Option - The Chemistry of ArtChapter 1 - Pigments Throughout History

Sources of Pigments in Early History

Colour

Pigments

Black

Charcoal burnt wood, animal bones, soot

White

Gypsum CaSO4.2H2OChalk - CaCO3 Kaolin - Al2Si2O5(OH)4

Red

Ochre crushed rocks

Cinnabar HgS

Yellow/Brown

Orpiment As2S3

Limonite FeO(OH)H2O

Green

Malachite Cu2(CO3)(OH)2

Grey

Galena PbS

Cerussite PbCO3

In early history, the pigments used were the minerals readily available in coloured earth and soft rocks.

Insolubility of Pigments

Pigments need to be insoluble in most substances for several reasons:

1. To ensure that the pigment particles remain on the painting after the medium has dried

2. To ensure that the pigments are not easily removed from the surface of the painting when exposed to rain

3. To ensure pigments used in cosmetics do not get washed away by perspiration

Prehistoric Use of Pigments Self Decoration

Aborigines:

Body art used red and yellow ochres, white clay and charcoal as pigments

Pigments were applied with fingers or brushed to produce symbols representing history and myths

Mainly used for ceremonial purposes

Egyptians:

Accentuated the eye as the main facial feature (symbol of the sun god Ra)

Cosmetics used include: kohl as eyeliner, green malachite as eye shadow, white lead to whiten the skin, red ochres on lips and cheeks and dyes as nail polish

Raw pigments were prepared by grinding on a stone slab and applied in powder form

Some cosmetics contained metallic poisons e.g. lead, mercury and antimony

Ancient Greeks:

White lead was used to whiten faces and red lead was used as blush on cheeks

Prehistoric Use of Pigments Preparation of the Dead for Burial

The Egyptians had a reverence for the passage of the dead into the afterlife and therefore used pigments for the dead in much the same way as they did for the living.

The main colours used included white, black, red, yellow and ultramarine blue and due to the lack of sunlight in the tombs, the colours remained vivid for long periods of time

Egyptian tombs (sarcophagi) were pained inside and out with representations of the dead and the life they lived

Mummified bodies were also decorated with geometric designers to ensure the continued survival of the owner

Cosmetic boxes of pigments were often buried with the dead in tombs to be used in the afterlife e.g. powdered gypsum, orpiment and haematite

Prehistoric Use of Pigments Cave Drawings

The Aboriginal cave drawings are considered to be amongst the earliest known paintings.

Few pigments were available, with the main ones being: chalk, ash, charcoal, limonite, kaolin and red, yellow and brown ochres

Binders used included egg yolk, tree resin, blood, urine and saliva

Traditional techniques were used to apply paints e.g. use of sticks and barks as well as body parts such as fingers and spraying with the mouth

Cave paintings were used to tell stories and pass on information e.g. tales of the Dreamtime

Health Risks of Heavy Metal Pigments

Ancient cultures including the Egyptians and the Romans often used heavy metals in the preparation of cosmetic products.

Antimony - Sb2S3 was used as a kohl, mascara or eye shadow. It is known to be carcinogenic and can cause nausea and vomiting in the short term

Copper - used in eye make-up in the form of malachite or as a kohl in black copper oxide form. It can cause anaemia, liver and kidney damage and intestinal irritation

Lead - Galena (PbS) was crushed up and used in black eye makeup. White lead (PbCO3) was also used as a face powder. Use may result in damage to the nervous system, mental retardation and even death

Mercury - often used in cinnabar (HgS) for lipsticks due to its vibrant red colour. Mercury poisoning can result in numbness, tunnel vision and brain damage

Structure of Paintings

Wood and canvas (the support) are normally unsuitable to paint on directly as they are too rough and absorbent and will therefore be prepared through layers of ground or by priming. Gypsum or chalk would be applied as a warm liquid and then later scrapped off as a white layer. On the ground, paint layers were added, which consisted of pigments mixed with a binding medium or oil. Paint layers were then protected by a cover of varnish.

Medieval Painting

An example of a medieval painting is Madonna and Child with Saints Jerome, John the Baptist, Bernardino and Bartholomew. The painter was Sano Di Pietro and it is estimated that the art work was completed in Sienna, Italy 550 years ago.

In this painting, a layer of linen was glued to the wood. Next, a layer of gesso grosso (CaSO4) was mixed in with animal glue, followed by finer layers of gesso sottile. Finer layers are highly polished to a smooth surface and fine black powder is added to fill in any holes.

Charcoal was used in the underdrawing. The gold leaf was adhered to the surface of the pain through oil, varnish or garlic juice. Mordant gilding was used in order to apply it over the paint. Pigments used included vermilion (red), ultramarine (blue), gypsum (white), Naples yellow and orpiment (yellow) and malachite (green).

Obtaining Colour

For paintings, colours are obtained through spreading several layers of paint and superimposing them, allowing the creation of complex colour effects. Pigments themselves are rarely mixed.

The same pigments were also used in the production of coloured glass. The oxides were finely powdered and added to the class mixture before melting e.g. cobalt oxide produces blue and manganese oxide produces a purple, both in accordance to temperature and pressure. Flashing involves coating clear glass with a thin layer of coloured glass to produce a lighter colour. Staining involves painting glass with silver nitrate before firing it in an oven. According to the number of times the glass is stained, a range of tones can be achieved.

Structure of Paints

Paint is formed through the colouring matter (the pigment) and a liquid (medium) which carries the pigment, allowing it to be spread over a surface.

A pigment is a substance in particulate form which is substantially insoluble in a medium, but which can be dispersed in this medium to modify its colour

The binder is the material used in paint that causes pigment particles to adhere to one another to the support. This provides a protective film, improving hardness and resistance to water

Examples of binders include:

Egg tempera - formed when egg yolk is mixed with a pigment to form a long lasting paint, used since the 14th century

Linseed oil - mixed with pigments in varying proportions to form oil paints, goes brownish yellow with age and is generally acidic

Acrylics - these are used in the majority of synthetic paints used today as they are durable, flexible and suitable for use on paper, panels and canvas

The Egyptians were advanced in using dyes, which are different to pigments in that the colouring matter is dissolved in solution. This provided new pigments for painting e.g. vegetable dyes were dissolved in water and fixed onto an insoluble white powder, which could be dried and extracted.

Lake pigments are manufactured by precipitating a dye with an inert binder. The binder used must be inert and insoluble and must be white or very neutral. Madder lake is a natural pink-red dyestuff formed form the root of the madder plant rubia tinctorium. Lakes are fugitive in that they will lighten in a relatively short time when exposed to light because the dyes are unstable.

Increasing Range of Pigments

The very first Aboriginal cave paintings only used red, yellow and black pigments as these were easily obtained in the forms of ochres and charcoal. As time progresses, new minerals are available due to improvement in extraction methods e.g. terra verte (green earth) was the first green pigment used and was only used after the iron silicate and clay mixture could be extracted.

The Egyptians and the Romans had better extraction technologies and therefore had a wider array of pigments available. Better purification techniques mean pigments are more easily isolated from ores.

More recently, there has been a shift from natural to synthetic pigments due to further advancements in technology. More and more organic pigments (carbon based) are being used.

Colours and the Periodic Table

Metals are present in each pigment and most of the metals involved are transition metals. The metals present in the pigment often have the same colour or colour range.

Chapter 2 - Structure of the Atom and Colours

Flame Colours

A flame test reveals that many metallic elements emit light of a distinctive colour when their compounds are heated in a flame or when their vapours are exposed to electrical discharge.

Name of Element

Cation

Flame Colour

Sodium

Na+

Yellow

Potassium

K+

Lilac

Calcium

Ca2+

Orange-red

Barium

Ba2+

Apple-green

Strontium

Sr2+

Red

Copper

Cu2+

Green-blue

Excitation of Electrons

According to the Bohr Model, electrons can move between energy levels by absorbing or emitting photons. A photon is an energy packet of electromagnetic radiation and its energy will equal the difference in energy between two orbits or energy levels.