Q: Recently, while hard soldering with a butane torch, I noticed an old lighter flint on a firebrick and began to play the torch on it. The flint became red hot, but didn鈥檛 melt. When I removed the flame, however, the temperature of the flint increased until it glowed bright orange for several seconds before cooling to dull red. Then, to my surprise, it slowly began to get hotter again until it glowed bright red. It increased in size and changed in texture to resemble crumbly black coke. What was going on?
A: Lighter flints are an alloy of iron and a lanthanide element, mainly cerium. These are extremely reactive metals and it is the exothermic oxidation of small, friction-heated particles ejected by the lighter鈥檚 grinding wheel which produces sparks.
The butane gas flame will exclude oxygen as it heats the flint but when removed oxidation and heat evolution follow rapidly. An oxide layer builds up and slows the ingress of oxygen, allowing some cooling, but the oxide coating then cracks and reaction and heating start again until the metals are consumed and only the mixed oxide cinder remains.
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A: Lighter flints are made from an alloy, usually between 60 and 80 per cent cerium mixed metal, 15 and 35 per cent iron and up to 5 per cent other metals, mainly magnesium. Its pyrophoric property was discovered in 1903 by Auer von Welsbach. The flint metal has a low ignition temperature of between 150 and 200 掳C and a high combustion energy density which makes it suitable for lighters. Cerium alone is too soft, so iron is used to harden it.
A butane torch is not hot enough to melt the metal and the flame also protects it from oxygen. However, as soon as the flame is removed, the cerium reacts with oxygen, releasing a lot of heat and forming almost colourless cerium oxide, which may even evaporate. The remaining iron, now porous, starts to react with the oxygen to form a special oxide Fe3O4 which is black, crumbly and of lower density than the metal, hence the volume increase.
The delay between the first and second glow phase is quite interesting. I suspect it is because the oxygen needs a bit of time to diffuse into the remaining iron. It would make a nice research project.