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Go and stop and go? Or just go?

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Jerrwel
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Re: Go and stop and go? Or just go?

Postby Jerrwel » Tue Jul 10, 2012 5:41 pm

Brad Walker wrote:I've noticed that many of the firing schedules that are in the canned programs, as well as some published schedules (such as the slumping schedule recommended by System 96), call for a ten to fifteen minute hold on the way up through the initial heating range. Usually the hold is around 500F, but can be as low as 300F or even multiple holds every few hundred degrees.


Brad: After purchasing almost a year ago, I just fired up my Olympic 3014GFE; I read the product manual and saw that there is a program guide which contains holds in all segments in all sample programs to include holds @ 250F, 500F, and 750F. Just an indication of how prevalent holds are becoming.
Jerry

dawnsud
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Re: Go and stop and go? Or just go?

Postby dawnsud » Tue Jul 10, 2012 8:40 pm

More holds====more wear on the controller and relays===more replacement, maintenance? Just a suspicious person.
Take care.
Dawn

Jerrwel
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Re: Go and stop and go? Or just go?

Postby Jerrwel » Wed Jul 11, 2012 2:27 pm

dawnsud wrote:More holds====more wear on the controller and relays===more replacement, maintenance? Just a suspicious person.


Detroit found out what 'built-in obsolescence' did to their products' reputations; I don't think kiln makers would benefit from that strategy. I'm thinkin' those holds will prevent user blunders in their projects and keep them involved in the medium.
Jerry

Morganica
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Re: Go and stop and go? Or just go?

Postby Morganica » Wed Jul 11, 2012 3:24 pm

I don't know if a hold is any worse on relays than a ramp up or down. The guys who fix/replace my controllers are usually more concerned about long schedules used in casting, which can wear out mechanical relays amazingly quickly.
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Babette (Shawn)
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Re: Go and stop and go? Or just go?

Postby Babette (Shawn) » Fri Jul 13, 2012 7:34 pm

In an earlier post regarding this topic there was a reference to Boyce Lundstrom. Boyce is a legend and he is a maverick in the glass world. I had the opportunity to take a workshop with him a few years ago and I think I have almost recovered. At the time of this workshop he was just starting to develop a new product line based around metal inclusions, dirt, household chemicals, and his "Raku" glass method. So Boyce ran this "special" workshop and gathered "students" so we could all experiment and he could see what we came up with using some of his ideas. A memorable event was when we used cars to run over thin metal to create embossing with plastic leaves and found objects. A year or so later Boyce came out with a book and products developed from these early experiments.

The class was very informal and Boyce was sharing his knowledge and ideas (many many ideas) and stories from the past. At that time Boyce was pushing the limits with his firing schedules...in this class Boyce would vent kilns too cool at 500F and pull hot pieces out of the kiln and wrap them in towels. He told us if you carefully reach into the kiln and flip the glass it would help to cool the glass more quickly. To this day I do not understand why we did not break pieces in that class. Needless to say this philosophy seriously messed me up. Glass was not my friend after that class. Boyce told us a story that once he was in a such hurry to get to a show he pulled a piece from the kiln before it had completely cooled, so he wrapped it in newspaper (his preferred method for insulating glass) and he hit the road only to find a short while later that he had a fire in the back seat of his car! Boyce teaches students to experiment and question the rules...so I came home and broke every piece of glass in sight. Pretty soon the pendulum swung and I slowed things down to a point where I am embarrassed to admit that one simple hold at 300F on the way up...is meager. So now I am in the process of bringing my schedules back up, this time with a plan to be both efficient while also reasonably cautious. Don't get me wrong, it was a great class and Boyce gave us all some wonderful marbles, I just wish he hadn't dropped a few bombs. I would take the class again, now that I know what I now know.

The moral to the story is: there are many reasons why there are ridiculous schedules floating around out there. Sometimes schedules work once or twice but eventually bad schedules fail but overly cautious schedules never fail.

PS I heard recently that Boyce is very seriously ill. [-o<
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Bert Weiss
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Re: Go and stop and go? Or just go?

Postby Bert Weiss » Sat Jul 14, 2012 9:41 pm

Babette (Shawn) wrote:The moral to the story is: there are many reasons why there are ridiculous schedules floating around out there. Sometimes schedules work once or twice but eventually bad schedules fail but overly cautious schedules never fail.


Shawn, like many rules, this one can be wrong. If your kiln has an area that is hotter or colder than 5 degrees C, than the rest of the kiln, and you hold too long at anneal soak, you will introduce this gradient in to the glass and make it impossible to anneal. If you had ramped down quicker, it could have been OK. It is a common misconception that you can't soak too long. (this does not apply to something as small as a pendant.)

Sorry to hear about Boyce.
Bert

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Lynne Chappell
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Re: Go and stop and go? Or just go?

Postby Lynne Chappell » Sat Jul 21, 2012 3:03 am

Now that's a concept I had never thought of. Uneven cooling means you should cool more quickly? Is it really true?

Personally I think that the holds in the canned programs are just there so that everything looks more the same. Rate, set point, hold. If every segment has numbers in 3 places it makes more sense to beginners (who after all are the ones using the canned programs).

Does Spectrum really have a slow ramp up to 300F? Weird. When I have pushed the limits on the heatup, my glass has always broken when the pyrometer says 900F. I never have a hold in the initial heatup unless there is something in there that has water in it (mold, freeze & fuse, etc).

Bert Weiss
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Re: Go and stop and go? Or just go?

Postby Bert Weiss » Sun Jul 22, 2012 11:16 pm

Lynne Chappell wrote:Now that's a concept I had never thought of. Uneven cooling means you should cool more quickly? Is it really true?

Personally I think that the holds in the canned programs are just there so that everything looks more the same. Rate, set point, hold. If every segment has numbers in 3 places it makes more sense to beginners (who after all are the ones using the canned programs).

Does Spectrum really have a slow ramp up to 300F? Weird. When I have pushed the limits on the heatup, my glass has always broken when the pyrometer says 900F. I never have a hold in the initial heatup unless there is something in there that has water in it (mold, freeze & fuse, etc).
Lynne This theory could be proven by embedding thermocouples in glass, in places where there is a strong uneven temperature gradient, and recording the changes as you hold for a long time at an anneal soak temperature. A typical uneven place would be close to the walls of a brick walled kiln.

The theory is simple. If you hold your glass too long in an uneven temperature environment, you will eventually introduce this uneven gradient in to the glass and make it impossible to anneal, as annealing takes place only when 100% of the mass of glass is within 5ºC inside the anneal range. There are definitely situations where a quicker soak will result in the intended goal of being all within 5ºC, in the right range.
Bert



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dee
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Re: Go and stop and go? Or just go?

Postby dee » Mon Jul 23, 2012 10:58 pm

the only time i've done extra holds on the way up/down is when i've included copper tubing for channels and then it's in the fire polish firing to avoid cracks around the tubing area. otherwise, i never had a need to stop and hold on the way up to 1100...
D
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Bob
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Re: Go and stop and go? Or just go?

Postby Bob » Tue Jul 24, 2012 3:13 am

Interesting thread.

I once asked a studio owner why they put a one minute soak in at 1000f and was told, and he was not joking, that it gave the molecules an opportunity to all get aligned. We had a "lively" discussion and I was unable to convince him that the one minute soak did nothing for the alignment of the molecules.

Just to be a contrarian.... I sometimes use a soak at 500F when I am fusing large wafers onto the top surface of glass. My experience was that the glass thermally shocked immediately beneath the wafer and I attributed this to the wafer acting as a thermal blanket. Since glass is a poor conductor of heat the glass immediately under the wafer was cooler than the surrounding exposed glass and the differential expansion due to the difference in temperature caused thermal shock. I tried slowing the ramp rate down and still got some (but less) breakage. The soak for about 20 minutes at 500F, in combination with the lower ramp rate, seems to have fixed the problem. I guess I could slow the ramp rate down even more and eliminate the soak but I reasoned that it would be more efficient to use a soak rather than slowing the ramp rate down.

I agree with all of the points that have been discussed above. This one case seemed to benefit from using a soak. It is on of those "if it works don't argue".

Cheers,

Bob

Nancy Juhasz
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Re: Go and stop and go? Or just go?

Postby Nancy Juhasz » Wed Jul 25, 2012 9:25 pm

I only do holds on the way up to 1100 if the piece is very thick and it is my second firing. I am a slow firer so I don't need holds. I use a bubble squeeze almost all the time just because that is what works in my kiln. Just because you can doesn't mean you should.


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