Oversizing Glass for slumping deep bowls

[ron]

How much bigger (if any) can you make the glass for slumping into a deep bowl(3"-4")? Right now I measure the outside diam. and then make my glass that size. When I slump the glass there is about 1/2" left all around the the outside of the mold. Can I increase my glass size by that 1/2"?
Thanks.
Ron

[tommckay]

I can only say when we have tried oversizing the glass drapes over the side of the mold. You have to accept the 'loss' of size in any particular mold. About 3mm overhang is the most we have successfully slumped but maybe someone knows otherwise.......
Tom

[Bonnie C]

Hi Ron,

I'm a member of the Research & Education Department at Bullseye and handle a fair amount of slumping projects in the Portland Resource Center studio. We recommend working as you have been for slumping into dish/bowl forms: measure and cut pieces to the top edge of the mold. It's tried & true.

That said, I've slumped pieces (6mm thick) that start off slightly larger than this edge and they've turned out fine - but probably over by no more than 4mm or so all around. Medium to larger diameter molds that support the entire perimeter of a piece combined with a bit of depth are generally more forgiving than molds where the glass rests on pointed corners or a wide rim. Ideally, the piece bends and draws to the interior of the mold at relatively low temperatures, before it gets hung up...

Cutting the piece a full inch larger in diameter with a 1/2" overhang all around sounds very risky and I'd suggest sticking close to your current method (if you make any changes at all). Hopefully, you'll get more anecdotes about oversizing successes and failures.

Just to get a better picture, what is the diameter of the mold?

Hoping this is helpful,
Bonnie

[morganica]

I have gone to about six inches wider than the opening of a deep bowl mold, but the ONLY way to do that I'm aware of is to physically extend the mold. If the overhang is only an inch or so, you can surround the mold with kiln furniture that is higher than the mold, top each post with fiber paper and lay the glass on the furniture instead of the mold. It will hold the glass up so it doesn't sag over the edge of the mold. The furniture spacing depends on the size, but the more glass you have outside the mold, the closer together you must put the posts, to prevent glass from sagging down between them and getting stuck.

If I want to extend the glass more than that, I build a fiberboard collar around the mold and use kilnwash to fill in the gaps. If I'm not going too wide the collar can be flat. If I want to really extend the glass I must build a sloping or multi-tiered collar so that it guides the glass down into the mold. Otherwise, the glass again gets stuck outside the mold and simply stretches and makes holes.

Also, the bigger the diameter of the glass hanging outside the mold, the more glass you're stuffing into that mold. If you go too wide, you start getting ruffles or really lopsided bowls. The molds aren't made for more than the diameter of glass and if I modify the mold, I pretty much count on doing some coldworking later.

[ron]

The mold I'm using is 11 1/2" wide and about 3" deep.
Thanks for all of the info.
Ron

[Bezaleel75]

I have gone to about six inches wider than the opening of a deep bowl mold, but the ONLY way to do that I'm aware of is to physically extend the mold. If the overhang is only an inch or so, you can surround the mold with kiln furniture that is higher than the mold, top each post with fiber paper and lay the glass on the furniture instead of the mold. It will hold the glass up so it doesn't sag over the edge of the mold. The furniture spacing depends on the size, but the more glass you have outside the mold, the closer together you must put the posts, to prevent glass from sagging down between them and getting stuck.

Is it like copying the mold of the kiln furniture? I have just read this post now and I’m sorry because I am just a starter in furniture making. I like to learn a lot on this because I heard it is good and easy source of income.