Amorphous Carbon and Charcoal

Amorphous carbon and charcoal are two important forms of carbon that lack a well-defined crystalline structure, distinguishing them from crystalline forms such as graphite and diamond. Both materials play significant roles in industrial processes, scientific research, and everyday applications due to their unique physical and chemical properties. Although both are non-crystalline allotropes of carbon, they differ in origin, composition, and method of preparation.

Structure and Nature of Amorphous Carbon

Amorphous carbon refers to carbon materials that do not exhibit a regular crystalline lattice. In this form, the carbon atoms are arranged irregularly, lacking the long-range order characteristic of graphite or diamond. Despite being termed “amorphous,” such materials may contain small regions with a degree of graphitic order or localised crystalline domains.
The structure of amorphous carbon generally comprises a mixture of sp² (graphitic) and sp³ (diamond-like) hybridised bonds, which determines its hardness, electrical conductivity, and optical properties. The ratio of these bonds varies depending on how the amorphous carbon is formed.
Amorphous carbon occurs in a variety of substances, including carbon black, soot, lampblack, coke, and charcoal. These materials are typically produced through the incomplete combustion or thermal decomposition of organic matter.

Preparation and Formation

Amorphous carbon can be produced by heating organic compounds such as sugar, coal, wood, or hydrocarbons in a limited supply of air. This process removes volatile components like hydrogen, oxygen, and nitrogen, leaving behind a carbon-rich residue.
Common examples of amorphous carbon and their methods of preparation include:

• Carbon black: Obtained by burning hydrocarbons in insufficient air; used in tyres and inks.
• Lampblack: Produced by burning oils or resins with limited oxygen supply.
• Coke: Derived from destructive distillation of coal; used in metallurgy.
• Charcoal: Formed from wood carbonisation; used as a fuel and adsorbent.

These materials differ slightly in purity and texture but all share the essential amorphous nature of their carbon atoms.

Physical and Chemical Properties of Amorphous Carbon

Amorphous carbon exhibits several distinctive properties:

• Appearance: Generally black, opaque, and soft.
• Electrical Conductivity: Lower than graphite but varies depending on structure and impurities.
• Hardness: Usually softer than crystalline carbon but harder forms exist, such as diamond-like carbon (DLC).
• Reactivity: More chemically reactive than graphite due to the presence of dangling bonds and disordered atomic structure.
• Density: Lower than that of crystalline forms.

When heated in the presence of oxygen, amorphous carbon readily burns to form carbon dioxide (CO₂) or carbon monoxide (CO), depending on oxygen availability. It can also reduce metal oxides to metals, making it valuable in metallurgical processes.

Charcoal: Formation and Composition

Charcoal is a well-known form of amorphous carbon produced by the destructive distillation of wood, bones, or other organic matter in the absence of air. During this process, volatile substances such as water vapour, methane, and other organic gases are driven off, leaving behind a porous black residue rich in carbon.
The process can be represented as:
Wood→limited airheatCharcoal+volatile gases (CO, CH₄, H₂, tar)\text{Wood} \xrightarrow[\text{limited air}]{\text{heat}} \text{Charcoal} + \text{volatile gases (CO, CH₄, H₂, tar)}Woodheatlimited air​Charcoal+volatile gases (CO, CH₄, H₂, tar)
The composition of charcoal typically includes 80–90% carbon, with small amounts of hydrogen, oxygen, ash, and moisture. Because of its porous nature, charcoal possesses a large surface area, giving it high adsorptive capacity.

Types of Charcoal

Charcoal is classified into several types based on its source material and method of preparation:

• Wood Charcoal: Produced by heating wood in the absence of air; commonly used as a domestic and industrial fuel.
• Animal Charcoal (Bone Char): Obtained by heating bones; contains calcium phosphate and is used in sugar refining and filtration.
• Sugar Charcoal: Made by heating sugar in the absence of air; one of the purest forms of amorphous carbon, often used in laboratory reduction reactions.
• Activated Charcoal: Formed by heating charcoal with oxidising agents such as steam or carbon dioxide; has a very high surface area and is used for gas purification, water filtration, and in medicine for adsorption of toxins.

Properties of Charcoal

Charcoal exhibits several key physical and chemical characteristics that determine its applications:

• Porosity: Highly porous, allowing for strong adsorption of gases, vapours, and dissolved substances.
• Low Density: Light and brittle.
• Combustibility: Burns easily in air, producing heat with minimal smoke.
• Adsorptive Power: Capable of removing colours and odours from solutions (basis of its use in purification).
• Reducing Ability: Strong reducing agent, used in metallurgical operations to extract metals from oxides.

Chemically, charcoal reacts with oxygen to form carbon dioxide or carbon monoxide depending on the air supply:
C+O₂→CO₂\text{C} + \text{O₂} \rightarrow \text{CO₂} C+O₂→CO₂ 2C+O₂→2CO2\text{C} + \text{O₂} \rightarrow 2\text{CO}2C+O₂→2CO
Charcoal also reacts with steam to produce water gas (CO + H₂) and with carbon dioxide to form producer gas (CO + N₂), both of which are industrial fuel gases.

Comparison Between Amorphous Carbon and Charcoal

Industrial and Practical Applications

Both amorphous carbon and charcoal have extensive industrial applications:
Amorphous Carbon:

• Used in the manufacture of carbon black, inks, and paints.
• Serves as a reducing agent in metallurgical processes.
• Employed in electrodes, dry cells, and carbon composites.
• Utilised in coatings such as diamond-like carbon (DLC) films for wear resistance.

Charcoal:

• Widely used as a fuel in domestic and metallurgical operations.
• Activated charcoal is used for water purification, air filtration, and gas masks.
• Used in medicine for treating poisoning and digestive disorders by adsorption of toxins.
• Applied in sugar refining, alcohol purification, and odour removal processes.