“I just finished slumping a dish and I got a big lump in the center of the bottom. This is not an air bubble, just a lump. What should I do to avoid this again?”
Several suggestions are possible.
Ensure there are holes at the bottom of the mould that allow air to get out into the kiln. Prop the mould up on stilts if the hole does not go directly out of the mould. Alternatively, drill a hole in the side to allow the air to escape from under the mould.
Firing for too long or at too high a temperature will cause the glass to continue sliding down. Having nowhere else to go, the weight causes the bottom to begin rising. This is a consistent experience across several kilns and with multiple users.
So keep the temperature down to the minimum required. To find that out, watch the slumping in stages (do not stare!). Look at the piece for a second or two every five minutes after you reach your desired temp.
If it already has slumped adequately, you are firing too high. Reduce your temperature in subsequent firings and watch to find what the required temp and time is. There is absolutely no substitute in slumping but to watch and learn what your mould and glass require.
If you are slumping at such a temperature to seal the glass to the mould, you are firing too hot anyway. Or put more positively, use a low temperature slump, that is, a slump at the lowest temperature to achieve the desired result over an extended period of your choice.
A low temperature slump will allow the glass to conform to the shape of the mould without softening so much that it takes up all the markings of the mould. Therefore, there are spaces for the air to escape from under the glass all the way to the top as well as through the air holes at the bottom. It also gives the most mark-free slump possible for your shape.