows that the boards already had a low cupping rate after a breakdown in both methods. The cupping rate increased
for the conventional drying, whereas it remained constant for the combined one after drying (0.48 mm). The values found for the
combined drying in this study are justified by the fact that the boards already had low moisture content due to pre-drying, when
the rates were low, and that the contractions of the boards were not expressive, causing this defect to not be very present. Besides,
the good stacking performed in the dry kiln also favored these low rates.
It is noteworthy that the maximum value allowed for this defect is 4 mm, according to NBR 14806 (2002). In both methods,
all boards presented values below the maximum one. Therefore, 100% of the boards were classified as suitable, with no defective
parts. Rosso (2006) and Stangerlin et al. (2009) did not verify pieces with cupping superior to 4 mm either, for C. citriodora and
E. saligna. The cupping rate may have increased in conventional drying due to the presence of boards close to the pith and of
tangential and radial faces in the same board.
Collapse
We attempted to standardize the quality of the dry sawn timber in the different treatments in order to avoid the concentration
of boards of a single region of the log in a single stack since the origin region of the board affects the presence of collapse in
Eucalyptus. Ananías et al. (2014) have found the greater presence of this defect in boards of Eucalyptus nitens from the transition
region between the heartwood and the sapwood. Thus, the results found in this study may be explained by the fact that collapse
occurs during the removal of capillary water (Table 5). Besides, it is directly related to the permeability of the lumber, which is
influenced by density and capillary diameter and obstructions, such as tyloses (Galvão and Jankowsky, 1985).
Despite the low temperature used until the complete removal of capillary water along with the high relative humidity for
conventional drying, the boards presented susceptibility to the manifestation of collapse, which is normal for lumbers such as the
Eucalyptus. Considering the drying potential of 1.90 and the initial temperature of 40°C, the use of lower temperature and drying
potential for this situation followed the recommendations of (Pratt, 1974; Northway, 1996; Ciniglio, 1998; Andrade, 2000; Keey
et al., 2000). Reducing the temperature and drying potential further would considerably increase the time of this stage and might
make it unfeasible. As a result, the low percentage of the collapse was satisfactory in both methods. It is worthy of highlighting
the final drying quality of the sawn timber.
The absence of collapse in the combined drying occurred due to the loss of capillary water in the lumber during air drying,
remaining only part of the impregnated water to be removed. Such a process is slower and does not require low temperatures in
the final stages. A collapse is a form of shrinkage that occurs in the drying process in most wood species, greatly reducing the size
of lumens (Kuo and Arganbright, 1978; Blakemore and Northway, 2009). It occurs in the initial drying stages in the presence of
liquid water above the SPF, whereas normal shrinkage occurs in the hygroscopic domain (Panshin and Zeeuw, 1980; Hart, 1984).
CONCLUSION
Overall, the results show that combined drying has reduced drying time in the dryer by 51% when compared to the
conventional drying method. It also had a higher rate of removal of impregnated water. The programs developed and applied for
Eucalyptus lumber in conventional and combined drying methods are considered to be soft. Combined drying presented better

29
Citation: Linéia Roberta Zen et al., 2020. Drying methods to evaluate the quality of Eucalyptus sawn timber. Australian Journal of Basic and Applied
Sciences, 14(2): 22-30. DOI: 10.22587/ajbas.2020.14.2.4
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