A common problem in firing bottles is that they may roll into one another and stick, making both bottles useless.
One way to overcome this is to let the bottle find its heavy point by setting on smooth and level surface. It will gently roll to one direction before slowly coming back in the other. When it stops this oscillation, the heaviest part of the bottle will be on the bottom. Mark the bottle in some way so you can move to the kiln in that position. If after this, it rolls in the kiln, then your shelf is not level.
Additional assurance against rolling is putting a small piece of thin fibre paper (1 or 2 mm)at each side of the point the bottle touches the shelf. Thinfire and Papyros are not enough to ensure there will be no movement. But the small bumps of fibre paper are enough to stop the bottle from rolling.
Sometimes you want a particular part of the bottle up or down, but it won’t stay in place. Then you need to put a slightly thicker piece of fiber paper against the bottle on each side. It is better if it is not Thinfire or Papyros as they tend to disintegrate above 400C, long before the bottle begins to distort enough to keep it in place.
Other materials you can use to prevent the bottle from rolling are crumbled chalk, whiting, kiln wash, or even a few grains of sand.
Preventing bottles from rolling in the kiln is about finding the natural heavy spot, or propping the bottle in place with a variety of heat resistant materials.