Colour

 This is the second of my editorials as mentioned earlier:

"...I am delighted to welcome you to this edition of the magazine which carries as its theme – “Colour”. This is my last magazine as editor and it is gratifying that the committee has allowed me to indulge myself with my favourite part of the industry. For colour is the most visible part of what we do; indeed it is the entire raison d'être for many architectural coatings and even the most robust anticorrosive coating on a structure like a bridge carries a colour that we are aware of long before we give any thought to the technical properties that coating may impart.

So what is this ‘colour’ that we see? Our eyes pick up reflected energy in the form of waves from the object we are observing and a series of rods and cones in our eyes translate these waves into what we perceive as colour. Those waves are originally generated by the sun, and this 6000^oC mass of matter is radiating energy into the universe at 300,000km/sec. Those waves of energy are in a range of between 10^-15 M to 10⁷M, however the earth’s atmosphere blocks out a large portion of these and those that do get through are called ‘light’.  Some of this light is invisible, ultraviolet at the short end of the spectrum (sub 400nm) and infra-red at the longer end (above 750nm), whilst the light we see every day as visible sunlight is in the range between these two. So we puny humans only see a very small proportion of what our sun generates.

                                              A Pictorial View of Energy Source:craigssenseofwonder.files.wordpress.com

And what of that visible light, and how we perceive it? When polychromatic white light falls on an object, one of several things will happen. If the light passes through the object with no change of direction, the object is transparent.  If it is reflected back completely at the same angle as it arrived, the object is a mirror. If it is totally reflected but scattered in all directions (diffused), the object is white; and no prizes for guessing that if it is completely absorbed the object appears black. The one scenario that particularly interests us with this edition of the magazine is that in which part of the wavelength striking the object is absorbed, with the remainder being reflected back as a mix of waves, at which time we ‘see’ a colour.

So a colourant is a material that absorbs (supresses) a specific wavelength of light and the resulting reflected mixture of waves becomes the visible colour. As an example, if the colourant supresses the waves in the 435-480nm range (indigo) the resulting colour we see is yellow. Any shift on which waves are being absorbs results in the shifting of the shade of colour we see.

                                                                Reflectance curves Source:http://imagebank.osa.org/

For the purposes of this magazine we are keeping things simple and only dividing colourants into pigments and dyes, and the basic way to sort these two out is to remember that dyes are soluble in the vehicle that carries them whereas pigments are insoluble. As with most things, there are exceptions to the rules and materials that belong to one group but behave like the other, but they can be dealt with on another occasion. Pigments and dyes have only been separated into individual disciplines over the last 100 years or so, and any overview of ‘colour’ from a historical perspective must necessarily allow a certain degree of overlap over the differing chemistries and nomenclatures.

The cave paintings of Altimira (discovered 1879) and Lascaux (discovered 1940) give us some insight into the earliest desires of human-kind to bring colour into their domestic world, and the colours used were oxides of iron and manganese along with Carbon Black (carbonised bones, shells, nuts etc) and Lamp Black (soot). Peter Walters covered this well in one of his “Painted Memories” of around a decade ago. He ran quickly from the Paleolithic frescos (Lascaux)of c.15,000 BC through the Greek and Roman era into the Medieval times and there is no need to repeat this here, but rather to encourage our reader to go into their back issues of Brushstrokes and re-read that article.

Meanwhile I will touch on of a few of what I find the more interesting vegetable and animal colours that weren’t mentioned in that original article.  Madder for example was used to dye cloth found in Egyptian tombs and the plant that produces this material by way of crushing of its roots was an important agricultural crop, particularly for France until well into the modern chemistry era. In 1868 the annual harvest in France alone was 50,000 tonnes, yielding 500 MT of dyestuff. So why was this so important? Look at any historic illustration of the British army and you’ll notice the red coats, which were dyed with Madder – as were the tunics of the Roman legions, robes of the medieval knights and all those in between.

I am of an age where most of my primary education was still very Anglo-centric, and we certainly spent more time on Julius Caesar’s conquest of Britain than any mention of Te Rauparaha’s sacking and conquest of Bank’s Peninsula. I have enduring memories of school-boy drawings of British warriors covered in woad, menacing Caesar’s legions, never knowing one day I’d have an interest in the colouring material itself. Woad is a part of the cabbage family and the sap of the leaves of this small plant contains a substance that rapidly turns blue in the presence of air.

                     Woad plant (Isatis tinctoria) Source:wikimedia.org/wikipedia/commons (& Woad Mel)

Indigo is produced by a number of plants within the Indigofera species all of which belong to the pea family. This has been the single most important dye produced from natural sources. As an interesting aside, had a synthetic substitute never been found, and the fashion for demin clothing in this shade still matched today’s demand, more than 90% of the surface of India would have to be planted in this crop to meet demand. It is lucky then that Badische Anilin- und Soda-Fabrik (BASF to the rest of us) who went within a matter of weeks of going bankrupt in their search for a commercially viable alternative, came up with one in 1885. In proof again that in our industry advances are less “Eureka!” moments than “Well I’ll be buggered, will you look at that?” it was only an accidentally broken thermometer over a  fuming sulphuric acid vessel that lead to the breakthrough. At the time, exports of Indigo from India were around 190,000MT p.a and within 20 years had dropped to 11,000 MT p.a and the price had halved.



Left:
Indigo plant (Indigofera tictora) Source:wikimedia.org/wikipedia

                                                                                                                                                                        BASF dye label Source:chinaforeignrelations.net

The final material that I want to mention in any detail before moving into the more modern era is Tyrean Purple. Perhaps not one that normally springs to mind, but nevertheless one of the most important and fascinating of the historical colourants. This material is produced by a shellfish, the Murex brandaris, as a glandular seepage.  

Murex brandaris Source:wikimedia.org/wikipedia

It is necessary to process 12,000 Muricidae to produce a gram of this valuable dyestuff. The shellfish must be caught in nets, cut into pieces, salted, boiled for several days and then after trial dyeing the decoction was either further concentrated or diluted. As one can imagine the effort involved in obtaining this dyestuff made it the most expensive of the ancient world, and the purple robes created were only for Emperors and high priests. Purple is still associated with status and the phrase “Born to the Purple” springs from this background.

Let us leap forward several centuries to the birth of coal tar chemistry. For our two regular readers of these editorials I must fulfil a promise made more than 12 months ago to once again mention Archibald Cochrane, the ninth Earl of Dundonald. You may recall that he was a researcher from around 1780 who attempted to produce pitch and tar from coal through destructive distillation, and built a tar works in the grounds of his family home, timed it badly as the English Admiralty made the decision to begin sheathing ships bottoms with copper and died in poverty in a Paris slum.

It was his initial work on coal tar, which has about 10% naphthalene content that led to the fledgling colourant field and its off-shoot industry, modern pharmaceuticals.

Archibald Cochrane, the ninth Earl of Dundonald

The ‘modern’ era of coal tar chemistry began with an 18 year old Englishman who was conducting experiments in the search for a synthetic quinine during the Easter holiday break of 1856. The entire sequence of events relating to why synthetic quinine was desperately needed in the British Empire is very well described by James Burke in his “Connections” series and I encourage everyone involved with our industry to at least read the chapter in his book entitled “The Longest Chain” or try and see the video of it. Tap me on the shoulder at the next meeting if you have any trouble finding either of these. Suffice it to say, there was another failed experiment and instead of disposing of the resulting black crystals from heating aniline sulphate and potassium dichromate, young William conducted some research into them. He found that they were soluble in alcohols and produced a rich purple colour suitable for dyeing wool and silk.

William Perkins 1852 Source imperial.ac.uk

The Victorians went crazy for the new colour and it was used everywhere from bunting at the World Expo in the Crystal Palace to the new ‘Penny Black’ postage stamp. By the outbreak of World War 1 more than 15,000 coal tar dyestuffs had been patented.

The British lost a commercial edge in their attitude to chemistry and research during this period though, with a distinct separation of ‘pure research’ and commercialisation. The Germans on the other hand, developed a system of Technical Colleges as the interface between academia and industry and built their chemical industry on a practical blend of commercial reality, chemical engineering and research. Our government and education system could do a lot worse than take a look at this model and understand how this aided the productivity of the developing German economy.

The next step was the development of the first pigment-like laked dyes and toners. Pigment manufacture takes place in an aqueous solution, so if we recall our earlier premise that a pigment is insoluble in the media in which it is carried, there needs to be some jigery-pokery goes on in the reactor vessels – once again not necessarily suitable for in depth discussion in this editorial. Suffice it to say that if we have two intermediates that precipitate naturally in each other’s presence, the resulting insoluble pigment is called a toner, and if those the two intermediates require the presence of a base to start the reaction, the resulting precipitate is a ‘laked’ pigment. In general toners are more concentrated and have higher tinctorial strength than laked pigments.

A couple of other key points to note in the development of colourants. The first stable phthalocyanine blue was produced by Scottish Dyes by accident, when they used a cracked enamel reaction vessel and allowed phthalic anhydride into contact with steel – and when this became commercial in 1935 it was the first time a new chemical class was released directly onto the market as a pigment without first having been a dye for textiles. Following on from our theme, the DPP colours developed in the mid 1980s by Ciba were also the result of investigations into waste products from pharmaceutical research.

So I think one of the main things that I’ve enjoyed from the colourant aspect of this industry is the continual generation of something positive out of adversity. There is always someone offering better fastness properties or a more useful shade or a more economic source of supply – or even in these times of severe shortages of so many colourants, any sort of supply at all. Yet for all that the market thrives and grows and colours generate all sorts of emotions and are positive marketing tools. In this edition of the magazine many facets of this will be covered and I hope you will find it as fascinating as I do..."