Pyroclastic flow lightning.

The grey dust is heat log debris from wood, used to light the fire in my shed. The very fine brown dust is peat, used to maintain a constant gentle heat once the metal of the fire is up to temperature. I use a "Henry" hoover to remove the dust. Simple and efficient.
While hoovering it is wise not to touch the metal of the fire and also keep one's hand on the plastic of the nozzle. 
If you have been amazed at the vivid lightning displays shown on news channels during a volcanic eruption then here is the reason why it occurs. The dust from my fire is caused to rub against other pieces of dust very rapidly during it's "soook" into the vacuum tube. Static electricity is created and there are audible clicks and flashes of blue light, just as in it's big brother, the volcano. Lightning is created in a Pyroclastic flow by the millions of tons of pumice and other rocks banging against each other.