Treatment of Paper with Persian Gum (Zedu) in Combination with Satureja khuzestanica Extract: Increasing Mechanical Strength and Fungicidal Effects

1.1 Background

Historical paper artifacts serve as invaluable records of the past, reflecting the historical evolution of different cultures and nations. The preservation of these works is crucial, as they embody the achievements and values throughout history. Conserving these artifacts for future generations requires a thorough understanding of their properties, which can be broadly categorized into material characteristics, such as paper structure and composition, and conceptual characteristics, such as the cultural and social significance of the works. Any alteration to these characteristics can undermine the preservation’s purpose, jeopardizing the authenticity and integrity of these artifacts.

Destructive factors affecting works on paper include physical, chemical, and biological agents, which lead to structural changes in the paper, increased acidity, brittleness, and yellowing over time. Therefore, the use of suitable strengthening materials, combined with treatments to introduce an alkaline reserve can be highly effective. This research focuses on the conservation of paper artifacts by reinforcing their strength using zedu for re-sizing which was altered to introduce an alkaline reserve into paper, ensuring that the integrity, form, or structure of the artifacts is not compromised.

Resizing in modern conservation practices serves three primary purposes: (1) increasing mechanical stability, (2) modifying the surface texture of abraded papers by reducing surface roughness and consolidating loosened fibers, and (3) preparing the paper substrate for inpainting. However, contemporary practices emphasize a more careful differentiation between improving mechanical stability and enhancing resistance to physical and chemical degradation. While protein glues were traditionally used for resizing, modern conservation commonly employs gelatin and cellulose ethers as sizing agents. Regarding application techniques, immersion methods – closely related to tub sizing used during paper manufacturing – along with localized or overall brush applications, were historically utilized for resizing. Spraying, introduced in the mid-20th century, is now recommended for resizing drawings to prevent friable media from being smudged (Hummert 2019).

1.2 Problem Statement

Primary responsibility of paper conservators is to prevent change or damage of paper-based artifacts through effective preservation strategies. The structure of paper typically comprises cellulose fibers, a reinforcing sizing material, and fillers to improve the writing surface. Due to their organic nature, cellulose and other polysaccharide components of paper are susceptible to damage in various environments. These damages may be physical, such as those caused my moisture and mechanical pressure; they may be chemical such as cellulose oxidation and acidity leading to yellowing or they may be biological, involving the effects of bacteria, fungi, insects, and rodents, all of which contribute to the mechanical and physical weakening of paper artifacts. Despite the advancement in materials science, significant gaps in research remain, particularly in exploring natural, locally sourced plant materials such as Persian gum (zedu). These materials hold potential as effective alternatives for various applications, especially in the context of conservation and preservation. Comprehensive studies are needed to evaluate the properties and efficiency of these materials, paving way for sustainable solutions in conservation science.

1.3 Research Aim and Objective

Given the objective of this research – specifically, the conservation of paper artifacts by reinforcing their strength – re-sizing with materials that at the same time introduce an alkaline reserve is proposed, provided that it does not compromise the integrity, form, or structure of the artifacts. The materials used in this process must possesses specific properties: appropriate particle size, color transparency, flexibility, desirable solubility, suitable viscosity for the artifacts, and an alkaline pH (Barrett 1992; Henry 1986, 1988). The aim of this research is to explore the potential of Persian gum as a sustainable and effective alternative for conservation and preservation applications. Despite advancements in materials science, the full potential and efficiency of Persian gum as a conservation material remain under-researched. This study seeks to evaluate its properties and contribute to the development of environmentally friendly and regionally significant solutions in the field of material conservation.

1.4 Literature Review

Numerous polymers are used for re-sizing or paper stabilization like gelatin, methylcellulose, and wheat starch. Several studies have been conducted around the world on these materials including Iran. In their research, Soheilipour and Azadi Bouyaghchi (2015) investigated reinforcing of paper artifacts using nano particles of tragacanth gum. According to their research, polysaccharides, including vegetable starches and gums, are among the most commonly used materials in textile and paper production. Gums enhance fiber connections when applied between them and are easier to produce and prepare than other natural polymers. A study by Ghorbani, Heidari, and Zabihzade (2017) found that the presence of biopolymers in the fiber network positively impacts the physical, mechanical, and biological properties of wood-based artifacts.

Biological agents, such as fungi, can cause irreversible damage including structural degradation of paper, weakening of fibers, and visual effects, all of which contribute to the degradation of the work and pose health risks to those in contact with artifacts. Identifying fungal types in archives has become a critical concern for conservators. Due to the varied responses of fungi to treatment methods, selecting an optimal treatment method requires precise identification of the fungus, understanding its mechanism, and assessing the form and depth of the damage. Ghahri (2005) investigated the remarkable ability of cellulose and collagen components in the structure of books to absorb, retain, and hold moisture from the atmosphere. This retained moisture creates an ideal environment for fungal growth, which can lead to the degradation and eventual destruction of archival materials such as manuscripts and documents. He identified Aspergillus, Penicillium, and Cladosporium spp. as the most significant paper-damaging fungi. Raeisnia (2010) has also studied the types of fungi that have significant impact on the degradation of paper artifacts in the collection of the National Library of Iran. These fungi not only weaken the structural integrity of the paper but also exacerbate the chemical degradation of cellulose, resulting in brittleness, discoloration, and loss of mechanical strength over time. Ghahri (2005) reviewed preventive methods, emphasizing the importance of controlling storage conditions, such as humidity and temperature, to mitigate fungal growth and maintain the stability of historical papers.

Referring to Persian historical texts and treatises, researchers have highlighted the potential of plants as preventive agents with the capacity to control biological growth. A study by Barkeshli, Ataie, and Alimohammadi (2008) demonstrated that the natural plant Henna, when used in the ratio of 1:10 as advised in historical recipes for paper dye, exhibits strong fungicidal properties against Aspergillus flavus. Khoubani et al. (2015) also investigated the antifungal properties of Kabikaj (Ranunculus sceleratus) extract for the conservation of historical works on paper, emphasizing the need for effective material with minimal risk. The antifungal properties of plants such as cumin, clove, pepper, sesame, camphor, thyme, cinnamon, savory, and garden sage have been affirmed in various studies. Similarly, Taheri et al. (2015) identified common fungi found in paper artifacts and studied the fungicidal properties of herbal extracts. In their recent study, Barkeshli et al. (2024) also examined the physical, optical, spectral imaging, and fungicidal properties of various natural vegetable and protein sizing materials used in Persian manuscripts, discussing their behaviour in detail.

1.5 Research Methodology

This study employed a systematic approach to evaluate the effectiveness of Persian gum nanoparticles in combination with other materials – calcium hydroxide, Satureja khuzistanica essential oil, and glycerol – on the mechanical and biological properties of paper, as well as their color stability. It is important to note that S. khuzistanica is listed as a threatened species by Kew Gardens (Kew Science: Plants of the World online). As such, this study focuses solely on experimental applications in a controlled environment and does not advocate for large-scale use of this plant. Future research may explore alternative sources or synthetic substitutes to address potential conservation concerns. The research methodology is structured as follows:

Material Preparation: Persian gum nanoparticles were synthesized and calcium hydroxide, S. khuzestanica essential oil, and glycerol were added to improve their positive effects on paper. These materials were selected for their known properties in enhancing mechanical strength, biological resistance, and minimal color alteration.

Sample Selection: Whatman filter paper was used as a model substrate to simulate the characteristics of historical paper artifacts exhibiting structural damage, porosity, and biological degradation. The samples were prepared as required for each analysis and analyzed both before and after treatment.

Experimental Design: The study was divided into three experimental phases:

1. Mechanical Testing: The paper samples were subjected to mechanical tests before and after treatment to assess improvements in tensile strength at room temperature.

2. Resistance to Biological Degradation: The fungicidal properties of the treatment was evaluated by exposing treated samples to A. flavus and other common paper-degrading fungi. The growth and impact on the paper fibres were monitored and recorded in the laboratory environment and in the incubator.

3. Color Stability Assessment: The optical properties of the treated paper were measured using spectral imaging techniques to determine the effects of the treatment on the paper’s visual characteristics.

Data Collection and Analysis: Data were collected from each experimental phase and analysed using statistical methods to determine the significance of the improvements in mechanical strength, resistance to biological degradation, and colour stability.

Comparison with Untreated Samples: To assess the effectiveness of the treatments, results from the treated samples were compared with untreated control samples. This comparison provided insights into the potential applications of Persian gum nanoparticles in conservation practices.

This methodology allowed for a comprehensive assessment of the potential of Persian gum as a conservation treatment for historical paper-based artifacts.

1.6 Material Properties

1.6.1 Persian Gum

The physical properties and rheology of gums were studied, highlighting its solubility as a material for increasing paper strength due to its nano-scaled size, neutral pH, and lack of color and opacity (Nayari et al. 2015). Persian gum, a natural polysaccharide from wild almond trees native to Iran, is particularly abundant in the Zagros forests. This gum is used in industry as a thickener, emulsifier, and stabilizer. Known scientifically as Amygdalus Scoparia Spach (Synonym: Prunus Scoparia Spach), this tree is one of the wild (mountain) almond species, which can be found along with other wild species in the Rosaceae family. The shrub is a vital part of Iran’s arid and semi-arid woodlands (Abbasi 2017).

Persian gum is a viscous exudate released from the tree’s trunk or branches in response to environmental stress or injury (Figure 1). While similar gums are produced by other species in the Rosaceae family, Persian gum from wild almond trees is the most commercially significant. It is known by various local names including gum Zedu, Zedo, Angum, Angom, Farsi, Shirazi, Arzhan, Arjhan, Jedo, Zed and Ozdu, gomme notras, and gum gharacia (Abbasi and Rahimi 2015). Persian gum is a transparent, odorless substance available in forms ranging from granules to powder, with colors from white to brownish red. Galactose, rhamnose, and arabinose are likely the major monosaccharides constituting Persian gum. Considering data obtained by gas chromatography coupled to mass spectrometry (GC-MS), it is concluded that Persian gum is mainly composed of arabinose (Ara) and galactose (Gal) with Ara:Gal ratio of 2:1. Traces of xylose (6.8 mol %), rhamnose (1.1 mol %), and mannose (0.3 mol %) may also be found (Fadavi et al. 2017). Fucose (Fuc), glucose (Glc), N-acetyl galactosamine (GalNAc) and N-acetyl glucosamine have not been detected (Dabestani et al. 2018). The white variety was used in this study. It has an anionic acid polysaccharide structure similar to other gums and contains soluble and insoluble components with a soluble-to-insoluble ratio of 70:30 (Barzegari et al. 2013).

Figure 1:

Persian gum and its formation on the tree. Source: Dabestani et al. 2018.

The gum was prepared in an aqueous solution which helps in deacidifying the paper and has shown desirable effects after aging. To optimize the gum for increasing the pH of treatment and providing alkaline conditions, hydroxides were utilized. Studies by Burgess and Duffy (1990) employed alkaline materials such as magnesium hydroxide, calcium hydroxide, magnesium carbonate, and calcium carbonate for the preservation of paper artifacts. Among these, calcium hydroxides demonstrated desirable properties, including the absence of by-products like saltpeter (potassium nitrate), which can form in other hydroxide-based treatments (Burgess and Duffy 1990; Kolar and Novak 1996).

Calcium hydroxide also offers additional benefits, such as the absence of fiber swelling, durability after aging, and compatibility with water-based solutions. It provides an alkaline deposit within the paper structure, which increases the pH of treatment and helps counteract acidity in paper artifacts (Bogaard 2001). While calcium bicarbonate is often preferred in modern conservation practices due to its better solubility and lower alkalinity, calcium hydroxide was chosen for this study based on its long-standing use in Iranian conservation practices and its availability. However, we acknowledge the ongoing discussions regarding the high alkalinity of calcium hydroxide and its potential limitations in certain conservation contexts.

Considering the low flexibility of polysaccharide-based treatments, it is necessary to add a suitable plasticizer to reduce structural interactions, improve flexibility, increase stretchability, and decrease the humidity transfer to the treatment (Ranjbar, Bahrami, and Joghataei 2013). Glycerol, a polyol with plasticizing properties, was selected due to its transparency, solubility in buffers, stabilization, and non-toxicity (Fazel et al. 2012). While glycerol is hygroscopic and can attract humidity, which is generally undesirable in cultural heritage applications, its use in this study was limited to experimental conditions where its stabilizing and plasticizing properties were prioritized. Future studies may explore alternative plasticizers with similar benefits but reduced hygroscopicity to mitigate potential long-term risks to treated artifacts.

1.6.2 Satureja khuzestanica

S. khuzestanica, commonly referred to as Marzeh Khuzestan in Persian, is a species within the Satureja genus, part of Lamiaceae family and the Nepetoidae subfamily (Figure 2). It is found in the southern and southwestern sections of Iran and is used as a mouth disinfectant and dental anesthetic in traditional medicine, as well as in the food and pharmaceutical industries (Abbasloo et al. 2023). In the present study, S. khuzestanica was used to provide fungicidal properties to the treatment. This plant contains more than 4.5 % essential oil (Khosravinia 2015). The main components of this plant include Carvacrol (90.8–94.16 %) P-cymene (1.26 %), and Gamma terpen (0.74 %). Gamma-terpinene is an essential oil with antibacterial and antioxidant properties found in some plants. Carvacrol, a thymol isomer with a similar aroma (Darvishnia, Rezaeinejad, and Delfan 2015), possesses antifungal and antimicrobial properties (Asaei, Bashiri, and Pajuhi 2012). The analysis of herbal extracts revealed that carvacrol plays a crucial role in the fungicidal properties of S. khuzestanica. The concentration of caracole is higher in S. khuzestanica compared to other savory species, as well as marjoram, thyme, peppermint, and olive leaf (Darvishnia, Rezaeinejad, and Delfan 2015). Phenolic-based extracts such as those found in S. khuzestanica are volatile compounds and stored in secreted glands, trichomes, or ducts within the plant. These aromatic compounds are often insoluble in water, have strong odors, and are less dense than water. Approximately 60 % of plant-based compounds exhibit antifungal properties, while 30 % are effective against bacteria (Gorran et al. 2015). A study conducted by Esmaeili et al. (2015) suggests that S. khuzestanica has antimicrobial power higher even than synthetic and semi-synthetic antibiotics. Due to their potent preventative potential, herbal extracts serve as excellent alternatives to hazardous chemical pesticides. They are considered effective, affordable, and environmentally friendly, with minimal side effects for both humans and the environment.

Figure 2:

Photo of Satureja khuzestanica, commonly referred to as Marzeh Khuzestan in Persian.

2.1 Materials

The primary material used in this research was Persian gum, selected for its specific composition, stability across different pH and temperature levels, local availability, and cost-effectiveness. The Persian gum from Arrajan village (located between Fars and Khuzestan) was sourced from a herbal’s shop at a local Bazar. A 5 % calcium hydroxide solution, purchased from Merck, was used to adjust pH levels. The extracts of S. khuzestanica were used as the fungicidal agent, while glycerol (C₃H₈O₃, M: 92.09 g/mol) was also used to impart flexibility.

2.2 Preparation of the Samples

The experimental process aimed to optimize the gum, combining it with calcium hydroxide, extract of S. khuzestanica, and glycerol, to create a treatment that provides reinforcement, deacidification, and fungicidal properties at the same time. The treatment was designed to enhance fibre integrity and prevent degradation of the paper. The following sub-sections outline the preparation of materials, application of treatments, and analytical methods used in the study.

2.2.1 Preparation of the Persian Gum Treatment

The calcium hydroxide solution for deacidification was prepared in the conservation laboratory of National Library of Iran by dissolving 5 g of calcium hydroxide in 1,000 ml of distilled water. The mixture was magnetically stirred for three days, after which the solution was filtered. The filtrate served as the deacidification solution. Subsequently, the calcium hydroxide solution was added dropwise to 100 ml of distilled water until the pH reached 10. The solution’s pH was carefully adjusted to ensure that when applied to paper samples, the pH of the samples did not exceed 8.5, as higher pH levels could cause excessive fibre swelling and alkaline hydrolysis. A final volume of 2.2 ml of calcium hydroxide solution was used to achieve the desired pH (Figure 3).

Figure 3:

Brief schematic diagram of the preparation steps for Persian gum nanoparticles.

According to a study conducted by Soheilipour et al. (2015), the proper method for extraction of nanoparticles from gums was reported to be as follows: Persian gum was powdered using a DEPOSE gristmill, and then powdered gum was sifted through a 20-mesh sieve. Different amounts of gum (0.2, 0.32, 0.4 and 0.42 g) were weighted and added to the calcium hydroxide solution. Glycerol, at 33 wt % of the raw gum, was incorporated to the mixture, which was then stirred with a glass rod and allowed to stand in a controlled environment (23 °C, 50 % rH) for 24 h to reach the maximum viscosity. The solution was heated on a magnetic stirrer (750 rpm, 40 °C) for 3 h to eliminate bubbles. The soluble and insoluble components were separated using a German Kenrolab centrifuge (3,500 rpm, 20 min). To improve and enhance the material properties, the nano-sized part of the solution was retained for further analysis, as nanoparticles are preferred over micro-sized material (Keramati et al. 2016). The 0.2 and 0.24 wt % gum solutions were excluded from further experiments due to their low viscosity, while gum solutions with concentrations higher than 4 wt % were excluded because of excessive viscosity, which could lead to plasticity in the structure. This plasticity makes it difficult to separate components and handle the material effectively. The study proceeded with 0.32 and 0.4 wt % gum solutions.

2.3 Methods

3.1 pH Measurement

The pH measurement results in the paper samples show that the reference samples were acidic, and the treated samples became alkaline due to the presence of calcium hydroxide in the treatment, and in addition, an alkaline reserve was also created in the samples, before and after treatment and after aging (Table 2).

3.2 Tensile Strength Measurements

Tensile strength increased in both vertical and horizonal directions, with the maximum increase observed at 1.45 MPa before aging and 0.59 MPa after aging. The decrease in resistance after aging is attributed to the acceleration of cellulose hydrolysis and the subsequent breakdown of cellulose bonds under high temperature and humidity conditions. The vertical tensile resistance of papers increased more than the horizontal resistance, indicating the effectiveness of the treatment in reinforcing the paper fibers.

Statistical analysis using Duncan’s multiple range test indicated a significant difference in tensile strength values between the treated samples and the reference sample, with a significance level of 0.0001, which is lower than the assumed error threshold of 0.05. The increase in tensile strength suggests that the treatment improved fiber-polymer adhesion, enhanced mechanical strength of the treated area, and generally improved the mechanical properties of paper samples. The treatment effectively reinforced the fibers, improved their interaction with material (zedu optimized), and prevented further degradation processes of the paper over time.

Previous studies from Soheilipour and Azadi Bouyaghchi (2015) and Mohammadifar et al. (2011) also demonstrated that gum materials used as coating and stabilizing agents could increase the resistance of treated samples and maintain their properties after aging. Additionally, the inclusion of glycerol in the treatment appeared to enhance the reinforcement capacity of the gum nanoparticles. This observation showed that glycerol improved the treatment’s modulus of elasticity which is directly related to the size, depth of penetration and the integration level of the particles, contributing to better overall mechanical properties (Figure 4).

Figure 4:

Tensile strength of the sample; TS1 (MPa), tensile strength of the sample containing 0.4 g gum, TS2 (MPa), tensile strength of the sample containing 0.32 g of gum, TS’1 and TS’2 are samples after aging. See also Table 1. List of paper samples as reference: Pa: Treated sample containing gum in the longitudinal direction or (MD). Pb: Treated sample containing gum in the transverse direction or (CD). Pc: Treated sample containing gum and glycerol in the longitudinal direction or (MD). Pd: Treated sample containing gum and glycerol in the transverse direction or (CD). Reference sample (MD): Untreated control sample in the longitudinal direction or (MD). Reference sample (CD): Untreated control sample in the transverse direction or (CD).

3.3 Fungicidal Effects of Gum Treatment Combined with Satureja khuzestanica Extract

Untreated paper samples showed significant bacteria and yeast growth before and after aging, along with the presence of Aspergillus and Rhizopus fungi. In contrast, treated samples exhibited no sign of bacteria or fungal growth, even after aging, demonstrating the effectiveness of the treatment (Figure 5). The fungicidal properties of the treatment are likely due to the chemical composition of S. khuzestanica, particularly the presence of thymol, carvacrol, and p-cymene. The treatment concentration of 500 and 1,000 μl/l were found to be most effective, with lower concentrations showing reduced efficiency. A concentration of 500 μl/l is recommended for practical applications to minimize potential color changes in the treated samples.

Figure 5:

Fungi, yeast, and bacteria growth after 10 days in samples combined with aqueous concentrate, arranged from left to right: a) a medium without fungi growth on gum treatment, together with savory concentrate in the treatment solution and paper samples, b) bacteria, c) yeast, d) bacteria and yeast, e) Rhizopus, and f) Aspergillus.

3.4 Colorimetric Measurements

After treatment with Persian gum and S. khuzestanica savory essential oil, minimal color change or yellowness was observed (Figures 6 and 7), with a ΔE of 0.6. As shown in the colorimetric results, when ΔE is between 0.2 and 1, opacity and color changes are not noticeable. Persian gum nanoparticles treatment and Satureja extract treatment has led to color changes at an acceptable level (Figure 8). The results indicated minimal changes in the overall color indices (L*, a*, b*) of the treated samples. The slight increase in color indices after aging was consistent across all treated samples and the reference sample, suggesting that treatment does not introduce significant visual alterations.

Figure 6:

Paper samples before treatment.

Figure 7:

Paper samples after treatment.

Figure 8:

Colorimetric data analysis of the samples. Tc (MD): Treated sample in the longitudinal direction.