Keeping shapes when fusing?

[Shades of Colour]

Hi All,  This is probably a stupid question but I need to ask.  I've been making a few coasters and these have come out fine as I have used two layers of equal size glass but I've just started to do some slumping and did a round bowl and have a square plate in the kiln as we speak.  My question is both have gone slightly wobbly in shape as I haven't put two layers of the same size ie the top layer smaller than the bottom.  Hope this is making sense!!  On the square plate the sides have pulled in slightly.  I've seen others fuse and theirs have come out straight!  I'm not sure what I've done wrong or how to correct it.  Any help would be really appreciated.  Thanks

[Margram]

I think you need to cut the top layer slightly bigger than the bottom one...

[jeannette]

Glass tries to be 6mm thick - the two equal layers are therefore successful. The results on dissimilar sizes are a bit unpredictable depending on your fusing schedule, but if you cut a smaller piece on top then the edges of the bottom rice are trying to draw in to get to 6mm - it's trying to raise itself, but it won't all pull in so you get wavy edges and sometimes that's what you want. If it's not, then you need to adjust your schedule to try and get sufficient melt without the sides pulling in.
This is a really good fact sheet about heating and layers and the expected results.
http://www.warm-glass.co.uk/images/pdfs/Bullseye%20TechNotes_04.pdf
There is also a nice app for the iphone/ipad devices from bulleseye or the fact sheets are on warm glass and other sites.
HTH

[Shades of Colour]

thanks guys for your feedback.  That is really helpful.  I've had a look through your link Jeannette, thank you.  Maybe I need to lower the top temperature slightly.  Im slumping the plate today to see how it turns out as the sides weren't pulling in too much.  I asked the question on facebook too and someone said to me "its a desinger piece" - hadn't tought of it that way.

Thanks again