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Big bubbles on flat fuse

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barclayb
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Big bubbles on flat fuse

Postby barclayb » Mon Jul 21, 2014 5:06 pm

I'm a little reluctant to ask this in case someone tells me to go do research - I've been doing that and I still don't know what's going wrong. This is a gorgeous piece of deep royal purple with rainbow irid, about 10.5" x 4.5". I was planning to slump it into a tray mold (am hoping to make an incense burner by drilling a hole for the incense stick).

big bubbles to post on warmglass.jpg
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So ... there must be air trapped between the glass and the shelf.

The thing is, I could fire blanks just fine straight on my kiln shelf until just recently.

Then my Skutt Hotstart's lid was dropping kiln brick dust onto the glass while it was still fluid enough for the dust to get incorporated in the glass. The pins had been falling out, the element (which is in the lid) was loose, apparently it was rubbing against the brick and dropping the dust. So I got new pins and wasn't able to push them in to move and secure the coil; the coil wire had gotten too stiff over time. So I ordered a new kiln lid and my husband drilled holes, etc., and installed it.

That's when the real trouble began. After various failed attempts at firing on flat molds (with the plan of making tiles), I decided to try firing on the shelf again. That was actually working well (and I found a solution for internal bubbles) with 5/25" square blanks until today with the 10.5" x 4.5" blank. With this result.

I've been using kiln wash instead of fiber paper because Id rather have as little exposure to particulates as possible.

I'd dried the kiln-washed kiln shelf at about 500 degrees for 20 minutes.

This is the schedule (from Bullseye TechNotes on reducing internal bubbles):
300/hour to 1200, hold for 20 min
25/hour to 1250, hold for 20 min
300/hour to 1490, hold for 10 min
9999/hour to 900, hold for 30 min
150/hour to 700
9999/hour

I've read that big bubbles seem to form in the 1350 - 1450 range (so if I'd tack fused, there wouldn't be bubbles, since the top temp programmed for that is 1325 - likely irrelevant).

The kiln is maybe 1mm - 2mm off level (if I raised the back legs that much). Is that enough to be a problem?

This is actually from a second firing. The first result was lumpy. I'd used powdered frit between layers of clear and purple irid, per Bullseye TechNotes, and it wasn't really an even layer (though it was thin). After the firing, I held the piece up to the light and looked from the back. There didn't seem to be many bubbles at all, and they were really small. (Yay!) The back was flat.

I fired again to see if the piece would flatten out (after kiln washing shelf and 500 degree drying cycle again).


Can someone give me some advice? Please?

It seems likely that there were biggish internal bubbles that I just didn't see, but I looked really well and it seems as if I'd have seen them if they were there.

I'm thinking that firing a blank of 1/4" thick clear would give me good information about whether bubbles are forming between the shelf and the glass.

Thanks much for any suggestions ...
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Brad Walker
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Re: Big bubbles on flat fuse

Postby Brad Walker » Mon Jul 21, 2014 9:52 pm

barclayb wrote:So ... there must be air trapped between the glass and the shelf.


That's it, exactly. The bubbles are not between the layers, they formed between the shelf and the underside of the glass. When the temperature got hot enough for the bubbles to expand, they came through the glass on top.

... decided to try firing on the shelf again. That was actually working well (and I found a solution for internal bubbles) with 5/25" square blanks until today with the 10.5" x 4.5" blank. With this result.


The larger the piece, the more likely you are to trap the large bubbles beneath the glass. Smaller pieces aren't large enough to trap bubbles.

I've been using kiln wash instead of fiber paper because Id rather have as little exposure to particulates as possible.


This problem won't happen when firing on fiber paper because the paper gives the air a way to escape instead of coming up through the glass. You only get the large bubble problem when using kiln wash. (Or thinfire paper, which isn't thick enough to allow the air a way to escape out the sides.)

This is the schedule (from Bullseye TechNotes on reducing internal bubbles):
300/hour to 1200, hold for 20 min
25/hour to 1250, hold for 20 min
300/hour to 1490, hold for 10 min
9999/hour to 900, hold for 30 min
150/hour to 700
9999/hour


Internal bubbles aren't your problem. It's bubbles between the glass and the shelf. So while this schedule will work in many situations, it could be modified to help with the large trapped bubbles. In situations where large bubbles are likely, you should go even slower from 1250 to the top temperature, perhaps 200 dph or so. And for two thin layers like your piece, 1490 is way too high a top temperature. Something around 1450 would have probably been sufficient and would have made large bubbles much less likely.

I've read that big bubbles seem to form in the 1350 - 1450 range (so if I'd tack fused, there wouldn't be bubbles, since the top temp programmed for that is 1325 - likely irrelevant).


1325 is pretty low for a tack fuse. Most people use a range from around 1350 to 1450. But you're right that the lower the top temperature the less likely you are to have the big bubble problem.

The kiln is maybe 1mm - 2mm off level (if I raised the back legs that much). Is that enough to be a problem?


I don't think this is the root of the problem.

Can someone give me some advice? Please?


In your case, there were several factors contributing to the problem.

1. You fired on kiln wash. Large bubbles only happen when firing on kiln wash.
2. The piece was larger. The larger the piece, the more likely the large bubbles.
3. The piece was fairly thin. The thinner the piece, the more likely you'll get the larger bubbles.
4. The top temperature was very high for this piece. The higher the temperature, the more likely the large bubbles.
5. You had new top elements. New elements are hotter, making the large bubbles more likely. And heat from the top also contributes to the likelihood of larger bubbles.

Any one of these might not have been enough to cause the bubbles, but all of them together most certainly made big bubbles more likely.

The solution is pretty straightforward. If you want to never have the problem, you could just fire on fiber paper, but you don't really have to go to that extreme. Just go slower from 1250 to your top temperature and make the top temperature lower. If you fire thicker pieces (such as two or more layers with items on top), you can usually safely go to a higher temperature because the extra weight of the glass will help push the air out before it can come through the top of the glass.

Hope this helps.

charlie
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Re: Big bubbles on flat fuse

Postby charlie » Tue Jul 22, 2014 2:10 am

[quote="barclayb"]am hoping to make an incense burner by drilling a hole for the incense stick.
[/quote]

well, now you don't have to drill holes.

barclayb
Posts: 24
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Location: Seattle, WA

Re: Big bubbles on flat fuse

Postby barclayb » Tue Jul 22, 2014 2:20 am

Brad, thank you so much! I very much appreciate your taking the time to be so thorough and helpful.

Charlie, you gave me my biggest smile all day. :)
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rosanna gusler
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Re: Big bubbles on flat fuse

Postby rosanna gusler » Tue Jul 22, 2014 10:40 am

also check that in all of the re doing of the lid the thermocouple is still in the right position. if it got shoved out, the controller will cause the kiln to over fire. r.
artist, owner of wanchese art studio, marine finisher

Terry Ow-Wing
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Re: Big bubbles on flat fuse

Postby Terry Ow-Wing » Tue Jul 22, 2014 2:05 pm

bubble on kiln wash seem to appear only if the kiln wash is too thick. I mix 10 to 1 and only use one or 2 coats over an existing layer. After the kilns wash shows wears I scrape and start over again with maybe 3 layers. hope this helps.
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barclayb
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Joined: Sat Jun 27, 2009 8:45 am
Location: Seattle, WA

Re: Big bubbles on flat fuse

Postby barclayb » Tue Jul 22, 2014 4:45 pm

rosanna gusler wrote:also check that in all of the re doing of the lid the thermocouple is still in the right position. if it got shoved out, the controller will cause the kiln to over fire. r.


Thanks, Rosanna! Will do, right now.


Terry Ow-Wing wrote:bubble on kiln wash seem to appear only if the kiln wash is too thick. I mix 10 to 1 and only use one or 2 coats over an existing layer. After the kilns wash shows wears I scrape and start over again with maybe 3 layers. hope this helps.


Ahhhh! I have thick layers of kiln wash slathered on top of thick layers. The kiln wash is also thicker. (I don't measure. Looks as if I should.)
from the whimsical and peripatetic world of
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(pronounced Barkley)
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Terry Ow-Wing
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Re: Big bubbles on flat fuse

Postby Terry Ow-Wing » Thu Jul 24, 2014 3:22 am

definitely measure - using the brush I use the 10 to one measurement when I use my air compressor to spray my shelves I use 20 to one and I don't really count how many layers I just look for an even coating of pink. I used to spray all the time. When I brush it's because I'm being lazy. But the first time I went back to brushing it on it was thicker and I got consistent bubbles the size of tennis balls - I freaked out!!! #-o but it was obviously my fault! I should know if you change one thing and you get problems it's that variable that caused the problems.
Terry Ow-Wing Designs
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barclayb
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Joined: Sat Jun 27, 2009 8:45 am
Location: Seattle, WA

Re: Big bubbles on flat fuse

Postby barclayb » Sat Jul 26, 2014 11:44 pm

Brad Walker wrote:The solution is pretty straightforward. If you want to never have the problem, you could just fire on fiber paper, but you don't really have to go to that extreme. Just go slower from 1250 to your top temperature and make the top temperature lower. If you fire thicker pieces (such as two or more layers with items on top), you can usually safely go to a higher temperature because the extra weight of the glass will help push the air out before it can come through the top of the glass.

Hope this helps.


Brad, it worked! Thank you so much.

Terry Ow-Wing wrote: But the first time I went back to brushing it on it was thicker and I got consistent bubbles the size of tennis balls - I freaked out!!! #-o but it was obviously my fault! I should know if you change one thing and you get problems it's that variable that caused the problems.


:? Thank you for sharing your experience; it helps me, to be able to learn from it. Aren't these things aggravating? It's great that you figured out what to change; cut yourself some slack.
from the whimsical and peripatetic world of
Barclay Elizabeth Blanchard
(pronounced Barkley)
servant of cats


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