My first big break! But not the good kind. What happened?

[Studiodunn]

My thinking is that I may have gone too fast on the way up @ 150/hr to 1000. This was a third firing and it was the first time I tried using the top elements on my new kiln.

Incidentally, after the second firing on this piece, I had some small stress fractures starting in the center through the clear. Since I am still learning, I came here looking through archives for a potential schedule to help take care of the cracks before they made it through the entire piece. I found a schedule posted by Lisa Allen years ago, modified it for System 96 and sure enough those cracks appear to have healed on this firing. So I was on the right track, but not quite...lol Ugh!

[Morganica]

You need to post your full schedules, size and thickness of the piece, the whole schmear. Often the problem really isn't that the ramp-up is too fast, but rather that the prior firing didn't anneal long enough. When that happens the next heat application often adds just enough stress to crack the glass. We need the full schedule, and the number of times the components were fired going into this firing, to really figure that out.

[Studiodunn]

The piece it 12 x 12...It has a clear base, with a thin white layer with fritty details.

First firing

100 900 15
9999 1400 5
9999 950 30 (Probably not long enough, but my logic was it would need a second firing anyways and I would anneal longer then. Oops!)
50 800
100 700
300 100

Second firing
(Added more layers of frittiness in attempt to correct color. Thickest sections approaching 1/2 in thickness, but not necessarily that thick throughout the piece)

100 900 15
200 1370 20
400 1450 5
9999 950 120
150 800 10
300 100

Third firing
(Notice some tiny fissures forming in center so I fired a third time to attempt to heal the damage.)


150 1000 10
400 1350 30
9999 990 60
50 950 60
50 935 60
50 700


Considering how to make a mosaic out of it at this point....Learning is expensive! lol But I would like to know where I went wrong to avoid future surprises...

[Kevin Midgley]

Buy Graham Stone's Kiln Companion book. Read it 3 or more times cover to cover.
Then you'll understand just how wrong your schedules were and you can devise ones that will work in your kiln.
Somebody's schedules offered here are not the answer to gaining an understanding of what needs to take place.
If you want to do nice art work as shown in the photos, you owe it to yourself.

[Tony Smith]

Seriously Kevin? Did YOU read Graham Stone cover to cover? What did he say that's applicable here?

I think you're schedule's actually pretty good. Since you noticed fissures, I would have kept to your earlier schedule of 100 deg/hr or less on warmup rather than trying to increase the rate to 150 deg/hr.

Tony

[Kevin Midgley]

Yes, Tony, I have read Stone cover to cover 3 times and gained greater understanding of the nature of glass each time.
If you haven't done so, you do not understand as much as you could do about glass.
The schedules themselves are about 10% of the book's value, imho.
It is the notes that go with them all that enable you to properly formulate schedules for your particular kiln.
For you to say the schedule was pretty good without reading Stone the 3 times and not knowing the particular kiln or the setups or anything else about the thermal masses involved......

[Studiodunn]

This was the first time I tried the top elements in my new kiln. It was the biggest piece I have tried in it. Working larger is new to me. My thinking was that the top elements would heat it more evenly given the fissure situation. It definitely makes more sense to keep the first ramp at or below the ones from the first and second firings. I'm actually happier with the design, than the execution in this case...I made two other condensed versions, simply using cut glass and irids., and not using frit. They both turned out nice after one firing (60 minute anneal) Simple and streamlined.

I do have Graham's book, but I have been trying modified schedules from other books/sources. I'll definitely hunker down and try and absord it again (and again) after this humbling lesson.

Thank you for sharing your thoughts...

[Tony Smith]

Yes, Tony, I have read Stone cover to cover 3 times and gained greater understanding of the nature of glass each time.
If you haven't done so, you do not understand as much as you could do about glass.
The schedules themselves are about 10% of the book's value, imho.
It is the notes that go with them all that enable you to properly formulate schedules for your particular kiln.
For you to say the schedule was pretty good without reading Stone the 3 times and not knowing the particular kiln or the setups or anything else about the thermal masses involved......

You didn't answer my question. WHAT was applicable in Stone's book to this situation?

Tony

[Kevin Midgley]

about the thermal masses involved......

If you'd read and understood Stone, it is all about the thermal masses and heat flow that determines optimal firing schedules.
You have to read Stone and think about what you are doing in order to determine what is right in your kiln.
EVERYTHINGin his book is applicable to this situation. Ignore it at your glass breakage peril.

[Brock]

I think the short answer would be, that by reading and understanding Stone's book, you would have the ability to devise a firing schedule that would not thermal shock your work.
Mantra: The more you know, the more you know . . .

[Tony Smith]

Let me try to be a bit more helpful than I was last night.

In a kiln, glass is heated three different ways: the first way is through radiation where heat from the elements is radiated directly to the glass (imagine the sun on your face); conduction is where the glass is heated by direct contact with the shelf or dam; and convection is where hot air circulates inside the kiln and heats up objects it contacts. When one side of the glass heats up faster than the other side, a thermal gradient is created and this gradient creates stress in the glass - the greater the temperature difference, the higher the stress. A temperature difference over a very short distance can create very high stresses. Temperature gradients can be reduced by heating evenly or by heating slowly.

There's a field called fracture mechanics which says that for a given defect size, there is a minimum stress that will cause that defect to propagate into a crack. The details aren't really important, but it says that when we have a known defect in the glass, we have to keep the stress down or the glass will crack. If we're going to heat the glass, we have to do it evenly, slowly or both.

If you picture what happens when you use top elements, the radiational heating from the elements heats the top surface of the glass first. The bottom of the glass is on the kiln shelf which is not as hot as the top of the glass. By slowing down, the edge of the shelf is also heated by radiation and that heat is conducted into the shelf under the glass which reduces the temperature difference and helps to heat the glass more evenly.

In your case, this was the first time you used the top elements, and you had a thermal gradient from the top to the bottom which caused the stresses to get high enough to propagate a crack. If you didn't have the fissures in the glass, you might have survived without a problem. Having defects in the glass are a warning sign to slow way down. So are rapid changes in thickness such as you might find with tack-fused features.

I hope this helps.

Tony

[Studiodunn]

Tony, that helps immensely. The way you explained it has really helped me begin to visualize where things went wrong. THANK YOU!

Making my way through Graham's book I have also discovered that heat soaking for 20 minutes every 250 degrees up to 1000 would have been beneficial as well.

Well, my wall of shame had to begin somewhere. If I didn't have one, it would mean I'm not trying. But I'm the type of person that will get mad enough NOT to let it happen again, if I can help it...lol

[twin vision glass]

NO shame at all. It is a work in progress and so very much to learn to finally understand all the things that happen in different kilns and different set ups.( kiln shelves out of Mullite will react differently to fiber board shelves) I am not with the thought of holding every 250. A longer gradual temp rise has worked much better for myself. The day I slowed down for each different project , was the day I started to save money on glass and time. Because I work fairly deep these days, I have found slowing even to 50 deg. per hour on heat up especially in a slump has helped a great deal, and bottom and top elements as well are super for even heating. You will have to learn your kiln and how it reacts , and Bullseye has some excellent tips for us all on their site. Each kiln is so different , and comes with it's own set up gifts and problems, so slow down until you are really confident in your schedules and write lots of notes and take photos and start a great glass scrap book of ideas. Then you have a reference to fall back on.
Leslie

[Rick Wilton]

Most people here will tell you that holds every 250 degrees is a waste of time. The point being is that there is not a right and wrong way to do this. If it works and holds up (ie annealed properly) then it's right. Find what works for you and go with it, anyone can give you there opinion, but what works for them may not work in your kiln.

[JestersBaubles]

IMO, my kiln, my experience, etc etc, and everything I've read, 900 for your first hold it a little too low. You didn't mention what kind of glass (or maybe it got lost in the noise about who reads what books), but Spectrum suggests 1100 these days.

Dana W.

[Mary Lou]

Why are you holding at 900 in your 1st and 2nd firing?

[Tony Smith]

There are artifacts in a lot of schedules that have lost meaning over the years. Some people hold at either 800 or 900 as a way to slow down near the quartz inversion temperature of clay. That temperature is actually closer to 1050 F. Personally, I think this isn't necessary, but cleaning up people's schedules could become a full-time profession.

Tony

[Mary Lou]

I know what it should be and I feel it is necessary but I'm asking studiodunn if he/she, knows why the hold at 900.

[Studiodunn]

I was holding at 900 per a firing schedule I found listed on Fusedglass.org to ensure burn off of Glass tac because of the fritty elements in this piece. Made sense to me...I'm happy to hear your thoughts on that.


Thank you Leslie and Rick for your encouragement and sharing insights about holds every 250 degrees on the way up...It's an interesting example of how trying it is to make sense out of "conflicting" information that is out there....Even information coming out of the glass gospel according to Graham

[Studiodunn]

And Mary Lou, my name is Brandie...I am a happily married mother of four children ages 7 through 12.