Cob Fireplace walls

[Rog]

Howdy folks,

Precurser:  Might I take this opportunity to say what a fantastic resource
this list is?  I've been following it for a few weeks having ploughed
through the early archives and you're an invaluable lot.

So anyway, I'm intending to get my cob cottage up and running over the
coming summer in readiness for moving in before winter (southern hemisphere
seasons, please).  I'm planning on building a slow-combustion heater/cooker
into my design, which will have its front, doors and cooktop made from cast
iron, with the remaining walls made from a cob mix minus the straw.
Well, at least that's the fantasy.  Does anyone have any tips on using
earth for fireplace walls?  What mix should I start experimenting with? 
Has anyone made anything similar?

Cheers, Rog.