Hi jb
Im sure its the the firelighters that cause the soot. I tend to use the instant light bags now instead.
I use one to get the thing going. I light the bag, drop it in and leave it alone for 5 - 10 minutes. It pays to stir the charcoal up at this stage to make sure all the paper has burnt. I then open the bottom vent fully, and load some restaurant charcoal (a must have) into the tandoor. I then put the lid on about three quarters, and rotate the tandoor so the vent is pointing into the prevailing breeze - if there is one of course. Usually I leave the tandoor well alone for at least an hour. Try it and see what you think!