4.3 Painting Processes: Grounds, Underpainting, Glazing, Scumbling, and Impasto

Painting Processes: Grounds, Underpainting, Glazing, Scumbling, and Impasto

The construction of an archival painting is an architectural undertaking governed by physical laws of adhesion, film flexibility, drying velocity, and optical refraction. Throughout art history, master painters rarely worked directly onto raw canvas in a single, unconsidered pass. Instead, they built paintings systematically in structured strata: sealing the substrate, applying an engineered ground, establishing a value underpainting, and modulating light through transparent glazes, broken scumbles, and tactile impasto. For visual art educators preparing candidates for the ILTS Visual Arts examination, mastering these multi-layered painting processes is essential for teaching durable studio craft.

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Substrate Sizing and Ground Chemistry

A painting's ground serves as the structural bridge between the physical support (stretched canvas, linen, or wooden panel) and the subsequent paint layers. Without proper sizing and priming, archival paint films inevitably suffer structural failure.

Sizing: Protecting Organic Fibers

Raw textile supports—such as unprimed cotton duck or Belgian linen—are composed of natural cellulose fibers. Vegetable drying oils (such as linseed oil) contain acidic fatty acids that undergo exothermic oxidation during curing. If unrefined drying oil comes into direct physical contact with raw cellulose, the acidic oil chemically burns and degrades the fibers over time, a catastrophic archival failure known as canvas rot:

• Rabbit-Skin Glue (Hide Glue): The traditional sizing agent prepared from animal collagen. Heated in a double boiler (never boiled, which breaks protein chains) and brushed onto raw canvas while warm. As it cools and dries, the glue contracts, tightening the canvas drum-tight while sealing the pores against oil penetration. However, animal glue is hygroscopic; it absorbs and releases atmospheric moisture, causing canvas to slacken and tighten in response to humidity swings, which eventually cracks brittle oil grounds.
• Polyvinyl Acetate (PVA) Size: A neutral-pH, chemically inert synthetic polyvinyl acetate size formulated with demineralized water (such as Lineco PVA). PVA size seals raw canvas without swelling or contracting with humidity, providing a stable, non-acidic barrier that will not yellow or embrittle with age.

Traditional Chalk Gesso vs. Acrylic Gesso

The word gesso is derived from the Greek gypsos and Italian for "gypsum" or "chalk." In contemporary studio practice, art educators must distinguish between two completely different chemical formulations:

• Traditional Chalk Gesso (True Gesso / Gesso Sottile):
  • Composition: Formulated by slaking refined calcium carbonate (whiting/chalk) or calcium sulfate (gypsum) in warm rabbit-skin collagen glue size. Applied in multiple warm coats, then scraped and sanded to a porcelain-smooth, brilliant white finish.
  • Physical Properties: Extremely absorbent, microporous, and crystalline. When oil paint or egg tempera is applied, the porous chalk pulls binder into its microscopic voids, anchoring the paint with exceptional mechanical adhesion.
  • Substrate Mandate (Rigid Panels Only): True gesso dries to an entirely rigid, brittle, non-elastic crystalline structure. True gesso must NEVER be applied to flexible stretched canvas. The slightest mechanical deflection or environmental expansion/contraction of canvas causes traditional gesso to shatter into a network of cracks and delaminate completely. True gesso is restricted exclusively to rigid wooden panels (such as poplar, birch plywood, or Masonite hardboard).
• Acrylic Gesso (Acrylic Primer):
  • Composition: A water-based emulsion of synthetic polyacrylic polymer resin, calcium carbonate chalk (which provides tooth and absorbency), and titanium dioxide (which provides high opacity and brilliant whiteness).
  • Physical Properties: Flexible, non-yellowing, water-resistant once dry, and non-hygroscopic. Acrylic gesso can be applied safely to both flexible stretched canvas and rigid panels.
  • Adhesion Rule: Oil paint can be applied safely over cured acrylic gesso. However, acrylic paint must NEVER be applied over an oil ground, as water-based acrylic emulsions cannot form a chemical or mechanical bond with non-porous, oily surfaces, leading to peeling and delamination.
• Oil Grounds: Formulated by grinding lead white (historically) or titanium/zinc white in drying linseed oil, applied over sized canvas. Oil grounds are non-absorbent and require several months to cure prior to painting. They provide an elastic, non-porous surface that allows oil paint to glide freely without immediate "sinking in" of the oil binder.

Underpainting Methodologies: Grisaille, Verdaccio, and Imprimatura

Underpainting is the foundational monochrome or low-chroma paint layer applied over the ground to establish the composition's tonal values, light hierarchy, and structural anatomy prior to color application:

Grisaille (Monochromatic Gray)

• Definition: Derived from the French gris ("gray"), grisaille is an underpainting executed entirely in neutral tones of gray, white, and black (or dark earth tones). The artist resolves all Chiaroscuro problems—core shadows, halftones, highlights, and spatial perspective—in pure value.
• Studio Function: By separating the construction of value from the complexities of hue and saturation, the painter ensures rock-solid three-dimensional volume. Once the grisaille is completely dry, subsequent layers of transparent chromatic glazes are washed across the modeled forms; the luminous white and light gray values underneath illuminate the glazes from behind, creating glowing optical color.
• Historical Precedents: Widely utilized in Renaissance altarpieces, preparatory studies (such as Leonardo's unfinished Adoration of the Magi), and academic painting at the French École des Beaux-Arts.

Verdaccio (Flesh-Tone Optical Foundation)

• Definition: Developed in Italian Gothic and Early Renaissance painting (codified by Giotto and Cennino Cennini), verdaccio is a monochromatic underpainting formulated from a dull, olive-green/brownish-gray mixture (traditionally yellow ochre, carbon black, white, and green earth / terre verte).
• Optical Mechanics for Portraiture: Verdaccio was formulated specifically as an undertone for human skin. Human flesh is not a solid opaque surface; it exhibits subsurface scattering, where light penetrates the translucent epidermis and scatters across vascular beds. By painting warm, transparent pinks, vermilions, and ochres over a cool, olive-green verdaccio underpainting, the underlying green optically neutralizes excessive redness, automatically generates cool subcutaneous vascular shadow transitions, and simulates realistic living skin.

Imprimatura (Stained Ground Baseline)

• Definition: An imprimatura (from Italian for "first layer") is a thin, transparent, luminous stain of earth-toned paint (such as raw umber, burnt sienna, or yellow ochre) washed across an entire brilliant white gesso ground.
• Psychological and Practical Value: A stark white canvas exerts severe visual glare, which distorts a painter's perception of value (making middle-value marks appear shockingly dark). The imprimatura instantly establishes a neutral middle-gray or warm mid-tone across the canvas. Highlights can be sketched in with white paint, shadows blocked in with dark earth tones, and half-tones left as exposed imprimatura, dramatically accelerating observational modeling.

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The Foundational Laws of Layered Oil Painting

When working in multi-layered oil painting, the artist must strictly obey three physical rules of paint film mechanics. Failure to observe these rules results in catastrophic surface failure, including craquelure (premature cracking), wrinkling, and paint flaking:

1. The Fat-Over-Lean Rule

• "Lean" Paint: Formulated with a low percentage of oil binder and a high percentage of volatile solvent (such as odorless mineral spirits or turpentine). Lean paint dries rapidly through solvent evaporation and minimal oxidation, yielding a hard, inflexible, porous, non-elastic film.
• "Fat" Paint: Formulated with a high percentage of oil binder (such as stand oil, walnut oil, or linseed oil) and minimal or zero solvent. Fat paint dries very slowly through oxidative polymerization, remaining elastic, supple, and expanding slightly as it cures over many months.
• The Rule: Build compatible layers so upper paint does not become rigid and fast-drying over a slower, more flexible underlayer. Use compatible paint layers and the manufacturer's medium recommendations. Do not turn fat-over-lean into fixed solvent-to-oil percentages: excessive solvent can underbind a layer, and excessive added oil can cause other failures.
• The Failure Mode: If a lean, fast-drying, rigid layer is applied on top of a fat, slow-drying, elastic underlayer, the top layer dries, hardens, and contracts first into a brittle skin. Over subsequent months, as the underlying fat paint continues to cure, expand, and contract beneath it, the brittle top skin cracks, wrinkles, and undergoes catastrophic delamination.

2. The Thick-Over-Thin Rule

Thin paint layers allow oxygen to permeate the film quickly, drying far faster than thick paint films. Thick passages of paint skin over on the exterior while remaining soft and uncured inside for years. Therefore, thin paint layers must always be applied in the lower strata, with thick, impasto paint reserved strictly for upper layers. Burying a thin wash over a thick, soft impasto stroke causes surface cracking.

3. Slow-Drying over Fast-Drying Pigments

Different pigments exert catalytic chemical effects on drying oils:

• Fast-Drying Pigments: Earth colors containing natural manganese and iron oxides (such as raw umber, burnt umber, and raw sienna), as well as Prussian blue, act as natural siccatives (driers), drying within 24 to 48 hours. These pigments must be used in lower underpainting layers.
• Slow-Drying Pigments: Pigments such as ivory black, lampblack, titanium white, and the cadmium family dry very slowly (taking 5 to 10 days to skin over). These pigments should be positioned in upper layers to ensure that underlayers cure ahead of them.

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Optical Modulations: Glazing, Scumbling, and Impasto

Once structural values are established, the painter modulates light through three classic techniques:

Glazing: Luminous Transparency

• Technique: Applying a thin, transparent, oil-rich film containing a very low concentration of transparent pigment (such as quinacridone magenta, phthalo blue, or raw sienna) over a dry, lighter-value underpainting.
• Optical Behavior: Light passes directly through the transparent glaze, strikes the reflective, light-toned underpainting beneath, and bounces back through the colored glaze to the viewer's eye. The resulting chromatic brilliance resembles light passing through stained glass, producing an optical depth, richness, and inner glow that cannot be matched by mechanically mixing opaque pigments on a palette.
• Historical Masters: Perfected by Jan van Eyck and Titian, who famously built flesh, velvets, and draperies through dozens of successive translucent glazes over modeled grisaille underpaintings.

Scumbling: Broken Atmospheric Light

• Technique: Dragging a stiff bristle brush lightly loaded with opaque or semi-opaque, light-toned paint (containing little medium/solvent) across a dry, darker underlayer. The dry brush skims across raised surface peaks, depositing broken, semi-transparent flecks of light pigment.
• Optical Behavior (The Rayleigh / Turbid Medium Effect): When light strikes small, opaque light particles suspended over a dark ground, shorter blue wavelengths scatter more intensely than longer red wavelengths. Consequently, scumbling light paint over a dark underlayer creates an optical, smoky, opalescent, cool veil without mixing any blue pigment. This optical gray is utilized to depict morning mist, translucent skin over veins, cloudy atmospheres, and glowing velvet highlights (mastered by J.M.W. Turner, Rembrandt, and Diego Velázquez).

Impasto: Tactile Physical Relief

• Technique: Applying thick, viscous, buttery paint with stiff hog bristle filberts, palette knives, or painting spatulas such that the paint physically projects from the plane in three-dimensional relief, retaining the exact imprint of tool marks.
• Physical Properties: Impasto transforms paint into a sculptural relief. The physical ridges catch directional room lighting, casting genuine micro-shadows that shift dynamically as the observer moves through the gallery. Impasto strokes are applied either alla prima (wet-on-wet in a single session, as in Vincent van Gogh's expressive works) or placed as structural highlights (such as Rembrandt's encrusted lace and gold embroidery) over completely cured, lean underlayers.

Classical Painting Processes Compared

The following table outlines the mechanical parameters, media formulation, optical behavior, and historical functions of primary painting processes: