COLOUR, PIGMENTS AND DYES SCIENCE, ART & NATURE

Ken Derham Halesworth U3A Science Group, April 2019 WHAT IS COLOUR?

• Colour is a characteristic of human visual perception. • This perception of colour derives from the stimulation of cone cells in the human eye by electromagnetic radiation in the visible spectrum. Wikipedia VISIBLE LIGHT A triangular prism dispersing a beam of white light. The longer wavelengths (red) and the shorter wavelengths (blue) are separated. Wikipedia

Violet light - Shorter wavelength, higher frequency has higher energy E = h v (Planck’s equation) WHY DO OBJECTS APPEAR COLOURED?

Objects appear different colours because they absorb some colours (wavelengths) and reflected or transmit other colours. The colours we see are the wavelengths that are reflected or transmitted.

White objects appear white because they reflect all colours. Black objects absorb all colours so no light is reflected. TRANSMITTED LIGHT PRIMARY COLOURS: ADDITIVE MIXING OF LIGHT • Mix Red, Green, and Blue light, you get white light. • Red, green, and blue (RGB) are referred to as the primary colours of light. • Mixing the colours generates new colours, as shown on the colour wheel. This is additive colour. • As more colours are added, the result becomes lighter, heading towards white. • RGB is used to generate colour on a computer screen, a TV, and other electronic displays. PRIMARY COLOURS: SUBTRACTIVE MIXING OF PIGMENTS • Mixing colours using paint, or ink, uses subtractive colour mixing. • The primary colours of light are red, green, and blue. • If you subtract these from white you get cyan, magenta, and yellow. • Mixing the colours generates new colours as shown on the colour wheel. • Mixing these three primary colours generates black. As you mix colours, they tend to get darker, ending up as black. • The CMYK colour system (cyan, magenta, yellow, and black) is the colour system used for printing. PRIMARY COLOURS AND MIXING OF COLOURS PIXELS • "Pixels" (short for "Picture Elements") are small dots that make up the images on digital displays (computers, TVs, digital cameras etc). • The screen is divided up into a matrix of thousands or even millions of pixels. • Typically, you cannot see the individual pixels, because they are so small. This is good, because most people prefer to look at smooth, clear images rather than blocky, "pixelated" ones. • A resolution of 640x480 is comprised of a matrix of 640 by 480 pixels, or 307,200 in all. That's a lot of little dots. • Each pixel can only be one colour at a time. However, since they are so small, pixels often blend together to form various shades and blends of colours. PIXELS

Schematic of a Pixel Matrix PIXELS

• Reproduction of an image created with 16 million pixels, each corresponding to a different colour on the full set of RGB colours. • The human eye can distinguish about 10 million different colours COLOURANTS: PIGMENTS AND DYES

• Most pigments are dry colourants, usually ground into a fine powder. • For use in paint, this powder is added to a binder (or vehicle), a relatively neutral or colourless material that suspends the pigment and gives the paint its adhesion. • A distinction is usually made between • a pigment, which is insoluble in its vehicle (resulting in a suspension), and • a dye, which either is itself a liquid or is soluble in its vehicle (resulting in a solution). • A colourant can act as either a pigment or a dye depending on the vehicle involved. PIGMENTS

Dan Brady - https://www.flickr.com/photos/11853009@N07/1382064216 , https://commons.wikimedia.org/w/index.php?curid=3534510 EARLY HISTORY OF PIGMENTS

• The earliest known pigments were natural minerals. • Natural iron oxides give a range of colours and are found in many cave paintings.

• Two examples include Red Ochre, anhydrous Fe2O3, and . hydrated Yellow Ochre - Fe2O3 H2O • Charcoal—or carbon black—has also been used as a black pigment since prehistoric times. • Pigments and paint grinding equipment believed to be between 350,000 and 400,000 years old have been found in a cave in Zambia HISTORY OF PIGMENTS • Before the Industrial Revolution, the range of colour available for art and decorative uses was limited. Most were mineral pigments or pigments of biological origin.

• Pigments from unusual sources such as botanical materials, animal waste, insects, and molluscs were harvested and traded over long distances.

• Some colours were costly or impossible to obtain, given the range of pigments that were available. Blue and purple came to be associated with royalty because of their rarity. ULTRAMARINE

• Ultramarine is a deep blue pigment which was originally made by grinding lapis lazuli into a powder. • The name comes from the Latin ultramarinus, literally "beyond the sea", because the pigment was imported into Europe from mines in Afghanistan by Italian traders during the 14th and 15th centuries. • Ultramarine was the finest and most expensive blue used by Renaissance painters. It was often used for the robes of the Virgin Mary, and symbolized holiness and humility. • It remained an extremely expensive pigment until a synthetic ultramarine was invented in 1826. ULTRAMARINE

Lapis lazuli Natural ultramarine Synthetic ultramarine blue USE OF ULTRAMARINE Masaccio, 1426 Sassoferrato, 1654 CHEAPER ALTERNATIVE

Azurite, Cu3(CO3)2(OH)2, i.e. copper(II) mixture of carbonate and hydroxide

• More widely available

• But not stable in air

• Presumably looked nice in a fresh painting but did not last SYNTHETIC PIGMENTS – PRUSSIAN BLUE • Prussian blue (Turnbull's blue) was the first modern synthetic blue pigment.

• Ferric ferrocyanide: Fe7(CN)18 or Fe4[Fe(CN)6]3 · xH2O SYNTHETIC PIGMENTS – PRUSSIAN BLUE

• Prepared as a very fine colloidal dispersion. Its appearance depends on the size of the colloidal particles (PB cf TB)

• Prussian blue pigment is significant since it was the first stable and relatively lightfast blue pigment to be widely used

• Easily made, cheap, nontoxic, and intensely coloured

• Prussian blue has attracted many applications. It was almost immediately widely used in oil, watercolour, and dyeing

• 12,000 tonnes of Prussian blue are produced annually especially for use in black and blue inks. USES OF PRUSSIAN BLUE IN ART The Great Wave off Van Gogh's Starry Night uses Kanagawa by Hokusai makes Prussian and cerulean blue extensive use of Prussian blue pigments CERULEAN BLUE

• Cobalt stannate (CoSnO3), introduced as a pigment in the 1860s. • Very stable and lightfast blue with limited hiding power. • Cerulean blue has a fairly true blue but it doesn't have the opacity or richness of cobalt blue. • Not recommended for use in watercolor painting because of chalkiness in washes. • In oil, it was particularly valuable to landscape painters for skies. SOME METAL-BASED PIGMENTS

• Aluminium pigment: Aluminium powder • Cadmium: cadmium yellow, cadmium red, cadmium green, cadmium orange, cadmium sulfoselenide • Chromium: chrome yellow and chrome green (viridian) • Cobalt pigments: cobalt violet, cobalt blue, cerulean blue, aureolin (cobalt yellow) • Copper pigments: Azurite, Han purple, Han blue, Egyptian blue, Malachite, Paris green, Phthalocyanine Blue BN, Phthalocyanine Green G, verdigris • Iron oxide: sanguine, caput mortuum, oxide red, red ochre, Venetian red, Prussian blue • Lead pigments: lead white, cremnitz white, Naples yellow, red lead, lead-tin-yellow • Manganese : manganese violet, YInMn blue • Mercury pigments: vermilion • Titanium pigments: titanium yellow, titanium beige, titanium white, titanium black • Zinc pigments: zinc white, zinc ferrite, zinc yellow TECHNICAL INNOVATIONS ENABLED… … ARTISTIC INNOVATION

• Claude Monet • In the Woods at Giverny: Blanche Hoschedé at Her Easel DYES AND BIOLOGICAL COLOURANTS

• Biological pigments were often difficult to acquire, and the details of their production were kept secret by the manufacturers. • Tyrian Purple is a pigment made from the mucus of one of several species of Murex snail. • Production of Tyrian Purple for use as a fabric dye began as early as 1200 BCE by the Phoenicians, and was continued by the Greeks and Romans • The pigment was expensive and complex to produce, and items coloured with it became associated with power and wealth. • Greek historian Theopompus, writing in the 4th century BCE, reported that "purple for dyes fetched its weight in silver at Colophon [in Asia Minor]." DYES

• The discovery in 1856 of mauveine, the first aniline dye, was a forerunner for the development of hundreds of synthetic dyes and pigments like azo and diazo compounds which are the source of a wide spectrum of colours. • Mauveine was discovered by an 18-year-old chemist, William Henry Perkin, who went on to exploit his discovery in industry and become wealthy. • His success attracted a generation of followers, as young scientists went into organic chemistry to pursue riches. • Within a few years, chemists had synthesized a substitute for madder in the production of Alizarin Crimson. • By the closing decades of the 19th century, textiles, paints, and other commodities in colours such as red, crimson, blue, and purple had become affordable WILLIAM HENRY PERKIN (1838-1907) • Discovered Mauveine in 1856 when attempting to synthesise Quinine from analine in his home lab in Cable Street DEVELOPMENT OF THE DYE INDUSTRY • Perkin patented Mauveine • With support of his father and brother, Perkin set up a small factory to manufacture analine purple in Greenford, west London, in 1957 • This expanded as new synthetic dyes were invented • In Germany other companies developed in this area, eg BASF, Hoechst, AGFA • By the end of the 19th century the German dye manufacturers had become dominant • This lead to shortage of dye for British army uniforms at the outbreak of war in 1914 DEVELOPMENT OF THE DYE INDUSTRY • Perkin patented Mauveine • With support of his father and brother, Perkin set up a small factory to manufacture analine purple in Greenford, west London, in 1957 • This expanded as new synthetic dyes were invented • In Germany other companies developed in this area, eg BASF, Hoechst, AGFA • By the end of the 19th century the German dye manufacturers had become dominant • This lead to shortage of dye for British army uniforms at the outbreak of war in 1914 COLOUR IN NATURE

• Chlorophyll absorbs energetic blue light and some red light • It reflects the green light • This provides the energy for photosynthesis • And is why most plants are green SPRING FLOWERS EXAMPLES OF COMPOUNDS THAT PROVIDE COLOUR

Basic chemical structure of General structure of a carotenoid Anthocyanins LATE SUMMER FLOWERS Absorption spectra of an Anthocyanin compared to Chlorophyll INTERFERENCE OF LIGHT IRIDESCENCE STRUCTURAL COLORATION

• The production of colour by microscopically structured surfaces fine enough to interfere with visible light, sometimes in combination with pigments. • What colour are Peacock feathers? • Peacock feathers are brown • but their microscopic structure makes them also reflect blue, turquoise, and green light, and they are often iridescent. IRIDESCENT COLOURS IN PEACOCK FEATHERS

• The head, neck and eye spot on the tail plumes of the Indian, or blue, peacock is a rich, iridescent blue. • This colour is created by a crystalline lattice of nine to 12 rods containing melanin, a colour pigment. These rods are spaced roughly 140 nanometers apart, a distance that causes light to reflect back at the viewer in wavelengths in the blue spectrum. • Green is created by a square lattice of roughly 10 rods spaced 150 nanometers apart. When light hits this structure, the wavelengths that are reflected back are in the green portion of the spectrum. • Yellow is formed by a crystal lattice composed of around six rods, each 165 nanometers apart.

LUMINESCENCE

• Spontaneous emission of light by a substance not resulting from heat; • I.e. a form of cold-body radiation. • Chemiluminescence, the emission of light as a result of a chemical reaction • Bioluminescence, a result of biochemical reactions in a living organism • Electrochemiluminescence, a result of an electrochemical reaction • Electroluminescence, a result of an electric current passed through a substance • Photoluminescence, a result of absorption of photons • Fluorescence, photoluminescence as a result of singlet–singlet electronic relaxation (typical lifetime: nanoseconds) • Phosphorescence, photoluminescence as a result of triplet–singlet electronic relaxation (typical lifetime: milliseconds to hours) LOCAL BIOLUMINESCENCE

Glow worm bioluminescing on Halesworth Millennium Green