Fritz. Do yourself a favour. There is stuff on the market called mouldable ceramic fibre. Basically it is chopped ceramic fibre mixed into a sticky mois clay base (White). After finishing your furnace, coat it with this. It will need to air dry for about 2 days (when packed 1 1/2 cm thick. It becomes hard and insulates very well. this causes the furnace to heat and cool at a slower rate largely reducing cracking caused by temperature fluctuations.
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