Need help with Creative Ceramics medium rectangle mold

[Deive Butvila]

My slump resulted in a raised hump and am not sure if it is the beginning of large bubble or if the glass just did not slump long enough. I need help understanding how I can best try to fix this piece and also avoid the problem for future projects.

I am using a JenKen fiber square kiln with top heating elements.
O

Under side

Topside
Top side

The mold is approximately 11.5 x 9.25 x .6.

The glass I am using: Uroboros radium clear over Bullseye elephant grey and Bullseye black opalescent with silver irid. The original fusing of the piece was very nice.

I used the following firing schedule:

330 1100 10
100 1250 50
Fast 960 60
150 750

The suggested firing schedule at Creative Ceramics shows 1330 as the slumping temp, but thought that was too high. Also, I now see Bullseye has a suggested 1225 slump temp for this mold, so think the temp I used was also too high.

Please advise on how to fix my piece and how best to slump in the future.

Thank you

The top side shows the raised hump (it looks like there's a bubble forming on the right side of the hump, but that is just photo glare).

The underside shows a bit of a ragged edge forming around the hump.

[DonMcClennen]

looks like your mold did not have a vent hole in it!!

[Valerie Adams]

Have to agree with Don. Without seeing your mold, it appears that as the glass slumped, it trapped air which caused the bubble.

[Deive Butvila]

looks like your mold did not have a vent hole in it!!

There are 2 unblocked vent holes on the base of the mold in opposite corners. Should I risk drilling a few more holes? I would hate damaging this new mold.

[Deive Butvila]

Have to agree with Don. Without seeing your mold, it appears that as the glass slumped, it trapped air which caused the bubble.

Is there a way to determine that this is a bubble rather than glass that did not slump?

[Vonon]

Did you raise the mold slightly off the kiln shelf? Air leaving through the vent holes may have been trapped under the mold. When slumping I remove the shelf which is propped on one inch kiln posts and use those posts to support the mold. This assures adequate air flow under and around the mold plus it gives a little more distance between the glass and the lid elements. This setup has worked well for me.

[Deive Butvila]

Did you raise the mold slightly off the kiln shelf? Air leaving through the vent holes may have been trapped under the mold. When slumping I remove the shelf which is propped on one inch kiln posts and use those posts to support the mold. This assures adequate air flow under and around the mold plus it gives a little more distance between the glass and the lid elements. This setup has worked well for me.

I used 1/2 inch kiln posts, perhaps I should try using 1 inch posts to help with the air flow.

Thanks.

[JestersBaubles]

330 1100 10
100 1250 50
Fast 960 60
150 750

I would essentially reverse the hold times for segments 1 & 2 and drop the temp a little:

330 1100 50
100 1225 10
Fast 900 60 << I believe Bullseye now recommends 900 deg for annealing
150 750

Dana W.

[Deive Butvila]

330 1100 10
100 1250 50
Fast 960 60
150 750

I would essentially reverse the hold times for segments 1 & 2 and drop the temp a little:

330 1100 50
100 1225 10
Fast 900 60 << I believe Bullseye now recommends 900 deg for annealing
150 750

Dana W.

Thanks Dana, I certainly will try this schedule next time, but do you think this schedule might also fix my unsuccessfully slumped piece? What would you do?

[JestersBaubles]

When I have seen "humps" or bubbles during slumping, it has been because of too much heat work. So that's either too high of temperature or too long of a hold. In your case, I wonder if it is a little of both (especially if you kiln runs "hot"). That is why I recommend dropping the slump temp and shortening the slump's hold, but lengthening the hold at 1100.

At 1100 and a 40 minute hold, you may find that the piece is completely slumped at the end or at least pretty close to being completely slumped and the 10 minutes at 1225 will finish it off.

Dana

[David Jenkins]

Could it have somehow slumped into the corners first, before center touched down, and thereby trapped air in the center? It wouldn't hurt to drill a 3/32" hole in the center, would it?

[The Hobbyist]

As it nears the target temp it is essential that you peek. Many times the slump is complete and needs no hold.

If you hold for a time after the slump is complete it can blow a bubble even though the air has escaped at the corners. There can still be a tiny amount in the center. Given time that can grow into a nice bubble.

PEEK..............................Jim

[Kathy K]

Just the subject I planned to post on.
I used a schedule of
200 1225 :20
AFAP 925 1:30
100 700 :01

Is there anyway to fix the bump in the middle. Not sure if I haven't had success in the past with this issue as I usually think I didn't hold it long enough. I am using an 8637 BE 12" Sushi mold.
Thanks for your help.
Kathy

[David Jenkins]

@Kathy:

Bullseye's guidelines for that mold are:

300/1180/5

so you'll be going slower, hotter, and longer. I'm just sayin' ...

[Stephen Richard]

In my view that bubble has to do with too high a temperature and too long a soak. Holes are not the problem, the distance from the floor or the elements is not the problem.

The problems relate to too high a temperature. this shape can be slumped at 630C in 20mins. Higher and longer allows the glass to move, but it is not fully plastic, so instead of becoming thicker, the glass rises at some point like a bubble. The fact that you have needles on the bottom is another indication that the firing is too hot. The second problem is that you did not observe what the glass was doing in the mould. Had you done so, you could stop the firing and advance to the annealing segment.

[Deive Butvila]

My slump resulted in a raised hump and am not sure if it is the beginning of large bubble or if the glass just did not slump long enough. I need help understanding how I can best try to fix this piece and also avoid the problem for future projects.

Thought I would provide an update to close the loop on my original question.
In order to 'fix' the bubble, I tried the following slump temps with very little result:

1200 hold 45 - no change to bubble
1220 hold 30 - no change to bubble
1225 hold 1hr 15 min - slight improvement
1225 hold 1hr 30 min - still only slight improvement

I finally took the bull by the horns so to speak and drilled a hole in the middle of the mold. The result is that I got a perfectly slumped plate in just 15 minutes at 1225!

Lesson learned - watch the slumping process to avoid the problem I created for myself in the first! Perhaps I would not have needed to drill another hole in the manufactured mold except for the fact that I had an obstinate bubble to bring down.

Thanks to all for very sound advice and suggestions.