Properties of Watercolor
Watercolor is like no other medium. It exhibits beautiful textures and patterns that reveal the motion of water across paper, much as the shape of a valley suggests the flow of streams. Its vibrant colors and spontaneous shapes give it a distinctive charm. And it can be applied in delicate layers to achieve subtle variations in color, giving even the most mundane subject a transparent, luminous quality.
For centuries, ground pigments have been combined with water-soluble binding materials and used in painting. The earliest uses of watercolor were as thin colored washes painstakingly applied to detailed pen-and-ink or pencil illustrations. The modern tradition of watercolor, however, dates back to the latter half of the eighteenth century, when artists such as J. M. W. Turner (1775–1851), John Constable (1776–1837), and David Cox (1783–1859) began to experiment with new techniques such as wiping and scratching out, and with the immediacy and spontaneity of the medium.
Watercolor images are created by the application of watercolor paint to paper. Watercolor paint (also called, simply, watercolor) is a suspension of pigment particles in a solution of water, binder, and surfactant.
Watercolor paper is typically not made from wood pulp, but instead from linen or cotton rags pounded into small fibers. The paper itself is mostly air, laced with a microscopic web of these tangled fibers. Such a substance is obviously extremely absorbent to liquids, and so the paper is impregnated with sizing so that liquid paints may be used on it without immediately soaking in and diffusing. Sizing is usually made of cellulose. It forms a barrier that slows the rate of water absorption and diffusion. For most watercolor papers, sizing is applied sparingly and just coats the fibers and fills some of the pores, leaving the paper surface still rough. A pigment is a solid material in the form of small, separate particles.
Watercolor pigments are typically ground in a milling process into a powder made of grains ranging from about 0.05 to 0.5 microns. Pigments can penetrate into the paper, but once in the paper they tend not to migrate far. Pigments vary in density, with lighter pigments tending to stay suspended in water longer than heavier ones, and thus spreading further across paper.
Staining power, an estimate of the pigment’s tendency to adhere to or coat paper fibers, also varies between pigments. Certain pigments exhibit granulation, in which particles settle into the hollows of rough paper. Others exhibit flocculation, in which particles are drawn together into clumps usually by electrical effects. (Since flocculation is similar in appearance to granulation, we discuss the modeling of granulation only in this blog.)
The two remaining ingredients, binder and surfactant, both play important roles. The binder enables the pigment to adhere to the paper (known as “adsorption of the pigment by the paper”). The surfactant allows water to soak into sized paper. A proper proportion of pigment, binder, and surfactant is necessary in order for the paint to exhibit the qualities desired by artists. (However, as these proportions are controlled by the paint manufacturer and not the artist, we have not made them part of our model.)
The final appearance of watercolor derives from the interaction between the movements of various pigments in a flowing medium, the adsorption of these pigments by the paper, the absorption of water into the paper, and the eventual evaporation of the water medium. While these interactions are quite complex in nature, they can be used by a skilled artist to achieve a wide variety of effects.
Watercolor can be used in many different ways. To begin with, there are two basic brushing techniques. In wet-in-wet painting, a brush loaded with watercolor paint is applied to paper that is already saturated with water, allowing the paint to spread freely. When the brush is applied to dry paper, it is known as wet-on-dry painting. These techniques give rise to a number of standard effects that can be reliably employed by the watercolor expert, including:
Dry-brush effects (Figure 1a): A brush that is almost dry, applied at the proper grazing angle, will apply paint only to the raised areas of the rough paper, leaving a stroke with irregular gaps and ragged edges. The drybrush effect occurs when the brush is applied at the proper angle and is dry enough to wet only the highest points on the paper surface.
Edge darkening (Figure 1b): In a wet-on-dry brushstroke, the sizing in the paper, coupled with the surface tension of water, does not allow the brushstroke to spread. Instead, in a gradual process, the pigment migrates from the interior of the painted region towards its edges as the paint begins to dry, leaving a dark deposit at the edge. This key effect is one that watercolor artists rely upon and that paint manufacturers take pains to ensure in their watercolor paint formulations.
Intentional backruns (Figure 1c): When a puddle of water spreads back into a damp region of paint, as often happens when a wash dries unevenly, the water tends to push pigment along as it spreads, resulting in complex, branching shapes with severely darkened edges.
Granulation and separation of pigments (Figure 1d): Granulation of pigments yields a kind of grainy texture that emphasizes the peaks and valleys in the paper. Granulation varies from pigment to pigment, and is strongest when the paper is very wet. Separation refers to a splitting of colors that occurs when denser pigments settle earlier than lighter ones.
Flow patterns (Figure 1e): In wet-in-wet painting, the wet surface allows the brushstrokes to spread freely, resulting in soft, feathery shapes with delicate striations that follow the direction of water flow.
Color Glazing One other very important technique in watercolor is the process of color glazing (Figure 1f). Glazing is the process of adding very thin, pale layers, or washes, of watercolor, one over another, to achieve a very clear and even effect. Each layer of watercolor is added after the previous layer has dried. More expensive watercolor paints are specially formulated to have a low resolubility, which not only allows thin uniform washes to be overlaid, but in fact allows any type of brushing technique to be employed over a dried wash (including dry-brush and wet-on-wet) without disturbing the underlying layers.
Glazing is different from ordinary painting in that the different pigments are not mixed physically, but optically—in their superposition on the paper. Glazes yield a pleasing effect that is often described as “luminous,” or as “glowing from within”. We suspect that this subjective impression arises from the edge darkening effect. The impression is intensified with multiple super imposed wet-on-dry washes. Figure 1 shows scanned-in images of real watercolors.
Brushstroke planning A painter can somewhat control the concentration and flow of pigment in a wash by carefully monitoring the relative wetness of brush and paper, knowing that spreading water carries pigment with it and tends to thin it out. Similarly, we control a glaze by adding incremental brushstrokes of pigment, and we control the direction of flow by increasing or decreasing water pressure wherever pigment is added.
Figure 1: Real watercolor effects: drybrush (a), edge darkening (b), backruns (c), granulation (d), flow effects (e), and glazing (f).
Absorption of Pigment
Using the example of a smooth wash as an example, the traditional approach to creating it is for a bead of paint to flow down from a previous brush stroke to the bottom of a new brush stroke as the wash progresses down the paper. The process of absorbing the paint during this process can be viewed using a three-layer model. From top to bottom, these three layers include:
Figure 3: The three-layer fluid model for a watercolor wash.
Backruns: Diffusing water through the capillary layer
Backruns occur only when a puddle of water spreads slowly into a region that is drying but still damp. In a damp region, the only water present is within the pores of the paper. In this situation, flow is dominated by capillary effects, not by momentum of the water/pigment.
Excerpted from Computer-Generated Watercolor, Cassidy J. Curtis, Sean E. Anderson, Joshua E. Seims, Kurt W. Fleischery, David H. Salesin - University of Washington, Stanford University, Pixar Animation Studios.