"What makes a flashbulb flash? Well, flashbulbs have, placed between their terminals, a piece of tungsten or zirconium filament. This wire is covered with an explosive primer paste. When current is applied, the wire heats up, igniting the paste, which then ignites the tin, aluminum or (in later years) aluminum wire (or wool). An oxygen atmosphere would increase the brilliance of the flash. Some bulbs were also filled with nitrogen gas to actually slow or delay the burn"

The statement above is from the link below.
http://www.darklightimagery.net/flashbulbs.html