In general the Flare is an illuminating tool, that only appears in Thief II: The Metal Age. It can be purchased for 25 gold or found during missions. Flares can also be made in Soulforge Cathedral by following the Flare Blueprints.
When lit, It will emit light on its surrounding and the player making both more visible. This effect lasts for around 17 seconds after which it disappears in a puff of smoke. During this time it can be thrown and picked up again. The Flare is able to light torches and gas lamps if it's thrown close enough and can also be used to detonate Explosive Barrels. Because of these qualities it might be seen as a non-explosive Fire Arrow although more cheap and noiseless.
An unlit Flare behaves like a tool but when lit it behaves like a piece of junk.
A canister of Flare Mixture will be manufactured.