Annealing question

[Chris Lowry]

I was reading about annealing and I can't believe the suggested times. The Graham Stone book was suggesting, 1 inch thick piece held at annealing temp for 12 hours.

I've been holding for 5 hours and have seen no trouble. Of course not cracking doesn't mean much but I have cut and ground my pieces with no trouble.

How long would you hold a 1" thick piece in a dam mold

[Babette (Shawn)]

Put it in the freezer, then put it out in the sun. Look at it under a polarized film. Test it to see if there is stress, but in the meantime don't sell it and attach a warning if you gift it. Think about if the risk worth a few additional hours of kiln time.

[Chris Lowry]

So what is your suggested time?

[Chris Lowry]

Was just looking at bullseye... They suggest 4 hrs hold for 1" thick.

I never understand why there is such huge differences in schedule times.

[Brad Walker]

How long would you hold a 1" thick piece in a dam mold

I would probably follow Bullseye's schedule, which calls for a 4 hour hold, following by about another 6 hours of controlled cooling in the annealing range. It's not just how long you hold, it's also how quickly you cool after the hold. You can see that here: http://www.bullseyeglass.com/methods-id ... slabs.html

Graham has a number of different schedules for 1 inch thick glass, but his standard 1" schedule (pg 62 of his book) calls for a 1 hour 40 minute hold at the annealing temperature, followed by a much slower cooling rate than Bullseye. Graham takes around 16.5 hours to cool through the annealing range, or around 18 hours total vs. 10 hours total for Bullseye.

The difference between the two is a matter of philosophy: longer hold/quicker controlled cool vs. shorter hold/slower cool. As with most differences of philosophy, arguments could be made for either side, but the pragmatist in me would tend to side with the shorter annealing schedule, which is Bullseye's.

Also, most kilns would take much longer than Bullseye's schedule to cool from 700F to room temperature, but the really important part of the annealing process is in the 1000 to 800 range. And, most importantly, I suspect your five hour hold was sufficient to anneal the glass, assuming it was a simple slab in a standard dam.

[Chris Lowry]

Thanks Brad, I have a hard time understanding Graham's book. I've been a glass blower for 28 years and never had trouble with my annealing but kiln work is new to me and didn't know if I was missing something. What you said makes sense and follows my common sense approach to glass.

[DonMcClennen]

I've been doing thick work (1 1/2" approx.) for several years and have always followed the Bullseye schedules for thick work as Brad suggested. Never had any failures with "90" or "96" pieces.

[Bert Weiss]

Thanks Brad, I have a hard time understanding Graham's book. I've been a glass blower for 28 years and never had trouble with my annealing but kiln work is new to me and didn't know if I was missing something. What you said makes sense and follows my common sense approach to glass.

There is an essential difference when annealing sheets and blown objects. A blown object is able to cool from all sides but the bottom. A sheet of glass on a kiln shelf is really cooling from one side. This is an entirely different challenge. Of course glass actually anneals in about 15 seconds, when the entire mass of glass is within 5ºC inside the annealing range. Getting there is the challenge. It requires evenivity.

Back in the early 80's I wanted to learn how to anneal thick float glass, and there was nobody to tell me how. I was fortunate to have access to an engineer at PPG research. He gave me a formula for developing anneal soak times relative to thickness. The catch was that the formula had a variable in it, and he didn't give me that value. At the time, I had Fusing Book 1 by Lundstrom. I also had messed around enough to guess at a value for that variable. I guessed well, and came up with a set of anneal schedules for float glass. Fast forward to the first Warm Glass Conference in Portland OR. By that time, I had a copy of Stone's book. Graham used a short soak at a higher temperature than I did, and then took a long ramp down. When I looked at our schedules together, we were taking about the same amount of time, but spending it at different temperature points. I asked Dan Schwoerer, Bullseye's CEO about this, and much to my surprise, he told me my strategy was better because glass anneals faster at the lower end of the anneal range. A few years later, Bullseye revised their annealing schedules for thick glass to reflect this strategy. I think you can trust their times. To develop these times they did a lot of testing with multiple thermocouples. Some were in the air, and there was one under the glass, one embedded in the center of the glass, and one sitting on top. They could track how much time it took to equalize the glass, and note how the glass temps compared to the air temps.

[Chris Lowry]

Another question here... I'm making thick slabs and then slumping them. If I have a little stress from the fusing part can I get it out by slowing down the slumping?

I was thinking of adding some more time to my fusing schedule but maybe I should just add time t the slumping schedule... What do you think. Any advantage to one or the other?

[DonMcClennen]

If you have stress remaining from your fusing you may experience breaking the piece when you reheat for slumping!

[Chris Lowry]

Yea... But can you actually release stress at the low temperatures of slumping?

On another note, if all the stress is out from fusing does that mean the slumping annealing schedule can be shorter?

Just interesting thoughts I don't know the answers too.


In blown work we assume that if you put a piece away with stress it takes 4 times as long to anneal. No math there just experience.

[Warren Weiss]

Yea... But can you actually release stress at the low temperatures of slumping?

Chris,
Slumping temps are high enough (over 900f) to require a correct annealing. The annealing hold at 900 f serves no other purpose other than getting a uniform temp from top to bottom and side to side of the piece. The thicker the piece, the longer the anneal hold. Annealing starts as you SLOWLY lower the temp to about 700/750. The rate of drop is again relative to both the thickness and uniformity of thickness of the piece. It has to cool uniformly or stress is induced. Remember, it can cool quicker from the top than from the bottom which is sitting on the hot kiln shelf. A bit tricky, but very important.

Warren

[Haydo]

Have you ever considered transferring the slabs to the slumping mould in the same firing, would save a ton in time with the cautious reheat.

[Chris Lowry]

So a hold at annealing temperature is to make sure you have uniform temperature threw out the piece.

When I'm fusing I'm dropping as fast as possible from 1500 to 890 then holding. It seems like it would take a long time to get even temperature. Would it be better to hold for an hour at say 1000, which might help a thick piece even out faster?

On the same idea when I'm slumping at a fairly low temperature it seems like it wouldn't take as long to get the whole piece back to even temperature.

For this reason annealing schedules seem like they should adjust not only for thickness of piece but also how hot it was before it got to annealing temperature.

[Peter Angel]

It may crack months or years later if you don't anneal it properly.

[Warren Weiss]

It would take just as long to get even at 1000 as 900. Then you have to add time to get it down evenly to 900 slow enough to keep it uniform. It doesn't matter how hot it once got. All of the stress is avoided starting at an EVEN 900 and going very slowly (80-100 f/hr., depending on thickness) to 750/700.

Warren