Accelerators and Retarders: What Helps, What Harms, and Why
Day 19 | The Chemistry of Binding Agents: From Egg Tempera and Linseed Oil to Modern Acrylic Polymers
When Michelangelo labored in the Sistine Chapel, he and his assistants faced the eternal race against time: plaster drying before their scenes could be realized in fresco. Conversely, in modern acrylic painting, artists find themselves wrestling against paint that dries too quickly to blend. From the Renaissance to now, the pursuit of the perfect working window—and the chemistry that controls it—has remained at the heart of the painter’s craft.
Chemistry of Accelerators and Retarders
Accelerators and retarders are additives that control the chemical processes of film formation and curing in binding agents. Accelerators (dryers, siccatives) increase the rate at which a paint sets—often by catalyzing polymerization or oxidation—while retarders slow these reactions, extending open time.
In oil paints, cobalt and manganese salts were early accelerators, encouraging cross-linking in linseed oil. Acrylic retarders, meanwhile, are often glycol-based and disrupt water evaporation or acrylic coalescence, buying precious time for blending.
Historical Notes & Famous Cases
The Renaissance saw the rise of metal-based accelerators: early treatises like Cennino Cennini’s Il Libro dell’Arte describe how to add lead white or manganese to help oil dry (Getty Publications). Impressionist painters, reliant on portability, used poppy oils for slower drying, but later—famously—turn-of-the-century artists like Edvard Munch sometimes suffered cracking from excess cobalt dryers (Tate Research). In modern acrylics, visual texture and open time are manipulated with fluid retarders, as showcased by Helen Frankenthaler’s soak-stain technique.
Pros and Cons: Summary Table
Conservation note: Imbalanced use can leave paintings vulnerable to mechanical damage or poor aging (Tate).
| Accelerators | Retarders |
|---|---|
| Rapid drying (good for impasto, layering) | Longer open time (ideal for blending, glazing) |
| Can cause embrittlement with overuse | May produce softer, tacky, or delicate films |
| Cobalt, manganese, lead salts (oils) | Glycols, gums, slow evaporating solvents (acrylics, tempera) |
| Can cause wrinkling or skinning | Risk of incomplete curing—makes cleaning harder |
How Artists Use Them: Practical Application & Workflow
- Oils: Add drops of cobalt drier to speed drying underlayers—cautiously; too much can cause surface skin and uncured interior (see Rijksmuseum conservation notes).
- Acrylics: Mix 1-5% retarder into paint, keep ventilation up; best in thin layers to avoid tackiness (Golden Artist Colors Lab).
- Egg Tempera: Gum arabic extends open time a few seconds. Use sparingly, as it weakens the film (see National Gallery Technical Bulletin).
Timing, Process Control, and Technical Evidence
Accelerators deliver dramatic effects: cobalt driers can turn days-long drying to single figures of hours (Tate Conservation Lab). But overdosing can "skin" a painting—where the top cures too fast, trapping solvents or uncured oil below. Retarders, when overused in acrylics, can result in sticky paintings days later (Golden Artist Colors), sometimes attracting dust or even mold.
Measured approach: Document test patches, cure at room temperature, and note environmental factors—humidity, heat, airflow all matter.
Best Uses & Key Takeaways
- Use accelerators: When rapid layering, impasto, or plein air require fast drying.
- Use retarders: For blending, glazing, or hot, dry climates where paint sets too soon.
- Avoid excessive use—most conservation labs point to overuse as a major factor in premature paint failure (Tate, National Gallery London).
In summary: Both accelerators and retarders are powerful tools in the artist’s chemical arsenal, but safe use relies on caution, testing, and an understanding of what is truly needed in your workflow.
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