In painting, especially on fabric supports like canvas or linen, sizing is a foundational step that directly affects the longevity and appearance of the artwork. Whether you’re working in oil, tempera, acrylic, or mixed media, properly sizing your support is essential.
Artists today can choose between traditional, animal-derived protein sizings and modern synthetic alternatives. Each type influences not only how paint interacts with the fabric but also how the fabric behaves physically over time. This article explores both categories, compares their benefits and drawbacks, and emphasizes the dual role sizing plays: as a protective barrier and a structural stabilizer.
What Is Sizing?
Sizing refers to the application of a sealing layer to fabric (typically canvas or linen) to:
- Chemically protect the fibers from paint—especially oil—which can deteriorate the material over time.
- Isolate the paint from the support to prevent absorption of too much binder
- Mechanically stabilize the fabric by stiffening it, thereby improving planar stability and reducing sagging, wrinkling, and distortion as the canvas ages and loses tension.
1. Traditional, Animal-Derived Protein Sizings
These natural glues have been used for centuries, especially in oil and tempera painting. While valued for their performance and historical continuity, they are sensitive to environmental changes and can change chemically as they age. Because they are animal-derived, they present an ethical barrier for some artists.
A. Rabbit Skin Glue (RSG)
Source: Collagen extracted from rabbit hides.
Use: Mixed with water according to a tested formula and gently heated, then brushed onto stretched fabric in multiple layers.
Advantages:
- Excellent oil barrier.
- Strong tightening and stiffening effect—ideal for maintaining a taut, flat surface.
- Deeply embedded in traditional practices.
Disadvantages:
- Brittle with age.
- May attract insects or microorganisms
- Highly hygroscopic (absorbs moisture).
- Requires careful preparation and application.
- Not ethically suitable for all artists.
B. Hide Glue / Gelatin
Source: Cow or other mammal collagen (often bones and hides).
Use: Prepared similarly to RSG, often used in panel gesso and tempera practices.
Advantages:
- Very good oil-blocking ability.
- Similar tightening and stiffening effects as RSG.
Disadvantages:
- Less elastic and potentially more brittle than RSG.
- Still moisture-sensitive and non-vegan.
C. Fish Glue / Isinglass
Source: Fish skin and bones; isinglass is made specifically from sturgeon swim bladders.
Use: Often used cold or warmed slightly; popular in conservation and fine work.
Advantages:
- More flexible than other protein glues.
- Cold-soluble and easy to work with.
- Excellent reversibility (especially isinglass).
Disadvantages:
- Weak tightening/stiffening properties.
- Still hygroscopic.
- Relatively expensive and harder to source.
D. Casein
Source: Milk protein, made soluble using borax or ammonia.
Use: Can function as both a sizing and paint binder in tempera and fresco.
Advantages:
- Tough and water-resistant once dry.
- Moderate stiffening.
- Matte, absorbent surface great for gesso and tempera.
Disadvantages:
- Not easily available in prepared form; difficult to make
- Needs fresh preparation and careful control of alkalinity.
- Less flexible than synthetics.
- Not suitable as a direct size for oil painting unless properly sealed.
2. Modern Synthetic Sizings
Modern synthetic sizings provide alternatives that are more stable, flexible, and not animal-derived. They are especially favored by acrylic and mixed-media painters.
E. Acrylic Polymer Emulsions
Examples: Golden GAC 100, Utrecht Acrylic Sizing.
Use: Brushed or rolled directly onto stretched fabric. Often followed by acrylic gesso.
Advantages:
- Excellent flexibility and durability.
- Moisture-resistant and non-hygroscopic.
- Easy to apply and vegan-friendly.
Disadvantages:
- Minimal tightening effect on fabric.
- Some conservators question long-term oil-blocking ability (though many tests support its efficacy).
F. PVA (Polyvinyl Acetate)
Use: Applied diluted as a sizing layer; often used under gesso or acrylic grounds.
Advantages:
- Chemically stable and inert.
- Good sealing properties.
- Flexible, easy to use, and vegan.
Disadvantages:
- Doesn’t shrink fabric.
- Less absorbent than traditional glues.
- Limited historical usage in fine art.
Why Planar Stability Matters
While the chemical barrier function of sizing is well known, its mechanical role as a fabric stiffener is just as vital. Sizing reduces fabric movement due to gravity, humidity, and handling, which:
- Keeps the surface taut and smooth for detailed work.
- Prevents cracking in paint layers caused by canvas slack or distortion.
- Provides planar stability (flat surface) as canvas loses tension.
This is provides critical support for brittle, aging paint films that can be damaged by movement.
Planar Control & Stiffening Comparison
| Sizing Material | Tightening / Stiffening Power | Planar Stability | Best For |
|---|---|---|---|
| Rabbit Skin Glue | ★★★★★ – Strong shrinkage | Excellent | Oil painting, traditional gesso |
| Hide Glue / Gelatin | ★★★★☆ – Strong | Very Good | Panel work, tempera |
| Fish Glue / Isinglass | ★★☆☆☆ – Mild | Moderate | Conservation, delicate surfaces |
| Casein | ★★★☆☆ – Moderate | Moderate | Tempera, fresco, mixed techniques |
| Acrylic Polymer | ★★☆☆☆ – Minimal | Moderate to Good (with gesso) | Acrylic, oil with acrylic ground |
| PVA | ★★☆☆☆ – Minimal | Moderate | Acrylic, conservation |
Complete Feature Comparison
| Feature | RSG | Hide Glue | Fish Glue | Casein | Acrylic | PVA |
|---|---|---|---|---|---|---|
| Oil Barrier | ★★★★★ | ★★★★☆ | ★★★☆☆ | ★★☆☆☆ | ★★★★☆ | ★★★★☆ |
| Flexibility Over Time | ★☆☆☆☆ | ★☆☆☆☆ | ★★★☆☆ | ★★☆☆☆ | ★★★★★ | ★★★★☆ |
| Tightening Effect | ★★★★★ | ★★★★☆ | ★★☆☆☆ | ★★★☆☆ | ★★☆☆☆ | ★★☆☆☆ |
| Moisture Sensitivity | High | High | High | Medium | Low | Low |
| Ease of Use | Low | Moderate | Moderate | Low | High | High |
| Vegan-Friendly | No | No | No | No | Yes | Yes |
| Historical Authenticity | High | High | Niche | Niche | Low | Low |
| Common Use Today | Moderate | Moderate | Rare | Limited | High | High |
Conclusion
Choosing the right sizing depends on your artistic goals, medium, environmental context, and ethical considerations:
- Use rabbit skin glue for historically accurate oil painting surfaces with excellent tension and absorption—but be cautious of environmental instability.
- Choose fish glue or isinglass for conservation work or delicate supports where reversibility and flexibility matter most.
- Go with casein in tempera or fresco practices where you want moderate stiffness and a matte, absorbent ground.
- Pick acrylic or PVA for modern, low-maintenance, vegan-compatible supports, especially under acrylic paints or synthetic gesso.
Whatever your choice, understanding the chemical and structural roles of sizing will help you build better, more durable painting surfaces.