John Constable's Hay Wain, one of the best-known Romantic paintings1, depicts an archetypal English, dreamy rural scene. A threatening sky, leavened only by smudges of watery blue, is ready to deluge the countryside at any moment.
In fact, the sky above the half-submerged cart could have come from many landscapes of that period. If Constable and his contemporaries had lived in Swansea - one of the wettest cities in Britain - there would be an easy, and specious, explanation for this fondness for rain-laden clouds. Constable was in all likelihood as constrained by his medium as he was by the climate or his imagination. There was simply not enough decent blue pigment to go around, and cloudless skies were a good deal more rare on canvas than they were in reality, even in Swansea. Had he lived in some other, sunnier climes under bluer skies, faithfully rendering the scenario depicted in the Hay Wain would have been rather more difficult.
Today's artistic expression is far less fettered. The intense blues of Dali's dreamscapes and Hockney's swimming pools exist purely because of the successes of chemists in mastering intensity, hue and, just as importantly, durability of colour. The modern artist owes the chemist a substantial and unspoken debt. The history of 'blue' is only one colourful strand of the story of how this debt was incurred, but a representative one.
How Colours Work
Colours exist in nature because substances preferentially absorb certain wavelengths of white light, and reflect the rest. Pigments are suspensions of a light-absorbing (or reflecting, depending upon your perspective) solid in a transparent medium, such as a lacquer. Blue pigments absorb yellow light: for this reason a blue car parked under a yellow sodium streetlamp is indistinguishable from a black one.
Blue is rare in living nature as it is a colour that is associated with organic molecules in alkaline conditions: few living systems have an alkaline chemistry. The morning glory flower, for instance, starts off the day an intense cerulean, fading to an insipid mauve at sundown as the alkalinity in the flower decreases.
Naturally occurring inorganic blue pigments also tend to be rare, but for other reasons. Before chemistry was well-enough developed as a science to reproduce colours, blue (as well as other colours) had to be dug out of the ground, in the form of lapis lazuli (literally 'blue stone'). This intensely blue, semi-precious stone has been known for millennia: the ancient city of Ur had a thriving trade in it in the 4th Millennium BC. It was believed that lapis was the sapphire alluded to in Biblical writings. It is still mined in Chile, Afghanistan, Burma, Colorado and Siberia.
Lapis was probably most used throughout the 14th and 15th Century in manuscripts and Italian paintings: the pigment was called ultramarinum (ultramarine), meaning 'from beyond the sea'. Being imported it was extremely costly, rivalling or even exceeding gold, and was often referred to as the 'blue gold'. Its use was subject to self-imposed rationing, and the less-expensive and inferior azurite was often substituted, which in itself was subject to shortages and often turned green. It was also a devil to process: the lapis had to be ground finely (not easy, given that it was a rock), mixed with melted wax, resins and oils, wrapped in a cloth, then kneaded under a solution of lye: the fine particles of ultramarine were then collected in a holding vessel while the retained residue held ash and colourless crystals. A - male - expert in this procedure wrote:
Know, too, that making it is an occupation for pretty girls rather than for men; for they are always at home, and reliable, and they have more dainty hands. Just beware of old women2.
The Chemist comes to the aid of the Artist
Needless to say, there was a need for a synthetic substitute that would not bankrupt fine artists, or their patrons, or expose them unduly to the predations of 'old women'. At the time that the hunt was on for such a substitute, the first half of the 19th Century, colour chemistry was poorly understood by even its most expert practitioners and serendipity had to play a major part in the discovery of synthetic ultramarine. The first hint to how this substance might be produced was in 1787. Goethe described how glassy masses of a blue material found in limekilns near Palermo were used locally as an ornamental substitute for lapis. 40 years later, Jean Baptiste Guimet, after observing a similar residue at the mouth of glass furnaces, won an open competition in Paris by demonstrating a process where coal, sulphur, soda and china clay, all cheap, commonplace substances, were heated to give synthetic ultramarine.
The value of artificial ultramarine was soon realised by contemporary painters: it sold for 400 francs per pound, as opposed to the three to five thousand francs for lapis lazuli. JMW Turner is said to be one of its first exponents, and its colour made its way into the paintings of the Impressionists. Later, Picasso would not have been able to so precisely express his feelings about death and loss in the lament of his Blue Period, had not the dark blue of synthetic ultramarine lent itself so vividly to this purpose.
Why Ultramarine is Blue
The gold flecks often found in lapis lazuli give a clue to its origins. The tiny gold flecks are iron pyrites, a mineral that forms only in the presence of sulphur. Ultramarine's major constituent elements, sodium, aluminium, silicon and oxygen, do not by themselves lend much colour to anything, let alone an intense blue. The colourless sodalite, a sodium aluminosilicate, results from mixing and heating compounds that contain these raw materials. Add sulphur, and the transformation to ultramarine is dramatic. The sulphur is the colour chemist's Philosopher's Stone, transmuting base mineral into something immeasurably more precious.
The sulphur is trapped in the crystal lattice in a special form. Elemental sulphur, the stuff that goes into fireworks, is a primrose yellow crystalline solid. The sulphur in ultramarine is an altogether more exotic beast. In elemental sulphur, which is stable, eight sulphur atoms are joined together in a ring. In ultramarine, three sulphur atoms are joined in a chain and carry a surplus, fidgety and highly reactive electron. It is this itinerant electron that intensely absorbs yellow light, and makes ultramarine very susceptible to the influence of acids, which decolourise it rapidly. This fragile, reactive blue molecule is trapped inside a transparent octahedral cage of aluminium, silicon and oxygen atoms. Nothing can get at it and, being bulky, it cannot get out of its cage: the chemical equivalent of an insect in amber, trapped when the molten sodalite crystallised around it and preserved almost indefinitely from environmental depredation.
Today, ultramarine is still produced and used as a pigment. If selenium or tellurium is substituted for sulphur in its manufacture, intense red and purple pigments are produced. Ultramarine was used in laundry whitener (the 'Blue Bag' of old).
The blue colour that we take for granted in this day and age is a comparatively recent invention. Ultramarine, as stated before, had its drawbacks: it was not resistant to acids at all. The main alternative to ultramarine, Prussian blue, is blue-black in colour and decolourised by alkali, and so was useless for frescos as it reacted with the plaster. Chemists had to wait for serendipity to act again on their behalf.
It was not until the turn of the 20th Century that the next breakthrough was made. In 1907, chemists at the South Metropolitan Gas Company in London were trying to react phthalimide with acetic anhydride: a highly insoluble deep blue material of unknown structure was formed. Then in 1927, chemists trying to make dicyanobenzene from copper cyanide and dibromobenzene also produced a similar material.
These chemists had inadvertently discovered the phthalocyanines3. These beautifully symmetrical and remarkably stable molecules bear a marked resemblance to the active portions of chlorophyll and haemoglobin, two naturally occurring pigments. In the centre of the molecule is a cavity just big enough to accommodate a transition metal, such as copper. Organic molecules are generally thermally unstable, but under the centripetal effect of the metal atom, things do not fall apart and the centre holds perfectly. Copper phthalocyanine is stable to 900°C, and can be precipitated from concentrated sulphuric acid without change. It is resistant to virtually anything that nature (and man) can throw at it.
Today, phthalocyanines are used in artists' pigments, paint, gas sensors, cancer therapy, detergents and even laser printer cartridges. They are cheap, easy to make, incredibly stable, non-toxic, easily processed and intensely coloured. In short, they are a colour chemist's Holy Grail. The only disadvantage with these compounds is that their most appealing characteristic as a pigment, their complete insolubility, renders them almost useless as a dye. The chance is high that any blue object you come across will be coated or impregnated with copper phthalocyanine.
Blue has always been the most elusive of colours to reproduce, and now, that it is so commonplace, it may seem that there is little left for chemistry to accomplish. This is only partially true. There are two ways of reproducing colour: either by absorbing its complementary colour (as pigments do) or by generating it directly. If blue was a difficult colour to reproduce in pigment, then it is an order of magnitude more difficult to produce it by generating blue light. This is mainly through energetic considerations: as light becomes bluer, the energy of its constituent photons increases, and the number of materials which can be excited to a high energy state and usefully convert that energy to light diminishes rapidly.
Computer manufacturers dream of the inexpensive, efficient and robust colour display, but the cathode ray tube, invented early last century, still reigns supreme. Liquid crystal displays are fragile and expensive, and very difficult to make. The most promising embryonic flat-screen technology is based upon Light Emitting Polymers (LEPs), which could be fashioned into huge, robust, bright displays and have been shown to generate blue light. These are produced in the time-honoured fashion by chemists working in conjunction with engineers and physicists to bring the technology to the marketplace. The Men in White Coats still have a role to play in making the world a more colourful place.