Scientific and technical issues

Selection of a pigment for a particular application is determined by cost, and by the physical properties and attributes of the pigment itself. For example, a pigment that is used to color glass must have very high heat stability in order to survive the manufacturing process; but, suspended in the glass vehicle, its resistance to alkali or acidic materials is not an issue. In artistic paint, heat stability is less important, while lightfastness and toxicity are greater concerns. The following are some of the attributes of pigments that determine their suitability for particular manufacturing processes and applications: Lightfastness and sensitivity for damage from ultra violet light Heat stability Toxicity Tinting strength Staining Dispersion Opacity or transparency Resistance to alkalis and acids Reactions and interactions between pigments. A final consideration is lightfastness, or the ability of a pigment to retain its original color appearance under exposure to light. This is usually indicated as a numerical rating, from I (high lightfastness) to III or IV (low lightfastness), on the paint tube or in the paint technical information available from the manufacturer. Lightfastness is a crucial issue with watercolors, because the paint pigment is not surrounded by a protective dried binder (as in oil or acrylic paints) but is left exposed on the surface of the paper. Watercolors acquired in the 19th century a market reputation for relative impermanence tha continues to suppress their price today, and painters who admire this medium will make choices to improve its market status: in fact, lightfast watercolor paints on archival papers are more durable than any oil painting on canvas. The most stable painting medium is pastel, but modern lightfast watercolors are now more stable than oil or acrylic mediums. Unfortunately, paint manufacturer lightfastness ratings are not always trustworthy. However, because they have been demonstrated to be impermanent in watercolors, certain pigments (paints) should never be used under any circumstances. These include: aureolin (PY40), alizarin crimson (PR83), genuine rose madder (NR9), genuine carmine (NR4), genuine vermilion (PR106), most naphthol reds and oranges, all dyes (including most "liquid watercolors" and marker pens), and paints premixed with a white pigment, including paints marketed under the names naples yellow, emerald green or antwerp blue. Most of these are colorants invented in the 19th century or before that have been superseded by far more durable modern alternatives, and these are usually sold as "hue" paints (e.g., "alizarin crimson hue" is a modern pigment that resembles alizarin crimson). Industry labeling practice is to include a lightfastness rating on the paint packaging, and painters should only use paints that have a lightfastness rating of I or II under the testing standards published the American Society of Testing and Materials (now ASTM International).