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Tar

physical science Maturity 7-9 Next article →

Tar is a dark brown or black viscous liquid. A viscous liquid is a substance that is thick and flows slowly. Tar is made of hydrocarbons and free carbon. These are chemical building blocks found in many organic materials. People obtain tar through a process called destructive distillation. This process involves heating organic matter to break it down into different parts. Tar can come from many sources like wood, coal, peat, or petroleum.

Birkenpech
Birkenpech

The process of destructive distillation works by using heat to change a material. When wood is heated this way, tar and pitch drip away from the wood. This leaves behind a solid material called charcoal. If you use deciduous tree woods, the process produces methanol and charcoal instead. Methanol is also known as wood alcohol. For certain materials like corn stalks, you can use a microwave to perform a similar process. This specific method of heating is called pyrolysis.

Corn Stover Tar from Pyrolysis by Microwave Heating
Corn Stover Tar from Pyrolysis by Microwave Heating

There are different types of these thick substances. Scientists often use the terms "tar" and "pitch" to describe them. People sometimes use these words interchangeably, but they have subtle differences. Tar is generally more fluid or liquid. Its thickness depends on where it came from and the temperature. Pitch tends to be more solid or viscoelastic. A viscoelastic substance behaves like both a liquid and a solid. Natural asphalt is a type of pitch that some call mineral tar.

Birkenpech
Birkenpech

Making wood tar is an ancient craft with a long history. People in ancient Greece knew how to produce it. In Scandinavia, it has likely been made since the Iron Age. For many centuries, wood tar was a vital part of the economy in Northern Europe. Sweden was a major exporter of tar for hundreds of years. In 1615, Sweden exported 13,000 barrels of tar. By 1863, that number grew to 227,000 barrels. The Royal Navy of the United Kingdom was one of the largest users of this material.

Tervavene 1910
Tervavene 1910

To produce wood tar, people traditionally used tar kilns. These are special ovens used for dry distillation. In Scandinavia, kilns were built near forests using limestone or holes in the ground. The bottom of a kiln is sloped toward an outlet hole. This allows the liquid tar to pour out. First, workers split pine wood into small pieces. They stack the wood very densely inside the kiln. Then, they cover the wood tightly with earth and moss. This prevents oxygen from entering the kiln. If oxygen enters, the wood might catch fire and ruin the batch. A fire is then lit on top of the kiln. The tar begins to pour out after a few hours and can continue for days.

Tjärdal 092
Tjärdal 092

Tar has many important uses in different industries. For millennia, wood tar was used to waterproof sails and wooden ships. It helped protect the wood from rotting. Today, it is still used to seal traditional wooden boats and historic church roofs. It is also used to paint the exterior walls of log buildings. In Finland, wood tar has a place in medicine because it is microbicidal. This means it can kill microbes like bacteria. People use tar water as a scent in saunas or as a flavoring in candy and alcohol. It is also used in cosmetics and anti-dandruff shampoos.

Tervavene 1910
Tervavene 1910

Some forms of tar are much more dangerous than wood tar. Coal tar and petroleum tar are considered toxic and carcinogenic. A carcinogen is something that can cause cancer. This is because these tars have a high benzene content. Because of this, coal tar is listed as number 1999 on the United Nations list of dangerous goods. However, in very low concentrations, coal tar is used as a medicine for skin conditions like psoriasis. In history, tar was also used for public punishment. This was called "tarring and feathering." A person would have hot wood tar poured on their skin and then feathers were attached to it.

Macarony Boston NMAH2003-25005
Macarony Boston NMAH2003-25005

Finally, tar is connected to many different parts of our natural world. You can find naturally occurring pitch in places like the La Brea Tar Pits in Los Angeles. In Alberta, Canada, there are large areas called tar sands. These are made of asphalt found in the ground. Even the term "Rangoon tar" refers to a specific form of petroleum. Whether it is found in the earth or made in a kiln, tar remains a very important material in human history and science.

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🖼️ Images & Media (5)
File:Corn Stover Tar from Pyrolysis by Microwave Heating.jpg
Corn Stover Tar from Pyrolysis by...
File:Tjärdal 092.jpg
Tjärdal 092.jpg
File:Birkenpech.JPG
Birkenpech.JPG
File:Tervavene 1910.jpg
Tervavene 1910.jpg
File:Macarony Boston NMAH2003-25005.jpg
Macarony Boston NMAH2003-25005.jpg
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