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npj Heritage Science volume 13 , Article number: 379 ( 2025 ) Cite this article
This study investigated the ancient coating technology of Sigiriya, a 5th-century Sri Lankan rock fortress, through a detailed literature review and compositional analysis. Key materials, including Sesame oil, Dorana oil, Hal Dummala, and plant gums, were subjected to compositional analysis through modern chemical studies. This study opens opportunities for future studies focused on environmentally friendly coatings, contributing to the development of sustainable and long-lasting materials, beyond simple historical replicas.
The deep historical importance of artistic expression through various media can be traced back to the beginning of human civilization. They are defined by unique and lasting characteristics 1 . Even in times when literacy was not prevalent, art served as a medium for individuals to express their imagination and communicate 2 . When exploring Sri Lankan paintings and its various facets, one can observe the gradual progression of art from prehistoric times to the present through existing specimens 3 . Sri Lanka is renowned for its extensive collection of Buddhist artwork, visible in rock caves that have been home to Buddhist monks since the third century BCE. These sites still carry the faint remnants of the art that was introduced with the onset of Buddhism in the country. The plastered surfaces of these rock-cave roofs were decorated with vibrant paintings that depict significant events from Buddha’s life and Buddhist Jataka tales 4 . However, providing a comprehensive exploration of Sri Lankan painting within a limited space poses a significant challenge 5 . Excluding architecture, Sri Lankan paintings represent the most compelling and intriguing aspect of Sri Lankan art.
The tradition of art based on ancient Buddhist stories continued uninterrupted until the 18th century CE. This period marked the integration of Buddhist art into Sri Lankan culture with the advent of Aryan speakers 5 . The arrival of “Arahath Sangathitta Maha Theranaiya”, a female Buddhist monk with a sapling of the “Maha Bodhi Tree”, was a significant event. This lady was accompanied by eighteen caste artists who rejuvenated and propagated the art form through Buddhist temples 6 , 7 .
The conventional inception of art began with depictions of Buddha and his life events, and it has creatively evolved across different eras, enduring till today. For ease of study, the history of Sri Lankan art can be segmented into distinct eras, taking into account factors such as style, techniques, technology, subject matter, and artistic appreciation. Artistic trends can be classified based on various dynastic periods, including the Anuradhapura period, Polonnaruwa Period, Medieval period, Kandyan period, and the Modern period 8 , 9 . Each of these periods corresponds to a unique phase of artistic evolution and development. The traditional art form has been molded by the creative paradigms of Sri Lankan art, adhering to the directives and needs of the reigning monarchs of those times.
The enduring aim of maintaining this tradition throughout history was to acquaint the masses with the basic teachings of the Master in an easily understandable way. This strategy was a crucial tactic used by the Buddhist clergy to communicate the virtues of the Master’s philosophy to the wider community. The evolution of Sri Lankan paintings saw changes in style and color scheme over time. However, despite these changes, the themes portrayed in the paintings consistently reflected the intended message, signifying a continuity in goals even amidst stylistic transformations 3 , 10 .
Most classical-style paintings predominantly focus on religious narratives 11 , with a few exceptions like the Sigiriya paintings. Sigiriya, or “Sinhagiri,” is an ancient rock fortress situated in central Sri Lanka. It comprises the ruins of an old stronghold built on a striking monolithic rock pillar in the late fifth century CE 12 . A unique feature of this fortress is its collection of 21 rock paintings depicting Apsaras (celestial singers and dancers), of which only 19 are currently visible 13 . Despite the limited number and scope of these paintings, they do not follow a single unified composition. However, the fragmented masterpieces found in Sigiriya provide a valuable resource for studying early Sri Lankan paintings, offering insights into various aspects, including artistic, iconographical, and chronological elements 3 . This paper delves into the investigation of the protective coating layers of Sigiriya paintings in Sri Lanka. The study aims for a thorough exploration of the scientific principles underlying the techniques used by ancient Sri Lankans. Historically written sources reveal that past artists had extensive empirical knowledge, enabling them to skillfully manipulate the properties of paints and varnishes.
Modern advancements in chemical understanding have allowed for a more comprehensive exploration of the interdependencies among coating systems used in past eras 14 . However, the goal of this study is not merely to replicate historical recipes. Instead, it aims to identify and understand relevant properties, creating a platform for researchers to contribute to the enhancement of environmentally friendly aspects associated with these coatings. This study seeks to unravel the complex science behind protective coatings, drawing inspiration from historical insights while pushing the boundaries of innovation.
When considering Sigiriya, it is an internationally recognized heritage site by UNESCO. It was officially listed under the operational guidelines’ criteria II, III, and IV by the World Heritage Committee in 2019. This significant landmark is located in the Matale District of Sri Lanka’s Central Province. It’s geographically positioned at a longitude of 80.760031, a latitude of 7.9571725, and stands at an Elevation of 265 m/869 feet 15 . Sigiriya is roughly 14 miles distant from Dambulla and about 17 miles westward of Pulastipura 15 , 16 . The massive rock, identified as a monadnock, is the centerpiece of an extensive urban layout 17 .
The murals on these stone surfaces were created in 480 AD. The ancient artworks discovered within a fissure on the western front of Sigiriya’s rock face have attracted considerable attention from both historical spectators and modern scholars studying these paintings. Currently, only 19 complete representations of women and remnants of three additional figures of women are preserved 17 . These remaining illustrations are solely located in fresco pocket A and fresco pocket B, two adjacent indentations in the rock surface 12 . The Sigiriya murals are positioned in a crevice near the elevated castle of Sigiriya, the lion staircase, and the polished plaster wall referred to as ‘the Mirror Wall’ or Ketapathpawura. The “Mirror wall” displays inscriptions from visitors spanning the sixth to 14th centuries. These inscriptions have led to various interpretations. The examination of Sigiriya paintings has been a central point in numerous discussions on the subject.
Archaeologists started focusing on these ancient paintings in 1890. The unique features of the site, such as the rock-face crevice, the elevated castle, the lion staircase, and the Mirror Wall with its inscriptions, sparked systematic investigations. Ryes Davis was the first to start these observations, followed by other researchers like H.C.P. Bell, Longhurst, Senarath Paranavitana, S. Bandaranayke, and lately, B. Priyanka, who carried out comprehensive studies. Their primary interest was to delve into the artistic and technical aspects of Sigiriya paintings, often drawing upon the writings found on the Mirror Wall 3 .
Scholars hold divergent views regarding the ancient painting techniques and the type of plaster employed in Sigiriya 3 , 13 . Two primary methods stand out: dry plaster (Tempera) and wet plaster (Fresco). While some, like H. C. P. Bell and Nandadeva Wijesekera, argue in favor of the use of wet plaster in Sigiriya paintings, others, such as R. de Silva and J. Weerasinghe, counter that there is no evidence of a wet plaster painting system in Sri Lanka. Analyst Jones Mark suggests that Sri Lanka might have employed a dry plaster method for drawing due to the absence of daily work restrictions. Nandadeva Wijesekera, who extensively studied the Sigiriya frescoes, asserts the use of at least three layers in preparing the painting surface 3 , 13 .
According to Seneviratne 18 , the coating layer of Sigiriya paintings consists of a water-soluble compound that forms a white fluffy substance when an excess of ethanol is added. This property has led to its identification as a vegetable gum. The binding medium also includes a drying oil, which is insoluble in water but soluble in ethanol, carbon tetrachloride, and chloroform. Somathilake 19 suggests that the use of an oil medium in ancient Sri Lankan wall paintings is justified by the outdoor locations of most murals, setting them apart from painting sites in India. Regarding the protective layer of paintings, Seneviratne 18 uses gas chromatography-mass spectrometry (GCMS) to reveal traces of wood apple gum and Dorana oil. The analysis concludes that the protective layer mainly consists of Dorana oil, implying that the paint layer mixed with Dorana oil may have degraded over time. Dhanapala proposes that the Sigiriya paintings were created using the Fresco Bueno technique, while Dr. O.P. Agrawal and Dr. N. Wickramasinghe argue that they represent the Fresco Lustro technique 20 . R.H. De Silva offers a different viewpoint, categorizing Sigiriya paintings as employing an “Oil-emulsion tempera with gum” technique 21 .
Much like the ongoing discussions about murals in India, there are varied viewpoints on the techniques used in Sri Lanka’s wall paintings. Initially, HCP Bell stated that the murals of Sigiriya, similar to those at Ajanta, were purely tempera paintings, where pigments mixed with a liquid vehicle were applied to a dry surface. However, Bell later changed his opinion, labeling them as true frescoes. Influenced by Bell’s views, AK Coomaraswamy also classified Sigiriya’s paintings as frescoes 3 , 5 , 12 . VA Smith, based on Bell’s initial assertion, maintained that the paintings were created in tempera on a dry surface. On the other hand, EB Havell contended that neither known tempera nor oil painting methods could endure tropical weather for nearly fifteen hundred years, as demonstrated at Sigiriya. Havell proposed that the technique used at both Ajanta and Sigiriya was a variation of the fresco method. Subsequently, DB Dhanapala and Nandadeva Wijesekara suggested that true fresco elements were more prevalent at Sigiriya than at later sites on the island, albeit adapted to local conditions. Wijesekara concluded that the process at Sigiriya was a fusion of true fresco and tempera, exhibiting characteristics of both techniques upon detailed inspection. As a result, he posited that the paintings at Sigiriya were not entirely true frescoes but rather a minor modification of the method conceived and implemented by artists of that era 3 , 5 , 12 .
Researchers have uncovered that the painting technique used was tempera, and a water-soluble medium was detected 13 , 18 . It’s clear that these methods closely mirror those used at Sigiriya and numerous other painting sites in Sri Lanka 3 . As a result, there is now a high degree of confidence, backed by restoration work and scientific studies, that the murals at Ajanta, Sigiriya, and similar sites were created using the tempera method on dry plaster. In some cases, the plaster layers are as thin as eggshells. Moreover, a study of polished sections of painted stucco distinctly shows that the pigments form a thin yet distinct layer on the surface, unlike the fresco technique. There is no noticeable diffusion of colors into the body of the plaster; instead, the pigments are carefully applied to the surface without seeping into the plaster. Features such as the form of the pigment, color flaking, paint-film lifting in cup-like shapes, and blistering effects collectively indicate that the painting technique used was not fresco buono, which is unaffected by water as the pigments do not soften 22 . This conclusion is further reinforced by the observation that, in most instances, the rock surfaces do not have the required smoothness for wet fresco application and tend to release moisture, rendering them unsuitable for the wet fresco technique 3 , 19 , 23 , 24 .
At this point, it’s important to highlight the crucial role that the binding medium plays in preparing the canvas and applying colors. Just as plaster defines a painting’s technique, the nature of the medium is consistently considered. The globally accepted classifications, such as oil, watercolor, tempera, and fresco, are typically based on the medium 25 . Regrettably, the binding medium at many sites in India and Sri Lanka has significantly degraded, partly due to autoxidation and insect-pest infestations. As a result, the bond between pigments and plaster has weakened, causing the pigments to peel off. In these situations, identifying the binding medium through chemical analysis becomes a daunting task, with nearly insurmountable factors preventing a definitive conclusion 26 . Scientific research indicates that in many murals, glue was the only organic binding medium, securely anchoring pigments to the ground, unlike the fresco method. Therefore, these findings strongly suggest a tempera technique at Ajanta and other locations 24 .
However, it’s important to note that the term “tempera” is often used to refer to any painting technique that involves mixing colors with substances such as egg white, egg yolk, casein, glue, or gelatine 22 , 25 . But it’s essential to clarify that authentic tempera specifically involves grinding the colors with only egg yolk. With this clarification, it becomes clear that the technique used at Ajanta and other locations resembled a form of tempera, almost akin to a fresco secco method where the pigment is applied to dried plaster 24 . Research has revealed that the Bagh technique also falls under the tempera category, with scholars highlighting the use of gum as the medium, similar to the murals of Ajanta. Moreover, the technique of wall paintings at Ellora seems to share similarities with Ajanta, challenging the previous belief by some scholars that Ellora’s paintings neither adhere to the tempera technique nor qualify as true frescoes due to the lack of any binding medium other than lime 27 , 28 .
Seneviratne’s 13 research indicates that Siriya coatings are achieved using a binding medium that includes an emulsion in water, merging a drying oil (Sesami/Dorana) and a vegetable gum sourced from wood-apple. This finding aligns with Seneviratne’s previous discoveries in 2013 and 2018 13 , 18 , where analytical methods like GCMS and FTIR were utilized. The following layer of the substrate is composed of a mix of freshly burnt lime, light-colored clay, and sand, integrating a combination of vegetable gum and oil in water. These insights suggest that the technique employed in Sigiriya paintings is tempera.
In this paper, first, an extensive review of preceding literature focusing on the protective layer of Sigiriya was conducted. Subsequently, the constituents believed to compose this protective layer were discerned from the literature review. Then, the identified key components underwent an exhaustive compositional analysis utilizing experimental techniques for examination and characterization. The acquired data were then employed to establish a comparative analysis between the properties of Sigiriya’s protective layer constituents and contemporary coating compositions. This comparative analysis involved the assessment of similarities and differences in terms of properties, durability, and protective capabilities. The findings derived from this analysis were subjected to thorough scrutiny, analysis, and interpretation. Finally, conclusions were drawn about the efficacy and uniqueness of the ancient Sigiriya protective layer constituents in the realm of modern coatings.
In light of the preceding literature, it is evident that the protective layer of Sigiriya is comprised of a blend of wood apple gum, Sesame oil, Dorana oil, and Hal Dummala. A comprehensive compositional analysis encompassing four key components: Plant gum, Dorana oil, sesame oil, and Hal Dummala, was conducted in this section. The properties of these constituents were meticulously examined to facilitate a comparative analysis with modern coating compositions.
In a wider perspective, plant gums can be classified into two categories: exudate and non-exudate gums. Exudate gums are produced by certain plants as a reaction to physical damage or as a defense strategy against microbial invasions 29 , 30 . Generally, plant gums are hydrophilic polymers that are complex mixtures of branched polysaccharides, mainly derived from the degradation of cellulose in a process termed “gummosis.” They are usually secreted from the bark or seed endosperm 31 . Conversely, plant resins constitute a varied group of binding agents, making it difficult to provide an exact chemical definition for these substances 32 . Plant gums comprise a complex mixture of organic substances, including alcohols, carboxylic acids, ketones, polycyclic hydrocarbon structures, and aromatic components. Many of these components are derived from terpenoids 32 . Due to their complex structure, gums do not have a specific melting point but display a softening point and a glass transition temperature.
Wood apple gum, notably important in Sri Lankan art, is the primary resin utilized. Ancient painters of Sigiriya used wood apple gum ( Feronia Limonia ) as a protective layer for the Sigiriya paintings, as revealed in literature 13 . This gum, a versatile byproduct of specific plant and tree metabolic processes, is derived from the wood apple tree native to the Indian subcontinent and Southeast Asia 33 . The wood apple tree is highly valued in the Indian subcontinent due to its multiple uses. Its fruit pulp, known for its glutinous and aromatic properties, is rich in vitamin C and is consumed raw, in jellies, or pickled. The leaves are used to treat indigestion. Furthermore, Wood apple gum, comparable to gum arabic, is used in varnishes and paints as a substitute or adulterant. The composition of the gum includes 35.5% arabinose and xylose, 42.7% d -galactose, and traces of rhamnose and glucuronic acid 34 , 35 . The gum is a clear, bright, brown-colored substance with various applications. Plant-sourced natural gums typically exhibit water solubility or absorb water to form a viscous solution 36 . Its main component is the myricyl palmitate ester (C 15 H 31 COOC 30 H 61 ). The tree releases gum through traumatic ducts that develop lysigenously in the secondary phloem and phelloderm of its bark 37 . Owing to its solubility in weak solvents and strong coating capabilities, it has become popular in the restoration of various types of paintings, including those on canvas.
Exudate gums, like Wood apple gum, are hydrocolloids that can either dissolve or disperse in water, exhibiting characteristics of suspension, dispersion, and stabilization. As a result, the application of plant gum exudates as agents for emulsification, gelation, thickening, suspension, binding, swelling, bulking, encapsulation, and flocculation in industries like food, pharmaceuticals, textiles, petroleum, paper, and cosmetics has seen a significant rise 31 .
In this study, wood apple gum was tested for solubility to further understand its performance in coatings. It helps to ascertain its hydrophilic or hydrocolloid properties and its solubility in various solvents. The gum, derived from the wood apple plant and distinguished by its characteristic brownish-red and solid state, was gathered from the center of the plant’s bark in mid-October, a period of steady gum secretion. The gum exudates were subjected to extraction processes that involved extraction at room temperature, followed by chemical purification and subsequent centrifugation.
The process of determining solubility followed the method described by Sarkar 38 , with minor adjustments. Specifically, samples were prepared solely at room temperature, excluding the use of a higher temperature (80 °C). In this context, 0.5 g of gum powder was added to 50 mL of solvent, and the resulting blend was stirred mechanically at room temperature (25 ± 1 °C) for a fixed period of 30 min. Following this, the gum solutions were centrifuged at 6000 × g for 30 min to remove insoluble elements. The residue was then moved to a petri dish and subjected to oven drying at 105 °C until a steady weight was reached. The solubility determination procedure was performed three times, and the average of these three readings was used for further data analysis, as detailed in Equation 01:
where C 1 = final weight of sediment; C 2 = initial weight of sample
Solubility test results were as shown in Table 1 .
The complete solubility of Wood Apple gum in water was confirmed through rigorous solubility analysis. The discovery of in situ preserved Wood Apple gum in Sigiriya paintings is unusual because of its solubility in water. The Wood Apple gum found in the Sigiriya paintings stands out precisely because it is soluble in water, which is an unusual and unexpected characteristic for a substance associated with the last preserved painting of ancient Sigiriya. This feature adds an intriguing dimension to the discovery, challenging conventional expectations and raising questions about the techniques used to transform a water-soluble resin into a water-insoluble form over 1500 years ago.
The term “Dummala” refers to a naturally occurring substance found in the topmost layers of the crust, usually beneath freshwater marshy areas, or as a dried resin similar to gums from the dummala tree ( Shorea oblongifolia ), native to Sri Lanka 39 . Locally, two types of dummala have been identified: Hal Dummala, which is directly secreted by the tree as a resin, and Bim Dummala, which is collected from the soil where it has undergone natural transformations over time.
With a history extending over 2000 years, Dummala has been used in Ayurvedic medicine and for making flares for exorcisms and processions. When extracted, Dummala appears as a peat-like carbonaceous material with grouped coarse-grained particles. The research conducted by Subasinghe 39 characterized Bim Dummala using X-ray diffraction (XRD) analysis to determine its phases, revealing its partially crystalline nature. XRF and XRD analyses in the study 39 revealed that Bim Dummala, sourced from the soil, contains magnesium carbonate, silica, and sulfur as its predominant inorganic components.
Studies 6 , 13 , 18 indicate that ancient painters utilized Hal Dummala as a binding medium in their artworks. Fourier-transform infrared (FTIR) spectroscopy analysis in study 13 , conducted within the 800–1800 cm −1 wavenumber range, revealed spectral similarities between Hal Dummala, natural tree resins, Sesame oil, and Dorana oil, suggesting comparable chemical compositions. Additionally, study 40 reported that Bim Dummala oil and Hal Dummala exhibit similar chemical characteristics. Furthermore, study 13 identified the presence of stilbenoid oligomers in Hal Dummala, a class of plant-derived phenolic compounds characterized by a C 6 –C 2 –C 6 structural unit.
Dorana oil, derived from the Dorana tree ( Dipterocarpus glandulosus ), is a well-known product. This oil, when combined with other organic substances, has been used to paint murals in ancient temples in Sri Lanka 41 . Dorana oil has been utilized as a binder for paint and a preservative coating known as valitti 42 . Seneviratne 13 noted that when clean Dummala powder is mixed with Dorana oil and boiled, it dries quickly and imparts a deep shine upon application to a surface.
Based on the FTIR spectroscopy conducted in Seneviratne’s study 13 , Dorana oil contains more C=C double bonds than other drying oils, resulting in a faster curing reaction. Seneviratne’s analysis 13 revealed that Dorana oil contains Sitosterol, Dipterocarpol, B-amyrin [C30H50O], Copalliferol A, and Copalliferol B. Drying oils are tri-esters of glycerol and unsaturated fatty acids. After drying, they form a robust film that is insoluble in water and many organic solvents. The curing process involves the polymerization of the fatty acid chains, stimulated by aerial oxygen, leading to the formation of epoxide intermediates. This reaction takes several months at room temperature, but the process can be accelerated with additives 43 .
Hubert and Jan Van Eyck are often lauded for popularizing oil painting and refining the technique in the first half of the 15th century AD. However, evidence indicates that the use of drying oils in art precedes them by several centuries. Thompson noted references to the use of drying oils for glazes in medieval times, and sources from the 8th century mention the use of a mixture of drying oils and resins as a varnish 44 . While the Van Eyck brothers did not invent oil painting, their expertise elevated the technique to new heights, making it the preferred method for future painters 43 . Given that Sigiriya was built in the fifth century, it’s clear that the use of drying oils in paintings was already in practice before the medieval period.
Sesame oil is a polyunsaturated semi-drying oil. It holds a significant place in the world’s vegetable oil production 45 , 46 . It is one of the oldest known oilseeds, along with coconut 47 . Derived from several species of Sesamum, a member of the Pedaliaceae family, sesame is a high-yielding, nonperennial oil plant. Despite containing high levels of unsaturated fatty acids (over 80% of total fatty acids), sesame oil exhibits remarkable resistance to oxidative deterioration compared to other edible vegetable oils 45 . In the industry, it is used as a solvent in injectable drugs or intravenous drip solutions, a carrier oil in cosmetics, and a coating for stored grains to prevent weevil attacks. It also synergizes with certain insecticides 45 .
Studies classify sesame seed oil as a semi-drying oil, a characteristic that enables the formation of a solid polymeric film through oxidative cross-linking 45 , 46 . It is a fundamental property for coating applications. This drying behavior is particularly beneficial when sesame oil is blended with other drying oils, as it facilitates a controlled interplay between film flexibility and mechanical durability, optimizing the overall performance and longevity of the coating system 48 , 49 , 50 .
Sesame oil exhibits physicochemical properties that make it a viable component in coating formulations, particularly due to its viscosity, density, thermal stability, oxidative resistance, and fatty acid composition. Its viscosity (0.0490–0.0525 Pas) facilitates uniform application and defect-free film formation, while its density (0.953 g/cm³) ensures compatibility with other coating constituents, enhancing the stability of multi-oil formulations 51 . The oil’s high thermal stability, with an initial degradation temperature exceeding 300 °C and a glass transition temperature (Tg) of 34.99 °C, enables coatings to maintain structural integrity under fluctuating environmental conditions. Additionally, its exceptional oxidative resistance, surpassing that of many vegetable oils, mitigates premature degradation, thereby prolonging coating longevity, which is particularly advantageous in heritage conservation, such as in the Sigiriya paintings 51 . Sesame oil’s high unsaturated fatty acid content, primarily oleic and linoleic acids, facilitates oxidative cross-linking reactions, leading to the development of a mechanically robust yet flexible film with superior adhesion, mechanical strength, and resistance to environmental stress 51 , 52 . These attributes make sesame oil a valuable component in both historical and modern coating applications.
Streitberger and Goldschmidt 49 define surface coating as a combination of film-forming materials, pigments, solvents, and other additives. When this mixture is applied and allowed to cure or dry on a surface, it forms a thin, often decorative film that serves a functional purpose. The composition of these coatings primarily consists of four components: a binder, a solvent, a pigment, and additives 53 . A graphical representation of these major components is depicted in Fig. 1
Typically, solvent-based coatings are composed of a polymer binder, which makes up about 25–35% by volume, dissolved in an organic solvent that accounts for roughly 50–75% by volume 54 . These mixtures also incorporate pigments or opacity agents, along with other additives like bactericides or rheology modifiers 54 . The polymers used in these coatings need to display adequate hydrophobicity to provide water resistance 55 . The film formation process in solvent-based coatings begins with the application, followed by the evaporation of organic solvents from the surface. This evaporation process brings the polymer chains closer, resulting in the formation of an interconnected network on the substrate surface 56 . Conversely, the use of water-based paints and coatings, often referred to as latexes, addresses the issue of high VOC emissions associated with solvent-based systems 57 . Water-based coatings consist of an aqueous suspension of polymer particles, with sizes ranging from about 50 to 300 nm, dispersed within an aqueous phase 58 . These coatings also contain additives such as opacity agents, pigments, and coalescing aids. The film formation process starts after the paint application, with the evaporation of water bringing the particles closer. This is followed by a complex film formation process where individual polymer particle chains diffuse across interparticle boundaries, leading to the slow merging of the particles into a unified film 58 .
When examining the binding media of ancient coatings, it’s important to note that the painting layer of Sigiriya comprises a water-soluble compound that emerges as a white fluffy substance when excess ethanol is added 13 , 18 . This is why it’s referred to as a plant gum. Another component of the binding medium was identified as a drying oil, which is insoluble in water but soluble in ethanol, carbon tetrachloride, and chloroform. At other painting sites on the island, the binding medium of the paint, like the binding medium in the ground, was typically an emulsion of plant gum and a drying oil. However, in a few paintings, the binding medium was found to consist only of plant gum. Seneviratne 13 discovered minute traces of Wood apple gum, Sesame oil, and Dorana Oil in the samples of Sigiriya paintings. It’s clear that if oil hadn’t been used as the medium for outdoor work, as seen in the paintings from the late medieval period of Sri Lanka, it’s hard to comprehend how they could have survived.
In the analysis of the constituent materials employed in the ancient coating technology of Sigiriya, it has been ascertained through this study that the Wood apple gum utilized in the paintings at Sigiriya exhibits complete solubility in water, presenting a fundamental challenge regarding how ancient painters managed to transform a film derived from an initially water-soluble resin into a durable, water-insensitive state post-application. In addressing this issue, recent advancements in materials science have yielded a variety of synthetic water-soluble resins. These resins possess inherent chemical reactivity, facilitating their in-solubilization. Upon dissolution in water, these resins form a homogeneous, single-phase solution with unlimited dilutability, distinguishing them from latex emulsions, which represent two-phase dispersions of resin-in-water, limited in their dilution capacity.
Here, the transformation, or in-solubilization, of the film deposited from such a solution is achieved through a chemical cross-linking reaction between polar groups present in the water-soluble molecule and an introduced reactive resin or drying oil. This reaction occurs under the influence of heat, oxidizing agents, and/or driers 50 , 59 . While certain air-drying variants of water-soluble coatings have been produced, the realization of optimal film properties typically necessitates a baking process. Commonly, baking schedules involve durations of ~30 min at a temperature of 300 °F (149 °C) 50 , 59 . This meticulous approach to film development underscores the intricate balance between material characteristics and the controlled application of external factors, ensuring the longevity and integrity of the coated surfaces.
Theoretical studies 6 and 13 consistently report that drying oils were typically heated or boiled with other ingredients during the preparation of coatings in ancient Sri Lanka. Also, studies 60 and 61 demonstrate that their prototyping of ancient coatings involved boiling the ingredients in the process of preparation, further validating the traditional practice of using heat to enhance the properties of the coating materials. Hence, one can posit that the ancient painters in Sigiriya engaged in a chemical crosslinking process involving Wood apple gum (water-soluble resin) and Dorana oil (drying oil), catalyzed by heat. This particular method ensured that the coating did not dissolve or degrade when exposed to water after being applied.
In the context of drying oils, the primary mechanism for forming a film involves oxidation, which includes processes such as isomerization, polymerization, and cleavage. Catalysts like zirconium, cobalt, and manganese, in the form of organometallic salts, play a crucial role in facilitating these reactions 62 . In study 13 , it is noted that the drying rate of oils can be enhanced by heating them to 150–160 °C in the presence of small quantities of lead, cobalt, or manganese salts. Coatings derived from drying oils undergo a curing process involving both oxidation and polymerization. This results in the formation of a loosely crosslinked and reasonably flexible structure, where adjacent molecules are linked through reactive unsaturated groups located in the middle of the fatty acid radical. The transformation of drying oils into a hardened state is commonly referred to as auto-oxidative polymerization 63 . Historically, the earliest instances of thermoset coatings can be traced back to drying oils and alkyds 64 . Broadly speaking, thermoset coatings refer to those in which the paint formulation contains resin precursors that undergo a reaction after application on the substrate. This reaction leads to the formation of a chemically crosslinked, thermosetting network 64 . A thermoset coating essentially constitutes a protective substance consisting of interlinked polymer chains securely bonded through robust chemical connections.
Based on the aforementioned analysis, it is plausible to posit that in ancient Sigiriya, Wood apple gum functioned as a binding agent, while drying oil served as a cross-linking additive. Drawing upon the assertions made by prior scholars regarding the technique employed, it is conceivable to concur with the classification of the employed method as the “Oil-emulsion tempera” technique. This conclusion is supported by the observation that when wood apple gum is dissolved in water, it exhibits complete solubility. Furthermore, when an aqueous solution is amalgamated with a drying oil and subjected to heat, cross-linking occurs, resulting in the formation of an interlinked polymer that imparts water-resistant properties post-application. The polymerization process of a plant gum and drying oil is elucidated in Fig. 2 .
Polymerization process of wood apple gum and drying oil.
The historical application of drying oils in painting was intricate, requiring specific processing methods. Throughout history, the practice of oil painting has been intricately connected to the incorporation of additives aimed at enhancing drying properties. Ancient texts on paint technology reveal that artists possessed significant empirical knowledge, allowing them to intricately manipulate the characteristics of paints and varnishes. This led to the evolution of oil painting as a comprehensive technological system, emphasizing the significance of material combinations and processing aids. Siccativation, especially, played a crucial role from the inception of this technique. Historical records indicate that artists possessed extensive empirical knowledge, skillfully manipulating the properties of paints and varnishes. Over time and across different regions, a variety of driers were employed. Contemporary chemical knowledge enables a better comprehension of the interrelationships within the systems of that era. The effectiveness of siccatives is contingent on the materials utilized, their raw material quality, and the processing approach. Present-day chemical understanding facilitates a deeper insight into the intricacies of historical systems. The effectiveness of these siccatives is influenced by factors such as the materials employed, raw material quality, and the method of processing.
The complexity of oil technology lies in the specific nature of its drying behavior, where the liquid starting material solidifies through the chemical process of “auto-oxidation.” This reaction follows a radical chain reaction with various defined steps, as detailed in several reviews 65 , 66 . Often, the drying process is too slow for an efficient working process for painters. However, artists historically employed empirical methods to influence individual reaction steps and enhance drying properties. It was well-known that different oils exhibit distinct drying properties 67 , 68 . The tradition of incorporating resins in drying oils dates back to the Carolingian period 69 , and many historical recipes subsequently include resin components in varying amounts. Additionally, the addition of resin offers the advantage of increasing the glass transition temperature of the binder system, resulting in quicker solidification of surfaces and films. Technologically, resins were particularly added as driers 70 .
When examining coating technology in Sigiriya, the utilization of various materials by ancient painters becomes evident. Notably, Hal Dummala resin, along with wood apple gum and Dorana oil, were employed in this context. Hal Dummala resin, a key component, contains magnesium carbonate, silica, and sulfur 39 . Silica, which is also found in Hal Dummala resin, is incorporated into coatings to induce changes in texture, modify absorbency, and create specific rheological effects 71 . Additionally, paints often include adulterants such as barium sulfate, calcium carbonate, and magnesium carbonate 72 . The incorporation of these adulterants serves multiple purposes, including cost reduction, weight minimization, and enhancement of durability. Furthermore, these additives contribute to the prevention of cracking in dry paint and aid in maintaining pigment suspension 72 . Considering the ancient Sri Lankan coating technology, it can be inferred that Hal dummala resin functioned as a siccative/modifier in the painting process. This amalgamation aligns with historical practices, reflecting a nuanced understanding of materials to achieve desired artistic and functional outcomes.
Considering the aforementioned findings, an additional component identified in the preserved coating of the Sigiriya paintings is Sesame oil. As previously mentioned, Sesame oil is classified as a semi-drying oil. Understanding the purpose of this semi-drying oil within the coating composition is essential. Referring to the ASTM standard terminology relating to paint, varnish, lacquer, and related products 73 , it is noted that semi-drying oils exhibit a slower drying rate, rendering them unsuitable for use in artist paints. Semi-drying oils tend to form soft, tacky films at ordinary temperatures, but their utility can be enhanced by blending or baking them with faster-drying oils or hard resins, resulting in the creation of superior coatings. When combined with a hard resin, semi-drying oils, similar to drying oils, contribute inherent flexibility, elasticity, and durability to the final film 50 . Experimental studies indicate that the addition of 10 to 20% sesame oil to waterborne varnish significantly enhances adhesion, surface hardness, and scratch resistance 52 . Its ability to accelerate the drying process, strengthen the polymeric film, and improve resistance to oxidative degradation and thermal fluctuations directly contributes to superior coating performance and longevity. Consequently, it is plausible to propose that ancient painters at Sigiriya utilized Sesame oil in conjunction with Dorana oil to craft coatings of exceptional quality, understanding the beneficial attributes of both components. Moreover, it can be inferred that Sesame oil may have been mixed with Dorana oil as a thinner. Unlike water, which is unsuitable as a thinning liquid for oil paints 74 , another oil serves as the optimal choice for this purpose. This dual use of Sesame oil in enhancing coating quality and serving as a compatible thinner aligns with the sophisticated techniques employed by ancient painters at Sigiriya.
The investigation into Sri Lankan paintings, specifically the fragmentary masterpieces of Sigiriya, unveils a comprehensive examination of the evolutionary trajectory of artistic expression from prehistoric eras to the contemporary period. Despite the challenges stemming from the scarcity and limited extant scope of paintings, the Sigiriya specimens serve as invaluable primary sources, significantly contributing to diverse dimensions such as artistic, iconographical, and chronological aspects.
The prominence of the Sigiriya kingdom as a zenith of technological advancements in ancient Sri Lanka stimulates a meticulous inquiry into the coating technology employed by its artisans. Preceding scholars have initiated inquiries into the constituent materials of Sigiriya paintings, thereby inciting scholarly discourse on the applied coating techniques. Expanding upon this groundwork, this research engages in an in-depth exploration of the coating materials, employing a scientific analysis to elucidate the rationale behind their selection and application.
The outcomes of this study disclose that ancient painters at Sigiriya demonstrated ingenuity in employing wood apple gum as a binder, water as a solvent for the gum, Dorana oil as a crosslinking additive solution, sesame oil as a thinner to enhance overall coating behavior, and Hal Dummala as a drier/siccative. Furthermore, this investigation posits that the ancient painters embraced an oil-emulsion tempera technique, a methodology crafted judiciously by considering the distinctive properties of the identified materials.
This research augments our comprehension of the materials employed in ancient Sri Lankan paintings and illuminates the sophisticated and intentional choices made by Sigiriya artists in crafting their masterpieces. The scientific scrutiny applied to the coating materials provides a wide perspective, fostering a deeper appreciation for the technical acumen of these early painters and contributing to the broader scholarly discourse on the intersection of art and technology in ancient civilizations.
Regardless, examining historical sources underscores the vast practical knowledge of previous artists in manipulating the properties of paint and varnish. Modern chemical progress has enabled a more profound exploration of historical coating systems. However, this study transcends merely replicating historical recipes; it aims to comprehend properties and establish a basis for enhancing the eco-friendly aspects of protective coatings. The perfect coating system would flawlessly merge the performance advantages of solvent-based coatings with the environmental, health, and safety benefits of water-based coatings, alleviating the disadvantages associated with each. This optimal system would be a uniform solution of dissolved polymer, negating the need for VOCs or additives to enable the formation of a seamless coating upon application. The research unravels the intricate science of protective coatings, drawing from history while pushing the boundaries of innovation.
No datasets were generated or analysed during the current study.
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The authors would like to express their gratitude to the Department of Civil Engineering, Faculty of Engineering, University of Moratuwa, for providing assistance throughout this study. 25% of this research was supported by a Senate Research Grant (Grant Number: SRC/LT/2021/29) from the University of Moratuwa. The authors are sincerely grateful for the funding provided, which was essential for the successful completion of this study.
Department of Civil Engineering, University of Moratuwa, Bandaranayake Mawatha, Katubedda, Moratuwa, Sri Lanka
R. M. K. M. Rathnayake, H. Galabada & R. U. Halwatura
Department of Engineering,NSBM Green University, Pitipana, Homagama, Sri Lanka
Department of Civil Engineering, ESOFT Metro Campus, Colombo, Sri Lanka
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R.M.K.M.R. wrote the paper and performed the research. M.S.M. and H.G. provided supervision. R.U.H. designed the research and concept. All authors have read and accepted the manuscript.
Correspondence to R. M. K. M. Rathnayake .
The authors declare no competing interests.
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Rathnayake, R.M.K.M., Mendis, M.S., Galabada, H. et al. An analytical review of ancient coating technology employed at Sigiriya, Sri Lanka. npj Herit. Sci. 13 , 379 (2025). https://doi.org/10.1038/s40494-025-01745-4
DOI : https://doi.org/10.1038/s40494-025-01745-4
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