“Creosote” Used in Wood Preservation, Explained
In the United States, the term “creosote” is pretty confusing. There are many substances referred to as “creosote,” which makes it hard to figure out which type has been used for over a hundred years to preserve wood infrastructure. Sometimes the type of creosote used in wood preservation is called “coal-tar creosote” to indicate that it’s formed by burning coal. Another common description, “coal-tar distillate,” is slightly more specific and explains that creosote is distilled out of coal tar.
But why say “creosote” at all? What does that unique word mean? “Creosote” is actually an adopted word, a slightly Americanized version of a German word “Kreosot,” which is probably why so many Americans are not sure what it means. However, its roots go even deeper than that: a German scientist named Carl Ludwig von Reichenbach created this term using the Greek root words “kreas,” which means “flesh,” and “soter,” which means “preserver”. The connection to Germany should come as no surprise, given that much innovation in wood preservation technology was achieved by German scientists in the 19th Century, for instance, Max Rueping (inventor of the Rueping Process) and Franz Moll.
Unlike the word, however, creosote is not man-made. It is not synthetic; rather, creosote is a 100% natural substance that is simply distilled out of coal during controlled heating processes that have been part of the industrial fabric of the modern world for well over a century. The first known use of the term Kreosot is in 1835, right around the time when industrial development was heating up across the West and wood preservation became more common. Only three years later, in 1838, the Bethell Process of pressurized wood-impregnation was invented.
Coal-Tar Creosote: Not To Be Confused With Desert Flora or Chimneys
There are two other common uses of the term “creosote,” which are distinct from the coal-tar creosote used to preserve wood infrastructure.
The first is the plant Larrea tridentata, which grows in the desert areas of the southwestern United States and northern Mexico. While the plant has nothing to do with coal-tar creosote, interestingly, it thrives in desert environments, just like wood infrastructure that has been treated with coal-tar creosote (for example, the ground-breaking Los Angeles and Salt Lake Railroad which cuts through the Mojave Desert).
The second is “chimney creosote,” which is simply the soot and tar that is left over from burning wood in a fireplace. Sometimes this creosote is referred to as “wood-tar creosote,” since it results from burning wood. When smoke does not quickly clear a fireplace and chimney, this substance can build up and cling to chimney walls. This type of creosote is composed of different compounds than the coal-tar creosote used to treat wood and is usually different in color. Wood-tar creosote is produced by the everyday burning of wood in residential fireplaces, where the byproducts of burning wood are released into the air—this is why it’s so important to have an effective chimney flu. In contrast, coal-tar creosote is not the result of an uncontrolled burning process inside a home. Rather, it is produced by a controlled manufacturing process in a regulated environment involving sealed ovens and distillation.
Coal and Coke’s Leftovers = Coal-Tar Creosote
Coal-tar creosote is an organic material that naturally occurs when coal is processed into coke. Coal, which comes in many types, is a sedimentary rock produced over millions of years through geological pressure and heat from the remains of plant life, or “ancient biomasses”. Although coal is a well-known fuel source, “coke coal,” or just “coke,” is its lesser-known, cleaner-burning cousin.
Coke’s largest source of demand has historically been from the steel manufacturing industry, which has since the late 1800s relied on coal as a fuel source for removing iron ore from rock. However, the non-carbon substances released by coal, for instance in the form of smoke, were undesirable for a number of reasons. To avoid this, a practice known as “coking” was developed whereby the non-carbon components of the coal are cooked off ahead of time, and then the resulting fuel— “coke”—was used in steel production. This burning of coal results in multiple byproducts, including the aforementioned coke (a glassy, mineralized-type of coal which is a purer form of coal higher in carbon content), gases that were widely used in the 19th century as “illuminating gas” for gas lights, and a liquid rich in organic substances called “coal-tar.” The coal-tar was then separated into several different “distillates,” one of which is the creosote used in wood preservation.
The coke production process broadened the application of coal, thereby preserving carbon-based natural resources and making existing steel production processes more efficient. Coal-tar creosote was already being produced in coke ovens—how fortuitous, then, that this naturally-occurring material turned out to be the most effective wood preservative in history.
Today, coal-tar distillates such as creosote are created using higher temperatures than previously used in coke ovens. This high-temperature process produces creosote that performs better, keeping wood preserved for decades longer than the creosote generated by coke ovens at the turn of the 20th century.
The Natural Protective Properties of Creosote and Other Coal-Tar Distillates
In the same way that organic substances in coal-tar helped preserve the ancient biomasses in the depths of the earth for thousands of years, coal-tar distillates have been recognized as having protective properties for over a century.
You may be surprised by coal-tar distillates’ variety of uses. For example, coal-tar distillates have been used as an ingredient in roofing tar, which protects homes, and small amounts of coal-tar have historically been used in skin products. Today, some shampoos designed to combat psoriasis and dandruff actually include .05% “coal tar” as an active ingredient. Coal-tar distillates are also used in everyday paints, dyes, and photographic materials.
To summarize, coal-tar creosote used in wood preservation is a mix of many organic, carbon-based chemicals which are derived from coal. Just like coal, coal-tar creosote is a natural resource created by the exertion of Earth’s heat and pressure on ancient plant biomasses over millions of years. To make use of its ancient protective properties, coal-tar creosote just needs to be distilled from coal.
This article originally appeared in the July/August 2023 issue of the Railway Tie Association’s Crossties magazine.