[Shadec (OP)]

Joel Hillman  asked the Naked Scientists:
   
Dear Naked Scientists...

I've been a little bit confused about something lately, in my house, we have an enclosed fireplace, which burns very hot.

I had some charcoal for sketching, and being the empiricist that I am I put some in the hottest parts of the fire, to see what would happen.

To my surprise, instead of combusting as I had expected it to, it turned an ashy white, if you'll excuse the pun. 

It has a similar weight, texture and size, but it turned pure white. I snapped the block in half and it was white inside as well as out, so it wasn't just transfer of ash.

My first thought was some sort of allotrope or something, but I don't think carbon has any like this, so I don't think that's it... I was wondering what you think?

Why did this charcoal turn white?!

Thanks

Joel 

What do you think?

[lightarrow]

Maybe it was not really carbon, or it was just in little percentage.

[SeanB]

Your sketching charcoal is a mixture of finely ground charcoal blended with a fine clay, to enable it to draw a consistent line and enabling it to be extruded into a rod for ease of handling. If you burn out all the carbon in a fire all that is left is the clay binder, which would be white to light yellow.

[Shadec (OP)]

Fantastic, thanks. Makes sense.
I guess I was naive enough to think that the charcoal was actual charcoal!

[CliffordK]

Perhaps a bit unrelated, but I think I've tried burning the electrode from alkaline batteries which appear to be primarily carbon, and I think they did absolutely nothing, remaining solid, and black colored.

[SeanB]

The graphite rod inside a primary non alkaline cell is pure graphite, but is so dense, and has been treated in a kiln to remove all other than graphite, that it will not burn unless you heat it up in an oxidising acetylene flame, or burn it in pure oxygen, where it will burn with a white light. In an alkaline cell the central rod is a graphite coated iron rod, more than a graphite one, as these cells can deliver a much higher current. It will not burn until well after it melts into a puddle.