Colour Chemistry: Cobalt Pigments Guide for Watercolourists

Cobalt blue, considered a true blue rather than greenish like cerulean, has applications for both landscape and portrait painting. Cobalt blue mixed with viridian and white creates excellent sky colors with haze effects. Nineteenth-century figurative artists commonly included cobalt blue on their palettes for flesh tones and other applications.

Cobalt blue (PB 28) emerged as a synthetic alternative to expensive natural lazurite, initially seen as a cheaper option. When synthetic ultramarine was developed, cobalt remained one of the most expensive pigments. It is a granulating inorganic paint with exceptional durability, resistant to hydrogen sulfide and UV light. Due to its ground pigment nature, it has strong covering properties. It is commonly used for sky painting on large formats where granulation is acceptable, but ultramarine is preferred for small formats. Cobalt blue occupies a middle position in the blue spectrum, neither too blue-green nor blue-violet, making it ideal for mixing with reds. It has higher saturation than ultramarine. It mixes well with virtually any color, producing noble smoky greens with ochre, vibrant greens with cadmium orange, and excellent purple tones with violet-red colors. As a light-reflecting pigment, it creates rich color reflections in shadows. When neutralized with iron, it produces grayish tones. Modern artists can neutralize pure spectral colors between themselves, a capability unimaginable to 19th-century artists who faced extreme costs for blue pigments derived only from ground mineral stones.

Inorganic pigments evolved through distinct phases: natural earth pigments (ochres, sienna, umber) from Paleolithic times, precious minerals (azurite, malachite, lapis lazuli) with toxicity concerns, and synthetic compounds developed from the 18th century onward. Early synthetic pigments like Prussian blue and Ultramarine Blue emerged through scientific discovery. Many early synthetics proved toxic or unstable, leading to bans on chromium-based pigments. Modern inorganic pigments include cadmium (yellow, red, orange), cobalt (blue, green, turquoise, violet, yellow), and manganese (blue, violet, black) families, with manufacturers using 'hue' or 'tone' designations for safer formulations.

The 19th century marked a revolution in pigment production with the invention of synthetic colors. Berlin blue (Prussian blue), invented in 1704 by Johann Jacob Diesbach, was the first synthetic blue pigment. Mauveine, invented in 1856 by William Henry Perkin, was the first synthetic purple pigment. Perkin, a 17-year-old student, accidentally created this color while attempting to synthesize quinine. Queen Victoria loved the color and wore a mauve dress to Prince Albert's funeral, causing a fashion craze. Perkin became one of the wealthiest people in the world by 1861. These innovations made pigments more accessible and affordable, though many early synthetic pigments were highly toxic.

Cobalt blue is an inorganic pigment traditionally made by heating cobalt with aluminum oxide at approximately 1,120°C, historically valued by artists for painting skies; when comparing four brands (Cotman student quality using ultramarine blue and white as a 'cobalt blue hue,' Jackson's professional using PB72 cobalt zinc illuminate blue, Daniel Smith using PB28 genuine cobalt blue, and Windsor & Newton using PB28), professional quality paints demonstrate higher pigment load and better performance, with Jackson's showing the most highly pigmented result and Windsor & Newton exhibiting the most granulation in washi swatches.

Modern chemistry enables creation of synthetic pigments that replicate or improve upon natural sources. Ultramarine blue was developed to replace expensive lapis lazuli, while synthetic inorganics like cobalt blue and cadmium pigments provide affordable vibrant colors. Synthetic organic pigments (phthalocyanides, quinacridones) offer enhanced lightfastness compared to natural organic pigments like rose madder, which are fugitive and fade over time. This advancement allows artists access to historically significant colors without relying on rare natural materials or sacrificing color longevity.

Inorganic pigments are derived from earth substances like metals and oxides (e.g., umber, sienna, ochre), making them grittier, more opaque, and prone to losing chroma when mixed with white, though they offer excellent light fastness; organic pigments are carbon-based compounds (e.g., quinacridone, phthalo, Indian yellow) that are transparent, higher chroma, and retain their color better when mixed with white, though they may have varying light fastness depending on the specific pigment.

Cobalt teal blue, also called cobalt turquoise light, is a light cyan pigment that differs significantly from blue. It creates much brighter and more powerful green mixtures when combined with yellow compared to ultramarine blue, which produces duller greens. This pigment serves as an effective 'killing' pigment for neutralizing intense reds, as it is complementary to cadmium red. Understanding the scientific distinction between blue and cyan is important for accurate color mixing.

Paint pigments fall into inorganic (mineral-based) and organic (carbon-based) categories. Inorganic pigments like cinnabar (mercury sulfide), orpiment (arsenic sulfide), and malachite (copper carbonate) have been used for millennia, offering fade resistance but sometimes toxicity. Titanium dioxide, the most industrially produced pigment globally (over 9 million tons/year), brightens colors and increases opacity. Organic pigments, derived from carbon chemistry enabling complex molecules, offer superior color depth but higher costs and varied light fastness. Examples include alizarin red from madder plants and phthalocyanine blue (commercialized 1930s). Creating desired colors requires blending multiple pigments, presenting formulation challenges as pigments may interact undesirably.

Modern synthetic organic pigments represent the pinnacle of industrial chemistry, offering intense colors less toxic than inorganics and more stable than historical organics. Key classes include: phthalocyanines (late 1930s, bright blues/greens, transparency, stability); quinacridones (1958, excellent lightfastness, transparency for watercolor, covering oranges, pinks, violets, earth tones); perinones (limited red shades, expensive); perilenes (scarlet to dark brown to near-black PBK 31); and pyrroles (discovered 1980s, intense orange, scarlet, red, carmine). These pigments leverage carbon atom versatility for almost unlimited color combinations, solving historical problems of fugitive organic pigments.

Single-pigment granulating paints (ultramarines, cobalts) can be added to regular watercolor anywhere. Mixing two granulating pigments causes them to cancel each other out, creating muddy results. Only transparent colors should be added to granulating mixtures - opaque pigments block granulation. Glazing transparent color over granulation creates contrast, while granulation over transparent background is unpredictable. Granulation should only be used in one layer - applying it on every layer causes quick muddiness and loss of transparency. For stones, granulation creates texture but must be limited. Clouds painted by placing granulating and regular paints side-by-side often disappoint. Time-constrained sketching (8 minutes) forces quick decisions without pre-planning, often producing unexpected successful solutions like dynamic single-color spots.

Advanced watercolor techniques involve understanding how specific pigments interact and granulate. Natural drying is essential; heat tools prevent proper pigment separation. Specific granulating pigments include PB29 (ultramarine), PY40 (earthy yellow), and PG7 (phthalo green). Mineral pigments like vivianite can be challenging to re-wet but still produce granulation. Artists can create custom granulating palettes by testing existing tubes to identify granulating colors. The process involves swatching, observing pigment separation, and organizing colors for specific effects. Combining granulating pigments in mixtures creates more pronounced textural effects. Artists should test granulation on larger swatch areas to properly evaluate color behavior.

Super-granulating and separating watercolors are created by mixing pigments with different particle sizes and weights—granulating pigments (like metal crystalline pigments such as cobalt, manganese, and iron oxides) settle and separate from non-granulating synthetic organic pigments (like quinacridones and phthalocyanines) or dyes when applied to paper, creating unique textured effects; this can be achieved through three methods: mixing two granulating paints, combining granulating with non-granulating paint, or blending watercolor with ink.

Three techniques for granulating pigments: painting dry paper for dense texture, wet-on-wet for flowing separation, and water drop blooms for unpredictable effects. Granulation occurs because heavy, irregular mineral particles don't dissolve completely in water. Earth pigments (ochres, siennas) and mineral blues/greens granulate naturally due to their coarse structure, producing muted tones. Synthetic organic pigments (pyrrols, quinacridons) are fine and smooth, creating vibrant colors without texture. Strategic use means applying granulating pigments to backgrounds and foliage where texture adds interest quickly, while reserving non-granulating pigments for detailed subjects requiring smooth control. Combining both types creates dynamic compositions with minimal effort.

Granulating watercolors contain large pigment particles that create texture and color separation effects. In wet washes, water disperses lighter particles away from heavier clumping pigments, producing distinctive granulation patterns. These colors are semi-opaque with two to three pigment ingredients and lift easily from paper without heavy staining, making them ideal for highlights and corrections. However, they are not ideal for layering. The best technique is applying a single, very wet wash and leaving it alone to dry. Cobalt pigments, particularly cobalt turquoise (PG-50), create the most pronounced color separation. Professional watercolors use a standardized pigment numbering system (PB29, PBr6) to identify ingredients, enabling artists to mix their own colors using single pigment paints.