getting rid of bubbles

[Lloyd]

Hi, I'm new to this site and glass casting and am wondering about soak times for getting rid of bubbles. I'm casting Reichenbach glass, pot drip into lost wax investment mold, small head about 1200g glass, billets broken into a couple pieces to fit in pot, 1525 peak temp, with 6 hr hold. Bottom of casting is fairly clear with bubbles in upper half. Considering 10 hr soak. Have heard of glass problems with prolonged heat so want to go long enough but not too long. Any suggestions?

[Morganica]

Hi, I'm new to this site and glass casting and am wondering about soak times for getting rid of bubbles. I'm casting Reichenbach glass, pot drip into lost wax investment mold, small head about 1200g glass, billets broken into a couple pieces to fit in pot, 1525 peak temp, with 6 hr hold. Bottom of casting is fairly clear with bubbles in upper half. Considering 10 hr soak. Have heard of glass problems with prolonged heat so want to go long enough but not too long. Any suggestions?

When you say "bubbles in upper half," do you mean bubbles in the upper half of the mold (which I'm assuming contained a cavity for an upside down head) as it sat in the kiln, or that when you broke the head out of the mold and turned it rightway up, the bubbles were at the top of the skull?

Assuming you meant top of the mold, there are still lots of variables, including your ramp up schedule, how you're curing the mold, the height of the drop and position of the hole in the pot, the size and position of the bubbles and the color(s) of Reichenbach casting glass you're using. Whether you'll hurt the glass with a longer soak depends on which color you're casting. Reds and rubies can go liverish with too much heat (or sometimes turn opaque, although I've not had that happen with Reichenbach). Opalines can lose their translucency and start looking chalky.

More process time will give all but the smallest bubbles time to reach the opening at the top and escape...but the ideal solution is not to generate the bubbles in the first place. Right off, and this sounds counterintuitive, I'd probably drop temps a bit, to maybe 1495F, and use a slow ramp so that the glass is very soft once it gets there. I wouldn't increase process times until I tried a few things to reduce bubble generation. For one thing, the faster the schedule, the more sudden the shift into flowing and the more likely that the glass will splash down and generate more bubbles.

If the hole in your pot is centered over the center of the cavity, so that the glass is freefalling all the way to the bottom of the deepest point, see if you can't move it toward one side or the other. You want the glass to strike the side of the mold fairly early and slide down along the plaster, so that it fills up from the bottom gently, pushing air out as it rises. (It's similar to the way you pour a glass of beer to avoid making too much foam) It's tough to do with heads, since they're essentially spherical; I've sometimes built my molds with the head at a slant to facilitate this, and accepted that I'll have a little more coldwork to re-level things.

I'd also fit a piece of billet into the bottom of your pot so that it's completely covering the hole and nothing drops through before the glass is really flowing.

That's about all I can offer without knowing more...hope that helps.

[Lloyd]

Cynthia, Thanks, these are the suggestions I was hoping for, and the explanations help me experiment in viable directions. You assume correct about mold orientation. I'm casting crystal, rhubarb, and bronze and don't see much difference in bubbles between them. I air dry my mold for several days, then dry it as part of my firing schedule: 100/hr to 200, hold 7hrs - 100/hr to 1250 hold for 2hrs - 600/hr to 1525 hold 5hrs. I'll try slowing my ramp from 1250 to 1495, 200/hr? and cover hole with billet, and keep hold at 5 hrs. Love your analogy of pouring beer! I'll try hitting the side instead of straight to the bottom. Thanks again, Lloyd