The Poison Problems: at Work and in the Home

Chapter 7

Poisonous Pigments and Dangerous Dyestuffs

IMAGINE LIFE WITHOUT COLOUR: it would be very dull indeed. Pigments and dyestuffs provide us with a means of using or copying some of the rainbow of colours found in nature. But, just like china in a china shop, they should be admired from afar and not touched.

Danger danger

Natural pigments come mainly from plants and minerals with very few coming from the animal kingdom. Plant pigments include chlorophyll for green; anthocyanins for reds, purples and blues; carotenoids for yellows, oranges and reds; and flavonoids for whites and creams. Plant pigments are not poisonous in themselves, although they may act as a signal that a plant is poisonous to those creatures that may wish to eat it (see Appendix III). In contrast, mineral pigments are often highly poisonous, as are many of today’s synthetic pigments and dyestuffs.

Most dyestuffs are intensely coloured organic compounds. Mordants, such as the basic hydroxides of aluminium, chromium and iron, are used to combine with and fix the dyestuff onto the textile fibres. Mordants need to be used in those cases where textile fibres cannot be dyed directly. Pigments that are formed by the interaction of a dyestuff and a mordant are called lakes. These dyestuffs can be intensely poisonous, so the profession of ‘dyeing’ can be as dangerous as it sounds.

An added danger is colour blindness, as full colour is vital to make use of coloured signals as warnings of danger. Normal colour vision is trichromatic: all three primary colours – red, blue and green – can be perceived. But some people are dichromatic: they can only perceive two of these primary colours. There are a number of different types of colour blindness, which are usually inherited but sometimes acquired. A world in which colour is severely limited is very rare, but some people, called monochromats, see only black, white and shades of grey. Their world is like watching permanent black and white television.

Colour blindness is fairly common in men, but much rarer in women. Some defect of colour discrimination is present in about one in 12 men but only one in 250 women. Red-green colour blindness is the most common type, which is usually inherited, although it may, rarely, be acquired, caused by a disease of the retina. Anyone with red-green problems needs to take extra care at traffic lights. Deuteranopia is a condition where reds, yellows and greens are confused.

This chapter looks at colour danger signs, uses of colour (and sometimes mistakes) and the rainbow of colours and their respective dyes and pigments.

Medicine in colour

A number of pigments and dyestuffs have been used medicinally for many years, either as a colorant or as the medicinal product itself. Brilliant Green is an antiseptic and, in the past, was used in combination with Gentian Violet (also known as Crystal Violet). This mixture was called Bonney’s Blue, named after the doctor who first reported its successful use, and was used for many decades for skin disinfection, as the treated area was then made readily visible.

However, some years ago Gentian Violet was shown to be carcinogenic and so there was an understandable decline in its use, both alone and in the Bonney’s Blue mixture. A number of other dyes also have antiseptic properties, including Malachite Green and Scarlet Red, a somewhat irritant antiseptic also known as Sudan IV. The use of the yellow dyes tartrazine and Sunset Yellow has diminished greatly over the years, following an increasing number of sensitivity reactions to them.

Multi-coloured mercury medicines

All medicaments have an official name, and many also have a common name. In the past, many of these common names were derived from the colour of the finished product, and many of these colourful medicines contained the highly poisonous mercury. Grey Powder, Blue Pills, Blue Unction, Yellow Ointment and Red Ointment all contained mercury. So too did Yellow, Red, Black and White Precipitate, these all being old-fashioned common names for various compounds of mercury.

Grey Powder and Blue Pills each contained about one third by weight of mercury. Grey Powder was given by mouth, and a dose of ten grains three times a day was given to treat smallpox. The British Medical Journal in 1910 reported that the results from its use were surprisingly good! Blue Pills were prescribed to treat syphilis, and were also said to be very effective in cases of cardiac dropsy, where digitalis alone had failed.

Yellow Ointment contained two per cent mercuric oxide (Yellow Precipitate) in Soft Paraffin. This was used for inflamed eyes, syphilitic sores and eczema. Blue Unction was a weaker form of this same mercurial ointment, but diluted with lard, which was a readily available form of fat, used to make most ointments in days gone by.

Until the mid 1960s, many medicines – particularly creams, ointments and lotions – were known by the Latin names that doctors, apothecaries and pharmacists had used in the past. This was a wonderful way to give the patient a fairly innocuous medicament, or even a placebo, with the grandiose Latin name hopefully filling the patient with great hopes for its efficacy. So we find that the impressive sounding ‘Unguentum Rubrum’ in plain English was simply Red Ointment.

This wonderful concoction contained two minerals of mercury, the oxide and sulphide, better known respectively as Red Precipitate and Vermilion. To make Red Ointment, both were mixed together with creosote and lard. Red Precipitate Ointment was much simpler to make: it was a mixture of one part Red Precipitate in nine parts of Paraffin Ointment. A hundred years ago, with no steroids and no antibiotics, this ointment, Unguentum Hydrargyri Oxidi Rubrum, was used for a wide variety of chronic skin problems.

The lotion of choice a century ago, used to bathe syphilitic sores and other skin lesions, was colloquially known as Black Wash. This doesn’t sound nearly as impressive as Lotio Hydrargyri Nigra, to give it its official pharmacopoeial Latin name. This lotion contained Calomel, another mercury-containing mineral, mixed with glycerin and Solution of Lime.

Blue and red nightmares

We take colour for granted and don’t question blue icing or yellow medicine. But when colourings start to accumulate in the body or change the colour of the urine or faeces, that’s when we understandably panic.

Brilliant Blue FCF is a permitted colouring in foodstuffs, and while blue food is not common, blue icing is used on many a birthday cake.

Brilliant Blue FCF features in a special liquid feed that has caused consternation. A child was unable to eat normally and so was fed a special liquid diet through a tube. Over a period of time the child received a large quantity of one particular enteral feed, which contained Brilliant Blue FCF colouring. The child slowly started to turn blue, but tests showed this was not from cyanosis, the usual explanation. The doctors then realised the culprit and a change of feed solved the problem.

Magenta, a dyestuff used in Castellani’s Paint, used to be used for certain skin complaints and as a food colouring. Today it is no longer considered safe, as cases of bladder cancer have been reported from those working in its manufacture. Magenta will reappear in just a moment.

When Dioralyte was first introduced in the 1980s as a treatment for the oral rehydration of children with sickness and diarrhoea, the original cherry-flavoured version caused problems.

When added to 200ml of boiled and cooled water, as per the instructions, there resulted a wonderful cherry-red coloured solution. The problem was that the same red colour reappeared in the child’s nappy some time later, causing great anxiety and consternation to parents and medical staff alike, with fears that the red-stained nappies might be due to internal bleeding. Once this ‘problem’ was brought to their notice, the manufacturers very quickly removed the red dye from the product. All flavours of Dioralyte have been colourless ever since.

Some dyestuffs are used not only as colourants in food and medicines, but also as marker dyes in diagnostic procedures. Indigo Carmine is used in amniocentesis and in some urological procedures while Methylene Blue and Patent Blue V are used in investigations of the lymphatic system.

White lead paint poison

As long ago as Roman times, Pliny wrote about lead in his Natural History. He knew, even then, that it was poisonous if swallowed. The Romans only allowed convicts and slaves to work in their lead mines, because they knew of its poisonous nature.

However, White Lead, chemically known as basic lead carbonate, has been used for thousands of years as a white pigment in paints and cosmetics. Artists have greatly valued it as a pigment for its vibrancy and intensity, but because of its poisonous nature it has been banned from sale within the European Union since 1994, except under special conditions. White Lead paint is now only allowed on Grade I and Grade II* listed houses in the United Kingdom for special historical reasons, and then only on the outside. Artists are now recommended to use a white paint made from titanium dioxide, which is so opaque that it is also used in correction fluid. Specially purified grades of titanium dioxide are even used as a white food colorant.

Apart from titanium dioxide, many other whites are available, including Bone White which is made from burned lamb bones, and Chinese White which is made from zinc oxide. Unfortunately, artists do not like the alternative whites on offer, including titanium dioxide, because they lack the vibrancy, sharpness and intensity of Lead White.

As an alternative to White Lead paint on the walls of buildings, lime washes have been used for centuries, but they need to be re-done each year. In some parts of the world, lime washing was once even believed to be the best precaution available against the plague.

Another poisonous pure white, antimony trioxide, is used today as an opacifier in paints and as a flame retardant in plastics because it is more effective than safer alternatives, in some applications.

Yellow peril

In some ways, yellow is a happy, sunny colour. In the world of dyestuffs and pigments, it often signifies extremely poisonous.

Chrome Yellow is the name given to the chemical lead chromate, while Primrose Chrome contains a mixture of this together with some lead sulphate. Yet another pigment, Cologne Yellow, is prepared by heating lead sulphate with potassium bichromate. All these chrome pigments are very poisonous.

In the past, cadmium sulphide was used as a pigment known as Cadmium Yellow, although depending on the amount of selenium and sulphur impurities present in the raw mineral, the colour could vary from yellow to orange to red to brown. It was once widely used in paints, artists’ colours, printers’ inks, vitreous enamels, rubbers and plastics but since the 1960s, when its poisonous nature became known, most of these uses have been phased out. Health and safety legislation now safeguards workers from excessive cadmium levels in the working environment.

Another poisonous yellow pigment is zinc chromate which, when mixed with Prussian Blue, gives zinc greens, a range of particularly stable and light fast colours. Zinc chromates are also used in rust-inhibiting paints.

Orpiment is a highly poisonous golden-yellow mineral, arsenic trisulphide, which has been used as a pigment since ancient times. Its modern name is simply a variant on the Latin for ‘gold pigment’, auripigmentum.

Naples Yellow is a light yellow pigment, originally held to be an Italian secret, which is manufactured from lead antimoniate. The mental illness and suicide of Vincent van Gogh may have been due, in part at least, to his frequent use of Naples Yellow, and to the lead and antimony it contains.

Gamboge is a bright yellow plant pigment derived from the resin of a tree that grows in Cambodia. This resin is a powerful purgative, causing severe griping if swallowed. In the past it was used as a medicine to expel tapeworms. Gamboge is not recommended for those artists who tend to suck their paint brushes.

A much safer yellow is luteolin, a yellow pigment extracted from the plant Dyer’s Greenwood, which has been used since Roman times. The leaves and flowers of the plant contain the yellow pigment. This pigment was once used in combination with the blue dye from Woad to produce an excellent green, known as Kendal Green, named after the town in Cumbria where this combination dyeing of wool was developed.

Yet another yellow plant pigment comes from Wild Mignonette, otherwise known as Weld, which has also been traditionally used to dye fabric. Craft workers still use these plant-based dyes, but with modern synthetic fabrics, newer chemical dyestuffs must be used.

Bright but biting orange and red

There are a number of safe orange and red pigments and dyestuffs originating from minerals and plants, but many others are poisonous, including the ones below.

When Chrome Yellow, mentioned above, is put into an alkaline solution it changes colour and becomes Chrome Orange, which is still just as poisonous as Chrome Yellow.

A bright red pigment, Realgar, is the poisonous arsenic monosulphide, and is found naturally in association with the golden-yellow mineral, Orpiment.

Red Lead, extensively used in making corrosion-resistant paints, is a chemical called trilead tetroxide. Once upon a time, all paints contained lead, but now, due to its toxicity, all lead paints are governed by detailed regulations, regarding their preparation, storage and application.

Vermilion, an artist’s pigment, occurs naturally as the ore Cinnabar, mercury sulphide. This same red pigment has been used for centuries in Chinese lacquer painting, as temple paint and in red ink, but this too has now been replaced in the interests of health and safety.

Another poisonous mercury-containing pigment is an iodide salt known as Iodine Scarlet, a deep red pigment which, interestingly, turns yellow when heated to 126^oC.

The exotic-sounding Dragon’s Blood is a resinous exudate from the fruit of some palm trees from Malaysia and Indonesia and is also obtained from the red sap from stems of the Dragon’s-blood tree, which only grows on the island of Socotra in the Indian Ocean. Dragon’s Blood is used for colouring varnishes and lacquers.

Solferino is the colour of rosaniline, a purplish red, so named as it was discovered soon after the Italian battle of Solferino in 1859. Rosaniline is the base of the fuchsin dyes, such as magenta, the hydrochloride salt of rosaniline. This was widely used in solution as a disinfectant for certain skin conditions in a preparation called Magenta Paint.

In crystalline form, Magenta is neither pink nor purple, but dark green, and only when it dissolves in water does it produce the typical purple-red colour. Magenta has also been called fuchsine, as the colour is similar to that of the flower Fuchsia, which was named after Leonard Fuchs (1501-66), a German botanist.

Mother Nature has fortunately given us many safe red pigments, such as Red Ochre. This mineral is also known as haematite or iron oxide, but most people know it by its more common name, rust. Calamine Lotion is a delicate shade of pink because of the presence of a minute quantity of this red pigment in the lotion.

The universal red, violet and blue pigments found in flowers, fruits and leaves, which are all water-soluble, are called anthocyanins. Madder is a plant that was used for centuries as a red dye. The orange-red pigment it contains is called Alizarin, which can be dissolved in alkaline solutions to give purple-red solutions, and can then be precipitated out as bright red, deep red, or violet lakes, by the use of metal salts as mordants.

Turkey Red is a fine, durable red that was obtained from the Madder plant in the past, but this colour can now be made synthetically.

Saffron is a species of crocus with purple flowers and brilliant orange-coloured stigmas, which are collected and dried, to be used as a (very expensive) natural dye. Safflower, otherwise known as Bastard Saffron, is a thistle-like plant whose dried petals are used both in cosmetics and also as a dye, which in acid solution gives a pinky-red colour. It is this particular colour that is the origin of ‘red tape’, as it is this dye that coloured the tape traditionally used by the legal profession to tie up their bundles of legal papers.

Cochineal is a red dye of animal origin made from the dried bodies of the females of a species of beetle, which come from Mexico and the West Indies. The deep red colour is due to the presence of carminic acid. The red lake pigment Carmine is obtained by extracting Cochineal with boiling water, and then precipitating the extract using alumina as the mordant. Cochineal is used widely to colour cosmetics, foodstuffs and medicines and is also used chemically as an indicator. Crimson is yet another rich red, used for thousands of years in Persia. It comes from the female bodies of another insect, called Kermes.

Other safe natural colourings include litmus, which is also used as an indicator, and is obtained from lichens after oxidation in the presence of ammonia. Litmus is red in acid conditions and changes to blue in alkaline conditions. Lycopene is the anti-cancer red carotenoid pigment found in tomatoes, rose hips and other berries, and Henna is a reddish orange pigment made from the leaves of a small oriental shrub of the loosestrife family, used for dyeing nails and hair and for skin decoration. Dyer’s Mulberry and Dyer’s Bugloss are both plants that have traditionally been used to produce pink and red pigments. Bixin is the principal pigment of a plant called Annatto. The violet/red crystals extracted from this shrub are used to colour foodstuffs with shades of red. Annatto will appear again in the chapter on Frightening Foodstuffs.

Passion for purple

Purple is an example of a colour obtained from both the plant and animal kingdoms, and another colour with both harmless uses and poisonous consequences.

Purple was the colour of the first synthetic dye, Mauveine. This was made accidentally by William Perkin in 1856. He was trying to make quinine, a feat of chemical synthesis not actually achieved until 1944, but ended up with a dark, sticky residue which, when dissolved in water, became a beautiful purple colour.

He originally wanted to call his mauve dye Tyrian Purple, but Tyrian Purple is a vat dye of great antiquity, which is made from Mediterranean shellfish called murex. This purple, in the past, was reserved for Roman emperors and royalty. So Perkin’s purple dye was eventually called Mauveine instead.

The Purple of Cassius, named after Andreas Cassius who died in 1673, comes from a colloidal gold reduced by a mixture of tin chlorides. This then results in a red-violet coloured solution due to the precipitation of finely divided gold on to the tin hydroxide in the solution. This exotic purple dye is still used in the manufacture of ruby-coloured glass.

Methyl Violet, also called Base Violet 1, is a violet-coloured aniline dye used to dye jute. It is also used as a bacteriological stain and as an indicator, but is perhaps best known as the purple dye used to colour methylated spirit. The dye is added deliberately to serve as a warning that it should not be swallowed, as it contains methyl alcohol, which is toxic and can also cause blindness.

Out of the blue

None of the well-known blue pigments are particularly poisonous, and some are even used in medicine.

Prussian Blue and Turnbull’s Blue are cyanoferrates formed by the addition of an alkali cyanide to a solution of an iron salt. Prussian Blue is a very dark blue, given its name because it was discovered in Berlin, when located in Prussia. Today this dye is used to treat thallium poisoning, as we have already seen in the chapter on the treatment of poisoning. Although Mr Turnbull did not discover Turnbull’s Blue, he gave the product his own name. He proudly named it Turnbull’s Blue for posterity, but he was merely the chemist who manufactured this particular pigment in 18th-century Glasgow.

Azurite is a blue mineral which is, chemically speaking, a basic copper carbonate, the synthetic version of which is called Blue Verditer and is used as an artists’ colour. Ultramarine is a vivid deep blue pigment, also much used by artists and originally made from the mineral Lapis Lazuli. Yet another blue mineral is cobalt oxide, used to make blue glass – the colour produced is called Cobalt Blue. Aquamarine is a greenish-blue semi-precious stone, which is actually a beryl-containing beryllium aluminium silicate.

Blue Vitriol is the old name for the beautiful blue hydrated copper sulphate. Many will remember growing crystals of Blue Vitriol in their schooldays. Because it is poisonous if swallowed, health and safety concerns in science lessons mean that today’s schoolchildren have a far less ‘hands on’ experience, and growing crystals tends to be only a demonstration by the teacher. Today, Blue Vitriol is known as the mineral Chalcanthite.

Methylene blue is an important dyestuff used in dyeing fast fibres and in calico printing. It is also used as a bacteriological and microscopic stain and as a mild antiseptic.

While red-blooded animals, including humans, have a respiratory pigment containing iron called haemoglobin, other respiratory pigments have evolved in some species. Haemocyanin is the copper-containing respiratory pigment of molluscs and crustaceans, which quite literally makes them blue-blooded. Indigotin, better known as Woad, is a very ancient permanent dark purple-blue dye, long used as both body paint and as a fabric dyestuff, which comes from a plant of the same name. Delphinin is an anthocyanin pigment found in delphiniums and larkspur, the seeds of which are intensely poisonous. This pigment is found in many other blue and purple flowers.

Green with envy

Like blue, there are many harmless green pigments, but unlike blue, there are quite a few poisonous ones as well.

Many copper and chromium salts are richly coloured; copper salts are blue and green while chromium salts can be green, red or yellow. When copper is exposed to the air, a green colour forms on the surface. This happens because carbon dioxide in the air reacts with rainwater to produce carbonic acid. This weak acid then attacks the copper, producing the green copper carbonate, commonly known as verdigris. The word verdigris means ‘green of Greece’ in Latin, although why this name is used to describe the patina of copper is unknown. While verdigris is not very poisonous, the similar sounding viridian definitely is. Viridian is a chromium salt and a brilliant green pigment, much used by artists, despite its poisonous nature.

There are a number of other green dyestuffs and pigments that contain both copper and arsenic, and although toxic, they have been used as medicines in the past, but today we have much safer, more effective alternatives. Scheele’s Green is copper arsenate, and in the past was used as a treatment for secondary anaemia and as an intestinal antiseptic.

Paris Green, Emerald Green and Schweinfurter Green are all basic copper ethanoate arsenates, which have been used in the past as insecticides for spraying fruit trees. But since these readily decompose to produce soluble arsenic compounds, which could poison anyone eating the fruit, their use is now severely restricted.

Lincoln Green is the name given to a brilliant green that, as its name suggests, was once made in Lincoln, and reputedly worn by Robin Hood and his merry men.

Malachite is another green, a native hydrated copper carbonate, used as an artist’s colour. Another basic dyestuff that contains neither copper nor chromium, called Brilliant Green, was used medicinally as a bactericide.

Brown and out

Most brown dyes and pigments are of natural origin and are generally regarded as non-poisonous. Umber, named after the Italian province of Umbria, is a mineral composed of iron and manganese oxides. This brown pigment, on heating, produces burnt umber, which is a redder shade of brown. Similarly, raw sienna is a yellowish brown, while burnt sienna is a reddish brown, both named after the city of Sienna.

Sepia is a dark brown pigment made from the ‘ink’ produced by cuttlefish when they are frightened. Vandyke brown, named after the artist, is a deep brown which is made from a mixture of lamp-black and ochre, while bistre is a warm brown pigment made from the soot of beech wood.

Black as night

Black is the new black. If you walk into any clothes store, you will see that there any many different shades of basic black. Black pigments and dyestuffs can be prepared from a variety of natural and synthetic substances, and each black dye reacts differently to the fabric fibres, with no two dyeing exactly the same shade of black. The nigrosines are synthetic dyestuffs which are all used as black pigments.

Pitch, a bituminous or resinous dark-coloured substance of fossil plant origin, is carcinogenic. Lampblack is another carcinogenic pigment, made from the soot of a fuel-burning lamp. Lampblack used to be mixed with parchment size or fish glue to create Indian ink. Ink was also once prepared from oak-galls, which contain gallic acid, or from burnt cork or charcoal, which are both derived from plants. Japan black is a quick-drying black varnish made from asphaltum and drying oil while ebony black is named after the black wood of a tree found in Africa, India and South-East Asia.

Finally, our discussion of black has to include lead pencils which, of course, do not contain lead at all, but are made of graphite, a form of carbon, which, being grey and slightly shiny, looks just like lead. This is why in the past it used to be called black-lead or plumbago.