Composition and Permanence

Rublev Colours Pigments

The Composition and Permanence tables in the description of Rublev Colours Pigments provide essential information on the composition, characteristics, and permanence of Rublev Colours dry powder pigments. Using the tables helps you choose colors based on how they will appear and how long-lasting the finished artwork will be, and by understanding exactly what’s in your paint, you can be sure to create the highest quality painting.

The term ‘permanence’ is used here to mean the quality of lasting or remaining unchanged indefinitely. This does not imply that it will remain unchanged forever, but rather for an extended time that cannot be measured.

Terms Explained

Pigment Names

Common Names

The common names of the pigment are provided in English, French, German, Italian, and Spanish.

Synonyms

In English and other languages, synonyms are provided for the pigment in historical and modern usage.

Nomenclature

The terminology includes the common name of the pigment or mineral, the name of the primary mineral or mineral equivalent of the pigment, and the geographical source of the pigment or mineral. If a pigment is synthetic, it is not derived from a native mineral, but the equivalent of the natural mineral is given.

Pigment Information

Color

The apparent color of the dry pigment.

Pigment Classification

This is a classification of the pigment type. Pigments can be classified as being either Synthetic or Natural and Organic or Inorganic. For example, Lead White is Synthetic and Inorganic. Yellow ocher is Natural and Inorganic.

Colour Index Name

Each pigment can be universally identified by its Colour Index Name. For example, Cobalt Blue is Pigment Blue 28, abbreviated to PB 28.

Colour Index Names of the pigments allow you to cross-reference the working properties in other sources, e.g., lightfastness, opacity, toxicity, etc. It is also necessary to fully identify some of the modern pigments. For example, the disclosure of a pigment as Naphthol Red is insufficient because there are over a dozen different types, differing widely in lightfastness and opacity.

Chemical Name or Composition

This row provides the chemical name or description of the principal colorant in the pigment or the pigment itself.

Chemical Formula

This is the common formula of the principal colorant in the pigment or the pigment itself.

CAS Number

This row provides the CAS Number of the principal colorant in the pigment or the pigment itself. The CAS Registry Number, also referred to as CASRN or CAS Number, is a unique numerical identifier assigned by Chemical Abstracts Service (CAS) to every chemical substance described in scientific literature (currently including those described from at least 1957 through the present), including organic and inorganic compounds, minerals, isotopes, alloys, and nonstructural materials.

Series

The Series number indicates the relative price of the pigment. Series 1 is the least expensive, and Series 5 is the most expensive. Series 7 are historical or rare pigments with pricing that ranges above series 1 through 5.

Lightfastness

Lightfastness is shown with an ASTM rating for the pigment in three paint vehicles: Oils, Acrylics, and Watercolors. The ASTM abbreviation stands for the American Society for Testing and Materials. This organization has set standards for the performance of art materials, including a color’s lightfastness.

To measure lightfastness using this system, colors are reduced to 40% reflectance by adding titanium white, except for watercolors, which rely on white paper. This means the amount of light reflected from the color swatch. The swatches are then tested in both sunlight and artificially accelerated conditions.

The swatches are measured with an instrument before and after exposure to light and the difference is calculated. Based on these calculations, swatches are assigned different lightfastness categories. For example, pigments with a mean color change of 4.0 or less ΔE*ab are assigned to Lightfastness Category I; pigments with a mean color change of more than 4.0 but not more than 8.0 ΔE*ab to Lightfastness Category II; and so on. In this system, I is the highest lightfastness available, and V is the lowest, though ratings I and II are considered permanent for artists’ use.

Where no ASTM rating is given for a pigment, it is labeled as ‘N/L’ meaning “Not Listed” this usually indicates that the ASTM has not yet tested the pigment. It does not necessarily imply a lack of lightfastness.

Physical Properties

Particle Size

The mean or average particle size of the pigment. Particles of pigments are an ensemble (collection) of different particle sizes, making them polydisperse, which means that the particles in an ensemble have different sizes. The notion of particle size distribution reflects this polydispersity. There are several ways of defining particle sizes, including median, geometric mean, and average. The size represented in this row is the average or mean particle size of the ensemble of pigment particles.

Density

Density measures how much mass is contained in a given unit volume (density = mass/volume). For pigments, it is usually expressed in g/cm3. Put simply, if the mass measures how much ‘stuff’ there is in an object, density measures how tightly that ‘stuff’ is packed together.

Hardness

If available, the Mohs hardness of the pigment is shown. The Mohs scale of mineral hardness is a qualitative ordinal scale that characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material.

Refractive Index

In this row, the refractive index of the pigment is indicated. In optics, the refractive index or index of refraction n of an optical medium is a dimensionless number that describes how light propagates through that medium. It is defined as n = c/v, where c is the speed of light in a vacuum and v is the phase velocity of light in the medium.

Oil Absorption

The quantity of oil required to thoroughly wet a measured amount of pigment to form a stiff paste when mechanically mixed. The oil absorption number or value of a pigment is the number of milliliters or grams of oil, usually refined linseed oil, used to bind together 100 grams of pigment under specified test conditions. The unit used is grams of oil for every 100 grams of pigment. The figure is not absolute but depends on the operator and the method of determination. The term “oil absorption” is frequently used as a measure of plastic viscosity. As such, it can lead to unpredictable results since oil absorption as measured bears little relation to finished mill-ground pigment dispersion. Oil absorption has the meaning of a qualitative sort, however, with regard to a preliminary mixer operation.


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