My glass cracked

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Judy Schnabel
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My glass cracked

Post by Judy Schnabel »

I've been playing around with pattern bars and had two successful fusings and slumpings using the following schedule:

Fusing:
250 to 1000
150 to 1250
400 to 1400 hold 20 minutes
999 to 955 hold 90 minutes
(forgot to put my normal 150 to 750)

Slumping
200 to 1170 hold 20 minutes
999 to 955 hold 90 minutes
150 to 750

Yesterday I put my fused glass into my slumping mold and before it reached 1000 I looked into the viewing window and thought I saw a "line."
I lifted the lid and this is what I saw.

http://www.picturetrail.com/gallery/vie ... &members=1
999 to 955 hold 90 minutes

The pattern bars in this particular piece did have cathedral glass mixed with opals.

Comments?

Judy
charlie
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Joined: Mon Mar 10, 2003 3:08 pm

Post by charlie »

thermal shock. heated too fast. maybe baffle and go would help. the points of the ends are significantly near the sides of a side fired kiln. this probably wouldn't have happened if the piece was round.
Judy Schnabel
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Joined: Mon Mar 10, 2003 9:45 pm
Location: SW FL
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Post by Judy Schnabel »

Charlie,

Thanks for your quick reply.

If that's the case, why were two previous firings successful? When I have a successful outcome, I try loading one kiln with the fusing and after that shift it to the kiln where I had the successful slumping. Sort of like once I get it right I'd better stay on a roll.

I have the pattern bars with orange in them fused to orange glass in the slumping mold right now, but started out at 150 dph.

Judy
charlie
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Post by charlie »

luck.

the base glass should have had a flaw it it? right on the edge for annealing in the previous firing? slight coe shift by accident during the making of the pb?
Andrew
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I do a slower drop

Post by Andrew »

My schedule for similar firings is a little different. I go up a lot faster and come down a bit slower:

600 dph to 1100
Soak 20 minutes
750 to 1470
Soak for 20 minutes
999 to 1000
150 to 960 hold for 30-60 minutes
100 to 955 hold for 60-30 minutes
250 to 800

My rational is that kilns tend to hover around a temperature Sometimes they overshoot or undershoot- They also vary at holding a particular temperature (depending upon the kiln). So you might intend to anneal at 955, but (depending on the kiln) your actual temperature may be varying 10 degrees either way. Another reason is that I don't trust that the numbers I see on the readout are absolutely calibrated. So I combine a shotgun annealing (slow pass through the range of annealing temperatures) with a soak bracketed around where it is "supposed" to be.


Andrew
Judy Schnabel
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Post by Judy Schnabel »

Thanks, Andrew.

I went out to adjust my decent and am following your suggestion.

Did you get to Sanibel this Christmas?

Judy
Cynthia

Post by Cynthia »

Judy Schnabel wrote:Charlie,

Thanks for your quick reply.

If that's the case, why were two previous firings successful?
I have a couple of thoughts as to why.

First you said that you forgot to plug in your cool down through to below the lower strain point. This could have made this piece more vulnerable to thermal shock.

Second, I see some relief in your picture of the pattern bar piece that cracked, and if that is the case, 200 dph is pretty fast for a piece with any relief. It's really hard to evenly heat the body of an uneven glass, so you really have to slow it down for that...and the annealing too. Turning off, or baffling your side elements would help with this also.

You are absolutely right to use tried and true schedules. But my third thought is that you aren't firing the same piece twice with this kind of work. So your tried and true's need to be thought of as your best efforts at a stable foundation that just might need to be tweaked. :wink: Because the inclusion glass has been fired, cut, fired again and then yet again to bend, you are firing a different composition each and every time. This inclusion glass is full of different colors, manipulated and cut and fired so much that you have to be very frugal with your schedules because this glass is more tempermental than cut bits of glass out of a sheet.

I have been doing a body of work that is similar to yours in that it is composed of previously fired and manipulated inclusion glasses fired onto an opal base, fired again to bend. I too have schedules that are tried and true for every stage of the work...yet once in a blue moon, A peice will thermal shock in the final bend. :?
I have the pattern bars with orange in them fused to orange glass in the slumping mold right now, but started out at 150 dph.

Judy
Lisa Allen is the Queen of pattern bar work in these parts. She fires with unbelievably conservative schedules. I assume it is on account of the unpredictable stability of the inclusion glasses. So your inclination to dial down that ramp rate is a good call. Also, think about your annealing schedules with this glass. YOu might want to err of the side of over caution with these beautiful pieces.
Tony Serviente
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Post by Tony Serviente »

Jusy-As to why it worked before and not this time; I have an item in production that is a high relief fused design on a 16 x 16" base. If I fire it in my top fired kilns I get a good slump every time. If I fire it in my top and side fired octagonal kiln I get about 7 out of 10. For my situation it is entirely a matter of element placement, and closeness to the glass. My top fired only heat more uniformly. Now, if I had been doing that item for the first time, and I fused up 10, and lost three in a row in the octagonal kiln, I might have given up, but the next seven would have been perfect. So...sometimes the success or failure events "clump", and you just have to fit that into a bigger picture.
Judy Schnabel
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Post by Judy Schnabel »

Thanks Cynthia & Tony as I wipe the tears from my eyes.

Seven out of ten? I hope I'm up to that.

Judy
Judy Schnabel
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Post by Judy Schnabel »

Update on this subject.

I fired two more platters with pattern bars and had good success.

http://www.picturetrail.com/gallery/vie ... &members=1

However, I tried the white again with the blue and green pattern bars and had the same results as before. The piece cracked into three pieces. This time I believe it cracked on the way down since I let it go through the complete firing cycle and the cracks were very sharp.

Judy
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