Unwanted asymmetry when slumping

[Sulie]

Hello, and greetings from Austria. I am very new to kiln forming and have run into a wall. I am trying to make small, simple, slumped dishes: my kiln is a Paragon Sc2 so space is limited to approx. 5" by 5". My fusing is going well, the pieces turn out as I intend, i.e. square. But when I try to slump into a mold the dishes turn out wider at the front than at the back. Some are also slightly curved in at the sides, but this doesnt bother me as the curves are at least symmetrical. I have double checked that the kiln is standing straight, both from front to back and side to side. I also do my best to place the fused tile straight in the mold.
I am using the following firing schedule (centigrade):
ramp at 260 to 535, hold for 20 min.
ramp at 500 to 750, hold for 30 min............... I fired my first dish a bit cooler than this, but it didnt slump til I refired it: both times coming out assymetrically.
afap to 515, hold for 30
ramp at 250 to 150, turn off

Another question: at the edges of the glass, where it has expanded and then contracted, I have a rim (about 1/8") of roughness from the mold: it's not particularly attractive and I'd be pleased if I could avoid it.
I realize that these are very basic questions, but I havent been able to find an answer anywhere. I'd be most grateful for the advice of those much more experienced than I.
Thank you in advance,
Sulie

[ElaineC]

Hello Sulie,

Could the uneven slumping be due to the temperature differential in your kiln between the front (near the door) and the back?

You don't say what type of glass you are using, but I also have an SC2 and here is my slumping schedule (in Centigrade) for Bullseye glass into a fairly shallow mould:

Seg Ramp Temp Hold
1 200 520 25
2 300 680 5
3 FULL 482 45
4 60 371 0

If slumping into a deeper mould, I increase hold 2 to 15 minutes.

[Sulie]

Hi Elaine, and thanks so much for your reply. You know, when my first dish came out crooked I immediately thought of the temp. difference and wrote to Delphi, from whom I purchased the molds. They said that a temperature difference couldnt cause a result like this, but I'm still not sure. (I work a lot in metal clay, and see the effect that temp differences can have) I even turned the dish around and re-slumped, but it didnt even out.
I have used both Bullseye and Spectrum and had the problem with both.
Thank you for sharing you firing schedule: I see it's cooler and shorter than mine. I will certainly try it; maybe I am simply "frying" the glass.....

[Stephen Richard]

In my view you both are firing to hot. It is much better to fire low and long. This also will help avoid the marks on the underside of the piece.
It sounds to me like this is a fairly shallow mould, so 630C for 20 - 30 mins should be enough. Observe when the glass is fully formed. If it is so before the whole of the soak time is used, then you can reduce the temperature on the next firing, but keep the length of the soak the same. As has been said here many times Low and Slow is the way to go.
There are of course, other things than can make for uneven slumps.

[Sulie]

Stephen, thank you very much. I will try a cooler firing.

[Bert Weiss]

After you slow down, level the mold, and level the glass. And, don't forget to remove the level

[Stephen Richard]

After you slow down, level the mold, and level the glass. And, don't forget to remove the level

[ElaineC]

Thank you Steve. I have been happy with my results so far, but will try the lower temp. schedule to compare the finished items.

[Sulie]

Hello: I just wanted to let you know that thanks to you kind folks, my problem seems solved. I used the temperature that Stephen suggested, and have 2 nice, square dishes as a result.
Thank you so much for answering my very basic question!