Kostas Tsoumas

Thank you for responding Fox, and yes reducing the width of the dense firebricks is a very good option.
I was actually checking that yesterday, and found some fireboards 1,4mm thick for pottery ovens. And on the outside I could put the ytong insulating bricks.
But there is also another consideration. The insulated fire breaks I have for the riser are quite low in alumina content and are prone to quick degradation due to the ash 's corrosive attack.. I found a very interesting discussion about this topic in donkey 32 Pro boards. So I am considering buying more insulated fire bricks, with high alumina content this time, and using these for the hottest parts of the core and leaving the previous ones for the cooler ones. They'll all have to be painted with a paint based on zirconia for protection. It is more expensive but mostly I'm not sure if even the higher alumina content ones, which are harder ,can withstand the impact with wood in the firebox.

Thanks for replying Thomas, and so quickly. heavy firebrick  is the option so far, but i suspect they are sucking a lot of heat at the first stage of the burn, and delay  good combustion in the riser. they weigh almost 80 kgrs (riser is made from insulating ones) . i would use these for the firebox too, but i fear abrasion due to knocking of the wood there, will damage them . So i thought i should protect their surface somehow.
i would prefer the system to be quick in reaching proper temperatures as it will be fired once a day probably, since i am in Greece.
Another option for the firebox i am considering these days is ytong bricks. they are rated for withstanding 1200C and fireproof, but haven't heard of anyone having used them. Thet are also very light, like the IFB's, they even look identical.

Hello to all. I am building a 180mm batchbox based on Peter's plans, with a floor channel. and was thinking of using metal for the firebox. And i just saw  Peter's answer for the temperature there reaching 850C. So, 3mm steel plate is going to survive this?