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Thermal shock or careless handling?

Posted: Thu Jun 06, 2013 6:01 am
by AileenK
Hi Everyone,
I have been reading this forum for quite a while now and have found it extremely informative. Now I need to put my problem out there and see what everyone thinks. I have been making bowls using a strip construction technique for about a year. I use System 96.
I have my work in various galleries, I also use my bowls at home. Earlier this year, I had some work in a gallery and I got the dreaded call, one of my bowls broke. Okay I thought, maybe it had suffered thermal shock and finally cracked a few months after it was completed as I hadn’t been very careful with allowing the bowl to cool properly before removing from the kiln. Lesson learnt, I will be more careful in future. So, I always allow the bowl to cool to room temperature before removing from the kiln, I always allow the bowl to sit for at least 24 hours before working on it.
I had some more work in the same gallery and today I got the call again, another bowl broken. A loud bang was heard and when the gallery owner went into the room to investigate, the bowl was broken. This bowl was made two months ago. I have only had two bowls break, both in the same gallery, pretty strange.
My annealing schedule is:-
510 degrees Celsius for 60 minutes , then 83 dph to 425 for a further 10 mins, 167 dph to 38. I remove when cool. The finished bowl is 8mm thick.
Do you think there may be a problem with the annealing or should I be suspicious of the gallery?
Thanks for reading my post.
Regards,
Aileen

Re: Thermal shock or careless handling?

Posted: Thu Jun 06, 2013 8:10 am
by David Jenkins
How thick is the bowl (strip width)? The drop from the 60 min hold seems kind of fast, too me - even for 6mm. I'm guessing if your piece is made of strips your blank is > 6 mm and hence would require a much slower drop than you're using.

But: I'm used to deg F., and Bullseye glass, so I may have things confused.

Re: Thermal shock or careless handling?

Posted: Thu Jun 06, 2013 9:48 am
by Morganica
One bowl might be careless handling. Two makes me suspect the glass. If it's transparent, what does it look like under a polarizer?

As David said, how thick is the glass? Is it the same thickness throughout? What is the full schedule? Has the piece been fused to a single, flat unit or can you still feel individual pieces of glass? What colors of glass are you using? Are you sure that all the glass is System 96? Had it been fired before you began cutting it into strips?

A picture would be very helpful.

Generally, if you hear a bang in the next room and find the glass in pieces, the glass was under a fair amount of stress that just gave way. One side might have warmed a bit more than the other, by gallery lights or the sun coming through a window, etc. The slight expansion that caused would be enough to pull the glass apart. Or someone might have touched it, just so.

Re: Thermal shock or careless handling?

Posted: Thu Jun 06, 2013 11:34 am
by Bert Weiss
I don't think well in ºC or in dph, so I didn't study your whole schedule. Your anneal soak at 510ºC is too hot in my estimation. I would soak at 490ºC. This could be causing your problem.

Re: Thermal shock or careless handling?

Posted: Thu Jun 06, 2013 12:13 pm
by Yardic Glassworks
Photos would be a great help but I think that you will find that this is an annealing issue. Your hold temp is appropriate for System 96, however IMO the hold is not near long enough. I work with strips in both System 96 and GNA and use an initial hold of 180 minutes, and a secondary hold of 60 minutes at the next level. Also, you indicated that these were bowls. Was this your fusing program, or slumping program?

Re: Thermal shock or careless handling?

Posted: Thu Jun 06, 2013 12:22 pm
by Morganica
The Glass Hole wrote:Photos would be a great help but I think that you will find that this is an annealing issue. Your hold temp is appropriate for System 96, however IMO the hold is not near long enough. I work with strips in both System 96 and GNA and use an initial hold of 180 minutes, and a secondary hold of 60 minutes at the next level. Also, you indicated that these were bowls. Was this your fusing program, or slumping program?

I've "strip bowls" with strips that were anywhere from 6mm to 30mm wide. I would tend to agree that the annealing schedule (not just the soak, the entire schedule) was probably not long enough, but until we know the actual thickness I don't think we can say hold for "X" minutes...

Re: Thermal shock or careless handling?

Posted: Thu Jun 06, 2013 12:47 pm
by David Jenkins
Tim, Bert, Cynthia: Do you think that anneal ramp speed is appropriate for what's perhaps a 12mm or so blank? Just seemed pretty fast, to me.

Re: Thermal shock or careless handling?

Posted: Thu Jun 06, 2013 12:49 pm
by rosanna gusler
first post says finished bowl is 8mm thick, r.

Re: Thermal shock or careless handling?

Posted: Thu Jun 06, 2013 1:13 pm
by David Jenkins
You're right - 8 mm. At any rate (no pun intended), isn't that ramp speed very high?

Re: Thermal shock or careless handling?

Posted: Thu Jun 06, 2013 5:13 pm
by Yardic Glassworks
Many of my strip bowls are about 9mm thick. I often make them from multiple blanks that are cut up and rearranged. In making the initial blanks I ramp down from the first hold sometimes as fast as 100 dpi, but slow down substantially on subsequent firings. I use a glass cutter rather than a tile saw to cut most blanks which is very telling if you have not annealed properly. My slowest firing by far is the final slump, both up and down. My schedules vary based on glass used as well, but also on how much time and money is involved. Being conservative, and patient, can be a good thing.

Re: Thermal shock or careless handling?

Posted: Thu Jun 06, 2013 5:38 pm
by Bert Weiss
There is only one simple test for a ramp up speed. Did the glass get hotter than the strain point without breaking. If it did, you didn't go too fast. As noted this speed should slow down as the project gets more complex. During the first firing, each individual strip is being heated and expands next to other glass. Once it has been fused, the whole piece expands as one. This is a far greater challenge.

Glass anneals faster at the lower end of the annealing range. Bullseye and Spectrum anneal in more or less the same range. I have no clue what Spectrum recommends, but 950ºF is too hot for an effective anneal soak. 930ºF, was the old number and 900ºF is the new number, hence my recommendation of 490º C.

Re: Thermal shock or careless handling?

Posted: Thu Jun 06, 2013 7:16 pm
by David Jenkins
I guess my concern was with the 83dph (I guess that's 83 deg. C?) ramp down. Shouldn't that be on the order of 48 deg. or so?

Re: Thermal shock or careless handling?

Posted: Thu Jun 06, 2013 8:13 pm
by Kim Manley
Am currently using 25 dph from 490 to 425(?) then 100 to 250. The question mark is because I am at work and can't actually go and check.

Re: Thermal shock or careless handling?

Posted: Thu Jun 06, 2013 9:05 pm
by AileenK
Thanks for all your very helpful answers, I really appreciate the time everyone has taken to reply. I will convert the Celsius to Fahrenheit so it makes it easier to read. I will try to include all the extra information that you have asked for. I guess I have a whole lot of things going wrong with my temperatures. With all the answers I have received, it seems to be definitely an annealing issue
The glass is system 96, I can’t get any other type in the area I live. I have used the Spectrum 96 firing guide, but results show it needs to be tweaked a fair amount.
I cut the strips to 3/8 inch. I use a mix of opal and transparent. It is the same thickness throughout. It is fused to a single flat unit. Unfortunately I don’t have a polarizer to check the transparent strips.
The Fusing program is
Seg 1 332 – 1150 hold 30 mins
Seg 2 232 – 1370 hold 17 mins
Seg 3 432 – 1480 hold 12 mins
Seg 4 9999 – 950 hold 65 mins
Seg 5 181 – 797 hold 10
Seg 6 332 – 100 – leave til room temp

After cold working, I generally fire polish as well.

The slumping program is
Seg 1 181 – 298 hold 15mins
Seg 2 332 – 1099 hold 20mins
Seg 3 181 – 1150 hold 15mins
Seg 4 432 – 950 hold 60 mins
Seg 5 181 – 797 hold 10 mins
Seg 6 332 – 100 leave til room temp

Having problems with attaching an image, I will figure it out eventually!
Thanks again
Regards, Aileen

Re: Thermal shock or careless handling?

Posted: Fri Jun 07, 2013 12:22 am
by Bert Weiss
for 10mm Stone says drop to 950ºF hold 25 minutes, 54dph down to 861ºF, 108dph down to 771ºF, 575 dph down to room temp. Note that the anneal takes place during the 1 hr 40 min second step, not the soak at 950.

The strategy I use is a bit different. I soak at 920 for 1.2 hours then take 1.2 hours down to 820. .8 hours down to 620 and .8 hours down to 320. (I don't program in dph so you have to do the math)

Bullseye starts at 12mm with this schedule:drop to 900ºF hold 2 hours, 100 dph down to 800ºF, 180dph down to 700, 600dph down to room.

Bullseyes 12 mm schedule takes 3 hours to soak and drop to 800. My schedule for 10mm takes 2.4 hours to make that soak and drop. I'm working 20ºF warmer. My schedule for 12mm takes 4 hours to soak and drop 100ºF.

I get it that this can be confusing. There are different approaches that can all get the annealing job done, but that said, if you do it wrong, you have too much stress left.

BTW if you can get a cheap pair of polarized sun glasses, pop the lenses, and you have a little polarizer to examine your clear strips with.

Re: Thermal shock or careless handling?

Posted: Fri Jun 07, 2013 1:50 am
by AileenK
Thanks Bert, will adjust my schedule accordingly. You've mentioned the Stone book a few times, think I might finally purchase one.

Re: Thermal shock or careless handling?

Posted: Sat Jun 08, 2013 6:06 pm
by Stephen Richard
" I will convert the Celsius to Fahrenheit so it makes it easier to read. I will try to include all the extra information that you have asked for. I guess I have a whole lot of things going wrong with my temperatures. "

Don't pamper to the lazy Americans, Alieen. They need to get with the rest of the world. :-)

And anyway there is a conversion tool at the top of each page of the site.
Perhaps Brad should put up a conversion between rates per hour and time to achieve target temperatures in addition.