Product Profile - Modern and Traditional Sizings Compared

One of the most important factors in ensuring the durability of a painting is good preparation of the support. Canvas, paper and panels can be prepared for lasting, professional results with a variety of time-tested, traditional and modern materials.

Size or "sizing" is a glue applied to cloth, paper and panels to stiffen, shrink and isolate the support in preparation for priming and painting. Isolation of the support is necessary to protect canvas and paper from direct contact with the acidic oil paint vehicle, which can cause yellowing and embrittlement. Sizing also supports vibrancy and adhesive power of paint by protecting against loss of vehicle into the support; some types may be used to block support-induced discoloration (staining of acrylic gesso from compounds in hardboard panels).

A flat, dimensionally stable surface helps promote durability in paintings on canvas. By providing rigidity, sizing ensures that even if the canvas loses peak tension, it remains reasonably flat and does not change dimension enough to compromise the bond between sizing, priming and paint.

Rabbit Skin Glue
Rabbit Skin Glue is a traditional animal-derived protein sizing. Natural materials like hide glue, casein (whey protein) and isinglass (sturgeon bladder jelly) have been used for centuries to size and protect painting supports. It's believed that painters discovered the protective effect of hide glue sizing upon observing the good condition of paintings on canvas-covered panels.

Modern Rabbit Skin Glue is prepared in-studio from dry granules according to a proportion proven to produce the greatest dimensional stability while promoting adhesion with the ground. Rabbit skin glue should only be used with oil or alkyd-based primers; it should not be used under acrylic gesso.

Rabbit Skin Glue (RSG) has a long history in the craft of painting, and canvases correctly prepared with RSG and white lead can last a very long time provided they are stored and displayed in a stable environment. This approach has its advantages and disadvantages, however. Craftsmanship and careful preparation are important- make sure to observe all package directions for product dilution and application, as well as all safety precautions.

RSG imparts a degree of rigidity to canvas difficult to match with synthetic alternatives (stiffness is important in protecting brittle, aging paint), but hide glue is hygroscopic; humidity will cause it to swell. Over time, this can lead to problems including cracking and paint loss. Animal glues can also be susceptible to molds and insect damage. Because of this, most artists today prefer convenient, ready-to-use synthetic sizings that offer some of the same properties as RSG with better durability.


Acrylic Canvas Sizing

Acrylic-based sizing offers excellent protection against oil absorption and imparts moderate to full stiffness on fabrics. Acrylic sizing is fully compatible with oil, alkyd and acrylic primers on panels and canvas, with the same expansion/shrinkage rate as acrylic gesso and paint. Acrylic sizing prior to priming shrinks, tightens and stiffens fabric and reduces the amount of gesso required for full coverage. Preparing panels with acrylic sizing prior to attaching canvas enhances sticking power when using acrylic gel medium as an adhesive.

Acrylic size can also be prepared from Acrylic Gloss Medium and other select acrylic painting mediums. Acrylic sizing (sometimes sold as "clear gesso") can accept thin to moderate applications of oil color directly for a "raw canvas" appearance, and can be used to prepare heavy papers for oil painting.

PVA Size
PVA size (polyvinyl acetate) is a water-based synthetic resin emulsion that can be applied to canvas and panel in preparation for oil, alkyd or acrylic priming. PVA imparts moderate stiffness and good protection from oil vehicle damage. PVA size is available ready to use; it can also be prepared by diluting PVA adhesives suitable for conservation/archival applications. Household and scholastic-grade PVA glues may not be sufficiently UV-resistant for use in permanent art. PVA sizing should not be used with acrylic gesso on panel because crazing of the ground may result.

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