'Structural' cob and 'building' cob

Hi all,

Working on the principle that the only stupid questions are the ones you didn't ask..

My understanding is that no fibre is added to the clay/sand/Rock mix used for the bench or Mass because it insulates/ will burn out if subjected to high temp.

I understand that fibre is added to the base mix of clay/sand for the top 2" layer of the mass for structural strength.

Q1: What I am not sure about, and have found conflicting images for, is whether I should be adding fibre into to the clay/ sand mix that I use to build up the mass around the firebox, burn chamber and riser (barrel area) or whether I should just add it to the 2" finishing/plaster layer?

Q2: Am I right in thinking that the mass around the burn area should be insulative to concentrate the heat into the chamber?

Q3: I am finding that the clay/ sand mix I'm using so far, (in burn area) is developing cracks, even before I fire it up. Is this normal, or is my mix wrong?

Q4: I've worked with clay and know that it's hard to stick wet clay to dryer clay and not have it crack along the join later (it helps if the surface is scored, kept damp and a slip applied) have others found it's best to complete the brickwork first and then build up the clay mass in one go?

Thanks in advance for any insights!

Lesley