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Thread: Casting in Glass (pic intensive)

  1. #1

    Casting in Glass (pic intensive)

    Hopefully, Zapins will see this. I cast a wall mounted object. Cast glass and sand cast aluminum. Here is the glass process, which is streamlined for semi-production mode:

    1 ) Cast refractory mold for the glass. I made the wax by sticking my hand in plaster that was just about to set. Later, I soaked the plaster mold of my hand and poured hot wax into it, let it skin up, and then dumped excess wax back into wax pot. This made a hollow box of wax with my handprint.

    2 ) The coddle mold is made from bent aluminum sheet. Plaster will not stick to this. Easy clean up with water. Made a dam between the walls and the board with clay. No clay needed at corners. Clamps work fine with no plaster leaks. Wear your mask! The refractory powder is 2 parts silica flour to 1 part plaster.

    3 ) Mixing the refractory mold. Did I mention the mix is 2 parts silica flour to 1 part plaster by weight? And then 1 part water to 2 parts powder by weight.

    4 ) I thought I invented this, but it's an old foundry trick. Brush your pattern. Gets rid of bubbles clinging to the wax.

    5 ) Refractory mold cleaned with Sureform and ready for dewaxing. I do not burn out. Wax wicks into the plaster if you do that. I steam it out.

    6 ) There's my dewaxer, a propane cooker. There's a tray to suspend the mold above the boiling water inside. Takes about 10-20 minutes to dewax. Pour boiling water on mold after steaming to get residue of wax off.

    7 ) Refractory mold clean and ready for the glass.

    8 ) Mold loaded in kiln. How much glass? I poured in water to the level I wanted. Weight of water x specific weight of glass = weight of glass. Ex: 100 grams of water, glass weighs about 2.5 grams/cc, water is 1 gm/cc, weight of glass 250 grams.

    Firing schedule for this piece: (It will vary by thickness). Took me years to dial in a crack free schedule. I'm handing it out for free. I must be a Commie!

    RT(room temp) to 400F @ 150F/hr
    Hold 400F for12-24 hrs. (Drives out water).
    400F - 950F @ 50F/hr (through the plaster shrinkage danger zone, too fast a run up and you get cracks).
    950F-1650F @ 100F
    1650F hold 12 hrs (lets glass flow into nooks and crannies and allows bubbles to escape)
    AFAP to 1250F
    Hold 1250F for 4 hrs (this is the hold to prevent shrinkage - glass can get suck void shrinkage like metal does)
    1250F - 900F @ 150F/hr
    Hold at 900F 8 hrs (anneal soak to eliminate strain)
    900F - 800F @ 6F/hr
    800F - 700F @ 12F/hr
    700F - RT @ 41F/ hr (anneal schedule from factory tech specs)

    9) Sand cast aluminum frame for mounting glass to wall.

    10) Finished product. Aluminum frame drilled and tapped, glass screwed in with wood frame, corner braces, and machine screw fasteners from back.

    11) This goes to 11.
    (Blog sometimes contains harsh language)

  2. #2
    Moderator greencheapsk8's Avatar
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    Aug 2008
    taranaki,new zealand
    very cool! i want one 8)
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  3. #3
    Senior Member Robert's Avatar
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    Sep 2008
    Charlotte, Nascarolina
    Very nice!
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  4. #4
    Senior Member machinemaker's Avatar
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    Jan 2008
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    Wow, great work.

  5. #5
    Great work, would love to do some glass but dont have that kind of control over my temps. Also I bet that takes some gas/electric? Have you worked out how much it costs to produce a piece that size?

  6. #6
    Outstanding results!

    That's a very elaborate firing schedule, I added up the hours, just over 4 days, I can see how that would take you years to develop. What do you use for temperature control?

  7. #7
    Wow. Great skills.

    I found that trying to find what I need and then make it work with what I have, is more trouble than designing what I want and doing it.

    "Quick decisions are unsafe decisions."

  8. #8
    Thanks for the kind words, gents.

    Quote Originally Posted by mantrid
    Great work, would love to do some glass but dont have that kind of control over my temps. Also I bet that takes some gas/electric? Have you worked out how much it costs to produce a piece that size?
    The electric kiln was a Skutt KM1227, with a Bartlet computer controller.

    One of the perquisites of being the studio manager is I get to use all the facilities and materials for free. If I were to break out the cost. Let's see. The glass I got 100 lbs for helping a glassblower fix his furnace. Figure (very reasonable) $50/hr for one hour = .50/lb. 2.4 lbs gives $1.24 for the glass. Plaster silica mix for the piece about $1.60. Wax is reusable so that doesn't count. Local electricity rate is around .08 kW/hr. Kiln uses 1.152 kW. Kiln ran 4 days. So $8.84 to fire it. Throw in 2 hrs unskillled labor at $5/hr. Cost of the glass = $21.68
    (Blog sometimes contains harsh language)

  9. #9
    Senior Member
    Join Date
    Dec 2008
    salt lake city
    And times that by 100x for retail prices.

  10. #10
    incredible process...and it hit me as an epiphany about the relative cost of my propane burner vs a electric kiln. Now I'm gonna have to see about kiln building... 8)
    Thx 4 sharing!!!

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