There a numerous web pages and books on how to build a Tandoor. One is Piers Thompson's " The Tandoor Site". He uses a pre-made clay liner, 700 mm tall, 550 mm wide, tapering to 300 mm, apparently made from unfired clay reinforced with fiber. This is placed on a base made of concrete with firebricks on top and the outside insulated with vermiculite, surrounded with a brick wall.
That sounds too large and complex for my apartment balcony. Traditional tandoors, such as Thompson's, use charcoal (placed in the middle of the floor of the oven). Modern ones in restaurants may be gas powered, with a gas ring in the bottom.
As I have a small gas BBQ, I thought the easiest approach was to build the tandoor as a pot to sit on top of the gas burner. Holes in the bottom would let the gas in and a hole in the top would let it out (and allow the food to be added).
My BBQ is designed to hold a 300 mm griddle. So a reasonable size for the Tandoor would 250 mm in diameter (about the size of a dinner plate or frying pan) and 300 mm high (long enough for short skewers to be hung). This would be big enough to cook in but small enough that the Tandoor can be put away in a kitchen cupboard.
As this will be a portable unit, the use of vermiculite insulation and brick support is not feasible. However, there is a material called Paper clay (or fiberclay), made from clay with paper fiber added. Bill Chalmers suggests this for making Tandoors. He suggests covering the outside with kaowool or vermiculite. But the paper clay should provide some insulation (the paper creating voids in the clay).
Perhaps an extra layer of shredded paper soaked in slip (watery clay), will do as insulation. Paper clay is not normally made from more than 50% paper as the strength of the clay is reduced. But if covered with a thin layer of high strength clay, the paper should be protected.
The construction process would then be to:
- Build the inner lining of the oven, as for a hand built clay pot. Allow to dry.
- Cover the outside of the pot with shredded paper soaked in slip. Allow to dry.
- Cover the paper with a layer of clay. Allow to dry.
- Test the oven empty. This should be hot enough to sterilize the clay, without necessarily firing it (over 138 °C rather than 1000 °C).
- Use it.