Struggling to get a nice round but thin

[short fuse]

Hi all, I am struggling to get the desired shape with a small piece of glass. I am firing a 40mm circle (3mm glass) and I want it to look like option 'A' when viewed from the edge. I also want the circle to turn into a perfect circle when viewed from above (so it gets rid of corners)

My results keep coming out like 'B' where the glass has pulled itself into the 6mm rule. It must be possible to get the 'A' finish as I have samples purchased from someone else so I think it may be a lower temperature for longer or something like that??? I have tried firing at 750, 760, 765 for 10 and 20 mins and they are all coming out like option B.

Is there anyone out there that knows a firing schedule to keep the majority of glass at 3mm but to get the edges to round off and curl up slightly like A 

[Zeldazog]

Have a look at the Tip Sheets and Technotes on the Bullseye website - there's some useful information there (although you didn't say what glass you're using, so remember their advice relates to their glass, and if you're using something else to adjust for that).  There's a very good one regarding what happens at different temperatures, with cross section views, but I can't recall which one it is off the top of my head.

As for getting it to form a perfect circle, you will probably find you need to start with a circle at that size and thickness.

[Pat from Canvey]

It may be that it's the cutting that's the problem and not the firing schedule. Try this, cut 2 perfect 40 mm circles then in one of them, cut out a smaller circle, say 30 mm to leave to leave a 10mm ring. Put the ring on top of the full 40mm circle and fuse. Don't worry if your 10 mm ring breaks in one or 2 places, just fuse anyway. Change the dimension of the top ring to suit what you want to achieve. Do you own a glass circle cutter? If not, you can still cut perfect circles using your usual glass cutter.

[Flyingcheesetoastie]

I've achieved this with 2mm float glass but a programme is going to be very dependant on the glass type and your kiln. It's also going to be a very subtle change so I would suggest getting in there at top temp to look and adjust the kiln accordingly.

Is it this raised edge that is essential or simply to fire polish and round the edges without an overall thinning of the width?