Guide to Traditional Natural Varnish Resins: Transparency, Gloss, Film Formation, Aging, and Application Selection of Dammar, Mastic, and Copal

1 Application Positioning of Traditional Natural Varnish Resins

1.1 Application Value of Traditional Natural Varnish Resins

Among natural resins, dammar resin, mastic resin, and copal resin have long been used in transparent varnishes, artistic protective coatings, traditional decorative coatings for woodware, and oil-painting media. Their main value lies in forming transparent, glossy films with a color-enriching effect, thereby improving the visual appearance of varnishes and supporting traditional restoration outcomes. The functions of these resins in coatings and varnishes are mainly reflected in four aspects:

① Improving the gloss and clarity of transparent varnishes.

Soft-resin varnishes such as dammar resin and mastic resin can form relatively transparent films, making underlying pigments, wood grain, or decorative layers appear clearer.

② Enhancing the visual depth of substrate color and texture.

A varnish film can reduce diffuse reflection on the surface, making wood grain, pigment layers, or substrate colors appear richer and fuller.

③ Use in artistic varnishes and traditional restoration materials.

Dammar resin and mastic resin have long-standing application value in painting varnishes, maintenance of traditional coatings, and the study of historical materials.

④ Providing mild surface protection.

These resins can form an isolating film on the surface, reducing the impact of dust, light contact, and surface contamination on the underlying layer; however, they cannot provide high-strength protective performance.

Traditional natural varnish resins should not be evaluated according to the standards of modern high-performance industrial protective coatings. Their key considerations are transparency, gloss, color-enriching effect, traditional application value, maintainability, and aging risk.

1.2 Basic Differences among Dammar, Mastic Resin, and Copal

Dammar resin, mastic resin, and copal resin all belong to traditional natural varnish materials, but they are not the same resin system. The three differ significantly in source, varnish type, film-formation mechanism, and difficulty of later maintenance.

Resin	Source	Varnish Type	Main Characteristics	Main Risks
Dammar resin	Exudates from plants such as those in the Dipterocarpaceae family	Volatile-solvent varnish	Good transparency, high gloss, relatively easy varnish preparation	Yellowing, embrittlement, and reduced solubility after aging
Mastic resin	Exudates from plants of the genus Pistacia	Volatile-solvent varnish	Transparent, color-enriching, and valuable as a traditional painting material	Yellowing, embrittlement, reduced solubility after aging, and increased difficulty of removal
Copal resin	Tropical plant exudates or semi-fossil resins	Mostly oil-based hard-resin varnishes, depending on source and degree of maturation	Harder varieties have higher hardness and notable value in traditional oil-resin varnishes	Complex processing, large source-dependent variation, and difficult restoration after aging

1.3 The Core Contradiction of Traditional Natural Varnish Resins

Freshly formed traditional natural varnishes have good transparency, gloss, and color-enriching effects. However, after long-term use, they are prone to yellowing, embrittlement, cracking, and reduced solubility.

Main Value	Specific Performance	Corresponding Limitation
Transparency	Underlying texture, pigment layers, or decorative layers remain relatively clear	Yellowing after aging affects the original colors
Gloss	Bright surface with enhanced specular reflection	Gloss decreases after aging, and the surface may become hazy
Color-enriching effect	Wood grain or pigment colors appear deeper and fuller	Yellowing is more obvious in thick films
Mild protection	Reduces the impact of dust, light contact, and surface contamination	Mechanical and chemical protection are limited
Maintainability	Some soft-resin varnishes can be removed with solvents before severe aging	Solubility may decrease after aging, making restoration more difficult

2 Optical Effects and Film-Formation Characteristics of Traditional Natural Varnish Resins

2.1 Visual Effects Produced by Transparent Films

Traditional natural varnish resins have long been used in painting and wood decoration mainly because their transparent films can alter the way light is reflected from a surface. An unvarnished pigment layer or wooden surface is usually relatively rough and tends to produce diffuse reflection, making colors appear grayish or pale. After a smooth transparent varnish film is formed, surface diffuse reflection can be reduced and specular reflection enhanced, making the underlying colors and textures appear deeper and fuller.

This effect is usually reflected in the following ways:

① Richer colors: the visual depth of pigment layers or wood grain is enhanced;

② Brighter surface: gloss is improved, and reflection becomes more concentrated;

③ Clearer texture: the transparent film reduces visual interference caused by surface roughness;

④ Stronger decorative effect: suitable for oil-painting varnishes, decorative wood varnishes, and traditional restoration coatings.

Therefore, the value of dammar, mastic resin, and copal lies first in their optical value, rather than in high-strength protective performance.

2.2 Film-Formation Characteristics of Soft-Resin Varnishes

Dammar and mastic resin are traditional soft-resin varnish materials and are usually soluble in organic solvents such as turpentine and petroleum-based solvents. After application, they mainly form transparent resin films through solvent evaporation.

The advantages of this type of varnish are relatively convenient preparation and application, transparent films, good initial gloss, and a certain degree of removability before severe aging. Their disadvantages are limited mechanical strength, insufficient long-term weather resistance, and susceptibility to yellowing, embrittlement, and changes in solubility under the influence of light, heat, oxygen, and film thickness. Soft-resin varnishes are more suitable for artistic varnishes, restoration varnishes, and decorative transparent coatings, and are not suitable as modern high-durability protective coatings.

2.3 Differences between Soft-Resin Varnishes and Hard-Resin Varnishes

Copal is a class of natural resins with substantial variation in source and degree of maturation. Among them, harder and more fossilized varieties are usually classified in the direction of traditional hard-resin varnishes and have historically been used more often in oil-resin varnishes. Many hard copals do not readily dissolve cold in common solvents and often require heat treatment or cooking together with drying oils.

Comparison Item	Soft-Resin Varnishes such as Dammar and Mastic Resin	Hard-Resin Varnishes such as Copal
Preparation method	Usually dissolved in volatile solvents	Often require heating or processing with drying oils
Film-formation method	Film formation through solvent evaporation	Film formation in an oil-resin system
Film characteristics	Transparent, glossy, and relatively easier to remove	Harder, more wear-resistant, and more difficult to remove
Main uses	Artistic varnishes, restoration varnishes, decorative varnishes	Traditional oil-based varnishes, historical materials research
Main problems	Yellowing, embrittlement, and limited solvent resistance	Complex processing and difficult restoration after aging

3 Dammar Resin: Transparency, Gloss, and Yellowing Risk

3.1 Source and Compositional Characteristics of Dammar Resin

Dammar resin, also commonly spelled damar in English, is a natural triterpenoid resin obtained from plants such as those in the Dipterocarpaceae family. After exuding from cracks or cuts in the bark, the resin solidifies into transparent to pale-yellow resin lumps. Dammar resin is usually soluble in turpentine, petroleum-based solvents, aromatic hydrocarbons, and some chlorinated solvents, while its solubility in alcohols is limited. Since the 19th century, dammar resin has been widely used in painting varnishes and is one of the representative materials among traditional natural varnish resins. It is commonly used in oil-painting varnishes, decorative wood varnishes, and traditional artistic protective coatings.

3.2 Gloss and Transparency of Dammar Resin

The main advantages of dammar resin varnish are good transparency, high gloss, and a pronounced color-enriching effect. Fresh dammar varnish can enhance the color depth of oil-paint pigment layers or wooden substrates, giving the surface a bright and full appearance. Dammar resin is suitable for applications requiring high gloss and transparent visual effects, especially traditional artistic varnishes and decorative transparent coatings. However, this initial visual advantage does not equate to long-term stability. Under the effects of light, heat, and air, dammar varnish gradually ages, and both its appearance and performance change.

3.3 Aging Problems of Dammar Resin

The main limitation of dammar resin is its limited long-term stability. Natural triterpenoid resin varnishes such as dammar and mastic resin undergo oxidation, yellowing, embrittlement, and cracking during long-term use. Common manifestations of aged dammar resin include:

① Yellowing: the transparent varnish gradually turns yellow, altering the colors of the underlying layer;

② Darkening: light-colored paintings or pale wood surfaces become darker in appearance;

③ Embrittlement: the flexibility of the coating film decreases, making it prone to cracking;

④ Surface haze: gloss decreases and transparency deteriorates;

⑤ Changes in solubility: later restoration or removal becomes more difficult.

Dammar resin is suitable for artistic varnishes and decorative varnishes. However, it should be used with caution in industrial transparent coatings that require long-term light-color stability, high weather resistance, or high chemical resistance.

4 Mastic Resin: Soft Gloss, Traditional Varnish Use, and Aging Limitations

4.1 Source and Basic Characteristics of Mastic Resin

Mastic resin is a natural resin obtained from plants of the genus Pistacia, especially Pistacia lentiscus. It is a traditional soft-resin varnish material and is mainly used in oil-painting varnishes, painting media, and traditional restoration materials.

In modern and contemporary painting varnishes, the proportion of mastic resin use has gradually declined, while dammar resin and later synthetic varnish materials have become more common. However, mastic resin remains an important reference material in historical materials research, traditional painting media, and specific restoration work.

4.2 Varnish Effects of Mastic Resin

Mastic resin can form a transparent and glossy varnish film and provides a good color-enriching effect on oil paintings and decorative wooden surfaces. Similar to dammar resin, mastic resin can enhance the color saturation of the underlying layer, making the surface brighter and visually deeper.

The value of mastic resin is mainly reflected in the following aspects:

① Forming the appearance of traditional painting varnishes;

② Enhancing the color-enriching effect of pigment layers or wooden surfaces;

③ Use in historical coating research and traditional restoration;

④ Maintaining a certain degree of removability before severe aging.

4.3 Aging Problems of Mastic Resin

Mastic resin varnish is prone to yellowing and embrittlement after aging, and reduced solubility may also occur. Common problems of aged mastic resin include yellowing, embrittlement, surface darkening, reduced solubility, and more pronounced color change in thick films. These problems can affect the appearance of the painting, decorative layer, or wooden surface beneath the varnish. The difficulty of later removal and restoration depends on resin composition, film thickness, degree of aging, and the sensitivity of the underlying layer. Mastic resin is more suitable for thinly applied varnishes, traditional painting-material research, or specific restoration scenarios. It is not suitable for thick application or high-durability protective coatings.

5 Copal Resin: Hard Traditional Varnishes and Processing Limitations

5.1 Source and Types of Copal Resin

Copal resin is a class of natural resins derived from tropical plants, including relatively recent plant exudate resins and semi-fossil resins that have undergone geological aging to a certain extent. Copal resins from different sources and with different degrees of maturation vary greatly in hardness, melting behavior, solubility, and varnish-making methods. When discussing copal varnishes, attention must be paid to the specific source, hardness, melting point, solubility, and processing method.

5.2 Hardness and Traditional Value of Copal Varnishes

Harder and more fossilized copal resins have traditionally been used to prepare harder and more wear-resistant oil-based varnishes. Compared with dammar and mastic resin, copal varnishes place greater emphasis on film hardness and the characteristics of traditional oil-resin varnishes. The traditional value of copal resin is mainly reflected in the following aspects:

① Use in traditional oil-based varnishes;

② Formation of relatively hard and solid resin films;

③ Processing together with drying oils to prepare oil-resin varnishes;

④ Use in historical coatings, classical furniture, and cultural-heritage materials research.

5.3 Processing and Restoration Limitations of Copal Resin

The main problem with copal resin is the relatively high difficulty of processing and restoration. Many hard copal resins are difficult to dissolve directly in common solvents and require heat softening, cracking, or cooking with drying oils before they can be made into varnishes. Heat processing may affect the color and viscosity of the resin as well as the final performance of the varnish.

Aged copal oil-resin varnishes are usually more difficult to remove. Their films are harder, and after long-term aging they may undergo more complex oxidation, crosslinking, and insolubilization, increasing restoration risks.

6 Aging Mechanisms of Traditional Natural Varnish Resins

6.1 Oxidation Is an Important Cause of Aging

Dammar, mastic resin, and copal resin undergo oxidation reactions when exposed to air, light, and heat for extended periods. Oxidation changes the molecular structure of the resin, leading to darkening, changes in polarity, changes in molecular weight, and deterioration of coating-film performance. For copal oil-resin varnishes, oxidation and crosslinking of drying oils, as well as structural changes caused by heat processing, must also be considered; these factors can intensify film hardening and insolubilization.

Aging Factor	Main Effect
Oxygen	Initiates oxidation reactions, leading to yellowing and structural changes
Light exposure	Accelerates photo-oxidation and reduces transparency and gloss
Heat	Accelerates embrittlement, color change, and deterioration of film performance
Film thickness	Thick films show more obvious yellowing, cracking, and removal difficulty
Resin composition	Determines aging rate, degree of yellowing, and changes in solubility

6.2 Yellowing and Darkening

Yellowing is one of the obvious problems of traditional natural varnishes. Varnish is originally used to enhance transparency and color depth, but after yellowing it changes the color relationships of the underlying layer, especially affecting light-colored paintings, pale wood, and cool-toned surfaces. In painting varnishes, yellowing can darken light-colored areas and interfere with blue, green, and other cool tones through the presence of a yellow varnish layer. In wood varnishes, yellowing can make pale wood surfaces appear yellowed and darker, weakening the original clear and transparent effect.

Yellowing usually becomes more severe over time, and it is more obvious in thick films than in thin films. Traditional natural varnish resins are more suitable for thin application and are not suitable for repeated thick application merely to improve protective performance.

6.3 Embrittlement and Cracking

As oxidation and structural changes progress, natural resin varnishes gradually lose flexibility. The film becomes harder and more brittle and may develop cracks. Embrittlement not only affects appearance but also weakens the protective function.

After embrittlement, varnish may show fine surface cracks, local detachment, haze, reduced gloss, and poorer transparency. Temperature and humidity fluctuations, excessive film thickness, deformation of the underlying material, and repeated varnish layering can all increase the risk of cracking. During application, the film thickness of traditional natural varnish resins should be controlled, and the varnish layer should not be treated as a high-strength mechanical protective layer.

6.4 Reduced Solubility and Increased Restoration Difficulty

One important value of traditional natural varnish resins is that they can be acted upon again by solvents, facilitating restoration and removal. However, after aging, the resin may undergo oxidation, polymerization, or crosslinking, causing its solubility to decrease.

Varnish Condition	Restoration Impact
Fresh varnish film	Relatively easy to soften with suitable solvents
Mildly aged film	Can be removed, but solvent strength must be controlled
Severely yellowed film	Removal becomes more difficult and risk to the underlying layer increases
Embrittled and cracked film	The underlying layer may be affected during removal
Crosslinked or insoluble film	Restoration difficulty increases significantly

7 Selection and Application Control of Traditional Natural Varnish Resins

7.1 Selecting Resins According to Application Purpose

Different traditional natural varnish resins are suitable for different application scenarios. Selection should be based on application purpose, appearance requirements, restoration requirements, and long-term aging risks.

Application Purpose	Preferred Choice	Reason for Selection
High-gloss transparent artistic varnish	Dammar resin	Good transparency and gloss; relatively convenient varnish preparation
Traditional painting-material research	Mastic resin	Consistent with historical varnish and medium research needs
Hard oil-based varnish	Copal resin	Relatively hard film, consistent with the characteristics of traditional oil-resin varnishes
Decorative wood varnish	Dammar or traditional oil-resin varnish	Focus on color enrichment, gloss, and decorative effect
Historical material reconstruction	Select according to the original process	Must match the original material and restoration principles

7.2 Controlling Film Thickness and Application Method

Traditional natural varnish resins are not suitable for thick application. Thick films involve more complex drying and internal stress, and they are also more prone to yellowing, cracking, and removal difficulty. When using this type of varnish, thin and uniform film formation should be prioritized. During application, the evaporation rate of the solvent should be controlled to avoid haze, brush marks, and surface unevenness. When applying multiple coats, the previous film layer should be allowed to dry sufficiently, and increasing thickness should not be used as a way to pursue high protective performance.

7.3 Paying Attention to Removability after Aging

For artistic varnishes and restoration applications, removability after aging is a very important evaluation criterion. High initial gloss does not necessarily indicate long-term suitability. If the varnish develops severe yellowing, obvious embrittlement, or reduced solubility after aging, later restoration will become significantly more difficult. When selecting these resins, attention should be paid to the following points:

① Whether the initial color affects the colors of the underlying layer;

② Whether resin impurities affect transparency and filterability;

③ Whether the solubility is suitable for varnish preparation and future removal;

④ Whether the gloss after drying meets decorative requirements;

⑤ Whether the varnish is prone to yellowing, cracking, or darkening after aging;

⑥ Whether it can still be safely removed with relatively mild solvents after aging.

7.4 Unsuitable Application Scenarios

Traditional natural varnish resins are not suitable for use as the main resins in modern high-performance industrial coatings.

Unsuitable Scenario	Main Reason
Long-term outdoor weather-resistant coatings	Prone to yellowing, embrittlement, and gloss loss
High water-resistance or high-humidity environments	Limited water resistance and wet-heat stability
Highly chemical-resistant coatings	Insufficient resistance to solvents, acids, and alkalis
Highly wear-resistant industrial coatings	Insufficient mechanical strength and long-term stability
Pale, highly stable transparent coatings	Relatively high risk of yellowing
Thick protective films	Prone to cracking and difficult to remove later

8 Representative Chemical Product Tables Related to Traditional Natural Varnish Resins

Table 1 Products Related to Traditional Varnish Resins and Film Formation in Oil-Based Varnishes

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Related natural transparent resin / optical mounting medium	8007-47-4	C304678	Canada resin	BioReagent	Can be used in studies of traditional transparent resins, optical mounting, transparency of natural varnishes, gloss, and refractive effects
Raw material for traditional oil-based varnishes	8001-26-1	L304664	Linseed oil	≥99%	Can be used in studies of oil-based varnishes, drying-oil curing, film formation in resin-oil varnishes, and aging of traditional coatings
Raw material for traditional oil-based varnishes	8001-20-5	T487290	Tung oil	—	Can be used in studies of oil-based varnishes, oxidative crosslinking film formation, water-resistant coatings, and traditional resin-oil varnishes
Fatty acid related to drying oils	463-40-1	L105575	Linolenic acid (α-Lnn)	Moligand™, ≥99%	Can be used in studies of drying-oil oxidation, yellowing of oil-based varnishes, crosslinking reactions, and coating-film aging mechanisms
Fatty acid related to drying oils	60-33-3	L100447	Linoleic acid	Moligand™, ≥97% (GC)	Can be used in studies of oxidative drying of oil-based varnishes, fatty-acid composition analysis, coating-film crosslinking, and aging evaluation
Fatty acid related to drying oils	112-80-1	O1456057	Oleic acid	European Pharmacopoeia (Ph. Eur.), ≥65%	Can be used in studies of fatty-acid composition in oil-based varnishes, flexibility of resin-oil varnishes, oxidative stability, and coating-film performance evaluation

Table 2 Products Related to Solvents and Preparation of Traditional Natural Varnishes

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Traditional varnish solvent	8006-64-2	T111671	Turpentine oil	AR	Can be used in dissolution, preparation, brushing/application, and film-formation experiments of traditional natural varnishes such as dammar resin and mastic resin
Traditional varnish solvent	8032-32-4	P116178	Petroleum ether	AR, bp 90–120 °C	Can be used in solubility screening of natural resins, varnish dilution, resin extraction, and studies of solvent-evaporation film formation
Aliphatic hydrocarbon solvent	110-54-3	H140380	n-Hexane	Anhydrous grade, ≥98%	Can be used in resin extraction, separation of varnish components, solubility testing, and pretreatment of natural resin samples
Aromatic hydrocarbon solvent	108-88-3	T399633	Toluene (regulated precursor chemical)	Anhydrous grade, ≥99.8%	Can be used in solubility testing of natural resins, varnish preparation, studies of dissolution behavior of aged resins, and sample pretreatment for analysis
Aromatic hydrocarbon solvent	1330-20-7	X112051	Xylene	Premium-grade reagent, ≥99%, xylene isomer and ethyl benzene	Can be used in dilution of natural resin varnishes, solubility studies, evaluation of coating-film solvent resistance, and resin sample processing
Ester solvent	141-78-6	E119698	Ethyl acetate	Anhydrous grade, ≥99.8%	Can be used in solvent screening for natural varnishes, coating-film cleaning tests, resin solubility studies, and removal studies of aged films
Petroleum-based varnish solvent	8052-41-3	S304692	Dry-cleaning solvent	—	Can be used in dilution of traditional varnishes, evaluation of natural resin solubility, coating-film cleaning, and restoration simulation experiments
Terpene alcohol solvent / modifier	98-55-5	A151628	α-Terpineol	≥95% (GC), mixture of isomers	Can be used in studies of terpene solvent systems, flow-control adjustment of natural resin varnishes, and evaluation of coating-film odor and volatilization behavior

Table 3 Products Related to Natural Resin Aging and Triterpenoid Structure Research

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Unsaturated triterpenoid structural reference compound	111-02-4	S109119	Squalene	Moligand™, ≥98%	Can be used in studies of oxidative aging of natural resins, structural changes in triterpenoids, yellowing mechanisms, and coating-film stability
Triterpenoid structural reference compound	17990-42-0	O305225	Oleanonic acid	≥98%	Can be used in compositional analysis, oxidation-product research, and aging evaluation of triterpenoid natural resins such as dammar and mastic resin
Triterpenol reference compound	545-47-1	L131628	Lupeol	≥95%	Can be used in triterpenoid component analysis of natural varnish resins, resin source identification, aging-product comparison, and coating-film research

Note: The above are representative Aladdin products. More product specifications can be searched on the Aladdin official website by “product name / CAS / catalog number.” Some products listed in the tables are related materials for natural varnish preparation, film formation, or aging-mechanism studies. Before practical use in cultural-heritage restoration, painting varnishes, or coating experiments, small-scale tests should be conducted based on resin source, purity, solubility, film thickness, substrate sensitivity, and the risk of removal after aging.

References

[1] American Institute for Conservation. Varnishes and Surface Coatings: Traditional Artists' Varnishes. AIC Conservation Wiki.

[2] Museum of Fine Arts Boston, CAMEO. Dammar.

[3] University of Tartu. Dammar and Mastic Resin.

[4] Dietemann P., Higgitt C., Kälin M., Edelmann M. J., Knochenmuss R., Zenobi R. Aging and Yellowing of Triterpenoid Resin Varnishes – Influence of Aging Conditions and Resin Composition. Journal of Cultural Heritage, 2009, 10(1): 30–40. DOI: 10.1016/j.culher.2008.04.007.

[5] Museum of Fine Arts Boston, CAMEO. Mastic Resin.

[6] Smithsonian Museum Conservation Institute. Painting Varnishes.

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