Purpose of Canvas Sizing in Painting
In traditional European oil painting, a size (or sealer) is applied to raw canvas or other flexible supports before the ground and paint. The primary purpose of canvas sizing is to isolate and protect the fabric from the oil binder in paint, which can weaken or “rot” the fibers over time (O’Hanlon 2025). Sizing also stiffens and tightens the canvas, providing a taut, stable surface for painting. Since the 16th century, as canvases replaced wooden panels as supports, artists developed various sizing materials and methods to improve the longevity and behavior of canvas paintings (Stols-Witlox 2014). This analysis focuses on historical sizing practices in Italian, Dutch, and French traditions — particularly the use of animal collagen glues — and examines their long-term mechanical and chemical behavior, preparation methods, and alternatives for contemporary use. We also consider related glue-paste preparations (such as the Spanish gacha) that were historically used as canvas sizing and lining.
Traditional Canvas Sizing Materials
Animal Collagen Glues (Hide and Bone Glue) as Canvas Sizing
Animal glue derived from collagen (found in hides, skins, bones, or fish bladders) was the predominant sizing material in European painting. In historical recipes from the 16th–17th centuries, nearly all canvas preparations include a glue-size layer (Stols-Witlox 2014). The glue — usually made by boiling animal parts — is rich in gelatin (collagen protein) and has strong film-forming properties.
Hide glues (notably rabbit-skin glue, often abbreviated RSG) were especially favored. These glues form a gel when cooled and create a continuous, tightening film as they dry. Rabbit-skin glue became synonymous with canvas sizing, though the term “RSG” is often used generically for any hide glue, not always literally from rabbits (Auf der Mauer 2025). True rabbit glue tends to have very high Bloom strength (a measure of gel strength around 500+), making it one of the strongest and most elastic animal glues (Auf der Mauer 2025). High-Bloom rabbit glue could therefore be used in highly dilute solutions for sizing. By contrast, bone glues are weaker (Bloom strength ~50–300) and often darker due to harsher extraction methods (Auf der Mauer 2025), and thus were less commonly used for canvas sizing.
Preparation and application: Dry hide glue (granules or sheets) is first soaked in water to swell, then gently warmed to dissolve into a liquid “size” (Auf der Mauer 2025). Overheating is avoided, as excessive heat can break down collagen and weaken the glue (Auf der Mauer 2025). Traditionally, the warm liquid glue was brushed onto the stretched canvas in one or more coats, allowing it to penetrate the fibers. Upon cooling and drying, the glue layer shrinks significantly. A free film of animal glue can shrink by over 70% from its swollen gel state to a dry state (Auf der Mauer 2025). This shrinkage pulls the canvas fibers taut, markedly increasing the fabric’s tension and stiffness. Indeed, rabbit-skin glue sizing is unmatched in its tightening and stiffening effect on linen or cotton canvas; it “tightens better than any others we have found,” as one materials researcher notes (Auf der Mauer 2025). The tightening is due to internal drying stresses (on the order of 14 MPa) that develop as the glue film contracts (Auf der Mauer 2025).
Once dry, the glue forms a transparent barrier film that prevents oil or ground layers from penetrating the canvas. This barrier was crucial: without it, drying oils would seep into the fibers and oxidize, embrittling or weakening the canvas over time (O’Hanlon 2025). Thus, animal glue size served both a structural role (stiffening the fabric) and a chemical role (protecting fibers from oil contact).
Variations of Animal Collagen Canvas Sizing: While rabbit/hide glue was ubiquitous, its exact usage varied. Some artists applied glue size alone, while others followed it with an animal-glue ground. For example, Italian recipes often called for multiple applications of a glue-chalk ground (gesso) over the initial size (Stols-Witlox 2014). Vasari (1550) describes canvases prepared with four to five coats of warm glue size, followed by a pigmented ground (Stols-Witlox 2014). Similarly, other 16th–17th-century sources describe a glue sizing layer, topped with several layers of gesso (chalk or gypsum bound in glue), and then an oil-based priming layer (Stols-Witlox 2014). In Northern Europe (the Netherlands, etc.), 17th-century painters often preferred a lean oil-based ground (frequently earth-colored)—but even then, manuals typically advised applying an initial glue size to the canvas before the oil priming (Stols-Witlox 2014). Nearly all 17th-century recipes for canvas, whether Italian or Dutch, include animal-glue sizing as the first step (Stols-Witlox 2014).
Responses