While Bert has far more experience than I, I will say that I have had good results slumping over the Ikea Blanda series of bowls, including the 14" one. But, it was (and continues to be) a bit of a learning curve. To eliminate the wrinkles, you need to go slow and anticipate that the glass will need to stretch some. Off the top of my head, seems to me the outside of the 14" Ikea bowl is 18" or so (from one rim, across the bottom to the other rim). I figure it will stretch a minimum of 4" (2" all around), so I make the blank no more than 13" (gives me a safety factor for a 2.5" stretch). I'll also add a doughnut of fiber paper (1/4" thick) that's 1/4" - 3/8" wide on the flat portion of this bowl, since the Ikea bowl doesn't have much of a bottom, this gives is a nice kick to allow the finished bowl to sit stable. Make sure the shelf is as level as possible, and that the glass piece is sitting level on the mold.
I slump it to 1250 (ramp up varies based on which size bowl and how much I really like the pattern), but I expect that it will take 2 hours at that temp to get the glass to lay flat. My kiln has a viewing portal, and I use that until it gets pretty close to done, but you do have to open and look to see all around the piece if you want it sitting flat to the mold. I've got a clamshell style, so I can see all around the piece easily. How "flat" you go depends on how patient you are and whether you want it without any crinkles. If you have one or two last sections that haven't gotten flat, I'll add some heat, but only if I have an 1" or so between the edge of the glass and the shelf, since the glass moves MUCH faster at higher temps. My wife likes some waves, so not all of the ones I make get the whole rim flat to the mold. I personally don't care for the look when the glass hits to shelf, so I always stop before that happens.
Opals and transparents flow at different rates, as do different colors, so don't expect to get a rim at the same height unless you are slumping a blank made from two solid pieces. Using streaky glass produces a bowl with a lot of movement on the rim, since the streaky colors having vary amounts of the different colors through the piece. As the piece flows, any pattern will tend to follow gravity, so be aware that intricate patterns will not look the same when doing this. Angles tend to soften, and lines want to straighten toward the rim. I went through a lot of trouble one time to do a bowl with pieces tilted 30 degress from perpendicular to the rim, and once slump, it looked pretty much perpendicular.
Also, if you're doing bigger bowls, consider building your pattern on a transparent piece, or doing a pattern on the inside as well, since you'll see a lot of the inside unless it's placed up high.
And one final note, I use BN on my stainless molds.
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