The terms tar and pitch are often used interchangeably, but they are chemically and physically distinct viscous substances. Both are dark, complex mixtures of hydrocarbons derived from various organic sources through intense heating processes. Distinguishing their properties requires understanding that pitch is a refined derivative of tar.
Defining Tar: Sources and Composition
Tar is a highly viscous, dark brown or black liquid obtained through the destructive distillation or pyrolysis of organic materials in the absence of oxygen. This process breaks down the organic matter into gases, a solid residue like charcoal or coke, and a complex liquid mixture that is the raw tar. The specific composition and viscosity of tar depend entirely on its source material, which is broadly categorized into wood, coal, and petroleum.
Historically, wood tar, often called pine tar, was the most common variety, produced by slowly burning resinous wood like pine roots and stumps. This process yields a relatively fluid, odorous substance, which was historically important for waterproofing ships, preserving rope, and treating wood.
Another significant source is coal tar, which is a byproduct of the high-temperature carbonization of coal to produce coke and coal gas. Coal tar is a dense, chemically complex mixture containing a high concentration of polycyclic aromatic hydrocarbons (PAHs), which are generally considered toxic.
Petroleum tar, more accurately termed asphalt or bitumen, is the heavy, non-volatile residue left over from the fractional distillation of crude oil. While chemically different from true wood or coal tars, it is often grouped under the general term “tar” due to its similar appearance and use in road paving.
Defining Pitch: The Refined Derivative
Pitch is the dense, semi-solid or solid residue that remains after raw tar has been further processed through distillation or heating. This refinement removes the lighter, more volatile oils and water content found in the initial tar. This removal significantly alters the physical properties, making pitch substantially different from the liquid tar precursor.
The resulting pitch possesses a much higher softening point and a drastically increased viscosity compared to the precursor tar. At room temperature, pitch is often hard and brittle, appearing like a solid, though it is technically a viscoelastic polymer that flows extremely slowly over time. For example, the pitch used in the famous Pitch Drop Experiment has been measured to have a viscosity roughly 230 billion times that of water.
This transformation from a fluid tar to a semi-solid pitch makes it a much more stable and durable material. When coal tar is refined, the residue is specifically known as coal-tar pitch, which is a major commercial product. Similarly, wood tar can be distilled to produce wood pitch, a denser product with greater binding and sealing capabilities.
Key Differences and Practical Applications
The most significant distinction between tar and pitch is their physical state, which directly dictates their suitability for various applications. Tar is characterized by its relatively high volatility and lower melting point, often remaining a viscous liquid even at moderate temperatures. This liquid nature makes tar suitable for applications requiring penetration, such as wood preservation, where it soaks into the material’s pores to repel water and inhibit decay.
Pitch, in contrast, is defined by its low volatility and high softening or melting point, often exceeding 150°C. This solidity and superior binding strength make it the material of choice for uses requiring structural integrity and weather resistance. For instance, coal-tar pitch is a widely used binder in the manufacturing of carbon electrodes for aluminum smelting and is used in roofing felt and shingles due to its durability.
Historically, the two materials were used together in shipbuilding. Tar was used to coat ropes and wood surfaces for general waterproofing, while the tougher, more pliable pitch was melted and used as a heavy-duty sealant for caulking the seams between wooden planks. In modern construction, petroleum-derived asphalt (bitumen) is the primary material for road construction, but coal-tar pitch remains important in specialized applications requiring high binding capability.