Varnishing oil paintings is a balance between the look you want today and the reversibility and color stability you will need years from now. Artists routinely ask which natural resin to choose (dammar, mastic, sandarac, or shellac), how long to wait before varnishing, how thin to apply, and what happens when oleoresinous media (dammar or mastic in oil gels) are used inside the paint.
The four studies reviewed here track the chemistry that drives those outcomes—formation and decay of hydroperoxides, rapid loss of conjugated double bonds, transient epoxides, and longer‑term growth of carbonyl and hydroxyl groups—using FTIR/FT‑Raman on curing oils (Mallégol et al., 1999; 2000a) and photo‑/thermooxidation of cured linseed oil (Mallégol et al., 2000b) alongside xenon‑light aging of spirit and oil‑based terpenoid varnishes (Ciofini et al., 2016). We translate these data into clear choices for final, removable spirit varnishes and for in‑paint oleoresinous media—when to use them, when to avoid them, and how to apply them to minimize yellowing, brittleness, and future cleaning risks.
Methods used in the studies
First, thin films (~25 µm) were cast on IR‑transparent windows to obtain clean spectra. Next, hydroperoxides (ROOH) and dialkyl peroxides (ROOR) were separated analytically by iodometric titration, with sulfur tetrafluoride (SF₄) derivatization to distinguish their contributions. Meanwhile, the varnish study aged brushed films on glass and primed canvas up to 500 h under solar‑simulating xenon light, tracking weight change, color (CIELAB ΔE), and FTIR band kinetics.
Core findings you can use
- Early curing is radical‑driven. Peroxy radicals add across double bonds as oils cure; conjugated double bonds drop quickly and short‑lived epoxides appear, then vanish.
What this means in practice: Fresh paint and resin‑oil mediums are chemically reactive for days to weeks. Keep layers thin, allow air circulation, and avoid UV/heat during this period. Do not apply a spirit final varnish until the paint is truly through‑dry. In oleoresinous painting mediums, expect fast “set” at the surface; avoid building thick, resin‑rich layers that can skin over while remaining soft beneath.
- Hydroperoxides (ROOH) peak early, then decline; permanent links are mostly ether and C–C. The ROOH maximum is higher under cooler curing, and peroxy bridges (ROOR) are only a minor part of the final network.
What this means in practice: Do not try to force‑cure with heat lamps; moderate room temperature and time are safer. Because early layers are ROOH‑rich, they are extra sensitive to light and solvents, so it’s best to delay varnishing. When using oleoresinous painting mediums, allow sufficient time between coats so oxygen can diffuse through the film and ROOH can decompose before you add another layer.
- Light drives yellowing and functional‑group growth more than heat at studio temperatures. In cured oils, photo‑oxidation increases carbonyl (C=O) and hydroxyl (O–H) groups; trace contaminants can amplify yellowing.
What this means in practice: Control UV at every stage: drying, storage, and display. Use UV‑filter glazing and keep illuminance modest. Expect natural‑resin spirit varnishes (dammar, mastic) to yellow with light exposure; plan for eventual removal and renewal if you choose them for their look.
- Varnish systems age differently. Spirit varnishes often lose a little mass first (solvent leaves), then oxidize; oil‑based varnishes gain mass as they take up oxygen. Some resins (colophony) discolor fastest; others (sandarac, shellac) crosslink strongly and become less soluble.
What this means in practice: As final varnishes, dammar and mastic provide a familiar gloss and saturation to paintings, but will yellow and become harder to remove over time; apply very thinly to cured paint and expect future maintenance fully. Avoid colophony in a final varnish if color fidelity matters. Sandarac and shellac can become difficult to remove as they crosslink—use only with full awareness of that trade‑off. In oil-painting mediums, resin‑rich mixes can stiffen upper layers, increase the risk of cracking on flexible supports, and complicate future cleaning. Avoid using resins in oil painting, or at a minimum, keep resin proportions low and layers thin.
- Spectral “early warning signs” map to visible symptoms. The first chemical sign of degradation is C–H loss (hydrogen abstraction), followed by growth/shift of O–H and C=O bands; peroxide‑related bands appear later.
What this means in practice: In the studio, you may see symptoms such as gloss dulling, warm color shift, and increasing brittleness over months to years. These changes signal reduced solubility of aged natural resins. Schedule periodic checks on works finished with spirit varnishes consisting of natural resins, and prefer modern removable synthetic varnishes when long‑term reversibility is the priority.
Spirit varnishes as final coatings for varnishing oil paintings
Artists commonly use spirit varnishes as the last, removable protective layer when varnishing oil paintings. The papers clarify how each resin behaves under light and heat, which directly affects gloss, color, and removability over time (Mallégol et al., 2000b, pp. 257–263; Ciofini et al., 2016).
Dammar (most common today)
- Pros: High initial gloss and saturation; familiar handling; readily soluble in mild aromatics/esters when fresh.
- Cons: Yellows under light; progressive oxidation decreases solubility; can bloom in humid conditions; becomes more brittle with age. FTIR and colorimetry show C–H loss and O–H/C=O growth during aging, consistent with yellowing and reduced removability (Mallégol et al., 2000b, pp. 257–263; Ciofini et al., 2016).
- Best use: If chosen for tradition/appearance, apply thinly to well‑cured paint; expect eventual replacement and plan for periodic condition checks (Ciofini et al., 2016).
Mastic
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