cutting glass for mold

[deerick]

Ok so I turned the mold over and cut the bottom layer of glass and proceeded to design and glue a number of pieces on top. Now I am thinking that once I fuse this piece it will expand a little and probably be larger than the 7x7 mold. Do I grind the edges some to make the fused piece smaller? What will the edges of the piece be once I slump it into the mold? I really want to avoid a third fusing or polish fuse before slumping. Thank you

[The Hobbyist]

If the assembly is only two layers thick it will not expand.

If it is thicker by a little it might expand very slightly. However, you should still be able to slump it without adjustingh the sides. Usually you can slump a blank that hangs over about an 1/8" without trouble.

If your assembly is more than a little over two layers then all the above doesn't apply.

Jim

[JestersBaubles]

I recommend grinding after the first firing, if the edges need to be straightened up.

Dana W.

[Morganica]

Ok so I turned the mold over and cut the bottom layer of glass and proceeded to design and glue a number of pieces on top. Now I am thinking that once I fuse this piece it will expand a little and probably be larger than the 7x7 mold. Do I grind the edges some to make the fused piece smaller? What will the edges of the piece be once I slump it into the mold? I really want to avoid a third fusing or polish fuse before slumping. Thank you

First, you shouldn't need a separate firepolish firing unless you just want one. You can generally slump and firepolish in the same step--grind the edges to a reasonably fine grit, then slump a bit hotter and longer than you normally slump. Depending one the quality of the edges going in, you'll get anywhere from a satin to glossy finish. For me, 220 makes a nice satiny sheen, 400 is full gloss. Obviously, since kilns and firing methods are different, you'll need to watch your kiln carefully and experiment a bit to get it right.

If your piece is 6mm thick throughout, it shouldn't expand. If it's got variable thicknesses that go beyond 6mm, it may not expand all that much, but the edges are liable to be uneven. They may need to be trimmed and neatened up whether they're too far over the edge or not.

As a rule of thumb, I can overlap 4-5mm on each side before the glass falling down and trapping the mold becomes a problem. There's just too much surface tension and thickness at that point. It also depends on the shape of the mold.

[Babette (Shawn)]

I may be wrong, and please correct me if I am. Because I live life on the edge I am constantly thinking about how much I can get away with...

Regarding slumping my conclusion is if you have a large mold you are slumping into you can overlap more. I did overlap an inch (all around) on Bullseye's large 22" round (diameter) mold successfully. I have used a blank that overlaps 1/2 inch on 11-12 inch molds. My theory is that because larger expanses drop more quickly (gravity) the center of the glass will start falling and pull the glass down into the mold before it has a chance to soften at the edge and snag. Rarely have I used a piece of glass that is exactly the size of the mold and it has worked fine for me. Of course you have to make sure the piece is centered and use a level to make sure the glass and the mold are level. *And the depth of the drop on the mold has to allow for the extra glass.

[Morganica]

I may be wrong, and please correct me if I am. Because I live life on the edge I am constantly thinking about how much I can get away with...

Regarding slumping my conclusion is if you have a large mold you are slumping into you can overlap more. I did overlap an inch (all around) on Bullseye's large 22" round (diameter) mold successfully. I have used a blank that overlaps 1/2 inch on 11-12 inch molds. My theory is that because larger expanses drop more quickly (gravity) the center of the glass will start falling and pull the glass down into the mold before it has a chance to soften at the edge and snag. Rarely have I used a piece of glass that is exactly the size of the mold and it has worked fine for me. Of course you have to make sure the piece is centered and use a level to make sure the glass and the mold are level. *And the depth of the drop on the mold has to allow for the extra glass.

I don't think you're wrong at all--you can get away with more than that in some situations. I do think it depends on the thickness of the glass and the depth and slope of the mold. If the blank was 6mm regular black I'd be more concerned than if it were 12mm of white glass, for example. But in general, if I'm going beyond a quarter inch in each dimension I take a careful look at the whole setup.