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I have a simple circular bowl mold much like THIS ONE. It has steep sides and settles to a relatively flat bottom. Here's my questions before I make a mess of things:

1) How much of a "lip" must I leave on my ready-to-slump glass? The thickness of the mold is just under 6mm and it is 6 inches across. The mold has no lip at all.

2) Will my glass circle shift when slumping? That is, if the glass does not match the edge of the mold all the way around, when it begins to slump, will gravity pull the edge to the bottom of the bowl while the opposite edge remains atop the mold?

3) The edges are steep and the drop is just over 2 inches (5cm). The glass will be stretched as it slumps and I'd like to start with glass that is more than 6mm thick.
Base glass thickness 6mm thick and 6 inches across (width of bowl mold).
Inner circle of glass, 3mm thick and about 5-1/2 inches across tack-fused to the base glass. Just narrow enough to start to fall into the mold if placed it on top.
Good idea, or not?
When slumping, should my fused discs be face up or face down? (larger disk below or above?)

I'm going to assume that because of the differing COE of ceramics and glass, that if any of my glass drapes on the outside of the bowl mold, that this would be a bad thing.

BillsBayou wrote:I have a simple circular bowl mold much like THIS ONE. It has steep sides and settles to a relatively flat bottom. Here's my questions before I make a mess of things:

Molds like this one -- deep with no lip -- are difficult to slump well. In most cases they're not really made for slumping glass, they're basically bisqueware that's sold as a slumping mold.

1) How much of a "lip" must I leave on my ready-to-slump glass? The thickness of the mold is just under 6mm and it is 6 inches across. The mold has no lip at all.

You can go around 1/4"/6mm beyond the edges of the bowl mold without getting into too much trouble. But I'd be hesitant to go more than that.

2) Will my glass circle shift when slumping? That is, if the glass does not match the edge of the mold all the way around, when it begins to slump, will gravity pull the edge to the bottom of the bowl while the opposite edge remains atop the mold?

The steeper the sides of the mold (and these are fairly steep) the more likely you are to have the glass shift during the slump firing. It's possible to have one side slide down the side and the other side stay higher. If you fire lower with a longer hold you can make that less likely, but it's still possible with molds like these.

3) The edges are steep and the drop is just over 2 inches (5cm). The glass will be stretched as it slumps and I'd like to start with glass that is more than 6mm thick.
Base glass thickness 6mm thick and 6 inches across (width of bowl mold).
Inner circle of glass, 3mm thick and about 5-1/2 inches across tack-fused to the base glass. Just narrow enough to start to fall into the mold if placed it on top.
Good idea, or not? ... When slumping, should my fused discs be face up or face down? (larger disk below or above?)

I'm not sure what you're wanting to do. Are you suggesting that you'd fuse the blank so that the top layer is larger than the bottom one? That's not something I've ever done, though I have beveled the edges on a lap wheel to make it sit better on top of a mold.

I'm going to assume that because of the differing COE of ceramics and glass, that if any of my glass drapes on the outside of the bowl mold, that this would be a bad thing.

Good assumption. If the glass drapes on the outside of the bowl mold, you could get stuck on the mold and need to break the mold or the glass to free the bowl.

Thanks for the reply.

With respect to question 3, my idea is to tack-fuse two disks to get a thicker disk for slumping. I had this weird idea that I would want to have a mold-sized disk and one that is just a bit smaller. Tack the smaller on top of the larger. Then the question becomes: Would it be better to slump this fused disk with the smaller disk facing up or facing down?

I'm thinking it would be better to have the disks the same size. Otherwise, I'll have a thin fragile rim on the bowl. Then its work work work to cut and grind and polish that down.

Your answer to the first question kind of seals it for me. If I tack-fuse two disks for a thickness of 9mm and a diameter wider (by a hair) than the bowl mold, I should get an edge that will hold on to the top of the bowl evenly as it slumps, and walls on my finished work that will have a sturdy feel to them.

The slow approach seems to be the key on the slump.

Also, try Brad's suggestion of beveling the back edge. This will help.

Dana

Just an idea, might it help to make a drop ring of equal size to the outer edge of the mold and put your glass on that for support as it starts to slump? Then it would probably not slide.

Another suggestion is to make a ring of fibre board to support the glass outside the mould. This assumes you make a larger disc than the mould diameter and use the fire board ring to support the "excess" glass. This also helps to reduce the risk of uneven slumping. However it will almost certainly leave you with an ourter flat rim. If this is undesirable, it can be cut off and cold worked.

You might want to do a shallow bowl slump of the piece before you slump it into the deep mold. The shallow bowl will center itself in the deep mold much better than a flat disc will. Even With the shallow mold, be sure to seam the edge that will be contacting the mold.

that bowl shape is not particularly elegant finished as the bowl will slump far down into it and be mostly flat with a small upturned edge and possibly an uneven bottom. As it was mentioned, it isn't really made for fusing and presents a lot of problems with shifting off center, which is a real heartbreaker. You might consider getting a different mold before you invest more in the project. One with a wider top- narrower bottom so light can shine through your creation, or even a drop out ring. Go slow on the slump if it is tack fused. That has more internal stress than a fully fused blank.