Whole books have been written on pigments classified by chemical types, but here we shall take only a very general look at them.

All pigments are either natural or synthetic and organic or inorganic chemicals. Many inorganic pigments are still dug out of the Earth's crust, crushed, washed and graded by size or made synthetically from other ingredients by a chemical process - apparently the same chemically, but often different in properties.

The difference arises because natural products are produced in the form found in nature, they may be contaminated with impurities, which are uneconomical to remove and crushing may produce a wide range of particle sizes outside the required range.

Synthetically produced pigments can be produced to the size and shape required by precipitation under controlled conditions and to a high degree of purity.

The chief family of pigments in which natural varieties are still of importance is the Iron Oxide family: ochres, umbers and siennas; red, yellow and black iron oxides.

ORGANIC AND INORGANIC PIGMENTS

There are now far more organic pigments than inorganic ones, due to the if efforts of chemists in synthesising new products, some of which now contain both metallic (inorganic) elements and organic structures. Many organic pigments are organic chemicals deposited on an inorganic (e.g. Aluminum Hydroxide) core. These pigments are called 'lakes‘. It is difficult to lay down rules for there are many exceptions, but pigments roughly fall into the following categories:

This list may give the impression that it is best to avoid organic pigments, but Factor 1 is a very powerful argument for using them (otherwise we may still be driving around in drab coloured cars).

The very best organic and organometallic pigments give the highest standard of performance and include the Phthalocyanine (Monastral) pigments, the Quinacridone (Cinquasia) pigments, the Perylene and Perinone pigments, the Dioxazine pigments and the high molecular weight Azo pigments.

Probably the most important property of a pigment is its colour. Colour is the subject of the next block, why we see red pigment as a red colour and why that colour is seen.

Pigments and dyes differ in that dyes are made to be soluble, producing colour in the same way as pigments, they are designed so that certain chemical groups will stick or associate with other chemical groups present on fibres, wools and cloths and used mainly in the ink and fabric dying trades. Pigments on the other hand are insoluble materials.

There is, of course, almost no limits to the kind of colour and effects that can be produced by pigments following the fashion. Currently pigments are being made which change colour with temperature. This could cause problems with identification, let alone trying to match the effects.

Fluorescent pigments appear to be brighter in colour than conventional pigments, this is because they are. They absorb light in the same way as conventional pigments, but conventional pigments lose that energy in the form of heat. Flourescent pigments lose that energy rapidly at longer wavelengths as visible light. Thus conventional pigments reflect light only in visible spectrum, flourescent pigments can reflect visible and U.V. light in the visible region.

Optical brighteners exploit this process by absorbing energy in the ultraviolet region and emitting in the visible region, so the fabric or textile appears brighter white.

Fluorescent pigments are then used on emergency vehicles because of their brighter colour and are more easily seen under poor lighting conditions.

Titanium Dioxide TiO₂ is the predominant white pigment used not only for paints and inks, but also for rubber ceramics and almost anywhere where a white pigment is required. It surpasses all other white pigments for brilliance of colour, stain resistance, opacity (refractive index 2.5-2.7) and chemical inertness. It is non-toxic and the particle size is 0.25-0.35 microns.

TiO₂ exists in two forms known as rutile and anatese which are different crystal structures, the rutile structure being more compact, leads to better durability.

The anatese structure, loses its structure rapidly and if the surface is rubbed gently, white pigment is removed (chalking).

There are large rutile deposits in Australia and purification is carried out by sulphate process or chloride process, the latter giving a purer product.

Carbon Black

These are the most intense blacks. They have a very fine particle size and consequently high oil absorption. They are produced by the incomplete combustion of hydrocarbon oils and gases. The fine particle size and excellent opacity (black absorbs all incident light), the pigment content in black paint is extremely low, about 3og of pigment per litre of gloss black.

Carbon blacks are difficult to disperse in organic paint media, but the process is assisted by the use of appropriate cationic or anoinic surfactants. On reduction with white, carbon black shows high staining power, but the undertone is an unattractive brownish colour.

Red Lead (Lead Oxide Pb₃ O₄)

Red lead is an orange red pigment with a high specific gravity and low oil absorption. It is produced by heating naturally occurring Litharge (PbO Lead Oxide) in an oxidising atmosphere and then cleaning impurities out.

Primers based on red lead dispersed in linseed oil have been used on structural steel work since the industrial revolution. They give excellent results, but had very slow hardening and drying properties. This, together with the high toxicity of the pigment has resulted in the decline of its use.

Lead Chromes

These are a group of pigments which produce a range of colours, from primrose yellow, lemon, golden yellow, orange red and scarlet. The wide variety in colour is dependant upon the addition of other chemicals to the basic structure.

As a group of pigments they have good opacity, tinting strength and weathering properties with low oil absorption. In common with other lead compounds they are highly toxic and their use is confined to industrial and maintenance paints.

One of the main uses of lead chromes has been in a mixture with Prussian Blue to give chrome or Brunswick greens or with phthalocyanine blues to give a brighter range of greens.

Iron oxides (Fe₂ O₃) are commonly used pigments in paints and range in colour from yellows, reds, browns to blacks, though generally they are not very clean colours. Red Oxide (Fe₂ O₃) used widely for primers for steel. Ochres contain less FeH₂ O₃ than red oxide and are really coloured clays. They are brown-yellow and mainly used as stains.

Umbers/Siennas.

They are impure oxides of iron containing small amounts of manganese and produce a brown-yellow and orange-red colour used mainly as stainers.

Chemically prepared sulphate of iron Fe SO₄ 7H₂O produce at low temperature Turkey reds, which are bright, good opacity and are a major pigment in red oxide paints.

At a higher temperature, produces light and deep Indian reds, a bluer tone than Turkey red and at high temperatures produce purple browns and chocolate browns used for light fastness and heat stability in special paints.

Prussian Blue K₃ Fe₄ (Fe (CN)₆)₃ 12H₂O

This, as you can see, is a fairly complicated pigment and is produced by adding a ferrous salt to a ferrocyanide and oxidising the precipitate. It has excellent fastness to light, is not dissolved by oil, solvent or water.

Phthalocyanine blues are characterised by brilliance of tone, light fastness I and chemical inertness. They are insoluble in all paint media.

Phthalocyanine greens are produced by replacing some of the hydrogen atoms in the peripheral benzene rings with halogen atoms e.g. chlorine or chlorine and bromine to produce a bluish or yellowish tone to the pigment.

Azo pigments contain the chromophore -N=N-, usually coupled to an organic structure.

More information on colour is given in the next module.