Uneven bowl slumping

[catshelby]

I am using a 12 1/2" Slumpy's mold, about 2" deep. It is set up on thin kiln posts to circulate heat better. I have tried two different pieces that are BE COE90, fully fused 2 3mm layers of Crystal Clear with a center design in streaky glass and frit. They are about 12 1/2" diameter. I am using a Skutt clamshell kiln.

Using BE's recommended slumping schedule as follows:
300 1220 .10
9999 900 .60
100 700 .01

Cooling to 100 degrees or less before opening kiln.

Slump is good, clear to the bottom of the mold. The problem is that the final piece has a variance in depth of 1/4" or more side to side. Happened on the first try and then the second piece was rotated to see if it would improve on the second piece. I have not noticed hot spots in the kiln before and am wondering if that's what I'm dealing with. New mold, new kiln wash, so am not thinking it is hanging up due to poor kiln wash coverage.

Not sure if these can be salvaged in any way. Would appreciate comments both on how to remedy slumping in the first place and if anyone has a suggestion of what to do with the two pieces that I have. Yes, I know...chop for scrap!

[Warren Weiss]

Check that the kiln shelf is level using a spirit level. Then check that the mold is level. Perhaps slow down by 50 d. above 1000.
Warren

[catshelby]

I did check the shelf and mold with a bubble level and all is where it should be.

[Warren Weiss]

Is the piece to be slumped uniform thickness (weight) all the way around? If it isn't I'm not sure how to compensate except perhaps to move the heavy side slightly further from the center.
Warren

[Stephen Richard]

Colours can affect it too. If you have dark or "heat absorbing" colours on one side, that will form first.

Often slowing the rate of advance from about 590C will give more control over the slump. It will take longer to achieve the slump, but it probably will be finished before 677C.

If the edges of the glass are on or over the edge of the mould it may have some effect too.

If the mould is steep, you need to consider whether to do the slump in two stages - a partial, followed by a complete slump.

[dawnsud]

You said you checked the mold and the shelf and found them to be level. Did you use a level on top of the glass after you put it in the mold?

[catshelby]

I did not use a level on the top of the glass disc. It would have not helped as I knew the glass contoured in the middle where the additional piece of design glass had been placed. The center design approximately was a rectangular shape and I have used this design successfully in many other sized plates without any slumping issues. These other plates did not have a lot of curve to them after slumping. This is the first time I've tried to do the design in a bowl.

For those that made comments about doing a double slump if the bowl was steep, thank you for the suggestion. This mold, however, was not a steep one and did not have any indentations inside, like some do. I chose it for the smooth interior as I didn't want to distort the bowl design.

One question I have about one of the comments. You mentioned that if the disc was sitting on the edge or extended over the edge, it could be the problem. The disc did not extend over the edge, but rested on it. I have done bowls this way before without issue and am wondering if I've just been "lucky". Do most people not rest the glass on the edge of the mold to begin the slump or was this a suggestion to overcome this particular design's problem with this particular mold???

Thank you all for your helpful comments.

[Brock]

Starting a blank on the edge of a mold can be problematic, as the blank can hang up momentarily on the lip of the mold. If you have the option, make your blank slightly smaller than the mold, so that the blank is resting on the sloping interior surface of the mold. That way, the blank is under compression and success is more likely.

[catshelby]

Thank you for the suggestion on a slightly smaller blank....it makes good sense to me because I do think something caused these to hang up on one side...will try again this weekend with a slightly smaller piece and let you know how it goes...

[jolly]

I'm probably too late to comment but isn't 300dph pretty fast for something that has been trouble? I only use float glass so others will know more but I might slow way down just before the glass starts to move. That would assure you have more even heating? Like I said I only use float glass so I may be way off on this.

[Nancy Juhasz]

Jolly
I don't slump any faster than 200 dph. Only thing I slump faster is plates because they are so shallow.
Nanc

[David Jenkins]

Didn't I read here once, long ago, that it might be helpful to bevel the bottom edges of the blank so that it could slip more easily without catching? Or did I just dream that up?

Also: Is the side that catches always in the same position, relative to kiln?

[Brock]

No, you didn't just dream that up.

[catshelby]

Update: Cut and fused another blank about 1/2" or more small than lip of bowl. Carefully checked the shelf to be level as well as the mold on top of it (on kiln furniture). I slowed the ramp and hold times on this last try , going at 250 dph. Still a lop-sided slump. Not a lot, but noticeable to me. I am tempted to take it to my lap wheel and grind the top edge until it is nearly even all around, then fire polish. Has anyone had any experience firepolishing in a slump mold???

The bowl mold is shallow and wide (12 1/2").

[The Hobbyist]

Yes, putting the bowl back in the mold and firing it hot enough to fire polish can create bubbles under the glass. They may make bumps in the top surface or just cavities on the underside.

It's a neat effect if you can control it.

Jim

[Jerrwel]

I am tempted to take it to my lap wheel and grind the top edge until it is nearly even all around,

As I am now experimenting with cold finishing processes, coldworking the top edge is exactly what occurred to me; but why not take the processing further to fully polish the top edge and skip the additional heat processing?

[David Jenkins]

You don't say whether you're leveling the glass before the slump - just the shelf and the mold. You said you had had previous successes - same kiln, shelf, and mold? Same (essentially) layup? Do you think the mold itself be made with a non-uniform slope?

Pictures are always helpful. Also, can you give the mold model/manufacturer?

[Warren Weiss]

When you mentioned that you leveled the mold, I trust that you leveled from side to side and front to back. If you changed the front to back you then need to redo the side to side.
Warren