Why??

[Theresa]

Every time I try to make anything larger than 8" in my new kiln I get this. I do not understand 12" dia. two layers. Glass and shelf fully dry.

400 to 1000 10min hold
300 to 1150 15 min hold
300 to 1250 10min hold
400 to 1470 20 min hold
Peeked in saw the bubbles so I shut off kiln.
This is the 5th time I have tried using 12" glass and I get huge bubbles. Each time I adjust my schedule nothing works. HELP!!
If I try another attempt and use kiln paper and it works does this mean I could have a problem with the shelf? Is that even possible?

[Tom]

I feel your pain.  Last year I was all gung ho about my new kiln and did some Christmas cheese plates that bubbled.  Only 1 out of 6 turned out right, and I was so disgusted, I quit using my kiln.  Havent used it since, but I am taking a very intensive 2 day class in Omaha next week on fusing, so I will be getting back at it.  I told the instructor my issues and he wants me to bring the "screw ups" to class so he can get an idea and maybe use it as a teaching tool.  Dont know what I will find out, but will pass on anything if he can help me.

Just dont give up like I did, I know someone will help you figure it out.

Good luck.

Tom

[Tre V]

I don't have a kiln and can't offer any advise, but Tom, the class sounds like a great opportunity!

[Kev]

Where is it in the schedule that you notice the bubbles forming?

What type of glass are you using? Bullseye or Spectrum?

You might try slowing down your ramp between 1150 and 1250 to 50 degrees per hour to allow more time for the air to escape before the edges seal together trapping any remaining air from escaping.

[Theresa]

Spectrum.

I peaked during the latter part of full fuse, saw the bubbles then.

I have a show this weekend but will give it another try on Monday. Will add a slower longer bubble squeeze and use paper on shelf.

Did not have this problem in smaller kiln, very frustrating.

[Theresa]

Tom, I'm debating on taking a class in June. If I keep having problems I may have to.

[Elizabeth]

I'm not in a position to offer advice on what went wrong - but perhaps a good class with an instructor who can help you sort out the peculiarities of your particular kiln might be the best - and in the long run, cheapest - way to go!   Good luck to you both, Theresa & Tom.

[Stephen Richard]

Every time I try to make anything larger than 8" in my new kiln I get this. I do not understand 12" dia. two layers. Glass and shelf fully dry.

400 to 1000 10min hold
300 to 1150 15 min hold
300 to 1250 10min hold
400 to 1470 20 min hold
Peeked in saw the bubbles so I shut off kiln.
This is the 5th time I have tried using 12" glass and I get huge bubbles. Each time I adjust my schedule nothing works. HELP!!
If I try another attempt and use kiln paper and it works does this mean I could have a problem with the shelf? Is that even possible?

Unless the bubbles always occur at the same point(s) relative to the shelf, you do not have a shelf problem.  You do have a very difficult lay up to fire though.  I'm not sure if the base is a single or double layer of 3mm glass. If it is one layer, then you have a conceptual problem, as that base layer needs to be 6-7mm thick to avoid bubbles.  lots of discussion on single layer fusing around the internet.  Shelf paper allows the air under the glass to migrate through it (the paper), so if it works it does not mean that you have a problem with your shelf, rather a problem with your firings.

However even with two base layers, these stacks provide a number of thick and thin areas.  Firing fast will allow the glass to conform to the shelf so well that the air still left under the glass will not be able to get out.  So some comments on your schedule (and I don't know how that will apply to the earlier schedules) follow:
222c/hr may be too fast for this lay up - you could have up to 5 layers or 15mm of glass.  To me this would imply a rise between 50C and 90C per hour - .25 to .5 of your current rate of advance. 
540C is a nowhere place to soak
621C is a place to start a slow ramp for a bubble squeeze - say 50C/hr to 650C and then a 30 min soak.  677 is the bubble squeeze soak recommended for Bullseye, 650C is the one recommended for S96.  After the bubble squeeze you can advance at the speed you have chosen without concern.
You may find the Bullseye tech note on glass and heat useful in designing your schedules: http://www.bullseyeglass.com/pdf/technotes_tipsheets/TechNotes_04.pdf
The information on the S96 site will be helpful too:
http://www.system96.com/Pages/ThickFiringGuideF.html
even though I do not agree with their concepts on firing.

My rule of thumb about uneven layers is to subtract the difference between the thinnest and thickest portion and add the difference to the thickest.  In this case this would mean that you should be considering schedules for 24mm - 1 inch.  So you could be considering an initial advance of as little as 30C/hr to the bubble squeeze, when you could increase to 90C/hr to your full fuse temperature.  This additional heat work of course means that you do not have to have such a high temperature to achieve a full fuse.  Normally I try to avoid staying at fusing temperatures for no more than 10 mins.

[Stephen Richard]

.......

Did not have this problem in smaller kiln, very frustrating.

Scaling up is not a linear process, it is a step change.
Different kilns have different characteristics.  What works with small often will not work with large.
Do you have any books?   Brad Walker's book - available at WarmGlass.com is very good.
have you had any classes?

[Becki]

Stephen, what are your thoughts on how firing black and white together affect her schedule?

[Stephen Richard]

Stephen, what are your thoughts on how firing black and white together affect her schedule?

I don't really have any.  I don't think there is a difficulty if fired slowly.  Certainly not in relation to the variations in thickness in this case.

Except when something very precise is required, the variations in "stiffness" between black and white are more theoretical than practical.  There are of course circumstances when the practical differences become important - especially when you want crisp lines.  But in that case use Bullseye stiff black along with one of their whites.

[Becki]

Theresa, If I were doing this piece I would slow my initial ramp up and add a long slow bubble squeeze, probably 50dph to 1250.  You could also probably fire your base first and add your elements in a second firing and have better results too.  Don’t get discouraged.  Just keep at it.

[Kev]

Now that sounds like a plan!  Keep in mind though that if you add the elements to a prefused base of 2 layers, the addition of the elements on top will most likely cause the piece to grow in size, which might be uneven, as it attempts to get back to 1/4 thickness.

[Theresa]

Thanks everyone. I will keep trying. Just annoying when what I have no problems with in my smaller kiln goes horribly wrong in the larger one. I am keeping notes and will eventually figure out what works. Hopefully sooner than later. I will try the slower ramp and longer bubble squeeze first.

Once the kiln cooled and I looked at the piece the bubbles seem to be starting from the botton and not between the two layers. You could see where another one was forming on the bottom.

[Debbie]

It makes you wonder if there are slight devit's in your shelf.  Can you use the other side?

[Stephen Richard]

As I indicated earlier in this thread, there are ways of observing whether you have low spots on you shelf - the appearance of bubbles at the same place relative to the shelf.  In this case, it is likely to the the combination of layup, schedule and kiln characteristics.
Larger kilns take longer to heat up and retain the heat better than small ones.  Often you can achieve the result at a lower temperature in a large kiln, because the glass has longer to get to the forming temperatures.  Of course, sometimes the reverse is true.   The point is that you have to observe each firing to learn how your kiln behaves under various circumstances.

[Theresa]

As I indicated earlier in this thread, there are ways of observing whether you have low spots on you shelf - the appearance of bubbles at the same place relative to the shelf.  In this case, it is likely to the the combination of layup, schedule and kiln characteristics.
Larger kilns take longer to heat up and retain the heat better than small ones.  Often you can achieve the result at a lower temperature in a large kiln, because the glass has longer to get to the forming temperatures.  Of course, sometimes the reverse is true.   The point is that you have to observe each firing to learn how your kiln behaves under various circumstances.

Not even remotely did I think about keeping the shelf in the same direction each time I removed it. I will now. Process of elimination.