Day 18 — Drying Versus Curing: Evaporation, Oxidation, Coalescence, and Time

Drying Versus Curing: Evaporation, Oxidation, Coalescence, and Time

Intensive Daily Masterclass — The Chemistry of Binding Agents

Three classic binders, three distinct pathways: Each relies on a fundamentally different chemistry for film formation. (Masterclass original visual.)

The Timeless Wait: From Smalt to Synthetics

It’s 1506. The golden light dancing on the wet oil glaze in The Mona Lisa is not just Leonardo’s mastery—it’s chemistry teased by time. Five centuries later, an acrylic dries within minutes, but centuries-old oil still slumbers, slow-curing deep beneath its crystalline glazes. Today, we untangle how binders truly “set”: is it mere evaporation, full chemical crosslinking, or something more subtle?

Understanding the Underlying Chemistry

Drying involves a physical process: water or solvent evaporates, leaving pigment and binder behind. Curing, in contrast, is a chemical transformation: molecules react (with air, or with each other), creating a durable film. Some binders, like acrylics, coalesce—a blend of both worlds. Each path shapes not only workflow but conservators’ interventions centuries later (Getty Conservation Institute, 1999).

Different binders, different set mechanisms. Most aqueous binders dry; oils cure; modern acrylics coalesce and partly cure. (Source: National Gallery London “Making Painting”, 2013)

• Evaporation: Tempera, watercolor, gum arabic. Solvent loss = solid film.
• Oxidative Curing: Oil paint. Binder molecules react with oxygen, crosslink, and harden over days to decades (British Museum).
• Coalescence: Acrylics. Water evaporates, acrylic spheres fuse into flexible film. Some minor crosslinking/curing follows (Golden Artist Colors).

A Brief History: Legends and Laboratory Proof

The Egyptians relied on fast-drying gum arabic. Renaissance masters stumbled on linseed oil’s slow oxidation, learning to layer “fat over lean” for archival stability—a principle Rembrandt exemplified in his Portrait of Jan Six (1654, Rijksmuseum), whose red glazes still cure below the surface (Rijksmuseum). In the 1950s, chemists formulated acrylic polymers, making high-speed drying feasible for muralists and conservators alike.

Tempera sets in hours (dries); oil may still cure after centuries (oxidizes); acrylics become touch-dry in 10–20 minutes (coalescence), cure fully in days. (Sources: Tate, Golden, Rijksmuseum.)

Case Studies: Famous Examples

Leonardo’s tempera grassa: Manipulated evaporation and curing for flexibility and depth (Art & Dossier, 2016).
Mark Rothko’s acrylic murals: Used rapid drying for clear, clean layering—a technical necessity for large scale color fields (Tate Papers 18).

Pros and Cons Table: Drying vs. Curing vs. Coalescence

Mechanism	Binders	Pros	Cons
Evaporation (Drying)	Egg Tempera, Gum Arabic	Fast; predictable; compatible with layering	Brittle surface; poor water resistance; little flexibility after dry
Oxidation (Curing)	Linseed, Walnut Oil	Durable film; flexible; rich optical qualities	Very slow; can wrinkle/craze; susceptible to yellowing
Coalescence (Fusion + Curing)	Acrylic Polymers	Fast film formation; water-resistant; flexible	Paint becomes irreversible quickly; less historical data

Studio Workflow & Practical Control

Timing is everything:

• Drying acceleration: Use gentle airflow or very slight warmth (never above 40°C for oil—risk of film damage).
• Slowing curing: Restrict oxygen, add clove oil (for oil paints), work in thin layers. See Tate’s Pre-Raphaelite studies for linar details.
• Acrylic best practice: Keep above 15°C during drying to prevent water entrapment (Golden).

Layering: Oils—always "fat over lean" (thicker, more oil-rich layers on top); acrylics—wait for touch-dry before recoating.

Correct layering tip: Oil "fat over lean"—each layer richer in oil than the last to ensure proper curing and avoid cracking. (Technique reconstructed from Cennino Cennini, Il Libro dell'Arte.)

Best Uses and Conservation Insights

• Egg tempera: Ideal for precise, detailed panel work; conservators favour due to predictable drying (Nat Gallery Bulletin).
• Oil: Unmatched in blending and deep glazing, but requires patient curing; highly valued in portrait and illusionistic tradition (Rijksmuseum).
• Acrylic: For speed and usability on unpredictable supports; modern formulations offer good flexibility and color retention but less studied for ultra-long-term aging (ICCROM Modern Paints).

Key Takeaways

• Drying: Physical loss of solvent (fast, stable, but can be brittle or soluble in water).
• Curing: Chemical crosslinking (slow, tough, risk of yellowing, needs oxygen—important for oil).
• Coalescence: Fusion of binder with some curing (fast, flexible, water-resistant).
• Conservation approaches vary with mechanism—always diagnose before treating!

Sources

1. Getty Conservation Institute
2. British Museum Research – Drying of Oil Paint
3. Golden Artist Colors – Drying vs. Curing of Acrylics
4. National Gallery London – Tempera Research
5. Rijksmuseum – Rembrandt, Jan Six (examination)
6. Tate Papers – Rothko’s Acrylics
7. ICCROM – Modern Paints (Unpuzzled)