Glass lid or nitrogen lance - Ever heard of these?

Discussion in 'General foundry chat' started by 0maha, Sep 7, 2022.

  1. 0maha

    0maha Silver

    Had a guy come by the shop yesterday who started talking about his experience in a bronze foundry from years ago. He's an old guy who freely shared that he didn't trust his memory, but thought these were interesting. Wondering if anyone has ever heard of either.

    This has to do with pouring bronze.

    1) Toss a handful or two of pieces of broken glass (he specifically said "whiskey bottles", which I thought was funny) into the crucible before starting the melt. As things heat up, the glass will soften, float to the top of the charge, and form a gas-impermeable lid. When you're ready to pour, just skim it off. This is supposed to cut down on gas absorption.

    2) Get a bottle of compressed nitrogen gas. Connect it to a hose, and put a thin, stainless steel tube on the end, forming a lance. When the melt is approaching pouring temperature, turn on the gas, plunge the lance into the melt, then stir it down around the bottom of the crucible. Supposedly, the nitrogen will help precipitate out any impurities in the bronze.

    Anyone ever heard of either of those? Ever tried them?
     
  2. Mantrid

    Mantrid Silver

    ive tried glass. it stuck to the crucible
     
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  3. crazybillybob

    crazybillybob Silver Banner Member

    I've done the glass thing with Brass to try and keep the zinc from burning off. Not sure how well it worked because I have no way to test before and after. But it didn't seem to hurt and it skimmed off ok. I've heard of it done with copper to cut down on oxidation. But Charcoal can be tossed on top for that.
     
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  4. Rasper

    Rasper Silver

    I use glass on my bronze melts. Bottle glass is safe to use. I use beer bottles. The point is that with silicon bronze you do not want leaded glass in your melt. (bottles are safe) Lead and silicon combine and can produce glass bubbles in your casting.

    The melted glass not only protects the melt from the atmosphere; it collects the dirt and trash and it all comes out in one glob when I skim.

    I have never had it stick to the crucible.

    Richard
     
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  5. Broken glass in the crucible was the secret of Sheffield steel, though they only used the crucible once as it was buggered after the long period in the furnace. There were some WW2 vintage films showing the process on Youtube. I scored some roasted dry unexpanded perlite (perlite is volcanic glass) from a local processor as that's what's used on molten iron and bronze as a slag coagulant to make it easy to skim off the crud in one lump. You can see it thrown on top of the melt in a lot of those small iron foundry videos online.


    https://forums.thehomefoundry.org/i...-bronze-brass-slag-coagulant.1901/#post-41563
     
    Last edited: Sep 7, 2022
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  6. 0maha

    0maha Silver

    Interesting. Thanks for the comment.

    Can you describe your process more fully? A bunch of questions come to mind:

    - How fine do you break up the glass (I'm assuming you do) before adding it?
    - I assume the glass is thrown away after the pour, correct?
    - When in the process do you add the glass?
    - When the melt is up to temperature, how fluid is the glass? (I'm guessing maybe a little runnier than peanut butter?)
    - What is the technique for deciding how much glass? My hunch is you want to just cover the top of the charge, and once you have that much, any additional glass is unnecessary.

    Also, what fuel is your furnace running?

    Thanks for the input! I'm very interested in this process.
     
  7. crazybillybob

    crazybillybob Silver Banner Member

    When I used it I dumped it in with the charge cold. I put in enough to cover the top of the crucible. Didn't throw a whole bottle in there maybe 1/4 to 1/2 (I was working with an A4 ish sized Crucible) I'm guessing that was over kill because it was 1/2" thick on top. When I pulled it off to pour it was like hot taffy, Stickier and stringier than peanut butter but flowed. My glass went right in the dross bucket then on to the tip because it was full of trash off the top of the melt.
     
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  8. Rasper

    Rasper Silver

    Bruce pretty much covered it.

    I put a beer bottle in a bucket and throw a rock at it. (With my eyes closed.)

    I run my furnaces on waste motor oil using Lionel designed burners: a simpler design is not possible.

    Richard
     
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  9. Foundry Rat

    Foundry Rat Silver

    I have seen that before with silica sand, very ugly mess. Use a cover flux to help preventing gas, after degassing with nitrogen.
     

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