Q. I have a question though before I`ll buy the materials tomorrow.. Can I also use fine sand as an insulation material? Vermiculiete is very hard to come around here (that is the Netherlands)
A. Hi Thomas, Thanks for the feedback and great question.
I haven't built one of these oven's using sand as insulation, so can't say for sure, but I will share with you my thoughts.
When I built my first pizza oven I used sand as part of the insulation as I had read it was a good insulator. I did give some insulation, and also kept the oven very hot, but a lot of heat was transferred to the outside. So much so that you could not touch the sides. Note a pizza oven is different to the tandoor oven, as you are typically have the pizza oven fired up for 4 or more hours.
Now coming back to your scenario, if you used sand in the tandoor for insulation, I would expect your inner oven to be hotter than mine, but I would expect the outside to be hotter. As to how hot I don't know. Currently I can pickup my oven when it has been alight for some time, I don't think you will be able to that if sand was the insulator.
I would be interested to see how you get on. I am now thinking about doing this myself and testing it out.
So have you tried sand for this build and what was your experience?