Production made bottles are not evenly thick all the way around. So they roll until the heaviest part is at the bottom.
One way to avoid any rolling in the kiln is to put the bottle on a level surface and let it roll to the heavy side. Mark the up side so that you can place it in the kiln with the heavy side down.
Some times though the seam lines – which will show in the final piece – are in the wrong place. To keep the seam lines to the side, you can place a small piece of 0.5 mm fibre paper against each side of the bottle at the base if you are working on a hard surface. On a softer surface, the weight of the bottle or a gentle push will sink it into the surface enough to avoid rolling.
The distance to place bottles apart is important to know so they do not become attached when several are slumped together.
You can slump test bottles of various diameters to determine their final width.
However, if you wish to calculate the distances, The width of slumped bottles is approximately 1.6 times the diameter of the original bottle. If you want the base of the bottle to be flat too, the final size will be wider, dependent on the amount of glass in the base.
You also need to leave some additional space than this calculated distance between bottles, as the final size is narrower than the size when hot. You should leave at least an additional 10 mm to each side of the bottle.
So a 50 mm diameter bottle will become 80mm wide. You need an additional 10 mm each side, so the spacing from the centre of one bottle to the next should be at least 100 mm.
The surface that you are slumping onto is important too.
You can place the bottles on a sand base that has been dusted with kiln wash powder. This has the advantage of allowing a gentle push into the sand to prevent rolling. But it imparts a grainy texture to the back.
You can get a smoother surface by using whiting or sifted plaster that has been screeded smooth. This also allows the bottle to be gently pressed into the surface.
You can use fibre papers as separators from the shelf, but they are relatively expensive.
Kiln wash works very well. It can provide a very smooth surface, but if you want more texture, you can sprinkle some kiln wash through a sieve over the shelf.