Why is this cracking?

[Sulie]

I have spent the last few months slumping just one layer and now hope to progress a bit.. This piece is 6 mm thick, consisting of 2 layers of 3 mm Bullseye. As you see it cracked miserably. Looking through the forum I suspect that my first temp is too hot, though I got this schedule from a book. (sorry, cant recall which) The layers didnt fuse together, so I am at a loss.
Temps are C.
ramp @ 448 to 501 hold 40 min
ramp @ 508 to 650 hold 60 min
ramp afap to 516, hold 45 min
ramp afap to 370 hold 30 min

I know what a newbie question this is, but I'd be so grateful if someone could help me out and tell me where my mistake is. Additionally, I'd like to ask: when I get a correct firing schedule can I re-fire this piece, or must I trash it and start again?
Many thanks and greetings from Austria,
Sulie

[Marty]

Sulie- First find that book, let us know who wrote it, then throw it out.
Next go here: http://www.warmglass.com/tutorial and check out the schedules, also in Brad's book, also on the BE website.

Basically, you're firing way too fast. You're inviting thermal shock on the way down as well- the afap ramp from 516C to 370C is brutal . You've also got to compensate for the black/clear issue.

[Marty]

One more thing- did you previously fuse the black and the clear before slumping?

[Sulie]

Hi Marty.. thanks very much! I didnt try slumping this, as this was my attempt to fuse before slumping.
I wasnt aware that there was an issue between black and clear; argh! The base is a white square, surrounded by black strips and then the whole thing is capped with a sheet of clear.
I have Brads book: I will read it again with better attention!
In your opinion, can I re-fire this with a correct schedule, or should I start over again with new glass?

[Marty]

You can re-fire it but you'll see that circular crack. It could be a design element?!

[Bert Weiss]

That circular crack looks to me like a compatibility issue between the black and white glasses.

[Judd]

That was my first thought too. This is all compatible glass?

[Bert Weiss]

Stresses from incompatibility tend to form around the perimeter of the 2 unhappy glasses. Stresses from annealing or thermal shock go across the interface and bend around.

[rosanna gusler]

incompatible glass. r.

[Glass Fever]

S/he said the glass did not fuse together, you're still looking at separate sheets of glass, so not incompatibility. Simple thermal shock from ramping too fast.

[Bert Weiss]

S/he said the glass did not fuse together, you're still looking at separate sheets of glass, so not incompatibility. Simple thermal shock from ramping too fast.

That is not what a thermal shock crack looks like.

[Glass Fever]

S/he said the glass did not fuse together, you're still looking at separate sheets of glass, so not incompatibility. Simple thermal shock from ramping too fast.

That is not what a thermal shock crack looks like.

They say the glass didn't fuse together, the top temp was only about 1200 F. And from the picture it just looks like a sheet of clear sitting on top and the cracks are in the clear sheet. If there was no fusing than it can't be incompatibility, so that pretty much just leaves thermal shock, right? What else can it be?

[Glass Fever]

I also would like to know were that weird schedule came from. The odd numbers make me wonder if it was translated from F to C, and maybe incorrectly.

[rosanna gusler]

the schedule allowed for a tac fuse. that is a classic compatibility crack as far as i can see. i wouls suggest full fusing that same stack and will bet it breaks back apart. r.

[Lori Schinelli]

Ok . My 2 cents.
The white resists movement because it is so stiff. The black is very likely to move quickly. If the clear sheet tack fused to the underneath layer,you have one sheet of clear that is being pulled in 2 directions. I think the crack was caused by the viscosity differences between the white and the black

[Bert Weiss]

Ok . My 2 cents.
The white resists movement because it is so stiff. The black is very likely to move quickly. If the clear sheet tack fused to the underneath layer,you have one sheet of clear that is being pulled in 2 directions. I think the crack was caused by the viscosity differences between the white and the black

Compatibility is a balance between viscosity and coefficient of expansion. They can both be different as long as there is a balance.

[Stephen Richard]

I agree with Bert, and think Rosanna's advice should be followed to determine if the glasses are compatible

[rosanna gusler]

so did you re cook it? r.

[Andrea R]

That circular crack looks to me like a compatibility issue between the black and white glasses.

Was thinking the same cause thats a wierd crack!!

[Sulie]

Hi everyone and thank you so much for your opinions. I'm sorry to be so late in responding: I didnt see the additional posts until just now.
I didnt re-fire the cracked piece, but I made a second one using the same glass as before. I used a firing schedule that I obtained from the fusing video on Bulleye's channel on Youtube, and it came out fine, no cracking. Therefore it seems to me that the problem was indeed due to thermal shock.
The photo is a bit fuzzy, but I'm rather pleased that I was finally able to fuse and slump more than one layer of glass.... "baby steps"! Thanks to all who replied!
regards,
Sulie