Creosote - Creosote Oils

Creosote Oils

For some part of their history, wood-tar creosote and coal-tar creosote were suggested to be the same substance — only in different purities — accounting for their common name; the two were determined only later to be chemically different substances. All types of creosote are composed of phenol derivatives and share some quantity of simple phenols, but these are not the only active element of any creosote. For its useful effect, wood-tar creosote relies on the presence of methyl ethers of phenol, and coal-tar creosote on the presence of naphthalenes and anthracenes; otherwise either would dissolve in water.

Creosote was first discovered in its wood-tar form in 1832 by Carl Reichenbach, when he found it both in the tar and in pyroligneous acids obtained by a dry distillation of beechwood. Because pyroligneous acid was known as an anti-septic and meat preservative, Reichenbach did experiments with dipping meat in a diluted solution of distilled creosote. He found that the meat was dried without undergoing putrefaction and had a delicious, smoky flavor. This led him to reason that creosote was the antiseptic principle contained in smoke, and he further argued that the creosote he had found in wood tar was also in coal tar, animal tar, and amber tar in the same abundance as in wood tar.

Soon after, in 1834, Friedrich Ferdinand Runge discovered carbolic acid in coal-tar, and Auguste Laurence obtained it from phenylhydrate, which was soon determined to be the same compound. There was no clear view on the relationship between carbolic acid and creosote; Runge described it as having similar caustic and antiseptic properties, but noted that it was different, in that it was an acid and formed salts. Nonetheless, Reichenbach argued that creosote was also the active element, as it was in pyroligneous acid. Despite evidence to the contrary, his view held sway with most chemists, and it became commonly accepted wisdom that creosote, carbolic acid, and phenylhydrate were identical substances, with different degrees of purity.

Carbolic acid was soon commonly sold under the name "creosote", and the rarity of wood-tar creosote in some places led chemists to believe that it was the same substance as described by Reichenbach. In the 1840s, Eugen Freiherr von Gorup-Besanez after realizing that two samples of substances labeled as creosote were different, started a series of investigations to determine the chemical nature of carbolic acid, leading to a conclusion that it more resembled chlorinated quinones and must have been a different, entirely unrelated substance. Independently, there were investigations into the chemical nature of creosote. A study by F.K. Völkel revealed that the smell of purified creosote resembled that of guaiacol, and later studies by Heinrich Hlasiwetz identified a substance common to guaiacum and creosote that he called creosol and determined that creosote contained a mixture of creosol and guaiacol. Later investigations by Gorup-Besanez, A.E. Hoffmann and Siegfried Marasse showed that wood-tar creosote also contained phenols, giving it a feature in common with coal-tar creosote.

Historically, coal-tar creosote has been distinguished from what was thought of as creosote proper — the original substance of Reichenbach's discovery — and referred to specifically as "creosote oil". But because creosote from coal-tar and wood-tar are obtained from a similar process and have some common uses, they have also been placed in the same class of substances, with the terms "creosote" or "creosote oil" referring to either product.