Rounded edges on slumped pieces

Stephen Richard

Sounds to me like you are slumping a single layer.  The rest of my comments are based on this assumption.

You need to know the effects of heat at various levels.  A good source for this is the Bullseye website, education section.  Lots of information there.  You could also visit my blog for my views. Rounding of the edges occurs at tack fusing temperatures (all your temps are in F, and I only think in C, so I won’t give any temps).  This is above slumping temperatures.  You can take the glass up to tack fuse in the mould, but two things happen.  You get a lot more mould marks on the bottom of the glass and secondly, you reduce the life of your mould.  Oh yes, a third thing – you need to put kiln wash on the mould again once you have gotten up into the tack fusing temperatures before you use the mould again – kiln wash sticks on the next firing once it has been to relatively high temperatures.

So your comment is correct – tack fuse first, then slump.   I think it is cold working the edges on this one, or have it as a learning exercise.


Now for the schedule!  I presume this was programmed into the controller.  It really does help to know why you are programming the kiln to do certain things.

400 to 750 hold 10; a soak at 400C is curious.  Some say it is to allow the glass to heat evenly through – in which case slow the rate of advance.

400 to 1000 hold 20; A soak at 540C is near the higher anneal point of Bullseye, but isn’t really any significant temperature, and why soak for 20 minutes there?  See my comments above.

400 to 1150 hold 15; Again a soak at 621C does not make any sense to me.

600 to 1240 hold 20; Ahh.  At last you are at a temperature that can do something.  670C is a good temperature for a slump that is narrow, of small diameter, or with light glass.  You can do a simple curve slump at 630C.

Full to 1000 hold 60; And why is there a soak at 540C?  This is not a good soaking point for Bullseye or S96 (not bad for float, though).  A 1 hour soak for 3mm glass is excessive.  It won’t do any harm, but uses electricity.  And the soak at 540C is not really useful for annealing.  Some say it is to allow the glass to equalise in temperature. Again see my comments above on that.  Secondly the soak at annealing soak temperature does the euqualisation thing.

90 to 960 hold 60; Yes, here we are at the annealing soak (516C).  this is where the glass equalises in temperature throughout its thickness.  So the length of the soak is related to the thickness of the glass.  By the way, you can go as fast as possible from the top temperature down to the annealing soak – no need for that slow drop of 50C/hr.  Also 30 minutes is adequate for 6mm glass to equalise in temperature, so would also be enough for a single layer.

120 to 750 hold 10; The 110C (200F) degrees below the anneal soak is where the annealing is done.  67C/hr seems to be much slower than needed.  80C/hr (140F) is what is recommended for 6mm glass, so would be adequate for the single layer.  Note that this means that your 120F/hr (or any other rate you choose) needs to go to 370C  (700F).

400 to 100.  I approve that you have controlled the temperature down to a reasonable temperature, even though not to room temperature  :-)  220C/hr is a reasonable rate to set the programmer to cool.  Your kiln will not of course cool that quickly at the lower end of the range, but that does not matter as no electricity will be used.


Apologies for the hectoring tone of the above, but now is the time to learn what the temperatures are doing to the glass and why you are putting the various rates and soaks into the programme.  There are some tutorials on this site to help too.  If you are using pre-set programmes, now is the time to dump them.  What you do need to do though, is get a manuscript book or other means of keeping records of what you did on each firing, and what the results were.  Again Bullseye has a useful form on their site.


Best wishes for an exciting time with kiln formed glass!


Stephen Richard

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