Answer:

 

Carbon monoxide produced of the oxidation of the carbon of the metal spreads across the porous zones of the silicon lining where it dissociates itself depositing coal.

 

2C metal+O2 (air)->2CO (gas) ->C+O2

 

What can happen?

 

It does not have effect on the refractoriness of the mass. However, in old or poorly conditioned coils it can cause arches between turns.

 

What to do?

 

1- Avoid overheating.

2- Do not put the graphite into the empty furnace

3- Increase the density of the area by improving material quality and / or vibration