Binding Agents Masterclass introduction: What a Binder Actually Does

The Chemistry of Binding Agents, introduction.

What a Binder Actually Does: Adhesion, Film Formation, Pigment Wetting, and Why Paints Behave Differently

Course note: This is Day 0 of an intensive daily course of at least 14 days. The planned curriculum is 30 days so the subject can be treated properly: chemistry, history, artist practice, conservation, and hands-on painting decisions.

A paint is not simply colour. It is a designed material: pigment particles plus a binder, usually with water, oil, solvent, surfactants, additives, or fillers depending on the system. The National Gallery defines a binder, or medium, as the material that holds coloured pigments together to form paint; in its collections this is often drying oil, but may also be egg yolk or animal glue. That single role, holding pigment together, controls flow, gloss, opacity, drying rate, layering, adhesion, cracking risk, and long-term ageing.

Original diagram: pigment particles in three different binder systems.

1. Pigment Is Not Paint Until the Binder Does Four Jobs

First, wetting. Dry pigment is a powder with enormous surface area. A binder or vehicle must wet those particles so they disperse instead of clumping. Poor wetting creates gritty paint, weak colour, uneven gloss, and fragile films.

Second, adhesion. The binder must grip the support or ground. Egg tempera traditionally performs best on rigid prepared panels; oils and acrylics tolerate a wider range of flexible supports, provided the ground is appropriate.

Third, cohesion. The binder must hold pigment particles to each other after drying. Too little binder produces underbound paint: matte, dusty, friable, and vulnerable to abrasion. Too much binder can make paint glossy, wrinkled, weak, slow-drying, or prone to yellowing, depending on the material.

Fourth, film formation. The paint must become a continuous solid layer. Different binders form films by different mechanisms, and this is the key practical aim of this lesson.

2. Three Binder Families, Three Different Drying Mechanisms

Binder	How it becomes a paint film	Practical feel	Main strengths	Main risks
Egg tempera	Water leaves; egg-yolk proteins and lipids bind pigment into a lean, hard film.	Fast setting, precise, short strokes, little blending.	Fine detail, luminous layering, crisp edges, durable on rigid panels.	Less forgiving on flexible supports; fresh mixtures can spoil; overthick layers can crack.
Linseed oil paint	Drying oil reacts with oxygen and crosslinks over time.	Slow, buttery, blendable, rich colour and gloss control.	Long open time, glazing, impasto, subtle transitions.	Slow curing; too much added oil can weaken or wrinkle films; poor layer order can crack.
Acrylic polymer emulsion	Water evaporates; polymer particles coalesce into a continuous film.	Fast drying, water-thinnable, flexible, variable with mediums.	Speed, adhesion, low odour, broad supports, glazing and layering.	Can dry too fast; excessive water or retarder can weaken film; dirt can adhere tenaciously to acrylic surfaces.

The word dry hides a lot. Egg tempera and acrylics lose water quickly, but acrylics also need polymer particles to coalesce properly. Oil paint is different: it does not merely lose solvent or water; the drying oil oxidizes and polymerizes. Winsor & Newton gives a practical rule of thumb that thin oil-colour films become touch-dry in roughly two to twelve days, depending on pigment and conditions. Acrylics can dry in minutes to hours; Winsor & Newton notes thin Galeria Acrylic films may dry in 10-20 minutes and thicker films can take an hour or more, while Professional Acrylic thin films are given as 20-30 minutes under typical conditions.

3. Practical Painting Consequences

If you want precision: egg tempera rewards planning. Use a rigid, absorbent ground, small brushes, and many controlled strokes. It is excellent for icons, fine detail, pattern, botanical precision, and luminous optical layering. Its fast set is a strength when you build form by hatching instead of wet blending.

If you want long blending: oil paint gives the longest open time of the three. It is best for wet-into-wet modelling, soft transitions, glazing, and thick physical brushwork. To accelerate oil drying, use thin layers, absorbent grounds where appropriate, fast-drying pigments, good air circulation, and alkyd mediums. Gamblin explains that alkyd resin, a highly polymerized oil, accounts for faster drying compared with linseed or safflower oils used alone. To slow oils, use thicker paint, slower pigments, less absorbent grounds, or appropriate amounts of drying oil, but avoid flooding paint with oil.

If you want speed and revision: acrylic polymer emulsion is the rapid worker. It suits underpainting, murals, graphic hard edges, collage-like layering, quick studies, and mixed-media structures. To slow acrylics, use manufacturer-formulated retarders or slow-drying mediums rather than improvising with household additives. Winsor & Newton specifically notes acrylic open time can be extended with Fluid Retarder; its product guidance also cautions that retarder itself is not additional acrylic binder and should be used only as needed.

4. Artist and Artwork Anchors

Egg tempera: The National Gallery lists Sandro Botticelli's The Adoration of the Kings, about 1470-75, as egg tempera on poplar. This is the kind of rigid-panel, finely layered use that made egg tempera central to European painting before oil became dominant.

Sandro Botticelli's The Adoration of the Kings, egg tempera on panel. 

Oil paint: The National Portrait Gallery describes oil paint as pigment mixed with oil, usually linseed oil, and notes that oil paint has been used by artists for more than 600 years. Its slow drying is exactly why it became powerful for blending, revision, glazing, and large-scale modelling.

Acrylic paint: The Whitney Museum notes that Helen Frankenthaler poured turpentine-thinned oil paint onto unstretched canvas in 1952 and began incorporating acrylic paints into her process in the early 1960s. The Metropolitan Museum of Art also identifies Andy Warhol and Helen Frankenthaler among early artists using acrylic as a modern medium.

Helen Frankenthaler's Off White Square (1973) at a gallery in Southampton, N.Y. 

Kena Betancur AFP via Getty Images

5. Best Practices From Day 1

• Match binder to support: rigid panels for traditional egg tempera; properly primed canvas or panel for oils; acrylic grounds or compatible supports for acrylics.
• Do not confuse touch-dry with cured: oil especially continues chemical change after it feels dry.
• Control thickness: thick layers dry or cure less predictably in every system.
• Use manufacturer mediums: drying control additives are chemistry, not magic. Use them sparingly and consistently.
• Record conditions: temperature, humidity, support absorbency, layer thickness, and pigment choice all change drying behavior.

6. Hands-On Exercise: Three Binder Behavior Swatches

1. Egg tempera or egg-tempera substitute: make a small hatched rectangle. Record how quickly edges lock down and whether later strokes disturb earlier ones.
2. Oil paint: make a small blend from dark to light. Record how long it stays workable and when it becomes tacky.
3. Acrylic: make a thin glaze and a thicker patch. Record touch-dry time, colour shift, edge quality, and whether added water changes strength or sheen.

Write the time, room conditions, support, ground, pigment, binder/medium, and layer thickness beside each swatch. This habit is how painters become technically fluent instead of relying on folklore.

Sources

• National Gallery, London, Binder glossary: https://www.nationalgallery.org.uk/paintings/glossary/binder
• National Gallery, London, Media glossary: https://www.nationalgallery.org.uk/paintings/glossary/media
• National Portrait Gallery, Tempera glossary: https://www.npg.org.uk/collections/explore/glossary-of-art-terms/tempera
• National Portrait Gallery, Painting media and techniques: https://www.npg.org.uk/schools-hub/painting-media-and-techniques
• National Gallery, Botticelli, The Adoration of the Kings: https://www.nationalgallery.org.uk/paintings/sandro-botticelli-the-adoration-of-the-kings
• Golden Artist Colors, Fluid Mediums / GAC acrylic polymers: https://goldenartistcolors.com/products/golden-artist-acrylics/fluid-mediums
• Winsor & Newton, acrylic drying times: https://www.winsornewton.com/en-ca/blogs/guides/drying-times-acrylic-paints
• Winsor & Newton, oil colour drying times: https://www.winsornewton.com/en-ca/blogs/guides/understanding-drying-times-for-oil-colour
• Gamblin Artists Colors, solvent-free oil painting: https://gamblincolors.com/studio-safety/solvent-free-painting/
• Getty Conservation Institute, Cleaning of Acrylic Painted Surfaces: https://www.getty.edu/conservation/our_projects/education/caps/
• Smithsonian Museum Conservation Institute, Caring for Acrylic Paintings: https://mci.si.edu/Caring-Acrylic-Paintings
• Whitney Museum of American Art, Helen Frankenthaler: https://whitney.org/artists/452
• The Metropolitan Museum of Art, Modern Materials: Plastics: https://www.metmuseum.org/essays/modern-materials-plastics

Next lesson: Egg tempera in depth: yolk chemistry, panels and grounds, Byzantine icons, Renaissance practice, modern revival, and why it rewards disciplined brushwork.