Lighthouses of Australia Inc. Bulletin

It could be said without too much exaggeration that kerosene caused a revolution in lighthouse technology. Before it became widely available, all sorts of illuminants were tried out. Whale oil predominated in the early 19th century, and when it became too expensive to procure, colza oil (otherwise called rapeseed oil) became most prevalent.

Even before 1860, it was possible to produce kerosene from shale oil or tar found on the Earth's surface in Europe, but its commercial viability came only after the discovery and subsequent exploration of underground deposits in Pennsylvania in the USA by E.I. Drake.

Kerosene accounts for between 10-25% of the total volume of crude petroleum, depending on its provenience. Kerosene, also called paraffin oil, is a mixture of hydrocarbons with a chain of 10 to 15 carbon atoms in the molecule, its composition again depends on its source.

Small spirit heater (primer) sits on the side of the mantle, its wicks covered by a protective cap.
It's position can be interchanged with that of the vapourised kerosene burner. The primer, when put in position, heats the inverted U kerosene feeder pipe above. To hold the heat longer, a copper retaining cap (not shown) would be fastened above the assembly. When the pipe was hot enough, the small amount of kerosene could be introduced through the fuel inlet, vapourised and lit at the burner. After that, and in quick succession, the mantle had to be put back on, the spirit burner removed and the kerosene input increased to a higher level. The kerosene then ignited, heated the mantle, and after it was put back in its proper place, also continued to heat and vapourise the incoming fuel. The heat retaining cap, no longer needed, would then be removed, and the light could keep on burning. The process required skill, and if not performed properly, the kerosene wouldn't vapourise until it reached the burner, where it would do so, uncontrollably, with an explosion.

Drawing: Thomas Tag

Kerosene is separated from the other hydrocarbons in crude oil by fractional distillation. The crude is heated in a retort to a high temperature at which most of the ingredients vapourise. The vapour then rises through a vertical column filled with glass beads where it repeatedly condenses and re-evaporates at descending temperatures. Eventually, the most volatile parts (petrol fraction), which boil at the lowest temperatures, rise to the highest and coolest part of the column. The kerosene fraction, which boils at temperatures from 140-230°C, is collected from the middle sections and the heaviest oil fraction, which boils and condenses at the highest temperature, is collected in the lowest part of the cooling tower.

Because the chemical composition of petroleum favours the heavy fractions which have limited use, oils are further processed by 'cracking'. Heated to a high temperature and exposed to a catalyst, the long carbon chains are split by the process, turning heavy molecules into lighter ones. Kerosene or petrol fractions are mass produced this way.

When refined, kerosene is a light, colourless or pale yellow liquid with a characteristic, not unpleasant smell. When discovered, it quickly replaced oil, not only in lighthouses, but in domestic lamps, and became the main product of the petroleum industry.

Because it was so light, kerosene burned with a brighter and cleaner flame, even when used on old-fashioned wick burners, but the true improvement came after the introduction of the incandescent mantle and pressurised vapour burners.

The mantle is a bag-shaped piece of loosely woven fabric that has been soaked in nitrates of radioactive thorium and cerium. The fabric burns off when it is first lit, and the residual fine net, now composed of only thorium and cerium oxides, is ready to withstand the high temperatures of the flame.

The mantle could be used with kerosene and was also successfully used with gas as a fuel. The burning flame would heat the mantle to a temperature of nearly 2000°C, making it glow white hot. The resulting light was not only brighter but also white instead of the usual yellow colour. Because the fuel burned at a higher temperature and with more oxygen available, it produced less smoke and fumes. There was no longer a need for a glass chimney, which was prone to sooting and cracking.

The process was improved even further when the fuel was pre-heated and vapourised before being introduced into the burner under low pressure. The drawing below shows how such a lamp functioned.

Pressurised kerosene vapour burners were a huge improvement and a large mantle combined with a Fresnel lens could put out as much as 1,000,000 candelas of light. The glowing mantle was a much more uniform and compact source of light than multiple large wicks, and that meant the lenses could better concentrate the light beam and send it further out. The fact that the pressure had to be maintained and the cylinders regularly pumped up by the keeper were the only disadvantage.

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