THE IMPACT OF CLIMATE CONDITIONS ON WOOD SHRINKING AND SWELLING AND ITS EFFECT ON FURNITURE DAMAGE (Case study in the Republic of Kosovo)

Abstract

In general, shrinkage and swelling are the two bad physical properties of wood. That is because the biggest problems that occur in wood-based products are precisely of this nature. During their service, wood and wooden products (especially joints of furniture) are subject of climatic exposed in different conditions. The shrinkage of wood occurs when its moisture content is below the fiber saturation point, and when climatic conditions (relative humidity and temperature) change. Dimensional stability of wood is one of the few properties that significantly differs in each of the three fiber directions. Depending from fiber directions (longitudinal, radial and tangential), shrinkage and swelling changes can occur in different parts of furniture that cause disruptions of join elements, cracking of attached parts, decay layers or protective lacquers and other damages to the furniture. The paper provides seasonal data on changes of relative humidity and temperature for some regions of Kosovo, extracted for a 38-year period. Based on these data there are calculated the maximum and minimum possibilities of shrinkage or swelling of beech wood (Fagus Sylvatica L) and oak wood (Quercus Petraea L). The study was conducted specifically by going to manufacturing entities of furniture for external and internal use in regions taken into consideration and are illustrated with specific examples the defects caused as a result of changes in these parameters. There are provided recommendations in order to reduce the defects occurring in wooden objects, when external or in some cases and internal climatic conditions change.

Full text

International Journal of Ecosystems and Ecology Science (IJEES) Vol. 12 (2): 525-530 (2022)
https://doi.org/10.31407/ijees ISSN: 2224-4980 https://doi.org/10.31407/ijees12.2
525
THE IMPACT OF CLIMATE CONDITIONS ON WOOD SHRINKING
AND SWELLING AND ITS EFFECT ON FURNITURE DAMAGE
(Case study in the Republic of Kosovo)
Rrahim Sejdiu1*, Ramadan Topuzi1, Muharrem Sejdiu2, Bujar Jashari1
1*University of Applied Sciences, Faculty of Architecture Design and Wood Technology,
Ferizaj, Republic of Kosovo;
2UBT College, Lagjja Kalabria PN., Pristina, Republic of Kosovo;
*Corresponding Author Rrahim Sejdiu, e-mail: rrahim.sejdiu@ushaf.net;
Received January 2022; Accepted February 2022; Published March 2022;
DOI: https://doi.org/10.31407/ijees12.225
ABSTRACT
In general, shrinkage and swelling are the two bad physical properties of wood. That is because the biggest problems
that occur in wood-based products are precisely of this nature. During their service, wood and wooden products
(especially joints of furniture) are subject of climatic exposed in different conditions. The shrinkage of wood occurs
when its moisture content is below the fiber saturation point, and when climatic conditions (relative humidity and
temperature) change. Dimensional stability of wood is one of the few properties that significantly differs in each of
the three fiber directions. Depending from fiber directions (longitudinal, radial and tangential), shrinkage and
swelling changes can occur in different parts of furniture that cause disruptions of join elements, cracking of
attached parts, decay layers or protective lacquers and other damages to the furniture. The paper provides seasonal
data on changes of relative humidity and temperature for some regions of Kosovo, extracted for a 38-year period.
Based on these data there are calculated the maximum and minimum possibilities of shrinkage or swelling of beech
wood (Fagus Sylvatica L) and oak wood (Quercus Petraea L). The study was conducted specifically by going to
manufacturing entities of furniture for external and internal use in regions taken into consideration and are illustrated
with specific examples the defects caused as a result of changes in these parameters. There are provided
recommendations in order to reduce the defects occurring in wooden objects, when external or in some cases and
internal climatic conditions change.
Keyword: wood moisture, shrinkage, swelling, climate, furniture.
INTRODUCTION
Dimensional changes accompanied by wood shrinkage and swelling are the main sources of problems in the
appearance and wood structure. The shrinkage and swelling occur as a result of wood moisture changing as a daily
reaction or seasonal relative humidity of the atmosphere, for instance, when the air is humid, wood adsorbs moisture
and swells, whereas when the air is dry, the wood loses moisture and shrinks (Eckelman, 1998).

Rrahim Sejdiu1*, Ramadan Topuzi1, Muharrem Sejdiu2, Bujar Jashari1
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526
Figure 1. Dependence of wood equilibrium
moisture against air relative humidity to 21.70C
Dimensional changes in the longitudinal direction between the fiber saturation point and oven dry are between 0.1
and 0.2% and are of no practical significance; however, in reaction or juvenile wood, these percentages may be
significantly higher (Arntzen, n.d.). The combined effects of shrinkage in the tangential and radial axes can distort
the shape of wood pieces because of the difference in shrinkage and the curvature of the annual rings (Fig. 4).
Generally, tangential shrinkage (varying from 4.4 to 7.8% depending on species) is twice that of radial shrinkage
(from 2.2 to 5.6%) (Green et al., 1999).
These properties occur when the wood is present below the fiber saturation point (30%). Various treatments can be
used to reduce these occurrences, but in general, these treatments cannot completely prevent wood shrinkage and
swelling. In addition, natural and artificial drying of wood does not prevent the wood to accept or release moisture.
Wood moisture content will always be equal with the relative
humidity of the air that surrounds it. This moisture content is
referred to as "Equilibrium Moisture Content" (EMC).
Wood equilibrium moisture equates to every level of
relative humidity. There is a correlation of the equilibrium
moisture content of wood with relative air humidity and
temperature (Rowell & Banks, 1985). Figure 1 shows
relation between wood moisture and air humidity. Wood
equilibrium moisture is made in function of relative
humidity and temperature to 220C (Eckelman, 1998). The
values equilibrium moisture content are almost similar in
all species of wood, so a customized value can be used for
all types of wood. In the woods that are dried below the
fiber saturation point, changes in moisture content with
only a few percent are enough to cause significant
shrinkage and swelling.
There is general agreement that, even and the trees of the same species may have different percentages of shrinkage
or swelling. It depends on the climatic conditions of the tree growth and other factors, which are related to the
properties of the wood (Stamm, 1935)(Green et al., 1999; Sejdiu et al., 2013). The amount of shrinkage varies
according to the type of wood by 30% moisture until oven dry conditions found in the coefficient of contraction,
which is shown in Appendix II. The change of shrinkage between types can be significant. Unlike maple, chestnut
shrinks 4.3% in the radial direction and 6.8% in the tangential direction.
This difference contraction between radial and tangential direction causes defects that appear all the time for
woodworking purposes. Minimizing wood shrinkage and swelling - According to the authors, there are many factors
that can count in shrinkage and swelling that occur in wood furniture. (Eckelman, 1998) count four factors:
• The wood is not dry uniformly throughout its volume before producing furniture. This contraction causes
excessive damage in furniture.
• Furniture construction is such that wood parts, which are mechanically linked, are not able to afford the
changes arising from the shrinkage and swelling.
• Excessive drying occurs with cracking boards.
• The object reveals visual differences in shrinkage and swelling elements.
Figure 2. The illustration of shrinkage and swelling in radial, tangential, and
longitudinal direction in logs and boards.

International Journal of Ecosystems and Ecology Science (IJEES) Vol. 12 (2): 525-530 (2022)
https://doi.org/10.31407/ijees ISSN: 2224-4980 https://doi.org/10.31407/ijees12.2
527
In this regard, other researchers counts and other different factors (Laboratory, 1957). These issues grow if the
furniture is constructed of wood, which is conditioned under inappropriate conditions for woodworking purposes.
To reduce the problems associated with moisture content of wood, it should be dried until its final service. For this
reason, after drying it should be stacked and stored into atmospheric conditions, which ensure the preservation of
this moisture. Keeping the wood dry will minimize the effects of shrinkage and swelling. It is impossible to know
the conditions in which the furniture will be used (missing information). Many manufacturers, for interior purposes
prefer to dry wood to 6% of moisture content, and some problems arise from excessive drying due to swell. It is
assumed that swelling causes more stable connection while shrinkage causes the expansion. In general there are not
enough information about the accuracy of the contraction and expansion of products “rule ± 2%” can be applied;
this rule allows the movement of dimensions to ± 2%. It is important to know that wood does not cause serious
problems along the fibers during contraction, while serious problems arise when the furniture is designed in such a
way that parts of wood are located in radial and tangential direction. Wood shrinkage and swelling cannot be
avoided except good drying and wet customized conditions with relative air humidity. Shrinkage and swelling rate
can be lower to some extent through the use of laminated wood, due to the careful selection of the adhesive material,
the radial direction use, the use of structural adhesives and the high degree of production technology. Wood is very
strong along the fibers (longitudinal direction), but it has a limited strength perpendicular to them. (radial and
tangential directions). "Thus, the furniture should always be designed in a way to have tolerance to expand (swell) or
to reduce (shrink) the dimensions in radial and tangential direction."
Aims of the study. The main goals of the study are listed below:
• Minimum and maximum value of wood equilibrium moisture used in studied regions
• Amplitude calculation of moisture equilibrium across seasons for the studied regions,
• Determination dimensions (oak wood and tree shrinkage and swelling which is used in furniture of these
regions).
• Frequent defects as a result of wood shrinkage and swelling.
MATERIALS AND METHOD
Climatic conditions in Kosovo
1
The climate of Kosovo is continental, resulting in warm summers and cold winters with Mediterranean and Alpine
influences (average temperature within the country range from +30°C (summer) to - 10° C (winter). However, due
to unequal elevations in certain parts of the country, there are differences in temperature and rainfall distribution.
December and January are regarded the coldest months, July and August as the warmest months of the year. The
maximum rainfall rate is reached between October and December.
In Kosovo, it may snow between November and March, even in the flat parts of the country. The largest amount of
rain could fall in the mountainous regions of Kosovo.
Based on climatic conditions, Kosovo can be divided into three climatic zones:
• Climatic area of Kosovo plain “Rrafshi i Kosovës”
• Climatic area of Dukagjin plain “Rrafshi i Dukagjinit” and
• Climatic area of mountains and forest parts.
The climatic area of Kosovo plain, which includes the “Luginën e Ibrit” Ibri valey, is influenced by continental air
masses. For this reason, in this part of the country, the winters are colder with medium temperatures –10°C, but
sometimes down to –26°C. The summers are very hot with average temperatures of 20°C, sometimes up to 37°C.
This area is characterized by a dry climate and a total annual precipitation of 600mm per year, approximately.
The climatic area of Dukagjin plain, which includes the watershed of the “Drini Bardhë” river, is influenced very
much by the hot air masses, which cross the Adriatic Sea. Medium temperatures during winter range from 0.5°C to
sometimes 22.8°C. The average annual precipitation of this climatic area is about 700 mm per year. The winter is
characterized by heavy snowfalls.
Sampling and method of data calculating
Data on temperature and relative air humidity were taken from Hydrometeorology Institute for the period of 138
years.
1
http://www.kosovo-mining.org/kosovoweb/al/kosovo/climate.html

Rrahim Sejdiu1*, Ramadan Topuzi1, Muharrem Sejdiu2, Bujar Jashari1
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528
Figure 3. Places where samples were taken for study are marked in red
For the calculation of wood equilibrium moisture data is used formula (Simpson, 1998):
Where:
h- is the relative humidity (in decimal) and the coefficients W, K, K1 and K2 depend on the temperature:
For temperature T in ° C
W = 349 + 1.29T + 0.0135T2
K = 0.805 + 0.000736T - 0.00000273T2
K1 = 6.27 - 0.00938T - 0.000303T2
K2 = 1.91 + 0.0407T - 0.000293T2
Calculation of dimensional changes
The amount of shrinkage or swelling in certain parts of wood that participate in the finished products can be
calculated by the formula:
Where
∆W = changes in product width; e.g. board,
W = the original width of the product,
∆mc = changes in the moisture content of the product. e.g. if board dries from 12% to 8%, it has changed in moisture content
SC = the adjustment coefficient of shrinkage is provided in Appendix II.
Changes in moisture content only for a few percent are sufficient to cause contraction and bulge of dried wood.
The value of shrinkage/swelling will grow with magnitude change of relative humidity in service products, which
could cause a serious problem if the wood material from which the furniture is produced is not dried in the place
where it is located. Some wood shrinkage and swelling can always be predictable; that is why it is important that
furniture should be prepared by suitable dimensions for dimensional changes as e result of climatic conditions. In
addition, the need to control moisture and eliminate defects that are associated with the problems of dimensional
stability increases with the value of furniture. Small cracks and flaws can be tolerated in low or medium quality,
whereas the cracks are completely unacceptable to the furniture of high quality.
Table 1. Average of wood equilibrium moisture content seasonality in Kosovo’s Regions.
Average e Equilibrium Moisture Content according to season for 38 years
Pejë
Winter
15.0%
Spring
12.0%
Summer
11.6%
Autumn
16.0%
Prishtina
Winter
15.7%
Spring
13.0%
Summer
12.1%
Autumn
16.5%
Ferizaj
Winter
16.4%
Spring
13.8%
Summer
13.2%
Autumn
17.9%

International Journal of Ecosystems and Ecology Science (IJEES) Vol. 12 (2): 525-530 (2022)
https://doi.org/10.31407/ijees ISSN: 2224-4980 https://doi.org/10.31407/ijees12.2
529
Table 2. Extreme values of equilibrium moisture content of wood for some Kosovo’s regions and average
(optimal humidity) wood drying for external uses.
Regions
Lower EMC %
Higher %
Amplitudes in %
Optimal equilibrium moisture
content of wood
Pejë
10.5%
17.3%
6.8%
13.9%
Prishtinë
11.1%
18.3%
7.2%
14.7%
Ferizaj
12.3%
19.1%
6.8%
15.7%
RESULTS
Data on table 2 shows that: Peja and Ferizaj regions has the same amplitude 6,8%, whereas region of Prishtina has
amplitude 6,5%. Also are shown averages of equilibrium moisture content which are considered as optimal
averages.
Examples of the use of wooden products and the possibilities of shrinkage and swelling
In the case of beech (Sejdiu et al., 2013) or oak (Sejdiu et al., 2016) wood board 320Mm width, cutting in tangential
direction and placed in an environment where the relative humidity varies 6,8% (as it is in the region of Ferizaj and
Peja) will changes in dimensions as below.
Table 3. Changes dimensions of beech and oak 32mm wide, which is used in tangential direction
in some regions of Kosovo.
Because of these wood dimension changes, issues that can appear are presented in a figure below
b
.
a
.
c
.
Figure 4. Illustration of possible defects as e result of shrinkage and swelling in:
a. Damage of outside table because of swell: b. Damage of inside door because of shrinkage and c. Joints damage
because of swollen wood
Regions
Amplitude
%
Dimension changes
for 32cm wide
Dimension changes in %
for 32mm changes
A possible changes of wood board
32cm width
Beech
Oak
Beech
Oak
Beech
Oak
+
-
+
-
Pejë
6,8
0,86
0,57
2,7%
1,8%
32,86
31,14
32,57
31,43
Prishtinë
7,2
0,91
0,60
2,8%
1,9%
32,91
31,09
32,60
31,40
Ferizaj
6,8
0,86
0,57
2,7%
1,8%
32,86
31,14
32,57
31,43

Rrahim Sejdiu1*, Ramadan Topuzi1, Muharrem Sejdiu2, Bujar Jashari1
________________________________________________________________________________________________________________________________________________
530
CONCLUSIONS
• Although the data are collected for three regions, we can conclude that these results can be taken as general
for the entire Republic of Kosovo.
• The amplitude of equilibrium moisture for external uses is quite large in all regions. Although the region of
Peja generally has better conditions for natural drying, moisture equilibrium amplitude of wood in this
region is equal to the region of Ferizaj, which is 6.8%.
• Prishtina region appear to be the region with higher amplitude 7.2%.
• The above data shows that larger movements in dimensions are in beech wood, which is known for its
dimensional instability.
Recommended subjects involved in the production of wood products for outdoor use to adjust the equilibrium of
moisture content of their products in accordance with the data, which are derived from the study, in the region of
Peja i.e. is 13,9%, Prishtina 15,1% and 15,7% in the region of Ferizaj.
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