Resins and Varnishes: Dammar, Mastic, Synthetic Resins, and Conservation Risk
Day 10: The Chemistry of Binding Agents
How shimmering Old Master glazes and sharp-edged modern acrylics share a surprising, sticky heritage—and why conservators debate their future.
Sources: National Gallery London, CAMEO (Museum of Fine Arts, Boston)
Dramatic Entry: When Art Needed a Shield
Imagine Venice in the 17th century: oil paintings, lush and luminous, exposed nightly to soot, smoke, and sticky hands in candlelit salons. Their salvation? Limpid films of natural resin varnish, painstakingly applied by artists or specialist "verniciatori". From Rubens to Vermeer, the greats trusted these glossy shields—yet centuries later, their choices pose chemistry challenges for conservators (see National Gallery, London).
The Chemistry of Resins and Varnishes
Resins—complex biological or synthetic materials—are dissolved in solvents (traditionally turpentine, now in some cases safer alternatives), then brushed or sprayed onto artworks as varnish. They form a protective, refractive, adhesive film with critical roles:
- Unifying surface gloss and saturation
- Shielding paint from abrasion and dirt
- Enhancing apparent depth via optical refraction
Natural resins like dammar (Dipterocarpaceae exudate, SE Asia) and mastic (Pistacia lentiscus sap, Mediterranean) are chiefly triterpenoid acids and alcohols; they cross-link very little, yielding soft, clear films but prone to yellowing (CAMEO).
Synthetic resins (notably Paraloid B-72, an acrylic copolymer) comprise tailored chains—often methyl acrylate/ethyl methacrylate—that form tougher, chemically-stable films, developed for conservation over the past century (Tate Papers).
Sources: Bomford et al, Conservation of Paintings (Yale, 1997); CAMEO
History and Notable Uses
Dammar and mastic were both imported to Europe in large quantities from the 18th century onwards (dammar somewhat later, 19th century: Getty Conservation Institute). Old Masters like Rembrandt and Rubens relied upon mastic (not egg white!) to glaze and finish major commissions (see Tate Papers), while 20th-century restoration disasters from yellowed natural varnishes sparked the scientific development of synthetic alternatives. Acrylic resins, first formulated in the 1930s, now feature in museum conservation worldwide [National Gallery London, Tate].
Famously, the cleaning of Leonardo da Vinci's The Last Supper (1495–1498) involved removing multiple layers of degraded mastic and oil-based varnish, revealing original color but also exposing the visual difference careful varnishing can make (National Gallery).
Comparative Table: Pros & Cons
| Resin/Varnish | Advantages | Drawbacks |
|---|---|---|
| Dammar | Excellent initial clarity, beautiful gloss, readily reversible | Yellows/softens with age, poor UV-resistance, can bloom in damp |
| Mastic | Rich gloss, unifies surface, easy to apply | Very prone to yellowing and cracking, softens in heat, susceptible to gases |
| Synthetic (e.g. Paraloid B-72) | Outstanding chemical resistance, minimal yellowing, used by conservators | Different visual look (less 'deep' gloss), trickier to reversibly remove without correct solvents |
Sources: CAMEO, Getty Conservation Institute, Tate Conservation Science
Studio Workflow & Application Tips
- Surface prep: Ensure the painting is dust-free and fully cured (oil paints: ideally 6+ months, acrylics: weeks). Test small areas first for sensitivity.
- Varnish preparation: Dissolve resin in appropriate solvent (1:2 ratio resin to solvent typical for dammar); filter to remove impurities. Commercial pre-solubilized acrylics (e.g. B-72 in ethanol) also available.
- Application: Use a wide, soft brush or gentle spray. Apply in thin, even layers. Let each layer dry 12–24 hrs (dammar/mastic) or as per synthetic manufacturer’s guidance.
- Timing/process control: Humidity and temperature strongly impact drying. Excess humidity causes “blooming” in natural resins; high temperatures can soften varnish. Synthetic resins are more forgiving but avoid direct sunlight which may speed drying too rapidly.
- Polish if desired: Some finishes allow gentle polishing after one week for added gloss/lustre (see Tate Conservation guides).
Sources: Tate Conservation, Getty Conservation
Best Uses and Conservation Evidence
Natural resins (dammar, mastic) are prized for small-scale traditional work where appearance is paramount and reversibility is planned; however, they are not recommended for outdoor or high-UV situations, or for works intended to last unaltered for centuries (Getty).
Synthetic resins (acrylic, e.g., B-72) excel in contemporary practice and conservation. In blind laboratory comparison, synthetic films remain clear and strong after decades of UV, while natural films yellow and degrade—leading most major museums to mandate synthetics for restoration (Tate).
Key Conservation Risk: Reversibility is crucial—the ability to remove varnish without damaging original paint. Natural resins are soluble in weak solvents early in life but harden and polymerize with age, complicating later removal. Some early acrylics, while stable, are stubborn to remove without harsh solvents (Tate Conservation Science Unit findings).
Key Takeaways
- Varnishes perform crucial optical, protective, and aesthetic roles in painting.
- Natural resins (dammar, mastic) are beautiful but impermanent—best for reversible, protected works.
- Synthetic resins (B-72, acrylics) offer stability and are widely adopted by conservators for their low reactivity, clarity, and removability with proper approach.
- All varnishing is a balance: visual effect, aging, and future conservation options. Test! Record! Label!
Sources
- National Gallery, London: Glossary - Varnish
- CAMEO (Museum of Fine Arts, Boston)
- Getty Conservation Institute: Dammar Varnish in Conservation
- Tate Papers: Synthetic Resin Varnishes in the Conservation of Easel Paintings
- Tate Papers: Old Master Paintings & Varnish
- Bomford, D., et al., Conservation of Paintings, Yale University Press, 1997.
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