10.6092/issn.1973-9494/12797
Lo scopo di questo studio è quello di evidenziare l'aspetto del deterioramento derivante dagli effetti degli agenti atmosferici sul legno di faggio utilizzato nell'ornamento delle grate delle finestre in legno (Mashrabiya) nella “Moschea del principe Hassan”, Akhmim-Sohag e nella “Moschea di Khawand Asalbay”, Fayoum, Egitto. Questo legno è stato accuratamente esaminato con un microscopio digitale e un microscopio elettronico a scansione (SEM) per diagnosticare i processi di deterioramento sulla superficie del legno. L'infrarosso in trasformata di Fourier (FTIR) è stato utilizzato per identificare i cambiamenti nei componenti principali del legno di faggio dovuti a fattori di alterazione. Il cambiamento cromatico della superficie in legno di faggio è stato misurato mediante spettrofotometro utilizzando il sistema CIE-L*a*b*. I risultati mostrano i cambiamenti fisici e chimici verificatisi nel legno di faggio degradato. Tali risultati hanno contribuito a sviluppare un piano di trattamento utilizzando materiali già sperimentati che non danneggiano i manufatti in legno.
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1973-9494
12797
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Mahmoud Ali
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Conservation Science in Cultural Heritage
weathering
Weathering of Outdoor Beech Wood and Methods of Conservation
Text.Serial.Journal
https://conservation-science.unibo.it/
20
Articles
2021-05-27
2021-04-27
2021-05-13
2021-05-27
167-180
CSCH
Mahmoud Ali
2020
10.6092/issn.1973-9494/12797
167
Conservation Department, Faculty of Archaeology , Luxor University, Egypt
1973-9494
weathering
en
180
Ali, M. (2021). Erratum: Weathering of Outdoor Beech Wood and Methods of Conservation. <em>Conservation Science in Cultural Heritage</em>. https://doi.org/10.6092/issn.1973-9494/12951
Weathering of Outdoor Beech Wood and Methods of Conservation
https://conservation-science.unibo.it/article/download/12797/12598
20
W       EATHERING OF OUTDOOR BEECH WOOD AND
        METHODS OF CONSERVATION

   Mahmoud Ali *
   Conservation Department, Faculty of Archaeology
   Luxor University, Egypt


   Keywords: beech wood, conservation, FTIR, outdoor, weathering



   1. Introduction

    Beech wood (Fagus sylvatica L) is one of the diffuse-porous hardwood types [1,2],
and is characterized by its medium-high density, high resistance to shock [3], and its
hardness and strength. For this reason, it is one of the most imported woods to have
been used in ancient Egyptian civilization throughout the different ages. It was used for
many purposes, such as in making statues, doors, windows, furniture and also in the
outdoor elements of archaeological buildings. These outdoor elements are directly ex-
posed to the damage resulting from various weathering factors which in turn lead to the
appearance of deterioration on the outer surface and over time can reach the interior
of the wood causing the loss of some of its surface parts.
    Weathering is a term used to describe the process of slow decomposition that occurs
in archaeological materials which are exposed to varying atmospheric conditions [4,5].
Weathering depends on many environmental factors such as moisture (relative humidity,
dew, rain, and ground water), solar radiation (ultraviolet light, visible light, and infrared)
and temperature [6,7]. These factors have a destructive effect on wood found in the ex-
ternal parts of archaeological buildings [8] and can cause complex damage [9] to the
surface layers [8], including changes in the appearance of the ornamental elements
and of the painted layers [10]. The impact of weathering begins with surface damage by
directly affecting the coherence of the wood fibres and color layers [11]. In addition,
weathering causes chromatic alterations, cracks and changes in surface texture [12,13].
This damage is the result of the joint effect of direct sunlight, water and erosion result-
ing from the wind, which contains sand particles [14,15]; this type of damage also causes
a loss of wood mass [16]. In addition, the spread of fissures on the surface of archaeo-
logical wood creates a suitable environment for the penetration of biological factors of
deterioration inside the wood [17].


   2. Materials and methods

     This study focuses on the beech wood used in the execution of wooden window
grilles made of turned wood (Mashrabiya) in two archaeological Mosques: the Mosque
of Prince Hassan and the Mosque of Khawand Asalbay (Figures 1 and 2). The Mosque


   * Corresponding author. dr.mahmoud.sayed82@gmail.com
                                                                        of Prince Hassan is located in the city of Akhmim in the Sohag governorate, Egypt, and
                                                                        was established by Prince Hassan bin Amir Muhammad in 1704 AD (1117AH); it was
                                                                        completed in 1709 AD (1121AH) [18]. The Mosque of Khawand Asalbay, the wife of
                                                                        Sultan Qaytbay, is located in the Fayoum governorate, Egypt. It was built during the
                                                                        reign of her son Sultan Muhammad Ibn Qaytbay in 903 AH / 1499 AD [19].
                                                                            The diagnosis of the damage and deterioration affecting these window grilles de-
                                                                        scribes their condition at the time of the research. The images and the results of the
                                                                        sample examination were obtained before any restoration work was carried out for these
                                                                        two mosques.
M. Ali - Weathering of outdoor beech wood and methods of conservation




                                                                        Figure.1 A. Prince Hassan Mosque; B. wooden window grilles made of turned wood (Mashrabiya);
                                                                        C. a cross-section of the beech wood (Fagus sylvatica. L).




                                                                        Figure.2. A. Mosque of Khawand Asalba; B. window grilles made of turned wood (Mashrabiya);
                                                                        C. a cross-section of the beech wood (Fagus sylvatica. L).


                                                                           2.1. Main weathering factors surrounding the two mosques

                                                                            Moisture
                                                                            Wood is a hygroscopic material that is easily affected by variations in moisture
                                                                        levels [20]. There are many sources of moisture surrounding the two mosques, such
                                                                        as rain, dew, frost, and relative humidity which increases with high temperatures [21].


                                                 168
These mosques are also situated close to agricultural land which naturally leads to an
increase in the amount of moisture. In addition, many ancient cities lack sewage net-
works which leads to an increase in moisture content and consequently affects the
wood in these buildings.
     The risk of damage from moisture is that it reacts with other deterioration factors
surrounding the archaeological materials, such as air pollution. In high levels of humid-
ity, some atmospheric pollutant gases turn into destructive acids for archeological wood
and for its color layers [8]. Examples of these gases include sulfur and nitrogen dioxides
which turn into sulfuric and nitric acids that cause the color of the wood to change [22].
Moisture, moreover, causes dust particles to adhere to the surface of the wood.

    Temperature
    The sun is a source of heat. Its high temperature leads to the destruction of outdoor
wood used in archaeological buildings that are constantly and directly exposed to the
excessive heat of high temperatures [23]. The swelling and shrinking of the wood, re-
sulting from the fluctuations in relative humidity and in temperature, lead to the separa-
tion of the tongue and groove joints, the separation of other wooden pieces from one
another, as well as to the overall dryness and fragility of the damaged wood [24]. Tem-
perature causes a decrease in wood substance and strength and is related to other
changes in wood polymers and structure [25].

    Solar radiation
    Most of the outdoor wood in archaeological buildings is directly exposed to the
sun with its harmful ultraviolet, visible light, and infrared radiation [26]. Solar ra-
diation is one of the main factors which causes notable changes in wood surfaces
[27], in addition to other physical, chemical, and mechanical alterations in its prop-
erties [26].
    The wood surface is affected by the photochemical deterioration caused by UV




                                                                                                Conservation science in Cultural Heritage
radiation [28]. It is a slow process that has an effect marked by only a few millimeters
on the damaged surface, while the fibres of the wood below the surface remain coher-
ent [29]. This process depends on the density, type of wood, and moisture content. The
changes observed on the beech wood surface (such as fragility, disintegration, color
changes, fibers dissociation, and roughness of surface) reflect the effect of UV radia-
tion on the chemical compounds. Lignin, which is an adhesive for cellulosic polymers
in the cell walls and the main component of middle lamellae [30], is severely affected
by UV radiation [31]. These rays cause the chemical bonds of cellulose and hemicel-
lulose to break down. Its effect on the cellulose can be observed in the following char-
acteristics: a loss of strength, a change in the degree of polymerization, its color, tend-
ing toward yellow, and sometimes brown, and an increase in its solubility in alkalis [26].
Infrared radiation produces a heat that leads to a loss of absorbed moisture in the wood,
which in turn changes the dimensions of the wood [32].


   3. Results and discussion

    Several examination methods and tests were carried out to better understand the
effect of weathering on archaeological beech wood by using digital USB Microscopy,
SEM Microscopy, Spectrophotometer OptiMatch 3100®, SDL Company, and FTIR.
The results are reported as follows.


                                                                                              169
                                                                           3.1. Physical changes

                                                                            The direct exposure of outdoor beech wood in these two mosques to weathering
                                                                        factors has had a visible effect on the appearance and on the surface of this wood. We
                                                                        can notice that weathering has led to several changes in the appearance of the wood
                                                                        surface, such as roughness, weakness and fragility (Figures 3 and 4). The weakness
                                                                        of the fiber bonding of the wood surface has led to its crumbling very easily. Cracks are
                                                                        widely spread in all parts of the wood and range between fine, medium, and large
                                                                        cracks (Figures 5 and 6). These cracks develop gradually with continuous exposure to
                                                                        weathering factors, so we find that the small cracks get bigger and become wider, thus
                                                                        enabling the side edges of the wood parts to be readily observed [33, 34,12]. The spread
                                                                        of these cracks in the surface of the wood, moreover, facilitates the permeation of the
                                                                        various weathering factors to the inner layers of the wood. The cracks therefore are
                                                                        also considered suitable areas for the spread of wood decaying fungi [35].
                                                                            Swelling and shrinking processes have led to the separation of the wooden joints
                                                                        especially when two pieces of wood are joined together in different directions [36]
                                                                        (Figure.5. D). The accumulation and adhesion of dust particles on the wood surface
                                                                        can also be observed. This is the result of the dust in the area surrounding the two
                                                                        mosques. Humidity and temperature also contribute to the adhesion of dust particles
                                                                        on the wood surface [37] (Figure.7).
M. Ali - Weathering of outdoor beech wood and methods of conservation




                                                                        Figure.3. Detail of the roughness of the surface. A-B) Mosque of Prince Hassan; C-D) Mosque of
                                                                        Khawand Asalbay.




                                                                        Figure 4. SEM micrographs. A-B) fragility and breakdown of the beech wood fibers, Mosque of
                                                                        Prince Hassan; C) loss of the bonding between beech wood fibers, Mosque of Khawand Asalbay,
                                                                        Fayoum. (A - 100 X; B – 130 X; C - 200X).




                                                                        Figure 5. A, B, C) Spread of various cracks and checks on the beech wood surface; D) separation
                                                                        of joints, Mosque of Khawand Asalbay.


                                                  170
Figure 6. Dryness and spread of fine cracks on beech wood surface; A-B) Mosque of Prince Hassan;
C-D) Mosque of Khawand Asalbay (Digital Microscope).




Figure 7. A, B, C, D) Adhesion of dust particles on the beech wood surface. B-C) Digital Micro-
scope; D) SEM micrographs 250X.


    Weathering causes color changes in the surface of the archaeological wood indicat-
ing that there are chemical changes. This process depends on several factors such as
the type of wood and the duration of its exposure to weathering factors. The color of the
wood surface turns gradually to yellow or brown, and as the degradation increases, the
surface color turns into a very thin gray layer [8,17,38,39]. This process is clearly evident
when the wood is exposed to weathering factors in dry climates or coastal areas [40].
The color change in wood surfaces occurs as a result of the photodegradation of chemi-
cal compounds in the wood, such as lignin and wood extractives by sunlight [8,38].




                                                                                                     Conservation science in Cultural Heritage
    Color measurement
    The (CIE-L*a*b*) system was used to measure the color changes of beech wood
caused by the effects of weathering. L* indicates the lightness of the surface (0 = black
to 100 = white), a* and b* clarify the chromaticity coordinates (+a* indicates the red
color and -a* indicates the green color), (+b* indicates the yellow color, and –b* indi-
cates the blue color). ∆E* clarifies the total value of color differences. The following
equations were used to calculate the differences in ∆L*, ∆a*, and ∆b* and calculate the
total color differences ΔE*:

    ΔL = L* - L*0 , Δa = a* - a*0 , Δb = b* -b*0.
    ∆E* = [(∆L*)2 + (∆a*)2 + (∆b*)2]1/2 [5,6,31,41-45].

    In this research, some samples of beech wood, taken from the two mosques, were
used to measure the color changes. We measured the change in color of the wood
surface to find the difference in color between the external parts (surface) that are ex-
posed to weathering factors and the internal parts (3mm from the surface).
    Table 1 indicates the changes in the values: L*, a*, b*, and ΔE* of the samples S1
(external surface), S2 (internal parts) taken from the Mosque of Prince Hassan), and
samples S3 (external surface), S4 (internal parts) taken from the Mosque of Khawand
Asalbay).


                                                                                                   171
                                                                        Table 1. Measurement of color changes of the beech wood surface (S1 and S3) and internal parts
                                                                        (S2 and S4)

                                                                          Sample        L*         ∆L*           a*           ∆a*           b*           ∆b*         ∆E*
                                                                           S2          55.11     -------      6.29       -----            13.32        -------     -----
                                                                           S1          48.07     - 7.04       1.68       - 4.61            5.68        - 7.64       11.37
                                                                           S4          55.30     -------      1.31       ------            7.35        -------     -----
                                                                           S3          49.17     - 6.13       2.09        0.78             6.08        - 1.27        6.31

                                                                            We can observe through this table that there is a clear decrease in ΔL* values
                                                                        (S1= -7.04) and (S3= -6.13) which indicates the darkening of the wood surface com-
                                                                        pared with the internal parts. A previous study indicated that the darkening of wood
                                                                        was the result of hemicellulose degradation caused by heat [46]. We can also notice a
                                                                        strong color change that can be easily observed by the naked eye on the external
                                                                        surface of the beech wood (ΔE* for sample S1=11.37) and (ΔE* for sample S3 =6.31).


                                                                           3.2. Chemical changes

                                                                            The effect of weathering on the main compounds of archaeological beech wood
                                                                        (cellulose, hemicelluloses, and lignin) can be observed by using FTIR spectra. The
                                                                        differences in the intensity, shape, and position of FTIR bands indicate the chemical
M. Ali - Weathering of outdoor beech wood and methods of conservation




                                                                        changes in the deteriorated wood [47]. To better understand the changes in the chem-
                                                                        ical compounds, we compared the beech wood samples from the two mosques using
                                                                        FTIR analysis. The surface samples – “S1 and S3ʺ, the internal samples – “S2 and
                                                                        S4ʺ) and beech wood which was not exposed to weathering factors was used as a
                                                                        control sample (S5) (Table 2 and Figures 8 and 9).

                                                                        Table 2. FTIR spectra results of beech wood samples.

                                                                         Wavenumber
                                                                                                 Functional group                                Assignment
                                                                           (cm−1)

                                                                          3410-3425     O-H stretching                            Cellulose, lignin and hemicellulose [48]
                                                                          1731 -1735    C=O unconjugated                          Xylan (hemicellulose) [49,50]
                                                                             1640       Absorbed O-H and conjugated C-O           Loss of water [51]
                                                                             1595       C=C stretching of the aromatic ring       Lignin [52]
                                                                             1508       C=C stretching of the aromatic ring       Lignin [53,52]
                                                                             1463       Asymmetric bending in CH3                 Lignin [52]
                                                                                                                                  Cellulose (crystallized and amorphous)
                                                                             1421       CH2 bending
                                                                                                                                  [54,52]
                                                                          1384-1346     C-H bending                               cellulose, hemicellulose [55]
                                                                          1330-1320     C–H and C1-O vibration                    In cellulose and syringyl units [48]
                                                                             1267       C-O stretching                            Lignin and hemicellulose [48,52]
                                                                             1158       C-O-C vibrations                          Cellulose [50]
                                                                           897-898      C-H deformation                           Cellulose [56]



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Figure 8. FTIR transmission spectra of the control sample (S5), surface (S1) and internal part (S2);
Mosque of Prince Hassan.




                                                                                                         Conservation science in Cultural Heritage
Figure 9. FTIR transmission spectra of the control sample (S5), surface (S3) and internal part (S4);
Mosque of Khawand Asalbay.


    We can notice a decrease in the O-H stretching band which appeared in some
samples in the range of (3410- 3425cm-1) with sharper peaks (S1 and S3 samples).
The decrease in the intensity of the water absorption band at (1640cm -1) resulted
from the loss of water from the wood after its continuous exposure to high tempera-
tures [49,57].
    Two bands at 1730-1735cm-1 were observed in the samples S3 and S4, but in
samples S1 and S2, the disappearance of the band of C=O stretching in hemicellu-
loses at 1733cm-1 was noted, compared with the control sample that had one band at
1733cm-1 which is due to the deterioration of the hemicellulose [4,6,58].
    It was clearly noticed that the intensity of the lignin bands at (1595, 1508, 1421 and
1267cm-1 ) decreased in both the S1 and S2 samples and was due to the effect of
weathering factors, especially UV light [41,59].
    The decrease of the C-O-C stretching bands at 1159 in all the samples indicate the
breaking of the cellulose chains [60,49,57]. The decrease in absorption at 897cm -1
bands (C1–H deformation of glucose ring) in all samples, refers to a diminution of the
amorphous form of the cellulose [61].


                                                                                                       173
                                                                           3.3. Treatment

                                                                           The treatment process includes cleaning and consolidation for some small sam-
                                                                        ples which were separated from the wooden window grilles (Mashrabiya) in the two
                                                                        mosques as a result of the effect of weathering factors.


                                                                           Cleaning
                                                                           The cleaning process is one of the most difficult steps in the treatment for two
                                                                        reasons:
                                                                           The spread of cracks full of dust and dirt on the surface.
                                                                           The extreme vulnerability of the beech wood.
                                                                           There are two kinds of cleaning: mechanical and chemical cleaning.


                                                                           Mechanical cleaning
                                                                            Mechanical cleaning is a very important process and any kind of damage to the
                                                                        surfaces, which are often very fragile and weak, must be avoided at all costs. During
                                                                        the accurate treatment of this archaeological wood, this process is carried out with
                                                                        great patience and a great deal of time. It is also an important step in detecting the
                                                                        extent of the erosion and damage [62]. The purpose of this process is to break down
M. Ali - Weathering of outdoor beech wood and methods of conservation




                                                                        the bonds of the accumulated dirt on the archaeological surface [63]. The digital micro-
                                                                        scope and SEM micrographs revealed that the surface of the beech wood was covered
                                                                        with a layer of dust. Due to the weakness of the wooden surface, we used some
                                                                        brushes and small scalpels with great caution to remove the dust from the surface and
                                                                        cracks. The results showed that this method was inadequate and did not remove all the
                                                                        dust from the surface [63].

                                                                           Chemical cleaning
                                                                            Chemical cleaning was used to remove the remaining dust adhering to the exter-
                                                                        nal surface of the archaeological wood by using chemical solvents to break the
                                                                        bonds that join together the atoms of dirt [22]. In this research, Ethyl alcohol was
                                                                        used to remove the dust by using brushing. It gave good results in removing the dust
                                                                        on the surface and made no significant impact on the appearance of the wood sur-
                                                                        face. A previous study showed that the use of ethyl alcohol has little effect on the
                                                                        structure of wood [64].

                                                                           Consolidation
                                                                           In general, the main objective of the consolidation process is to bring back co-
                                                                        herence and to stabilize the archaeological wood deteriorated by chemical, me-
                                                                        chanical, and biological factors [65]. It helps also in the restoration of some me-
                                                                        chanical properties of the damaged wood. Due to the fragility of the surface, the
                                                                        beech wood samples were consolidated with paraloid B.72 dissolved in acetone
                                                                        (3% w/v) [66] using soft brushes. The purpose of the consolidation is to ensure the
                                                                        adhesion of the separated fibers and parts that have become detached from the
                                                                        wood surface (Figure.10).


                                                   174
Figure 10. Some samples of the wooden window grilles made of turned wood (Mashrabiya).
A) Mosque of Prince Hassan; B) Mosque of Khawand Asalbay, before treatment processes;
C) Mosque of Prince Hassan; D; Mosque of Khawand Asalbay, after treatment processes (Digital
Microscope).


      4. Conclusions

     The exposure of wood in archaeological and historical buildings to weathering fac-
tors is an inevitable reality. The situation and conditions, therefore, need to be con-
stantly monitored and examined, even after all the treatment processes have been
completed. In this research, the deterioration of the wooden surface resulted from the
exposure of beech wood to moisture, temperature, and intense sunlight. By using the
scanning electron microscope (SEM), the USB digital microscope, and spectropho-
tometer and FTIR analyses, this research clearly highlighted the changes in the beech
wood surface, such as the spread of several cracks, its roughness and fragility, and the




                                                                                                  Conservation science in Cultural Heritage
changes in color. The highest rate of color change appeared clearly on the beech wood
used in the Mosque of Prince Hassan, as the color tended toward dark brown and is
the result of hemicellulose degradation. The FTIR results showed the deterioration and
deformation of the main compounds of the archaeological beech wood (Cellulose,
Hemicellulose and Lignin) by the weathering factors. This deterioration and deforma-
tion appeared in both the surface and the internal samples (up to 3mm below surface).
     The purpose of the treatment process on several small samples, which were found
detached from the windows, was to benefit as much as possible from the research re-
sults. These results enabled us to select suitable methods and materials for the success-
ful treatment of the deteriorated archaeological wood, instead of using modern paints or
varnish and replacing some of the archaeological parts with new ones, which would have
inevitably led to distorting the wooden artifacts and the loss of their archaeological, his-
torical and artistic value. This is, however, a situation which can be observed in some
historical mosques that have been treated in previous periods. In this research, we sug-
gest appropriate scientific methods for correctly treating beech wood.


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   Biographical notes

    Mahmoud Ali is a lecturer in restoration and conservation of wood at the Conserva-
tion department in the Faculty of Archaeology, Luxor University, Egypt. He obtained a
Master degree in 2011 and Ph.D. degree in 2015, in conservation of archaeological
wood at the Conservation department, Faculty of Archaeology, South Valley University,
Egypt.


   Summary




                                                                                               Conservation science in Cultural Heritage
     The aim of this study is to highlight the appearance of deterioration resulting from
weathering effects on the beech wood used in the ornamentation of wooden window
grilles (Mashrabiya) in the Mosque of Prince Hassan, in Akhmim-Sohag and in the
Mosque of Khawand Asalbay, in Fayoum, Egypt. This wood was accurately examined
with a Digital microscope and Scanning Electron Microscope (SEM) to diagnose the
deterioration processes on the surface of the wood. Fourier transform infrared (FTIR)
was used to identify the changes in the main components of the beech wood resulting
from weathering factors. The chromatic change of the beech wood surface was mea-
sured by spectrophotometer and the results were calculated by using the CIE-L*a*b*
system. Results showed the physical and chemical changes that had occurred in the
degraded beech wood. The results helped to develop a treatment plan using already
experimented materials that do not harm the wooden artifacts.


   Riassunto

    Lo scopo di questo studio è quello di evidenziare l’aspetto del deterioramento deri-
vante dagli effetti degli agenti atmosferici sul legno di faggio utilizzato nell’ornamento
delle grate delle finestre in legno (Mashrabiya) nella “Moschea del principe Hassan”,
Akhmim-Sohag e nella “Moschea di Khawand Asalbay”, Fayoum, Egitto. Questo legno
è stato accuratamente esaminato con un microscopio digitale e un microscopio elettro-


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                                                                        nico a scansione (SEM) per diagnosticare i processi di deterioramento sulla superficie
                                                                        del legno. L’infrarosso in trasformata di Fourier (FTIR) è stato utilizzato per identificare
                                                                        i cambiamenti nei componenti principali del legno di faggio dovuti a fattori di alterazio-
                                                                        ne. Il cambiamento cromatico della superficie in legno di faggio è stato misurato me-
                                                                        diante spettrofotometro utilizzando il sistema CIE-L*a*b*. I risultati mostrano i cambia-
                                                                        menti fisici e chimici verificatisi nel legno di faggio degradato. Tali risultati hanno
                                                                        contribuito a sviluppare un piano di trattamento utilizzando materiali già sperimentati
                                                                        che non danneggiano i manufatti in legno.
M. Ali - Weathering of outdoor beech wood and methods of conservation




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