Glass breaking during slumping.

[CrazyKiln]

 :'(Yesterday I slumped a round piece of glass (28cm) into a bowl mould (28cm x 7cm) and was very upset when I opened the kiln to discover the glass had broken.

The piece was x 2 layers of 3mm tekta with glasslines (bubble and normal), powders, coarse and fine frits. Much of the glass was just the tekta. It appeared fine after the fuse firing.
The programme I used for slumping was:
Seg 1 167 - 520 hold 25 mins
Seg 2 330 - 650 hold 40 mins
Seg 3 9999 - 482 hold 60 mins
Seg 4 56 - 371 End.

The glass was still a little warm from the fuse firing when I put it into slump. Could this be the reason why it broke, even though I have done that before with other pieces and with no problems? This was the first time I had used this particular mould. Finally, I am presuming the break happened during the initial heating phase as the edges are smooth.
Any suggestions on what went wrong much appreciated.

[ajda]

I assume you annealed as part of the initial fuse firing? If not, you'd be starting the slump firing with an unstable piece that would be more likely to crack as it heated up. Also I assume all powders/frits were also Bullseye, so fully compatible with the tekta? I don't know about glasslines - haven't tried it myself...

[CrazyKiln]

Yes,  piece was annealed properly and all frits and glasslines by Bullseye.

[flame n fuse]

where were the breaks? a photo would be useful

[Glyn Burton]

Did it crack on the way up or the way down? If its a cooling crack it will be sharp on the edges

[CrazyKiln]

On the way up, as the edges are smooth but there is one sharp jagged shard - very tiny but a bit confusing.

[flame n fuse]

Your programme is a bit different to ours, but not drastically so. You say that it was a new mould. Did you use thinfire or bat wash? Is there any possibility that the glass stuck somewhere and then cracked because part of it couldn't move? Were there a lot of bubbles in it after the first fuse?

[CrazyKiln]

There were no significant bubbles after the fuse firing and I used batt wash . I usually do 5 coats, applying it in different directions - maybe I missed a bit. It is the first time I have slumped into a deepish mould, so maybe I need to slow the initial heating stage down. Thanks for your thoughts.

[CrazyKiln]

Hope this shows the photo I put on FB.



Edited by Dawn to allow linkk to image to work

[flame n fuse]

yes 7cm is quite deep, good point.   Have you looked at this link? http://www.bullseyeglass.com/images/stories/bullseye/PDF/other_technical/mold_tips_cone_bowls.pdf

also is there a possibility that part of the rim of the glass got caught over the outside edge during the slump?

[flame n fuse]

Hi, just showed your FB photo to my husband who does a lot more fusing than I do. He suggests that you might be getting uneven heating with the deep mould, and recommends that you slow down the heating on the way up. This is especially likely to happen if you are using a kiln with elements in the roof, and if the shelf is quite close to them.

[CrazyKiln]

Thank you so much. My kiln does have elements in the lid and I am aware it has a cold spot too. I also used slightly higher stilts to sit the mould on than I usually would (laziness - I couldn't be bothered to go and get the shorter ones). I have a ceramic kiln too, with the elements in the sides. I used to do all my glass work in this before I bought my glass kiln. would I be better off firing this mould in my ceramic kiln?

[Jane C ♫]

Deep moulds need a much slower ramp-up, not only because of the depth of the mould but also because the glass is much closer to the element.

[flame n fuse]

I'd try a slower ramp first before giving up.

[CrazyKiln]

Thank you all for your advise. I am slumping again today, so fingers crossed.