Puzzled....

[lbailey]

I work with a local artist that makes kaleidoscopes. She is tired of the stained glass/solder look so for the past several years we have been draping BE glass into 6-8" long arcs (not quite a half circle), which she then glues together for a nice oval tube for her scopes. We also make simple rectangular bodies by draping a single layer of BE over a SS form that is 10" long, 3" wide and has 3" sides. (Think of an upside down "U"). She then glues a flat piece of matching glass to the open side and uses that for a scope body.

Even though the SS form is 3" on the sides we have always cut the glass for 1.5" sides, which means the glass is cut 6" wide. I make sure the form is level and use a Sharpie to mark reference points so that the glass is centered and square on the form. We're sailing along and all is working as planned, have made a bunch this way. Slumping temps vary a little based on color but we've figured that out, (white needs more, etc.) The SS mold has been sandblasted to give it a little tooth, I use a couple of light coats of Zyp as a separator.

Troubles began lately when we decided to increase the height of the sides to 2". The glass is now cut 7" wide, which allows 3" for the top and 2" for each side. Using the same mold, same set up, same schedule (see below), same kiln one side is consistently 3-5mm higher than the other. All of these draped bodies require a bit of cold working to true them up, but nothing like this. I've run this 4 times to double check level, witness marks on the glass, schedule, etc, with the same result.

Its as if the glass is slipping on the mold before the sides fall, but how could it occur so consistently? Always one side longer in the 3-5mm range and on square with the opposite side. What the heck is going on?

Kiln is an Olympic SQ14GFE, firing the sides only, schedule for a single sheet of Tekta:

200f dph 1100f 10 min
100f dph 1200f 10 min
AFAP 900f 60 min
150 dph 700f Off

Thanks guys

[Kevin Midgley]

The answer is in understanding the heat work in your kiln. Something seemingly tiny is unevenly providing heat work.

[Tony Smith]

Many people don't realize that the air temperature in the kiln isn't necessarily the same as the glass temperature. The glass temperature is affected by conduction to the mold, convection with the air and radiative coupling to the elements. You're not going to change the air or the mold, so the only variables you have to control radiative coupling are distance from the elements, or blocking view of the elements. I would start by moving the side that's moving too quickly away from the elements. To Kevin's point, start by moving away a tiny amount... maybe a half inch, the reevaluate and adjust.

Tony