Tempering Glass

Discussion in 'Foundry tools and flasks' started by oldironfarmer, May 6, 2019.

  1. oldironfarmer

    oldironfarmer Silver Banner Member

    Since I seem to be using up all my tempered glass for flask windows I thought I'd try to temper some. How hard can that be?

    First try was today, I put a piece of 1/4" plate glass on some kiln furniture.


    Ans started a program to heat it to 1148F in 180 minutes. Jason had suggested 175F per hour but this is thin, and only 2" by 4".


    Top number is actual, bottom is setpoint. It overshot about 8F until the fuzzy logic learned the system, then it was about 2F.

    I was at the neighbor's doing some electrical troubleshooting when it reached 1148F. No worry, I had it on hold after then for 60 minutes. When I got back to it, it had cooled to 950F. I realized I had not programmed it right to simmer at 1148F. Since it had already been there, I ran it back to 1148F and executed hold.


    Then it took more than a half hour to get ready to pull the glass out. I had my son in law and his grandson in law lined up to shoot the hot glass with an air jet from each side.

    What went right:

    Nobody got hurt.

    What went wrong:

    Glass had draped over the furniture.

    Glass was stuck to the furniture.

    Glass was not tempered.

    Glass began cracking soon after we chilled it with air jets.


    Questions I have:

    Should I have pulled it out as soon as it hit 1,148F to avoid sag. Maybe that would not help. I know commercial furnaces move it continually through on rollers. I thought for automation but it may be to keep the glass flat by continually supporting it at different places. It has to soak long enough to get it all to equilibrium.

    What could I support it with that would not stick? Stainless steel?

    Jack said glass used to be hung in tempering furnaces. I may have to come up with a clamp arrangement I think will stand 1,148F.

    None of the glass was tempered. I broke a lot of it with a hammer.
  2. I've got no idea what won't stick to it, graphite maybe. When I used to do this for optometry lenses we never got it hot enough to make it soft, you could make a complete ground and polished lens before heat treating it. You could try soaking the glass in molten potassium nitrate for a few hours to reduce internal stresses of the glass which might make it harder to crack from thermal shock. The potassium nitrate diffuses potassium into the glass which replace the larger sodium atoms and reduces the internal stresses of the glass. We had an electric heated salt bath that looked like a tiny chip fryer which the lenses were lowered into.
  3. crazybillybob

    crazybillybob Silver Banner Member

    liquid tin. Glass floats on tin.
  4. Jason

    Jason Silver Banner Member

    Kiln wash should prevent it from sticking to stuff inside the kiln or a $50 can of boron nitride. If your glass slumped, you got too hot OR soaked too long at that temp.... The tough part here is we don't exactly know what that glass you have is and that dictates procedure. As you conduct your research how to make this work, be on the lookout for words like, softening point and strain point. Keep in mind andy, we hear the word quench and that sounds like a fast process. With glass, there is nothing fast. You may need tons of air, or a gentle breeze... I don't know. Another thought, I would be more tempted to kiln wash a shelf with a few coatings of the stuff, and lay that glass FLAT on it. That should help it heat up nice and evenly on all parts of it. That kiln paper might be something else you could try and skip the wash.

    I'm seeing some kind of thin wire bbq rack with a fan below and a fan above blowing on the piece. While at the same time, slightly shifting the piece to reach the areas blocked by the wire. I don't think clamping and trying to suspend it is going to work. You would have major temperature differences in that area me thinks.

    Mark, sounds like you've some firsthand knowledge how this is done with the magic of chemistry. How thick was the glass you saw getting annealed? Then was there any further cooling required?

    One thing for certain, there is more brainpower here on this forum than in congress, so I have full confidence Andy will sort this out.
  5. Jason

    Jason Silver Banner Member

    OH... Cut up some small glass coupons.... 2"x 2" and practice with those. It's going to take some serious trial and error I bet.:D
  6. The lenses I worked on were mostly under 5/16" thick, the 1970's old machine I used for air tempering lenses (to make prescription safety glasses) just had two 3/16" thick wire rods that you sat the edges of the two lenses on, rotated the oven timer to set the heating time and let it run, once the heating was over the lenses emerged on the wire rack and stopped over two upwards facing air nozzles that had low pressure air puffing out on the bottom of the lenses. In thew middle of the unit pictured but hard to see are two upwards facing air nozzles.


    Molten salt baths are used to make stuff like Corning's Gorilla glass and other products, it can treat almost any practical glass thickness by varying the treatment times. The demonstrations of salt bath treated glass usually involved bouncing house bricks off large picture windows 1/4" thick. The ultimate strength would be to make glass from potash instead of soda to get a glass with no internal stresses. Probably the most heat resistant option is going to be to get some borosilicate glass cookware like a microwave oven glass tray or electric frypan lid. Borosilicate/Pyrex has a much lower thermal expansion coefficient so it'll be less likely to crack with sudden heating.
  7. oldironfarmer

    oldironfarmer Silver Banner Member

    Thanks for the replies, guys!

    I know I let it stay at temperature too long.

    What is kiln wash? Would Satanite do in a pinch?

    I have some stainless steel pipe, I may try that.

    I did have 3" left over from one piece of glass so I had two 3x4 coupons for testing. One is ready to go.

    The piece of glass I busted tonight was a halogen light lens and it had two dimples where it was picked up. I guess if it is soft enough at 1,148F you can dimple it to hold it.

    Round two will be laying on some SS tubing and I'll pull it about when the controller gets it to 1,148F.

    If we have a collective brain power greater than Congress, we could still be pretty dumb.
  8. Jason

    Jason Silver Banner Member

    You have a point there bud. Kiln wash is used by the pottery and glass weenies to keep stuff from sticking to shelves and furrniture. It's a powder, add water, mix and paint a thin layer on. Repeat a few times. You can get it amazon cheap. I don't know if satanite will work.
  9. I've been thinking about tempered/toughened glass where you cool the outside faster than the inside: this just creates a surface skin under compression and an inner core under tension. In other words the compression increases the strength of the outside but the glass as a whole is under massive amounts of mechanical stress. I think having a low expansion glass like borosilicate would last longer with extreme heat differences. Ideally you could score some fancy sheet of fused silica from Ebay for next to nothing as it would probably last for multiple experiments without breaking.
  10. oldironfarmer

    oldironfarmer Silver Banner Member

    I have used one piece of borosilicate glass. Lasted longer but still broke. I can't find fused silica larger than 3" square.

    I may try borosilicate with an air cooling jet, but blowing aluminum all over is no fun either.
  11. The expansion co-efficient is causing the glass to shatter so a material with a low coefficent is going to help, the trouble is that it's all expensive. Fused quartz expansion coefficient is less than 1/6th of borosilicate and 1/14th of soda glass, with no guarantee that it won't shatter anyway. I wonder if laminated glass backed up by by a normal glass sheet would work long enough to let the aluminium solidify: the laminated glass shatters but more or less stays in position. Those clamps you use in your video may also be a source of stress, if you could use some aluminium angle to make a frame to slide the glass sheet into it so it sits in place.

    You can cut laminated glass to size by scoring both sides and breaking along the score to expose the glue core and then splash some ethanol/metho on the crack to soften the glue core and let it separate.
  12. DavidF

    DavidF Administrator Staff Member Banner Member

    Look on Facebook marketplace and look for a free woodstove that has a glass in the door.
    Best glass you will ever find for what your doing. Forget what type it is though..
  13. oldironfarmer

    oldironfarmer Silver Banner Member

    Jack will give me laminated glass if I want. May have to try it. I try to leave the clamps loose, but they need to be tight enough to hold the liquid aluminum in the mold.

    I can buy borosilicate for $5 each and have ordered ten. Unfortunately I would have a tough time going to get a free stove for $10 worth of glass. Now if they would bring it to me...

    I still want to learn to temper it. I noticed the marks on one halogen light lens where it was held. Two dimples in the flight reflection.

  14. Petee716

    Petee716 Silver Banner Member

  15. oldironfarmer

    oldironfarmer Silver Banner Member

  16. DavidF

    DavidF Administrator Staff Member Banner Member

  17. oldironfarmer

    oldironfarmer Silver Banner Member

    My Pyroceram came in yesterday and it's 10-1/2" by 11". I could use 5-1/2" by 10-1/2" very nicely and have two flasks rammed up that way. $40. I puckered up thinking about shattering it. Their site says yo can cut it like regular glass. I got over my fear and cut it today. Like they said.

    Mark's castings likes this.
  18. Jason

    Jason Silver Banner Member

    And you expect this stuff to survive?
  19. DavidF

    DavidF Administrator Staff Member Banner Member

    this is the stuff that is in my fireplace insert. you can put a blow torch on one side of it and then hit it with cold water on the other side and it wont crack...
  20. Jason

    Jason Silver Banner Member

    This I gotta see. I wonder if the aluminum will stick to the glass. Maybe Andy can pull it off after he gets the video footage, before it freezes.

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