It looks like it typically had a current passed though it to ignite zirconium threads.
What properties of rhenium made it the best metal to use for this?
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$\begingroup$ Well, I think this is a dumb question, but what is a "flashbulb"? Are you asking about the photography flashbulb one? But, then that WP page doesn't mention "rhenium" anywhere... $\endgroup$– Gaurang TandonMar 16, 2018 at 7:14
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3$\begingroup$ @GaurangTandon For the youngsters out there: there was an era when photographs were taken using photographic film. For much of this era electronic means of producing bright light (xenon electronic flash devices and white LEDs) were rare. So people used chemical means of creating flashes of bright light (originally magnesium powder, later a variety of enclosed bulbs containing things like zirconium wire the could be ignited electrically or mechanically). Some of these clearly used rhenium igniters. ;-) $\endgroup$– matt_blackMar 16, 2018 at 10:49
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1$\begingroup$ Given the price of rhenium, it is quite likely the igniter is an alloy. However, Wikipedia does suggest that rhenium filaments for, e.g. mass spectrometers, are more stable then alternatives in air/oxygen atmospheres. Combined with the refractory nature, one could imagine that getting the filament really hot really fast in air (so the zirconium threads burn rapidly) is pretty stressing on the filament, and rhenium would be a good choice. $\endgroup$– Jon CusterMar 16, 2018 at 13:06
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$\begingroup$ A friend suggested that perhaps rhenium was chosen for its spectral output when burned. These bulbs were around at the beginning of color photography, so good color rendering index would have been desirable. I will need to test the spectrum of these compared to, say, burning iron or aluminum! $\endgroup$– bigjoshApr 2, 2018 at 15:23
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$\begingroup$ Rhenium is also an extremely rare and expensive metal. This type of flashbulb was single use, disposable; and professional photographers used them by the dozens. So any rhenium content would have been extremely small. Something like a plating on another element. Now, most lightbulbs are either evacuated, or filled by an inert gas. Flashbulbs are instead filled with oxygen, to get the brightest possible burn from their fuel. Now here I speculate: prolonged storage in an oxygen atmosphere would likely be corrosive, leading to failures. The rhenium may be an anticorrosion plating for reliability. $\endgroup$– SecurigerAug 30, 2018 at 12:34
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1 Answer
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New tungsten alloys containing rhenium were developed as a filament material in flashbulbs for the purpose of improving their reliability. Weak batteries (anything but rare in the carbon-zinc era) could easily result is a slightly-delayed flash or no flash at all, spoiling the photo in either case.
An important characteristic of batteries is their internal resistance. This impedance is small in a fresh dry-cell battery but increases as the battery wears out, eventually reaching a value high enough to make the battery unusable. The rhenium acts to "increase the cold resistance of tungsten wire filament so as to permit sufficient power to be obtained over large variations in impedance."
As the prior post suggested, anti-oxidation properties of the rhenium also helped greatly reduce the chance of a filament burn-out and consequent flash lamp ignition failure.
Because photography wasn't cheap back in the 50's and 60's, the vast majority of cameras and flash units sat in closets at least nine-tenths of the time. Even unused, their carbon-zinc batteries could go bad in three years. Considering the many millions of flash pictures attempted with nearly-dead batteries in that era, this reliability feature looks like an excellent idea.
For details, please see United States Patent No. 3,123,993 granted 3/10/1964 to George W. Cressman, Chagrin Falls, and Louis A. Demchock, Jr., Cleveland, Ohio, assignors to General Electric Company.
