A number of useful moulds for slumping do not have flat bottoms. There are a number of possibilities to have the bowl sit firmly without grinding the bottom flat. Remember that you do not need to surround the whole bottom to give the bowl stability.
Some of these include things like:
• A rubber “O” ring, although they usually come in black only.
• Thin slices of wide-diameter tubing.
• Wok support rings.
• Plastic tubing with a small joining dowel allows you to make any size. You can then paint it with the appropriate colour.
• Macramé, embroidery and curtain rings can be suitable.
• You can make them using hole saws. Cut out the big ring first so you can use the pilot hole to line up the smaller hole. Then bevel the inside to fit the bowl.
• Use three bumpons on the bottom. Be sure that the bottom of the bowl is perfectly clean, dry and free from oils. Then use some weight pressing on the bumpons for a day or more so that they stick permanently. You can do this by turning the bowl upright and fill it with some heavy objects.