Stainless steel mold questions

For discussion of processes related to using Bullseye glass, including kilnforming and kilncasting, torchwork, blowing and stained glass.

Stainless steel mold questions

Postby Colleen1 » Fri Jan 14, 2011 1:14 pm

Another mold question!
So stainless steel molds should only be used for draping only, no slumping? I bought a stainless steel tray at a second hand store and was going to try it out, with a single layer of glass. Should I drill holes? Anything that you can share would be great! Also, I noticed in these pictures it looks like fiber paper was used under the molds and kilns posts under the mold, I don't use either and have no problems....
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Re: Stainless steel mold questions

Postby bertglass » Fri Jan 14, 2011 9:07 pm

Stainless steel molds can be excellent for slumping. The deal is that stainless steel shrinks more than glass. However if the mold is drafted in shape, the mold can not hold on to the glass. I have molds that are spherical in shape. These molds, when all the way cooled, will hold on to the glass. Either I demold the glass before it is cool. or invert the mold and reheat. At some point the glass will fall out.

SS molds work best for slumping when they are designed to be slump molds. Often mixing bowls have sides that are too steep to be good for glass work. Stainless steel woks make excellent bowl molds. Eastman woks have a flat bottom which makes a bowl that sits nicely. Amazon sells them. Fusion Headquarters makes several excellent ss slump molds. These will last a lifetime with minimal care.

Personally I have seldom seen drape bowl molds work well. I always got an ugly wrinkled mess. The less you ask the glass to drape the better it works. A baby moon hubcap shape works well. A classic bowl shape wrinkles and deforms, for me.

I don't think you need to drill for a drape though. For slumping you do need to drill some holes. Otherwise hot air will get trapped and try and blow bubbles.

I recommend several coats of bullseye kiln wash. Heat the mold to 1200 and then cool to 500 and spray on the wash. When it is too hot, the wash bounces off. When it is too cool, the wash runs. When the steel cools too much, reheat and spray until you don't see the steel through the dry wash.
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Re: Stainless steel mold questions

Postby ejgiebel » Sat Jan 15, 2011 10:43 am

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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Re: Stainless steel mold questions

Postby Colleen1 » Sat Jan 15, 2011 4:30 pm

Thank you, Bert and Ed!
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Re: Stainless steel mold questions

Postby charlie87 » Thu Aug 25, 2011 5:50 am

Stainless Steel Molds for slumping can be used in several different ways, but the most common are slumping INTO and slumping OVER the mold. Slumping INTO the mold. Probably the most widely used way to slump, this approach allows the glass to sag into the mold. Shapes frequently formed in this manner include bowls and platters. The molds used in this way need to have small holes in the bottom in order to allow the air to escape when the glass slumps. Slumping OVER the mold. This approach, in which the glass is allowed to fall over the outside of the mold, generally uses molds made of stainless steel. Because bowls and vases made using this technique tend to be characterized by their wavy sides (like folds of cloth), slumping over the mold is sometimes called "draping."

One very important thing to keep in mind is that molds that are used for slumping INTO (rather than OVER) need to have holes drilled in the bottom. This is to allow air to escape from the mold as the glass slumps into place. Without the holes, air will be trapped underneath the glass and it will not be able to slump properly.
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Re: Stainless steel mold questions

Postby jerid » Sun Aug 28, 2011 7:00 am

ok, maybe a silly question, but what is BN?
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Re: Stainless steel mold questions

Postby Chris J » Sun Aug 28, 2011 10:09 am

Boron Nitride. It is a spray you use instead of kiln wash on your stainless steel molds. It is easier to use because you do not have to heat the mold get it to stick.
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