SR 50(2) 19-22

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SCIENCE REPORTER, FEBRUARY 2013

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CHAGANTYKRISHNAKUMARI

Feature

Article

GORGEOUScolourful floral designs in the

front of Indian homes are an invitation

to those entering the house. Whether it is

a festival or a special occasion, this social

artifact is widespread in villages and towns

in South India, though we come across

simpler floral designs in the front yards daily

also. The bamboo-mud walls of thatchedroof houses are smeared with cow dung

mixed with water and designs are drawn

on walls with powdered limestone. Some

people use red mud instead of cow dung

for walls and altars to house religious idols.

The designs enhance the beauty of Indian

villages.

Now, how is it done? Well, the village

women folk sprinkle cow dung mixed and

stirred thoroughly with water early in the

morning before sunrise in the front yard of

their hutments. While the surface is still damp,

holding a handful of flour in their right hand,

they uniformly drop the rice flour or chalk

powder to make a line. With their thumb

firmly planted on the ground, they keep

moving their hands to make the curve or the

dots to get beautiful designs. The patterns

could be geometric or mathematical line

drawings around a matrix of dots. When

the water evaporates from the ground,

the designs stick to the greenish yellow

background, ultimately adding beauty to

the surroundings. This is how the womenfolk

very easily paint the bile pigments from

the dung using water as vehicle. Without

Pigments of various kinds are ubiquitous in our lives.

Can you think of some pigments, and how they are

useful to us?

Left to right: Anthocyanins play a role in ripening of fruits; Pumic is used for making painting materials;Flavonoids are found in dark chocolate and some fruits, Octopus squirt ink composedprimarily of concentrated melanin

Rangoli: Colourful designs

Pigments are materials that change the colour of transmitted or reflected light due to selective

wavelength absorption. The classification on the basis of colour is easier. In this classification they

are categorised as white pigments, transparent pigments, and coloured pigments.

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Feature Article

Dyes and Pigments

Both dyes and pigments are coloured

compounds, they impart a colour to

a material, so the term colourant is

often used for both. Then what is the

difference between these two?

Let us take a rusted iron implement.

The reddish dust accumulated on the

iron article is what we call rust. This

is actually a result of oxidation of iron

by atmospheric oxygen in presence of

moisture that produces reddish brown

iron oxide on its surface. Collect this

rust and grind it with a bit of sugar,

you get a fine red powder. Put it under

a microscope and you will see white

and red granules separately. One fact

becomes clear: sugar and rust are

not mixed with each other although it

appears to the naked eye.

Mix a bit of the rust in few drops

of water and dip a piece of white cotton

cloth. The cloth becomes reddish brown.

Dry and then rinse it, the particles of

iron oxide fall off from it. Iron oxide isinsoluble in water. Mix alizarin in water.

It dissolves. Dip a white cloth in the

solution; the cloth gets a permanent

red shade. Iron oxide is a pigment and

alizarin is a dye.

The major difference between dyes

and pigments is solubility. Dyes are

usually soluble in water. Once a dye

is dissolved in water, the material to

be dyed can be immersed in the dye

solution. As the material soaks up the

dye and dries, it develops a colour.

Pigments are generally not soluble inwater, oil, or other common solvents.

They disperse in the medium as very fine

particles. Before applying to a material,

they are first ground into a fine powder

and thoroughly mixed with water, or oil

or any other liquid. The liquids used to

mix the pigments are called vehicles.

Though the pigment does not dissolve in

the vehicle, after the vehicle dries up the

pigment adheres to the material intact,

adding colour and shine to it.

Pigments in flowers make them

attractive

Plants absorb sunlight using the

green pigment Chlorophyll

Near Tuscany in Italy, an orange-red mud called

Sienna is available. The Telugu speaking people of

south India call it Kichchili rangu mudda. Villagers

in India paint their earthen flours with this red mud.

Its yellow-brown colour comes from ferric oxides.

Raw sienna is heated to remove the water from the

clay. Burnt sienna is an iron oxide pigment that is

reddish-brown in colour. Ochre, umber, and sienna

are the first pigments to be used by humans, and

have been found in many cave paintings.

The most heavily studied betalain is betanin,also called beetroot red after the fact that it

may be extracted from red beetroots

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much training effort they bring into action

complex molecular structures similar to that

of porphyrin, in their skillful floral designs.

Near Tuscany in Italy, an orange-

red mud called Sienna is available. The

Telugu speaking people of south India call

it Kichchili rangu mudda. Villagers in India

paint their earthen flours with this red mud.

Its yellow-brown colour comes from ferric

oxides. Raw sienna is heated to remove

the water from the clay. Burnt sienna is an

iron oxide pigment that is reddish-brown incolour. Ochre, umber, and sienna are the

first pigments to be used by humans, and

have been found in many cave paintings.

Our ancestors used to extract a red

substance from the roots of Manjistha plant

and dye their cotton cloths with it to get

a purple colour. The botanical name of

Manjistha isRubia cordifolia, often known as

Indian Madder. Apart from using its root for

medicinal purposes, Indian Madder is also

used for dyeing as it contains a chemical

known as alizarin. Cow dung contains a

carbon compound called bilin, a bile

pigment, whose brownish yellow colour

gives the faeces its characteristic colour.

Alizarin is a dye, where as iron oxide and bilin

are pigments.

Classification of PigmentsPigments may be classified into inorganic

and organic groups. They may also be

classified as natural and synthetic.

Pigments are materials that change

the colour of transmitted or reflected light

due to selective wavelength absorption.

The classification on the basis of colour

is easier. In this classification they are

categorised as white pigments, transparent

pigments, and coloured pigments. Among

the white pigments white lead, zinc oxide,

and titanium dioxide are noteworthy. Earlier

mostly lead oxide was in use but as lead is

toxic its use has been banned. Zinc oxide

is the most popular white pigment. Today,

titanium dioxide is widely used in a variety of

applications.

Transparent pigments are added to

varnishes, paints, polishes, and coatings

to increase the bulk, and so these are

also called extenders. Commonly finely

powdered calcium carbonate, magnesium

silicate, barium silicate etc are in use.

The earliest known coloured pigments

were natural minerals. Natural iron oxides

give a range of colours and are found

in many Paleolithic and Neolithic cave

paintings. Red Ochre and the hydrated

Yellow Ochre, carbon black have been

used as pigments since prehistoric times.

Indian red, Spanish red, Persian Gulf redpigments are noteworthy red pigments that

are all oxides of iron only.

Lapis lazuli, abbreviated to lapis, is a

relatively rare semi-precious stone largely

formed from the mineral lazurite. It has been

prized since antiquity for its intense blue

colour as it takes an excellent polish and

can be made into jewelry, carvings, boxes,

mosaics, ornaments, and vases. Powdered

lapis was used as eye shadow by Cleopatra.

It was also ground and processed to make

the pigment ultramarine for tempera paint

and, more rarely, oil paint. Its usage as

a pigment in oil paint ended in the early

19th century when a chemically identical

synthetic variety, often called French

ultramarine, became available.

Ferric ferrocyanide is used as

blue pigment it is also called Prussian

blue. Among organic blue pigments

phthalocyanine blue is very important and

popular. Sienna is a natural yellow pigment.

Chrome yellows, cadmium yellow, hensa

yellow, and benzedene yellow are a few

popular synthetic pigments that are strong

and bright. Among green pigments chrome

greens are worth mentioning. A mixture of

chrome yellow and Prussian blue gives a

green pigment. Phthalocyanine green is an

organic green pigment.

Pigments in IndustriesPigments are used in paints that are oil and

resin based, automotive finishes, emulsion

paints, distempers, aqueous based paints

like lime; all kinds of printing inks like inks

for printing metal foils, lacquer etc; finishesfor leather and textiles; for colouration of

plastics and cement; colours for cosmetics

and paper.

Today the role and application of

pigments have increased manifold. There

would hardly be any industries left where

pigments do not play a substantial role.

The challenge is now to discover pigments

that are capable of not only long-lasting

applications but also are environmentally

safe.

A pigment is not used just to enhance

the beauty; there are many functional

advantages of its use. Examples of such

uses include anti-corrosion pigments,

metal pigments, luminous pigments,

special pigments etc. Corrosion of metals

by atmospheric oxidation is prevented by

coating them with pigments. For example,

iron articles like beams are protected from

being rusted by coating them with red lead

(lead oxide), zinc yellow (zinc chromate)

and metallic lead pigments. Pigments not

only give the paint its colour and finish, but

also serve to protect the sur face underneath

from weathering as well as helping to hold

the paint together. Special pigments can

be used to give metallic finishes (for car

bodies), to be hard-wearing (for road

markings) etc.

When near ultraviolet light strikes

on luminous pigments, visible light gets

transmitted. Hence, materials painted with

luminous pigments dazzle bright, enhancing

their beauty. Luminous pigments may be

phosphorescent or fluorescent. Recent

advances in self-emitting light technology

enable some luminescent pigments to emit

In industries, thermochromic pigments change the shade of a painted colour at a specific temperature

and return back to the original colour as the pigment is cooled.

These are now incorporated into special ink and printed on to plastic films to create thermometers or

temperature indicators the battery test strip is a good example.

When the battery is in good condition, current flows through a printed resistor under the thermocromic

film and heats it to cause a colour change.

Betalains give rise to the

distinctive deep red of beetroot.

The composition of different

betalain pigments can vary,

giving rise to breeds of beetroot

that are yellow or other

colours, in addition to the

familiar deep red.

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light for a significantly longer duration after

charging by exposure to daylight, white

lamplight or UV radiation.

Fluorescent pigments, often used

for security applications, emit light

when excited by an energy source.

Phosphorescent products, typically referred

to as afterglow products, continue to

emit light after the energy source has been

removed. Security marks and features

that are not visible under normal light are

made with luminescent pigments and they

can be easily verified using ultraviolet light

or a luminescence sensor. Luminescent

pigments are widely used in making

security stamps, code tapes, credit cards,

licenses etc.

The type of luminescent pigment

selected often is based on the particular

application needed. Zinc sulfide, rare

earth-doped strontium aluminates,

rare earth oxide sulfides, alkaline earthsulfides, silicates, phosphates and organic

heterocyclic compounds etc are used in the

production of luminous pigments. Pumice is

a volcanic rock that is solidified frothy lava

typically created when super-heated, highly

pressurized rock is violently ejected from a

volcano. Porous stones of this type are mixed

in pigments used for painting materials so as

to give them a rough surface. This prevents

them from being slippery. In some coatings,

the pigments are mixed with glass beads.

When light flashes fall on such surfaces

the beads reflect the light, making thesurroundings clearly visible.

Thermochromic pigments change

the shade of a painted colour at a specific

temperature and return back to the original

colour as the pigment is cooled. These

are now incorporated into special ink

and printed on to plastic films to create

thermometers or temperature indicators

the battery test strip is a good example.

When the battery is in good condition,

current flows through a printed resistor under

the thermocromic film and heats it to cause

a colour change.

Pigments in Living OrganismsMelanin is the main pigment found in

mammals. It is responsible for the colour

of hair and fur. There are different types

of melanin producing a wide range of

colours, from black to sandy to red. Melanin

pigments in the skin of animals may also

serve to protect tissues from ultraviolet

radiation.

Pigmentation is also used in many

animals for protection, by means of

camouflage, mimicry, or warningcolouration. Squid, octopi, and cuttlefish are

well known for the jets of ink they squirt out

in clouds at their attackers. Octopus squirt

ink composed primarily of concentrated

melanin. The colour of the skin attracts the

other sex and helps in sexual reproduction.

Chameleons change colour to signal, fend

off rivals, or attract a mate.

Pigments also play an important role

in the growth, development and protection

of living organisms. A plant absorbs

sunlight primarily using the green pigment

chlorophyll, and is responsible for the

green colour of foliage and leaves. More

importantly, this pigment by enabling plants

to produce oxygen during photosynthesis

is critical to sustaining our life on earth.

Chlorophyll and several other pigments

such as beta-carotene are involved in this

light reaction.

Flavonoids are the yellow plant

pigments seen most notably in lemons,

oranges, and grapefruit. The name stems

from the Latin word flavus, which means

yellow. Many of the foods that we eat,

including dark chocolate, strawberries,

blueberries, cinnamon, pecans, walnuts,

grapes, and cabbage contain flavonoids.

These chemicals lower cholesterol levels

and many have antioxidant properties.

Flavonoids include red, purple, or blue

anthocyanins, as well as white or pale yellow

compounds such as rutin, quercetin, and

kaempferol.

Anthocyanins play a role in the colours

of ripening fruit. They are found in most other

plant parts and in most genera. Anthocyanin

pigments take their colour from the range

of red, purple, or blue, depending on their

pH. Blueberries, cranberries, and bilberries

are rich in anthocyanins. Anthocyanins are

partly responsible for the red and purple

colours of some olives.

Betalains give rise to the distinctive

deep red of beetroot. The composition of

different betalain pigments can vary, giving

rise to breeds of beetroot that are yellow

or other colours, in addition to the familiar

deep red. Pigments in flowers and fruits

provide attractive visual cues for animal

pollinators and seed dispersers to locate

their targets, thus helping to propagate their

breed. Bilins are bile pigments formed in

many organisms as a metabolic product

of certain porphyrins. Although found in

mammals, they can also be found in lower

vertebrates, invertebrates, as well as red

algae, green plants and cyanobacteria.

Bilins can range in colour from red,

orange, yellow or brown to blue or green.

The brownish yellow pigment Bilirubin gives

feces its characteristic colour. It consists

of an open chain of four pyrrole-like rings

(tetrapyrrole). In heme, by contrast, these

four rings are connected into a larger

ring, called a porphyrin ring. Bilirubin is

responsible for the yellow colour seen in

jaundice sufferers and bruises due to its

accumulation in their blood; otherwise

normally it is thrown out of the body through

urine or excreta. This pigment has also been

found in plants.

The bilirubin in plants does not come

from breaking down hemoglobin. (Inanimals hemoglobin is broken down to

heme, and then converted to bilirubin.)

Chlorophyll molecules have similar ring

structures to those of heme, and it appears

that breaking down Chlorophyll can also

yield bilirubin.

Fungi, some microorganisms and

some plants posses the ability to either

bend towards sunlight or move away from

it for their survival. They have a simple on-

off switch for such a response to light that

is due to the presence of photo pigments

of the photoreceptor protein phytochrome,which is again a type of bilin. An example

of an invertebrate bilin is micromatabilin,

which is responsible for the green colour of

the Green Huntsman spider.

Dr. Chaganty Krishna Kumari retired as Reader

& Head, Department of Chemistry, S.C. Womens

College, Kothagudem. She taught chemistry to

under graduate students in English and Telugu for

36 years. Address: 204, Saradagopalan Apartment,

St. No. 8, Himayatnagar, Hyderabad-500029, AP.

Email: [email protected]

Mix a bit of the rust in few drops of water and dip a

piece of white cotton cloth. The cloth becomes reddish

brown. Dry and then rinse it, the particles of iron oxide

fall off from it. Iron oxide is insoluble in water.

Mix alizarin in water. It dissolves.

Dip a white cloth in the

solution; the cloth getsa permanent red shade.

Iron oxide is a pigment

and alizarin is a dye.

SR 50(2) 19-22

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