Cracked while slumping

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    While slumping (first time) a 12″ bowl( 3 layers -clear base,mini cabs touching with frit in between topped with clear) I checked on the bowl’s progress at around 850, I noted that half the glass was beginning to slump and there was a crack where the other half wasn’t. I followed 3layer slumping schedule for the spectrum glass with the 100/hr ramp time. Ceramic mold new with several layers of kiln wash. Mold is elevated so it sits high in the kiln but i believe it isn’t less than 3in from the lid element.  Glass was dry.  Is it because I peaked?  How can I fix this? Can is re-fuse/slump?   Thanks!!


    The only reason something breaks while temperature is on the way up is thermal shock.  We have a good tutorial on thermal shock on this site here:

    If the piece was not properly annealed on the previous firing that could contribute.

    It is possible that peaking might have done it – but you would have likely heard the break if that was the case. From your description it sounds like it happened before you looked.

    Always fire as low in the kiln, and as far away from elements, as possible since that gives you the most even heating.

    Yes – you can refuse and reslump.

    Helios Kiln Glass Studio



    Thank you for your response.  I thought the 100/hr was low enough. Perhaps it was the annealing previously as one half slump first. It did not stick to the mold.  Do you firing with body elements would have been better?  I didn’t hea a crack just saw half slumping w/o the othe half!


    Thank you!

    Stephen Richard

    Occasionally, splits can occur on the bottom, while the top is still in one piece.  this usually happens when the rate of advance is just a little too fast.  On the piece that is shown in this blog –

    the clear capping piece allowed the darker glass to split while the top remained unbroken.  This kind of uneven heating of glass of different colours shows that the rate of advance needs to be pretty slow on these – much slower than normal.

    I looked up the S96 site and could not find a schedule for slumping 3 layers.  Slumping a 9mm piece should be possible at 55C/hr, but all the way up to about 630C with no soaks until then, so that everything is heating gradually and evenly.  With a 300 mm span you shluld not need much more than 20 mins for it to fall and conform to the mould, unless is it a highly detailed one.

    Stephen Richard

    blogs at: and

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