The fire itself does not burn what it catches on fire, it however, spreads and acts like conventional fire, aside from the mentioned above. What it "burns", is chakra. Prolonged exposure will burn even the chakra network, destroying it and subsequently causing massive damage to the target's body. For the fire to begin, the Kaname will offer some initial chakra as a toll. But, from there, if something with chakra is caught on fire, it needs no more chakra from him after this. While it burns chakra however, it causes a severe pain in the burned limb. The flame takes on an unusual white color, and can only be put out with if he decides to, or when he shuts off his dojutsu.