The first scientific study of the adsorptive properties of charcoal was made by the Swedish scientist Carl Wilhelm Scheele in the late 18 th 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 19 th 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 materials 4. Gas was first used in the second battle of Ypres in April , 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 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 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. In the s and s, 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 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 s 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 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. The photograph shown here was taken during her time in France, which by all accounts was a very happy one 5. 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 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. 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 o C.
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 o C, the chars could not be transformed into crystalline graphite, even at o C. 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 7 , 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, C 60 , whose structure is shown on the right.
They were first identified in 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 graphite 8. 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 nanotubes 9.
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 In this work, non-graphitizing carbons prepared from polyvinylidene chloride and sucrose were heat treated at temperatures up to o C.
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 C 60 can be extracted from wood charcoal 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 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. HALL and K. KING: "Protection - the black art? There are a few types of charcoal used by artists to create a drawing.
These types of charcoal include "vine" and "compressed". Vine charcoal usually consists of burnt willow wood. Vine charcoal is easily spread on a surface and is very easy to erase. As a consequence it is generally makes a lighter mark when you draw than compressed charcoal and easily smudges. Which may be a benefit. Compressed charcoal is held together by a gum binder and is darker than vine charcoal.
As a result, it is harder to erase, harder to smudge, but makes a darker mark. Compressed charcoal may come as a round stick, a square stick, or in a pencil. Vine charcoal is almost always a round stick. Some compressed charcoal is pigmented.
This is the case with white compressed charcoal. When using charcoal to draw, a few tools will need to be at your disposal. First, you may need a variety of different types of charcoal- vine, compressed, pencil form. You will also want to have a kneaded eraser.
Kneaded Eraser - A kneaded eraser is a special type of eraser that is designed to lift the material off of the surface. Kneaded erasers work especially well with charcoal. Blending Stumps - You may also want to have a blending stump. A blending stump will allow you to have full control over the blending and smearing of the charcoal.
You can create a blending stump by tightly rolling up drawing paper to a point. If you are drawing on a flat surface, it's also a good idea to have a paper towel handy. You can lay the paper towel between your hand and the surface of the drawing, so that your hand doesn't smear your work. Vine charcoal and compressed sticks should be held differently in your hand than a drawing pencil.
It will vary from artist to artist, but my suggestion is to hold the charcoal with your thumb and forefinger with your palm facing the surface of the paper.
For detailed marks, a charcoal pencil can be used and held just like a traditional pencil. More on grips for holding a pencil can be found here. When charcoal drawings are finished, the artist may chose to "fix" the charcoal in place. Because charcoal drawings are often very dusty, this is a common practice. There are different types of fixative. There is workable fixative which allows for some work to done on the artwork after it has been "fixed".
There is also final fixative, which is used when the drawing is finished. Most artists, like myself, chose to only use workable fixative, since you may chose to go back to drawing and work on it at a later time.
Fixative comes in a spray can and is applied by spraying it onto the artwork. The best way to do this is in a well-ventilated area with the spray can held about a foot away from the artwork. Hairspray can be used in place of fixative but it can darken the surface so it is not recommended. Learning how to draw with charcoal is just like learning how to draw with any medium. It takes practice. So don't be discouraged at first. Charcoal drawing is different than drawing with a pencil. Most people are already used to drawing with a pencil, so it may take some time to become accustomed to charcoal.
But don't worry, you will get better with time. The drawing begins with a loose contour line sketch using a charcoal pencil. Areas of high contrast are noted with a line. Next, powdered charcoal is spread over the drawing and worked into the surface with a mop brush and a paper towel.
Darker areas are strengthened with the charcoal pencil.
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