Deep slumps cannot be done in one slump. Usually, multiple slumps are required to get an even rim with even thickness along the sides.
Special three stage moulds have been developed for deep slumps. The set is expensive even if you have the shallow starting mould already.
When deep slumps are tried in a single stage, uneven sides, hang ups at the edge, needling at rim, and distortion of the image are common in addition to some thinning and significant distortion.
Do it yourself
This leads to investigating whether it is possible or reasonable to try do it yourself methods.
The DIY process involves using two moulds and filling the deep mould with powdered separator.
· First stage – slump the glass blank into a shallow shape first. The starting diameter of the blank will need to be about one third larger than that of the finished vessel. This can be determined by measuring the diameter of the deep mould and adding one third. This means that if your deep mould is 300mm, you will need a 400mm diameter starting disc and an equivalent size of mould. Fire this slump at your standard slumping schedule for large shallow pieces.
· Second stage – Add powdered kiln wash or whiting to the deep mould. Fill the mould to half or two thirds of the volume. Smooth a shallow depression in the powder. It should rise to meet the curve of the mould shoulder, even if it does not fully match it. This firing is probably the most critical in the DIY process. The shallow shape will be considerably larger than the diameter of the mould on which you are placing it. This means that you must fire slowly and you should peek frequently. As the glass begins to slump, the outer edge will begin to rise at first. As soon as the outer edge begins to relax, you must advance to the annealing segment. If you allow the rim to sag, it will not sit very well in the mould at the next stage.
· Third Stage – This may require more than one firing to achieve the intermediate shape. In preparation, remove about half of the powder from the previous firing. Shape the remaining powder to a smooth curve. Fire the glass, again watching and advancing to the anneal when the rim begins to flatten. If the glass has not touched the powder at the bottom, you will need to do another firing.
· Fourth stage – Remove all the powder from the deep mould. Place the glass and fire.
Keep the kiln wash powder for future use. Its composition will not have changed as you have not fired it to tack fusing temperatures. Dispose of the whiting, if you used it. It may be fine for further use, but since it is cheap, it is not worth the risk of it sticking to the glass in subsequent firings.
Remember that long – low and slow – slumps are required at all stages of creating a deep slump. As a comparison, think about the hours required for a free drop to form and still keep the glass at the shoulder thick. Deep vessels require long hours of watching just as aperture drops do.