Reaction with carbon Usually maximum time we see then after cooking by gas there is no black spot on the pot. But continually burning can affect the pot surface and makes black powder. 

My question is: 


  What is the most acceptable reason that black powder(carbon) take places on the pot surface. Is it related to the burning temperature? or fuel reacts with the pot surface?
For combustion to occur you need 3 things at the same place: fuel, oxygen and heat. 

If the air intake is dirty or not big enough, you get a flame that has an orange color, and on a cooler surface deposits C before it has a chance to burn.

The reaction with the pot is only a physical one of heat transfer and maybe C deposition.