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Thread: Borosilicate Annealing Schedule Discrepancies

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    Icon5 Borosilicate Annealing Schedule Discrepancies

    Disclosure: Long post – buckle up. I’ve tried to do this all accurately with references – I could have easily made a mistake.
    Searching for annealing schedules from what I would deem trusted sources produces a wide variety of key temperatures, ramp times, hold times, and overall cycle times.

    I understand that there are many ways to achieve the same goal with annealing. Also different kilns and different shape glass structures may require special cycles. However, I believe it would be of great benefit to the community to produce an accurate general table of annealing schedules / annealing cycles for borosilicate glass ranging in thickness from 1mm up to 152mm thick.

    What surprises me, is how dramatically different the information about annealing is from one source to another.
    This leads to the questions of: Is there an accurate set of formulas for calculations? Which source should be used as a trusted guide for annealing? How can we get semi-accurate schedules for a given thickness of borosilicate?

    It is hard for me to find multiple sources for annealing information about thicknesses of borosilicate greater than 0.5 inch (12.7mm) so I will use that dimension as the comparison thickness from each source. Below are general kiln annealing cycles which range from 12mm to 12.7mm thickness.

    Northstar for 0.5” (12.7mm) thick boro:

    Source: http://northstarglass.com/users-manual/annealing/

    1050F (566C) for 2 hours
    925F (496C) for 1.7 hours
    850F (454C) for 0.5 hours
    700F (371C) for 0.5 hours
    500F (260C) for 0.3 hours
    Crash
    Total Hours: 5 hours (300 minutes)

    No ramp times listed. No times for initial heating. Northstar provides calculations to determine hold times, but unfortunately the formula doesn’t reflect accurately into the chart of times they provide.
    (Example: the above 925F hold should be for 2 hours and the 500F should be for 0.5 hours according to their formula.)
    The times depart greatly from their formula the greater the thickness of the glass.

    Simax 12mm thick boro:

    Source: http://www.simax.com/en/section/32-simax-glass-mass.html
    (towards bottom of page)

    Ramp from 20C to 550C at 8C per min (66.25 minutes)
    Ramp from 550C to 560C ?
    Hold at 560C (30 minutes)
    Ramp from 560C to 490C at 0.6C per min (116.67 minutes)
    Ramp from 490C to 440C at 1.6C per min (31.25 minutes)
    Ramp from 440C to 40C at 8C per min (50 minutes)
    Crash
    Total Minutes: 294.17 minutes (4.90 Hours)

    No ramp time listed between 550C to 560C.

    Bandhu Dunham - 0.5 inch (12.7mm) thick:

    I do not own Bandhu Dunham’s books but I have used the chart provided on this forum by nodice:
    Source: http://www.talkglass.com/forum/attac...4&d=1206061572

    Ramp from room temp 68F (20C) to 1050F(566C) at 107.50F per min. (9.13 minutes)
    Soak for 15 minutes at 1050F (566C) (15 minutes)
    Ramp 1050F (566C) to 910F (488C) at 18.75F per min (7.47 minutes)
    Ramp from 910F (566C) to room temp 68F (20C) at 72.5F per min (11.6 minutes)
    Crash
    Total Minutes: 43.2 minutes (0.72 Hours)

    Pyrex corning 7740 - 0.5 inch (12.7mm) thick:

    Source: ASGS Fusion Issue #3 published in 1958

    Room temp (20C) to 555C at 5C per min (107 min.)
    Hold 30 min. at 555C (30 min.)
    555C to 495C at 0.8C per min (75 min)
    495C to 445C at 1.6C per min (31.25 min)
    445C to room temp (20C) at 8C per min (53.125 min)
    Crash
    Total minutes: 296.375 minutes (4.94 hours)

    Schott Duran 12mm thick:
    Source: http://www.vidrasa.com/eng/products/duran/duran_pf.html
    (bottom of page)

    Room temp (20C) to 550C (?)
    Hold 30 min. at 550C (30 minutes)
    550C to 480C at 0.8C per min (87.5 minutes)
    480C to 400C at 1.6C per min (50 minutes)
    400C to 20C at 32C per min (11.875 minutes)
    Crash
    Total minutes: 179.375 (2.99 Hours)

    Some notes:
    It is interesting that the times to anneal roughly the same thickness of glass (12mm to 12.7mm) range from 0.72 hours to 5+ hours among the 5 above listed sources.

    It is also interesting that the referenced article for Duran glass mentions that annealing time at 550C should NOT exceed 2 hours or there may be a negative result to the chemical stability of the glass, yet the Northstar table recommends 24 hours at annealing temp for a 6 inch (152.4mm) thick piece.

    What do you all think?

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    Annealing is not a set temperature or time. It is range. There are many ways to get there and I may be able to properly anneal a piece in 1 hour that may take you 5 hours. So many variables the only logical solution is to check your work with a polariscope. You can easily make a DIY version for a couple bucks or buy a full fledged color version for hundreds of dollars.

    I know you are looking for a black and white chart but it just doesn't work like that. I can suggest a annealing cycle but like I said what works for me may not work for you.

    One thing to note which is true holding borosilicate glasses at high temperatures for long times can degrade the glass. This isn't something new but really rarely comes into play considering our tolerances.

    Im sorry I can't be more helpful. If you have a specific question I'll do my best to provide you an answer.

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    Default Re: Borosilicate Annealing Schedule Discrepancies

    Annealing is part science, part art, and part black magic. And I'm being serious.

    Every piece is different, with different wall thicknesses, different color combinations, and different masses. All you can do is get close and check your work with a polariscope.

    And I haven't even touched on some of the green based glass that has COE variations...

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    Default Re: Borosilicate Annealing Schedule Discrepancies

    Mike, does a polariscope work for colored glass too, or only clear?

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    Default Re: Borosilicate Annealing Schedule Discrepancies

    Concur. Only way is to check with a polariscope.
    But the ramp down from the "annealing" temp to the "strain" temp is the most crucial.
    Allowing the glass to slowly gain rigidity as the molecules slow down evenly.
    Here's my general schedule for .5", which is a combination of all that shit you posted above, plus
    A lot of trial and error involving a notebook and a polariscope:

    565C - 2 hours
    Ramp down at a rate of 55C an hour
    510C - 1 hour
    Full ramp down
    496C - 30 mins
    Full ramp down
    454C - 30 mins
    Crash to room temp

    I check my work with a polariscope and haven't seen stress since my divorce.
    It should be noted that each batch of glass may even have a slightly different chemical make up, so the annealing temp and strain temp may be slightly off from one to the other, plus: most times that you see these temps they're round numbers
    Which seems convenient, no?
    More proof that the temps are subjective.
    Quote Originally Posted by itssteve View Post
    Hey jimmi if this deal were to fall through (which I doubt will happen) I'll let my lynx go for a couple cooter pics of your sis. Pm me

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    Default Re: Borosilicate Annealing Schedule Discrepancies

    If I'm firing decals, then I bring it up to 610C, venting the door of the kiln 1/4" from 300C to 400C, hold at 610C for twelve minutes, then down to annealing temp at the rate that the kiln naturally drops, then the above schedule.
    Quote Originally Posted by itssteve View Post
    Hey jimmi if this deal were to fall through (which I doubt will happen) I'll let my lynx go for a couple cooter pics of your sis. Pm me

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    Default Re: Borosilicate Annealing Schedule Discrepancies

    Quote Originally Posted by Sparko View Post
    Mike, does a polariscope work for colored glass too, or only clear?
    Not on full opaque color.

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    Default Re: Borosilicate Annealing Schedule Discrepancies

    my annealing schedule is as follows...

    finish piece.
    pray it doesnt crack
    put in kiln
    pray it doesnt crack
    Run all night
    take it out
    check for inevitable crack


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    Default Re: Borosilicate Annealing Schedule Discrepancies

    ahh i see your problem:

    finish piece.
    pray it doesnt crack
    put in kiln
    pray it doesnt crack
    Run all night
    pray it doesn't crack
    take it out
    no crack!


    see the differences there? you forgot to pray before going to bed.
    you will shank me later

    Quote Originally Posted by FifDeez View Post
    I like the idea of burning water. Sounds mystical even tho I understand it completely.

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    Default Re: Borosilicate Annealing Schedule Discrepancies

    I wonder if the Duran guide regarding not exceeding two hours was referring to .5" or whatever.

    IIRC borosilicate clear glass "anneals" at 1050 degrees F, anything below this temp it is not technically "annealing." This may account for varying info regarding cool-down procedures.

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    Quote Originally Posted by Foxy999 View Post
    I wonder if the Duran guide regarding not exceeding two hours was referring to .5" or whatever. IIRC borosilicate clear glass "anneals" at 1050 degrees F, anything below this temp it is not technically "annealing." This may account for varying info regarding cool-down procedures.
    Annealing is not a temperature and can not be defined that way. You can relieve stress at 1050f but think about it if you take it out and set it one the bench what do you think will happen? Annealing is a range of time and temperature. It's a process that cools the glass down to the strain point in a controlled manner to not reintroduce stress in the ware.

    Yes the Duran guide is referring to scientific ware but it is true for any thickness. In my experience you won't notice any degradation until many many hours at holding temperature (1050f). I see it in thin ware that has been repaired many times.

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    Default Re: Borosilicate Annealing Schedule Discrepancies

    I appreciate the comments from everyone.

    I found this article that might be valuable to others trying to make their own calculations and annealing schedules customized to their work.

    Casting - Practical Annealing - lecture given by Dan M Watson
    http://www.gafferglassusa.com/index....echnical_id=18

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    Default Re: Borosilicate Annealing Schedule Discrepancies

    Boro will anneal below 1050
    It just takes longer
    Quote Originally Posted by itssteve View Post
    Hey jimmi if this deal were to fall through (which I doubt will happen) I'll let my lynx go for a couple cooter pics of your sis. Pm me

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    Default Re: Borosilicate Annealing Schedule Discrepancies

    Do you want an honest answer?

    I've been lucky enough to visit several shops.

    Here is what the majority does , and yes, I've seen fully worked pieces treated like this, pieces with 40+hours put into them, over 100 worked sections, 5 pounds or so of premium color ....

    Set the kiln to 1050.

    Fill it with work through the day.

    Take an hours or so to clean the shop, have a couple beers and a few puffs.

    Turn it off and leave to cool slowly overnight.


    That's it. One thing tho, make sure any holes are plugged with fiber, or this method won't work.

    This is what I do, for pieces up to 13mm thick

    Set the kiln to 1050,full ramp. If doing repair, put in cold oven.
    Set 1050 at full ramp for 9 hours, with a hold programmed.
    When it beeps, my workday is done. Last piece in. I try to schedule my day so the last 2 hours of work are smaller/thinnest. Give it at least a 15 min soak. Then..
    1150 (full ramp). soak for an hour or whatever I want struck.
    (Optional, if i want a deep purple I add in 30 min of 800, then 30min of 1175*careful )
    1050 full ramp , 2-3 hours
    At this point my phone alarm goes off, if there are pieces that need to be restruck, of flame struck, that's when i do it. Then start the 1050 cycle again.
    850 full ramp 1.5 hour
    500 full ramp 1/2 hour
    Let sit closed til morning
    That's what I do.

    If Im working 1.5"-2" marbles with single amber purp inside, this is my usual setting.

    1050 til ovens full.
    1050 for 3 hours
    1160 for 2 hour
    1000 for 15min
    1175 for 45 min
    1050 3 hours
    850 1 hour
    600 1 hour
    Keep closed at least 1hour.(all ramps at full)


    Production pieces with normal wall thickness and striking colors.

    1050 work garage.
    1050 3hours
    1000 15 min
    1160 1 hour (+ flame strike if applicable)
    1050 2 hours
    850 1 hour
    600 1 hour
    Closed athe least 2 hours

    For clear, 13mm thick or less
    1050 3-4 hours
    850 1 hour
    650 1 hour
    500 1 hour
    Leave closed.

    Boro beads striking color 9mm thick

    1050 2 hours
    1000 15 min
    1165 1 hour
    1000 15 min
    1150 30 min
    1000 10 min
    1050. 2 hours
    850. 1 hour
    650. 30 min
    500 1 hour
    I try to time my annealing so when the next workday starts the pieces are "muffin warm" and ready to pack.

    IMHO ramps are most important for fusing and soft glass stuff, correct if I'm wrong.

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    Default Re: Borosilicate Annealing Schedule Discrepancies

    What's the light box made with 2 pieces of polarized glass that allows you to see strain in clear glass called?
    Polariscope?

    OP, make one if these when you can , you will probably be amazed at how little time it actually takes to take the stress out of borosilicate.

    Also, flame annealing before garage is good practice, I've had many, many pieces crack ....just as I was about to open the oven, even while the piece was in the kiln and I was about to lay my handle down .

    IMHO there is no sure fire calculations to get the "best" annealing cycle, even the same glass strains differently as its worked (blacks and blues pick up molecules ). You just have to feel it out, adjust as necessary, and from time to time sacrifice a Virgin to the Glass God's
    I recommend sacrificing a Virgin Pina Colata. (After she's dead add rum and drink as necessary)

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    Default Re: Borosilicate Annealing Schedule Discrepancies

    Quote Originally Posted by somewhere View Post
    Annealing is not a temperature and can not be defined that way. You can relieve stress at 1050f but think about it if you take it out and set it one the bench what do you think will happen? Annealing is a range of time and temperature. It's a process that cools the glass down to the strain point in a controlled manner to not reintroduce stress in the ware.
    I was using the term loosely hoping to show that with the double quotes I used. You're right, the definition is a verb and includes "cooling down."

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    Default Re: Borosilicate Annealing Schedule Discrepancies

    Quote Originally Posted by JLF View Post
    It is also interesting that the referenced article for Duran glass mentions that annealing time at 550C should NOT exceed 2 hours or there may be a negative result to the chemical stability of the glass, yet the Northstar table recommends 24 hours at annealing temp for a 6 inch (152.4mm) thick piece.
    I did a little looking around and there doesn't seem to be any scientific research that confirms/denies that the chemical stability of the glass changes. One research paper even went so far to say that the homogeniety of the boron-oxygen bonds in glass remained mostly unchanged.

    However, some other research I looked into claims that there is a very small change in the refractive index of the glass, depending on the annealing schedule used.

    Additionally, it is important to note that Duran glass was initially made for laboratory glassware blowers. The intended use of this glass is to be able to withstand high temperatures, and very aggressive chemical environments. This glass is also put into very sensitive instruments that can make measurements all the way down to a part per billion level. It was not made to be used as art. So in that light, I think the data sheet is oriented more towards people who intend on using this glassware for labwork. This is probably why they recommend to not anneal for very long because:
    1. The reactive properties at long annealing times have no scientific literature (that I could find) and therefore might have some unforeseen consequences. This might not always be a violent reaction. It might be something very unnoticeable like leeching boron into whatever is poured in, throwing off sensitive scientific measurements in spectroscopy or contaminating an experiment.
    2. The refractive index of the glass may change, which can cause sensitive spectrometers that use this glass to give incorrect readings.


    That's just my take on it, though. I think for the purposes of art, there shouldn't be an issue. I haven't seen anyone (yet) complain that a long annealing time has had a severe impact on their work.

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