A-level Applied Science/Colour Chemistry/Dyes/Pigment
In the colouring of paint, ink, plastic, fabric and other material, a pigment is a dry colorant, usually an insoluble powder. There are both natural and synthetic pigments, both organic and inorganic ones. Pigments work by selectively absorbing some parts of the visible spectrum whilst reflecting others.
Pigments in paint must also make paint opaque, thus increasing a paint's hiding power. Opaque paint will also protect the substrate from the harmful effects of ultraviolet light.
Some pigments are toxic, such as those used in lead paint. Paint manufacturers replaced lead white with a less toxic substitute, which can even be used to colour food titanium white (titanium dioxide) which was first used in paints in the 19th century. The titanium white used in most paints today is often coated with silicon or aluminum oxides for better durability.
Some newer paints—called prism paint—can produce effects where the colour changes depending on the angle (orientation) at which it is viewed. Modern banknotes, specifically the newer higher denomination notes, have this effect on them. This effect is produced by having pigment molecules that are long and thin and are meant to dry in a specific orientation, with different ends of the molecule being different colours.
Pigments also thicken the consistency of the paint.
Dyes and pigments
A distinction is usually made between a pigment, which is insoluble, and a dye, which is either a liquid, or is soluble. There is a well-defined dividing line between pigments and dyes: a pigment is not soluble in the vehicle (or matrix) while a dye is. From this follows that a certain colorant can be both a pigment and a dye depending on in which vehicle it is used. In some cases, a pigment will be made by precipitating a soluble dye with a metallic salt. The resulting pigment is called a "lake". Fugitive pigments are non-permanent pigments.
Biological & organic pigments
- Heme/porphyrin-based: chlorophyll, bilirubin, hemocyanin, hemoglobin, myoglobin
- Light-emitting: luciferin
- Carotenoids: alpha and beta carotene, anthocyanin, lycopene, rhodopsin
- Xanthophylls: canthaxanthin, zeaxanthin, lutein
- Photosynthetic: chlorophyll, phycobilin
- Organic: Pigment Red 170, phthalocyanine, Phthalo Green, Phthalo Blue, Alizarin, Alizarin Crimson, crimson, Indian Yellow, indigo, quinacridone, Quinacridone Magenta, woad.
- Resin: gamboge
- Polyene enolates: a class of red pigments unique to parrots
- Other: hematochrome, melanin, Phthalocyanine blue, urochrome, Van Dyke brown
Important pigments (according to AQA) are highlighted.
- Carbon pigments: bone black (also known as bone char), carbon black, ivory black, vine black, lampblack, Mars black
- Cadmium pigments: Cadmium Yellow, Cadmium Orange (both cadmium sulphide), Cadmium Red (cadmium yellow plus cadmium selenide), Cadmium Green (cadmium yellow plus viridian).
- Iron pigments: Caput Mortuum, oxide red, red ochre, Sanguine, Venetian red, (all iron (III) oxide in various forms), iron (III) hexacyanoferrate (II) (Prussian Blue or iron blue).
- Chromium pigments: viridian (chromium (III) oxide), Chrome Yellow (lead (II) chromate (VI)), Chrome Green (viridian, or chrome yellow plus Prussian Blue),
- Cobalt pigments: Cobalt Blue (cobalt (II) oxide), Cerulean Blue (cobalt (II) stannate)
- Lead pigments: Lead White (basic lead carbonate, a.k.a. Cremnitz White, Foundation White), Naples Yellow (lead (II) antimonate), Red Lead (trilead tetroxide)
- Copper pigments: verdigris (copper acetate), Paris Green (verdigris plus copper arsenite),
- Titanium pigments: titanium white (titanium dioxide)
- Sulphur pigments: ultramarine (lazurite, a complicated sodium aluminium silcate with sulphur; artificial versions are Ultramarine Green Shade and French Ultramarine), vermilion (mercury sulphide)
- Zinc pigments: Zinc White (zinc oxide), zinc chromate, zinc potassium chromate
- Clay earth pigments: sienna, raw sienna, burnt sienna, umber, raw umber, burnt umber, yellow ochre
- Powdered metals: lead, zinc, aluminium.
Corrosion-resistant paints contain powerdered metals or trilead tetroxide or zinc chromate.
Cadmium, lead and mercury are highly toxic and are rarely used in modern paints.
Titanium dioxide is extensively used for both house paint and artist's paint, because it is permanent and has good covering power. Titanium oxide pigment accounts for the largest use of the element. Titanium paint is an excellent reflector of infrared, and is extensively used in solar observatories where heat causes poor seeing conditions.