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  #26  
Old 08-07-2018, 12:29 AM
Chris Lowry Chris Lowry is online now
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Hey Peter, thanks for the ideas, it could very well be me. I have been playing around with changing my technique doing what your talking about. It could be my drip off leaving new bubbles for my next gather as well.

I do get touch down bubbles especially on top of black frit. Although you can tell if they are touch down because they are right on the previous gather making them hard to pick.

Mine tend to be centered in the new gather, plus I often see them wrap around the piece as Iím gathering.
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  #27  
Old 08-07-2018, 05:56 AM
James Ennis James Ennis is offline
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Is snowflake just not pellets? I cant say faster, as i have never melted in a
small wire kiln but Greg Englesby liked to melt it in his 250 lb sic electric furnace,
but we refine at 2350, and I believe he may have gotten away with as cold as 2300. we like the beer bags to keep dust down, its not bad if you are carefull
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  #28  
Old 08-07-2018, 07:05 AM
Josh Bernbaum Josh Bernbaum is offline
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Quote:
Originally Posted by Chris Lowry View Post
Thanks but not totally my question.

We do turn down to 1930 every night.

Is that too low?
Probably not too low if you pass that point at which antimony does it's "flip."
But if it's lower than necessary, then I'd say that's just more energy and $ needed to get the temp back up those extra degrees again. In my 200# moly furnace with SP pelletized, I squeeze at 2000 after trying originally with 1950, and seeing no difference if I remember correctly. I let it remain at 2000F for at least 12 hrs. which is prob. overkill for my pot size but I usually have the time which I know is a luxury. I also potato religiously.

Not much has been mentioned yet in this thread with regards to "raking" before your bigger gathers. If the bubbles are on or close to the surface this can help. I made a stainless rake years ago (maybe 7-8" of stainless angle stock welded to a 1/2" solid stainless rod), we keep it in a clean bucket of water next to the furnace and use it a lot before important gathers. I don't know if it works 100% but it is def. doing something to minimize unwanted things in gathers. A glass rake would be even better I think, as that pulls the surface of the glass out of there, but I've found that even just pushing the junk to the back can help keep it out of your way enough.
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  #29  
Old 08-07-2018, 07:24 AM
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Quote:
Originally Posted by James Ennis View Post
Is snowflake just not pellets? I cant say faster, as i have never melted in a
small wire kiln but Greg Englesby liked to melt it in his 250 lb sic electric furnace,
but we refine at 2350, and I believe he may have gotten away with as cold as 2300. we like the beer bags to keep dust down, its not bad if you are carefull
********
Snowflake has a ton less water. Far finer material.
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Old 08-07-2018, 10:36 AM
Chris Lowry Chris Lowry is online now
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Thanks Josh Iíve been curious about raking, Iíve never done it but it seem like it might be worth a try. Do you go very deep or are you trying to to skim as little as possible?

We started turning down the furnace at night thinking it might save some money but not sure it really does... any thoughts?
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  #31  
Old 08-07-2018, 12:47 PM
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Raking helps with small attenuation bubbles from gathering. I'd pay far more attention to what peter Bowles suggests.
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  #32  
Old 08-07-2018, 07:18 PM
Josh Bernbaum Josh Bernbaum is offline
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Originally Posted by Chris Lowry View Post
Thanks Josh Iíve been curious about raking, Iíve never done it but it seem like it might be worth a try. Do you go very deep or are you trying to to skim as little as possible?
No we just skin the surface, maybe sinking the edge of the metal around 1/2" deep. Again, I don't know if it helps all the time but I do feel like it helps sometimes and can't really hurt unless someone bangs an element or something so we just do that here before any larger gathers.

At the very least I'd think you should try this to see if it's just schmeg from previous gathers or if it's a bigger issue like the two Peters suggest it may be. Use stainless.
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Old 08-07-2018, 07:46 PM
Eben Horton Eben Horton is offline
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Itís my opinion that crucibles can create cords. Bubbles ? No way unless you have a Swiss cheese pot.

I suggest adding an extra hour to your cook and fine time before you turn down to squeeze.
That is assuming that the bubbles are from the charge. I always reheat my post for 3 seconds before I gather on it and magicly my gathers are always bubble free.

If Iím making a serious piece I will have who ever is helping me scan the pot with a punty rodís shadow to see if there are any bubbles from my last gather and if there is we make a glass rake or just gather them out.
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  #34  
Old 08-08-2018, 06:58 AM
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Mullite pots can cause bubbles. Mullite is 76% alumina - 24% silica. When the pot wears, it's the silica consumed primarily leaving a honeycomb like structure for the wall and it does indeed have the capacity to trap bubbles. Mullite pots are not really made anymore, the last ones being from Ipsen Ceramics decades back. The standard now is 90% alumina or AZS ( but not in the US) . LaClede continues to make some clay pots. Composite pots can cause real troubles that use AZS ground up with a clay binder.

I find the notion of the reheat on the glass prior to gathering to be really interesting. I periodically make stems and gather the foot onto a small ball at the base of the glass. It's incredible easy to trap a bubble there while gathering yet the glass in the pot is just fine.
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  #35  
Old 08-08-2018, 07:19 AM
Eben Horton Eben Horton is offline
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The bubble from a cold gather has always made me scratch my head.. the reheat makes a big difference.
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  #36  
Old 08-08-2018, 08:41 AM
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It does seem to me that if the gather on the pipe has any chill marks on it that it's an invitation for a bubble trap if laid down somewhat on its side in the melt.

Then I also think about that interesting phenomenon I occasionally talk about of gas coming out of solution. It does seem to me that bubbles gathered from the pot usually seem to shrink some.
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  #37  
Old 08-08-2018, 11:21 AM
Jordan Kube Jordan Kube is offline
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If they are big bubbles you are putting them in there somehow. There's a nice formula in Modern Glass Practice the describes the rate of rise of bubbles of a given size. Pin heads basically never rise and have to be reabsorbed into the melt. Large bubbles rise and eventually pop or just remain at the surface as frogs eggs. There aren't any large bubbles left in the glass after a proper melt cycle.
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  #38  
Old 08-08-2018, 03:10 PM
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well, rate of rise is based in viscosity and temps. It does seem to me that a proper melt leaves a sea of bubbles that are probably about9-10 mm across, smaller if the melt was not really hot ( lowering viscosity). When you turn that mass down, three things seem to be happening simultaneously.

Bubbles rise
Glass shrinks as it cools and puts a load on the bubbles.
Bubbles go into solution.

This stuff take some time. Glass shrinks about three ten thousandths for every one degree C it drops and vice versa for rise in temp. we have a lot more trouble measuring that once the glass loses its rigidity. Even more interestingly, it's best measured only once per sample or at least that's my experience.

My favorite attribution for bubbles comes from an obtuse monograph from Corning identifying various stones, defects , etc in glass. It summed up one case study saying

"Once the worker was fired for throwing his asbestos glove in the furnace at the end of shift, the problem ceased"

There are days I just love this shit. We started off with High Temp pots under the 60 watt light bulb looking for confessionals. As Steve Weinberg once said "C'mon, it's only glass after all."
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  #39  
Old 08-08-2018, 07:55 PM
Peter Bowles Peter Bowles is offline
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My thinking about gathering on a too cool gather (and I've given this a lot of thought) is that as that cold surface touches the glass in the pot, the temp differential between the two surfaces is sufficient to effectively freeze the new glass creating a seam or a ridge. And its the size of that ridge that traps bubbles as it turns through the glass as the gather is made.
The quick heating of the surface of the parison reduces the likelihood of the ridge being big enough to trap air. Same goes with pushing deeper into the pot before turning, or turning slower whilst gathering.
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