|One of Karl Harron's deep slumped bowls|
Deep slumps require multiple stages to get even drops without thinning the sides. There are several makers of staged slumping moulds which allow progressively deeper slumps in a series of firings into deeper moulds.
If you have a steep-sided mould, you will find slumping directly into the shape difficult. There will be uneven slumps, thinning of sides, hang-ups, etc., among your attempts to achieve the slump in one firing. It is possible to mimic this series of moulds without buying the whole set.
To avoid these difficulties, you can build up the inside bottom of the mould by placing powdered kiln wash in the bottom and smoothing it to a gentle curve. You should aim for a gentle shape as in a ball mould.
After the first firing, remove some of the powder, placing it in a clean container. Shape the remaining powder into a deeper slump than the first one.
It takes some time and practice to achieve a smooth even curve. You can ease the shaping process by cutting the intermediate shapes from stiff card. This can be rotated to achieve an even curve in the powder. Remove any excess powder and do a final rotation to give the powder a final smoothing. Place the glass back on the mould and fire.
It may be that you will need to repeat this several times to get the full slump. Separate template curves need to be cut for each slump if you are doing more than one intermediate slump. It does depend on the steepness of the mould sides and the depth of the slump as to how many stages are required. Sometimes the slump can be achieved in only two stages.
After firing the powder, pour it back into your kiln wash container, as it still is good for mixing to apply to shelves, moulds etc.
This method is useful for any mould that is too deep for achieving the slump in one firing, and without buying intermediate moulds. Remember the final result will be smaller than the size of the deep mould, as the span of the glass becomes less with each deeper slump.