Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK.[A version of this article appeared in Interdisciplinary Science Reviews 1999, Vol.24, No.4, pp.301-306]
This site considers the ways in which Man has made use of charcoal down the ages, as a fuel, as an artistic material and as an adsorbent. The scientific study of charcoal's structure is discussed, and the role played by Rosalind Franklin highlighted. Current ideas on the structure of charcoal are summarised.
"We want ours to burn good and slow," said Young Billy. "If he burns fast he leaves nowt but ash. The slower the fire the better the charcoal".
Susan was watching carefully.
"Why doesn't it go out?" she asked.
"Got too good a hold," said Young Billy. "Once he's got a good hold you can cover a fire up and the better you cover him the hotter he is and the slower he burns. But if you let him have plenty of air there's no holding him."
Man's use of charcoal extends back as far as human history itself. It was first used more than 30,000 years ago to make some of the earliest cave paintings. Much later, charcoal played an important role in what might be considered mankind's first technology, the smelting and working of metals. In more recent times, charcoal has remained a technologically important material, primarily as a result of its adsorptive properties. The use of activated charcoal in gas masks during World War I saved many thousands of lives, and today charcoal is used on an enormous scale for the purification of air and water. From a scientific perspective, charcoal is also of great interest since we are beginning to achieve a detailed picture of its atomic structure for the first time. In this article I want to look at charcoal from the point of view of art, technology and science.
What is Charcoal?
The dictionary defines charcoal as "The black porous residue obtained by the destructive distillation of animal or vegetable matter in a limited supply of air". In fact charcoal, or more correctly char, can be produced from a range of synthetic materials, such as polymers, as well as from natural sources. The basic atomic structure of the char is independent of the precursor, although the larger scale morphology may differ. It is important not to confuse charcoal with other forms of impure non-crystalline carbon such as coke and soot. Although coke, like charcoal, is produced by solid-phase pyrolysis (usually of bituminous coal), it is distinguished from charcoal in that a fluid phase is formed during carbonisation. The structure and properties of cokes and chars are quite different, as discussed further below. In the case of soot, this is formed in the gas phase by incomplete combustion rather than by solid-phase pyrolysis, and it has a microstructure quite distinct from either coke or charcoal. It will not be discussed here.
Early Charcoal Production
The origins of charcoal production are intimately bound up with the beginnings of metallurgy approximately 5000 years ago. Attempts to smelt metals using wood fires could never have been entirely successful, since it would have been impossible to achieve sufficiently high temperatures. When plain wood is burned there is a large quantity of water driven off, plus assorted volatiles, and this limits the temperature of the fire. Burning charcoal, on the other hand, produces a much higher fire temperature (well over 1000oC), with little smoke: ideal conditions for metal smelting and working. Oxide ores of copper were first reduced with charcoal in about 3000 BC, initiating the era we know as the Bronze Age. Iron is more difficult to smelt than copper, requiring higher temperatures and a greater blast of air, and was first achieved in about 1200 BC, marking the beginning of the Iron Age.
The earliest method for producing charcoal probably involved the "pit kiln" process, in which wood was slowly burned in a shallow pit covered with soil. However, this eventually gave way to the more efficient and manageable above-ground "forest kiln" method. Charcoal was still being produced in this way until the 1950s in Forest of Dean and elsewhere. The basic arrangement used is shown in above. The first stage in constructing the pile is the raising of a centre log, around which a shaft is left to act as a chimney. Wood is then stacked around this to form a hemispherical heap, which is then covered with earth or turf. Around the base of the pile some small air inlets are opened. To light the kiln, some kindling is thrown down into the central shaft, followed by a burning torch. When the wood is alight, the hatch is closed and the intensity of the fire is regulated by opening and closing the air inlets at the base. Controlling the input of air ensures that the wood smoulders rather than bursting into flames, and in this way it is slowly converted into charcoal, a process which typically takes around ten days.
All iron production until about 1700 was based on the use of charcoal. However, as metal production increased, deforestation became a significant problem throughout Europe, and attempts were made to find an alternative to charcoal. Coal was found to be unsuitable, because the impurities which it contains (especially sulphur) are transferred to the metal.
Around 1709, the English iron worker Abraham Darby found a way of making cast iron using coke, produced from plentiful bituminous coal. As a result of this innovation, demand for charcoal fell substantially, but it continued to be produced on a small scale, mainly for cooking.
Charcoal as an artists' material
As already noted, charcoal was being used by artists long before its use in smelting metals. Cave paintings made using charcoal and other materials have been dated as early as 30,000 years BC 1. However, the most famous cave art, at Lascaux, Roufignac, and other sites in France, is rather later than this, dating from 15,000 BC to about 9000 BC. A beautiful charcoal drawing of a mammoth from Roufignac is shown on the left. These pictures would have been made using charred sticks taken from a fire rather than intentionally-produced charcoal. When the caves a were first discovered the paintings were very well preserved. They had been protected from environmental erosion, and the stable levels of moisture and temperature within the caves provided an ideal environment for the preservation of pigments. However, when the caves were opened to the public in the late 1940's, the presence of enormous numbers of visitors soon disturbed the delicate environment of the cave, and the paintings began to deteriorate. The colours faded and a green fungus grew over the pigments. The caves were closed to the public in 1963, and replicas were constructed for visitors.
In more recent times, charcoal has remained a popular medium for artists. We know that it was used widely in the Renaissance for making preparatory drawings, but few of these have survived. This is because charcoal marks on paper are relatively impermanent, as discussed below. At the end of the 15th century, methods were developed for "fixing" charcoal drawings by immersing them in baths of gum, and as a result many charcoal drawings from the 16th century have come down to us. In most cases these were probably preparatory sketches, but it seems that some artists began to regard charcoal drawings as finished works in their own right. One of these was the German Albrecht Dürer (1471-1528). Dürer is famous for his ink drawings and woodcuts, but also made many charcoal drawings, among them an expressive portrait of his mother (right). Most major artists since the Renaissance, from Rembrandt to Degas have used charcoal in one way or another, frequently for portraiture, where its expressionistic qualities can be used to the full. In the 20th century, notable works in charcoal have been made by Matisse and Picasso, whose Artist before his canvas is shown on the left.
In considering the attraction of charcoal as an artists' material, it is interesting to compare its characteristics with those of graphite pencil. Although both charcoal and graphite are forms of carbon, their properties are very different. Graphite has a layered crystal structure with very weak chemical bonds between the layers. As a result, a graphite pencil slides easily over paper leaving marks which result from the shearing away of tiny shards of graphite from the parent crystals. A graphite pencil can also be sharpened to a fine point, enabling accurate, fine lines to be drawn. In practical terms, charcoal would seem to be less suitable as a drawing material. It does not have a layered structure like graphite, but instead has a rough texture which does not glide smoothly over paper. Marks made with charcoal result from the deposition of tiny carbon particles into depressions in the fibrous paper surface. As already noted, these marks are less permanent than those made with graphite pencil, having a tendency to "dust-off." Nevertheless, many artists prefer charcoal, partly because of its blackness. Graphite pencils can never achieve as dark a black as charcoal, since pure graphite is grey and metallic in appearance, rather than black. The tendency of pencil marks to become shiny upon repeated application is also undesirable. Stylistically, charcoal encourages a free, expressive style, since fine lines are nearly impossible to draw. It can also be deliberately smeared or smudged to produce moody, atmospheric effects which many artists have found highly appealing.
David Nash is a contemporary artist who uses charcoal in his work. Nash makes large wood sculptures to which he applies a flame gun, transforming the colour and texture of the wood to the intense richness of charcoal. An example of his work is "Three Forms (Pyramid, Sphere, Cube)", shown on the right.
Charcoal as an adsorbent
The use of charcoal as an adsorbent, like most of its other applications, has a very long history2. Egyptian papyri from around 1500 BC describe the use of charcoal to adsorb malodorous vapours from putrefying wounds, and there is an Old Testament reference (Numbers 19:9) to the ritual purification of water using the charred remains of a heifer. The first scientific study of the adsorptive properties of charcoal was made by the Swedish scientist Carl Wilhelm Scheele in the late 18th century. Scheele was a brilliant chemist who identified the elements chlorine and barium and prepared oxygen two years before Priestley. He described how the vapours adsorbed by charcoal could be expelled by heating, and taken up again during cooling 3: "I filled a retort half-full with very dry pounded charcoal and tied it to a bladder, emptied of air. As soon as the retort became red-hot at the bottom, the bladder would no longer expand. I left the retort to cool and the air returned from the bladder into the coals. I again heated the retort, and the air was again expelled; and when it was cool the air was again adsorbed by the coals. This air filled 8 times the space occupied by the coals."
During the 19th century, work on the adsorptive properties of charcoal continued at a fairly low level. There were still relatively few applications for charcoal as an adsorbent, apart from specialised areas like sugar refining, and little incentive for research. It was the use of poisonous gas in World War I which created an urgent need for effective adsorbent materials4. Gas was first used in the second battle of Ypres in April 1915, when the Germans released chlorine over the Allied trenches. The British and French troops were completely unprepared for this new weapon, the only protection being a piece of damp cloth tied over the face. Subsequently, slightly improved defence against chlorine was achieved by using cloth saturated with chemicals such as photographers Hypo solution. However it was clear that a far better form of protection was going to be needed. The first true gas masks were made using wood charcoal which was activated chemically. Subsequently, research in the USA showed that charcoal made from coconut shells had the best characteristics for use in gas masks, since its more open macroporous structure allowed for a more rapid flow-through of air.
The British deployed gas-bearing shells in September 1915 at the Battle of Loos, and the use of gas continued on the Western Front until the end of the war. However, the effectiveness of gas as a battlefield weapon was limited by its vulnerability to changes in wind direction and by increasingly effective gas masks. The use of mustard gas, which began in 1917 was partly an attempt to defeat the new charcoal gas masks, but again was only partially effective. In World War II, the British authorities feared that gas would be used to attack civilian targets, so gas masks were issued to the entire population. In the event, the feared attacks never materialised.
Today, activated charcoal is used on an enormous scale in both vapour-phase and liquid-phase purification processes. It is still used widely in respirators, as well as in air-conditioning systems and in the clean-up of waste gases from industry. In the liquid-phase, its largest single application is the removal of organic contaminants from drinking water. Many water companies in Europe and the USA now filter all domestic supplies through granular activated carbon filters, and household water filters containing activated carbon are also in widespread use. Other applications include decontamination of groundwaters and control of automobile emissions. As a result of its commercial importance, charcoal has been the subject of a huge amount of research in both industrial and academic laboratories. Despite this, many important questions remain, not least about its detailed atomic structure.
The structure of charcoal
The work of Rosalind Franklin
In the 1920s and 1930s, X-ray diffraction was used to determine the structures of a huge range of inorganic materials. Graphite was one of the first structures to be solved, by John D Bernal in 1924. Non-crystalline carbon materials, such as soot, coke and char, presented more of a challenge. It was established that these carbons, like graphite, contained hexagonal carbon rings, but the way these were linked together remained unknown. Some workers suggested that char might have a three dimensional network structure lying somewhere between those of graphite and diamond, but there was no direct evidence for this. The distinction between char and coke was also not understood. The field remained in some disarray until the classic work of Rosalind Franklin in the late 1940s and early 50s.
Rosalind Franklin is, of course, far better known for her work on the structure of DNA than for her work on carbon. However, before she moved into biology she made a major contribution to our understanding of coals, carbons and graphite. Franklin studied chemistry at Newnham College, Cambridge, graduating in 1941. She then joined the British Coal Utilisation Research Association (CURA) in London, which was carrying out a major research programme, important to the war effort, on the efficient use of coal. Franklin's research focused on the porosity of coals, and she was awarded a Ph.D. for this work by Cambridge in 1945. After the war she went to Paris to work with Jacques Méring at the Laboratoire Central des Services Chimiques de l'Etat. The photograph shown here was taken during her time in France, which by all accounts was a very happy one5. From Méring she learned the techniques of X-ray diffraction and used them to study a range of carbon materials. Franklin's work during this period resulted in a number of outstanding papers which are still frequently cited in the literature.
In one of these, published in Acta Crystallographia in 1950 6, she described XRD studies of a char prepared from the polymer polyvinylidene chloride. By rigorous quantitative analysis of the diffraction data, Franklin was able to propose the first reliable model for the structure of a char. In this model, 65% of the carbon in is contained in individual graphite layers, highly perfect in structure but only about 1.6 nm in diameter, with the remainder of the carbon being disordered. Earlier models, based on three dimensional network structures, were shown to be incorrect. This was followed by a detailed study of the effect of high temperature heat treatments on the structures of cokes and chars, which probably constitutes her most important work on carbon. This work was made possible by the availability of an early induction furnace at the French Laboratoire de Haute Temperatures. Using this furnace, she was able to heat the carbon samples at temperatures up to 3000oC. It would be expected that these very high temperature treatments would convert the disordered carbons into crystalline graphite, which is known to be the most thermodynamically stable form of solid carbon. So Franklin's results came as a surprise: while the cokes could be graphitized by heat treatments above about 2200oC, the chars could not be transformed into crystalline graphite, even at 3000oC. Instead, they formed a porous, isotropic material which only contained tiny domains of graphite-like structure. These results demonstrated, for the first time, the key distinction between cokes and chars.
Franklin summarised her studies of graphitization in a lengthy paper for Proceedings of the Royal Society, published in 19517, which is one of the classics of the carbon literature. In this paper she coined the terms graphitizing carbons and non-graphitizing carbons to describe the two classes of material she had identified, and proposed models for their microstructures, which are shown on the right. In these models, the basic units are small graphitic crystallites containing a few layer planes, which are joined together by cross-links. The structural units in a graphitizing carbon are approximately parallel to each other and the links between adjacent units are assumed to be weak as shown in (a). The transformation of such a structure into crystalline graphite would be expected to be relatively facile. By contrast, the structural units in non-graphitizing carbons, are oriented randomly, as shown in (b), and the cross-links are sufficiently strong to impede movement of the layers into a more parallel arrangement. Although these models do not represent a complete description of graphitizing and non-graphitizing carbons, since the precise nature of the cross-links is not specified, they provided for many years the best structural models available for these materials.
The atomic structure of chars and the reasons for their resistance to graphitization are still the subject of intense research, nearly 50 years after Franklin's work. However there is a growing belief that the key to the problem may lie in the discovery of a new class of carbons known as fullerenes. Fullerenes are a group of closed-cage carbon particles of which the archetype is buckminsterfullerene, C60, whose structure is shown on the right. They were first identified in 1985 by Harry Kroto, of the University of Sussex, and Richard Smalley, of Rice University, Houston, and their colleagues, during experiments on the laser vaporisation of graphite8. Subsequently it was found that they could be prepared in bulk using a simple carbon arc, and this stimulated a deluge of research which led to the discovery of a whole range of new fullerene-related carbon materials including nanoparticles and nanotubes9. The distinguishing structural feature of these new carbons is that they contain pentagonal rings in addition to hexagons. These pentagons produce curvature, and Euler's law states that the inclusion of precisely 12 pentagons into such a lattice will produce a closed structure.
The discovery that carbon structures containing pentagons can be highly stable led to speculation that such structures might be present in well-known forms of carbon. At first, this speculation centred on soot particles, whose spheroidal shapes immediately suggest a possible link with fullerenes. However, there is also growing evidence that microporous carbons may contain fullerene-like elements. The first indication of this came in a high-resolution electron microscopy study published in 199710. In this work, non-graphitizing carbons prepared from polyvinylidene chloride and sucrose were heat treated at temperatures up to 2600oC. It was found that the high temperature heat treatments produced a structure made up of curved and faceted graphitic layer planes, including closed carbon nanoparticles, which were apparently fullerene-like in structure. This suggested that fullerene-like elements may have been present in the original carbons, and subsequent studies using a variety of techniques have provided support for this idea. Eiji Osawa and colleagues at the Toyohashi University of Technology in Japan have also demonstrated that C60 can be extracted from wood charcoal11. As a result of these studies, many workers in the field now believe that charcoal has a structure made up of fragments of randomly curved carbon sheets, containing pentagonal and a heptagonal rings dispersed throughout a hexagonal network, as shown on the left. However, this idea is by no means universally accepted.
Charcoal may seem a mundane material, but as we have seen its unique properties have been valued by man throughout history. Its use as a fuel was crucial in the development of metallurgy, and its qualities as an artistic medium have been appreciated from the earliest times. Today activated charcoal is of enormous importance in the purification of water and air. The science of charcoal has been studied for over 200 years by such outstanding figures as Wilhelm Scheele and Rosalind Franklin yet it still remains only partially understood. We have made important advances recently, but there is still much to learn.