I got a friendly email from someone also thinking of building an oil drum tandoor. Others might be interested in my responses to his questions

Q1. Sorry to be a pain, but I am going to make a Tandoor, and have read your Oildrum Blog with enthusiasm. I am assuming that you got your clay from Bath Pottery Supplies, but can you tell me which clay you decided on & why ?

I used raku clay. I was advised that it has excellent thermal shock characteristics and hardens at a moderately low temperature.

Q2. I felt that your liner should have been thicker (3 cm at least) but did it crack further ?

The tandoor has been extremely stable with no further signs of cracking. I
think the thickness of the clay was ok. If it is too thick the tandoor will take
longer to heat up

Q3. Hindsight is a wonderful thing, what changes, if any would you make if starting again ?

• The joints between the clay panels are crucial. This is where
  most of the cracking came from. It is worth spending a bit of time
  practicing making high quality joins
• I wouldn't put so much concrete in the base - it made the whole unit very
  heavy
• Get a decent lid for it. With a lid it heats up much quicker and gives
  better control of the heat

Good luck!