Firing 90 Coe Bullseye with 90 Coe Wissmach

[S.TImmerman]

I'm sorry I don't have photos, I'll try to give the best description I can.  I cut two circles, one a blue 13 in (bullseye) and one a 14 inch black circle (wissmach ) . I put 4 dots of glue and centered the blue in the center of the black. So I would have a black rim on the blue. I decided to flip fire for crisp lines. The blue (smaller) on the bottom with the black (larger) on top.  I'm using a skutt clam shell kiln

My schedule :
300dph to 1150 hold 45 min
50dph to 1240 hold 45 minutes
100dph to 1370 hold 20 min
400 dph to 1460 hold 10 min
AFAP to 950 hold 60 min
150dph to 800 hold 10 min
250dph to 650 no hold


When I opened the kiln there was a huge 3 inch wide by 2 inch high  looking volcano (popped bubble) that sent black flakes all over other things in the kiln. Looking into the popped bubble there was black glass on the bottom . It looked like the black wissmach separated and popped.  The bottom (blue) had such a beautiful finish .. No evidence of the disaster on the black side.
It also had unpopped smaller bubbles all around the rim. Undoubtably the ugliest glass I've ever made.  well, except for that weasel  I was commissioned to make in stained glass.    

Did I trap air by putting a larger piece on top?  I've done this so many times to make smaller  bowls with a rim the same color of the underside.  
Could the black wissmach have separated ? The bottom of the 2in high pop was black..not blue like the bottom.  

I'm embarrassed to admit but I tried to fix it ( it was going to be a gift after I painted the blue side with a black design)  I sandblasted both sides and the black side (with the volcano) I broke off the popped bubble and brought it back flat ... I flipped it and Refired it (same schedule) and this time the blue (smaller circle) was on the top and it had 2 very obvious lumps on it. This is the largest bowl I've attempted to make ..where did I mess up? If I flip it over (yet again) maybe, just maybe could it fixit? Or should I start over?

Any help would be greatly appreciated 
Thank you, kindly
S.T

[The Hobbyist]

My guess is that the two 45 min holds allowed the black top glass to seal over the bottom all the way around the edges and this trapped air leading to you bubble. Crappy Wissmach glass may also be a contributing factor.

Jim

[Morganica]

Went to their site and can't find any technical information about firing schedules and such. BarryK is there with a PDF testimonial saying it's the best glass he's ever used, so maybe he can chime in with a suggested schedule or three.

It does sound as if you trapped a bunch of air that blew out pretty quickly. What are all the holds for?

[S.TImmerman]

I got the schedule here, and have never fired anything that big so I really didn't know. Without the holds on the way up, do you think that schedule world work? I did refire (flipped it over) it and it was a waste as there was a valley where that nasty blow out was. I could not believe how much glass it blew all over when it burst! First time Ive has a blown bubble.

Thanks so much for chiming in !

[Faye Malench]

Jim is most likely right - the edges sealed. That long schedule is overkill on black glass. It turned to mush. Shorten your 45 minute holds to 20 - skip the 100 dph after reaching 1240. After the blank soaks at 1240, you are well past the strain point and can go at 400dph to process temp.

[Barry Kaiser]

Went to their site and can't find any technical information about firing schedules and such. BarryK is there with a PDF testimonial saying it's the best glass he's ever used, so maybe he can chime in with a suggested schedule or three.

It does sound as if you trapped a bunch of air that blew out pretty quickly. What are all the holds for?

Quite true.
Remember that I only fire small pieces (up to 2x2). Also, most of my firings only go part way to full fuse. Much of the glass I use is clear and black. I do find the Wissmach superior for my use to Tekta (and other glass for my specific use. For one thing, I am one of those guys that get the intermittent haze with Tekta (fired on thinfire). Also, I find the Wissmach somewhat softer than other clear glass and the surface finish ends up marginally smoother than other clears. Finally, the price makes a heck of a difference.
I don't pretend to disseminate schedules for larger fusings. With very rare exceptions, I haven't fused anything larger than 4x4 in 10 years.
If you need firing schedules for what I do fuse, I have a tutorial on my site which gives almost all of them.
Barry

[S.TImmerman]

Jim is most likely right - the edges sealed. That long schedule is overkill on black glass. It turned to mush. Shorten your 45 minute holds to 20 - skip the 100 dph after reaching 1240. After the blank soaks at 1240, you are well past the strain point and can go at 400dph to process temp.

Faye, Thank you so much for the schedule. I'll try it out tonight. I really want to make functional ware - I've had no problem with smaller pieces -seems the bigger you go the more persnickety the schedule is.
S.T