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Wednesday, 12 August 2009

Coal tar before gas

Perhaps the substance, which most readily comes to mind when talking about gas industry by-products, is coal tar. The next few chapters will look at production of tar by the early industry and at what indications there are for the profitable use of it. It is not proposed to look at the more sophisticated chemical products made after the mid-century but at the simpler mixtures which preceded them.

Coal tar was known before the gas industry began to produce it. It had been well promoted and there is no reason to believe that it did not sell well within its market.

WHAT IS COAL TAR?

When newly made gas emerges from the retort it is full of impurities and needs to be cleaned. Normally the gas is channelled from the retort, through a condensation process and then into wash water. One result is the production of a 'tenacious bituminous fluid called tar'.

It is perhaps useful to look at a definition of tar from a dictionary of 1758:

TAR. A gross liquor issuing or extracted from various trees, exceeding useful upon many occasions, especially for the smearing of cordage and planks belonging to ships'2

What was known as 'tar' before 1800 was not a by‑product of coal. It was a wood based product traditionally from the Baltic - and sometimes called 'Stockholm' or 'Archangel' tar from its place of origin. Some industrialists knew that a similar substance could be made from coal in theory but they had not found it easy to manufacture. They did not want to make gas, but to produce tar for a market they knew existed. Supplies from the Baltic were under increasing pressure during the Napoleonic wars and it was important to find a supply of tar from a source that was not under threat. Coal, or 'mineral', tar seemed to be the answer.

One industrialist who had tried to make coal tar was John Champion from Bristol, whose father's experiments with gas manufacture in Pimlico have been described above. He had registered a patent for tar manufacture in 1799. James Watt's friend John Roebuck, the Birmingham vitriol manufacturer, had also tried. Other experiments had been sponsored by landed estate owners ‑ the Clerks of Penicuik and the Fitzwilliams at Elsecar.

Archibald Cochrane, Ninth Earl of Dundonald, was happy to point out that they had all failed.He had, to some extent, succeeded in coal tar manufacture.

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