Slate Gray Pigment

Slate Gray is a quiet, stony neutral with low tinting strength, making it ideal for atmospheric passages, architectural tones, and underpainting. It reduces chroma gently in mixtures—cooling and graying colors without the harshness of a strong black—and typically yields a natural, matte surface character in paint films.

Slate Gray is a naturally occurring gray mineral pigment made by finely grinding slate—a metamorphosed mudstone/shale. Slate is not a single mineral, but a rock made up of very fine crystalline components. Analytical results for this material indicate that Slate Gray consists primarily of mica (muscovite/illite-type) with chlorite (clinochlore-type) and quartz, along with minor iron-bearing phases that contribute to its gray tone.

Slates typically range from light to dark gray, sometimes with a faint greenish or bluish cast depending on the relative proportions of mica, chlorite, quartz, and iron-bearing minerals. Because the component minerals are extremely fine-grained, individual particles are commonly below the resolution of standard optical microscopy.

Origin and History

Slate is a generic geological term for a fine-grained metamorphic rock that has developed a strong, planar cleavage, allowing it to be split into thin sheets. Its mineralogy varies with the composition of the original mudstone and the degree of metamorphism. Most slates contain aligned micaceous minerals (which produce cleavage), very fine quartz, and minor accessory minerals that may include iron oxides, iron sulfides (pyrite), carbonaceous material, and trace carbonates.

Finely ground slate has a long history of industrial use as an inert mineral filler and extender in coatings, putties, and filling compounds. In artists’ materials, it is valued for its subtle, stony gray hue, low tinting strength, and restrained, natural appearance.

Note on “Slate Black” and related terms: “Slate Black” is sometimes used as a common name for slate-derived gray/black pigments. Historically, some “shale black” materials were produced by calcining bituminous shales; these are distinct from a simple ground-slate pigment and can contain higher levels of carbonaceous matter.

Source

Our Slate Gray pigment is derived from slate deposits in the Ardennes region of France, likely from the La Richolle quarry in the Rimogne district.

Permanence and Compatibility

Slate Gray is composed of stable, inorganic minerals (phyllosilicates and quartz). On this basis, it is expected to be highly lightfast and generally compatible with other artists’ pigments in oil, acrylic, watercolor, and other common binders.

Oil Absorption and Grinding

The oil absorption of Slate Gray is 43 g linseed oil / 100 g pigment. It is a medium-grain pigment that mulls readily into oil and disperses well in water-based binders.

Toxicity

Slate Gray is considered to have low toxicity in normal use. As with all powdered minerals, avoid inhalation of dust and unnecessary skin contact. Use appropriate ventilation and a properly fitted dust mask or respirator when handling dry pigment or sanding dried films. Conforms to ASTM D4236.