large bubbles

[Jan Quade]

I’m new to fusing. I just bought an evenheat set pro. I want to do a tack fuse not a full fuse.  I tried small projects and they turned out fine. I have a 12″ square with a John Deere emblem in the center and a 3/8″ border set in slightly from the sides. Most of the glass is only the one layer. The program I used first was:  275dph  to 1000 hold 10 minutes    325 dph to 1350 hold 15 minues   full to 960 hold 1 hr  The second attempt was 275dph to 1100 hold 20 min   325dph to 1350 hold 20 min full 960 hold 1 hr Both firings produced hugh bubbles (you could almost yet your cereal in them) in the areas where the glass was only one thickness.  The second attempt I looked in after the first ramp just as it was starting the second ramp and the bubbles were there again.  I saw in a previous discussion on your web site that changing the dph  from 325 to 50 in the second ramp would take care of this. I don’t think this would help because the bubble formed in the first ramp. My shelf was primed and curred and my project was very dry. I’m very discourraged and afraid to try it again. Please help.  Jan

              

[Stephen Richard]

As you now know, scale does matter.  What can be done at a small scale does not directly transfer to a larger scale.

The first problem you have created for yourself is using only one layer as the base. Glass has a surface tension which means that it tries to become 6-7 mm thick. One layer is only half that.  As it thickens at the edges, it traps the air under the other parts of the glass, and as the glass softens the expannding air bubbles come up through the thin parts of the glass.

The design is the second problem. The weight of the border makes it even more difficult for the air to get out from under the glass.

The third problem is that you do not have a bubble squeeze in your schedule. You soak 10′ at 538C – not necessary.  You do need a soak at or above 650C – this is the bubble squeeze. It is also the slump temperature, so you can determine what the bubble squeeze should be for your glass by what the slump temperature is.  Your soak at 747C (tack fuse range) seems long to me, but you are the one who can observe and determine when the glass has tacked.

Fourth problem. As your separator is batt (kiln) wash, then you need to change to fibre paper for single layer pieces, as that allows more air out.  Thinfire may be enough, but you can put it over 0.5 mm fibre paper for a smoother finish, or sprinkle powdered batt wash over the fibre paper and smooth it with a plasterer’s float or a piece of window glass.

Five. Look at the glass manufacturer’s website.  Both Bullseye and Spectrum give basic firing schedules that work.  I don’t understand why kin manufacturers don’t simply refer to the sites to give their customers good advice, instead of the pre-programmed stuff.

 

 

Stephen Richard

blogs at: http://www.verrier-glass.blogspot.com/ and  http://www.glasstips.blogspot.com/

[Jan Quade]

Richard, Thanks so much for all the info. I bought fibre paper and tried another project which turned out fine. It was about the same size and covered 3/4+ with a second layer. My schedule was as follows:  275dph to 1000 hold 20 min,   100dph to 1350 hold 20 min, 9999 to 960 hold 1 hr. I slowed way down the speed going to 1350.  I want to try the John Deere emblem again. I read that two pieces of glass the same size have trouble getting the air out, but you can prop up the second layer with a small piece of glass and it disappears when fused. Or I could try just the one layer again but without the border. I could skip the border on  the double layer also. I have the schedule suggested by System96 which has 6 segments. I can use that but they recommend 200dph to 1350 and I could slow that down to 100dph because that worked in the other project. I also read that a bubble squeeze could be held as long as 2-3 hrs for projects that pose problems. So I believe that means I could hold at 1000 for a much longer time. Please tell me what you suggest and again thanks for sharing your knowledge and time. Jan

[Stephen Richard]

Jan, my comments are in a different colour spread throughout your post. Oh preview shows there is no different colour.

“Stephen, Thanks so much for all the info. I bought fibre paper and tried another project which turned out fine. It was about the same size and covered 3/4+ with a second layer. My schedule was as follows:  275dph to 1000 hold 20 min,   100dph to 1350 hold 20 min, 9999 to 960 hold 1 hr. I slowed way down the speed going to 1350.  I want to try the John Deere emblem again. I read that two pieces of glass the same size have trouble getting the air out, but you can prop up the second layer with a small piece of glass and it disappears when fused.”

I have fused two sheets of glass up to 800 by 500 mm and had no difficulty in the air getting out from between layers.  The small pieces of glass will not completely dissapear.  They will have the appearance of little bubbles.

“Or I could try just the one layer again but without the border.”

Yes that would make it easier on the single layer.

“I could skip the border on  the double layer also.”

You don’t need to, if you have a bubble squeeze.  I emphasise that is not at 1000F ( a temperature above which the glass will not be temperature shocked – called the strain point).  Even S96 state that is at 1150F.  That is the temperature you need to soak at.

“I have the schedule suggested by System96 which has 6 segments. I can use that but they recommend 200dph to 1350 and I could slow that down to 100dph because that worked in the other project.”

Unless you know why it worked on one project and not another, you don’t have a good basis for decision.  Once you are past the bubble squeeze, it does not matter much how quickly you go (with certain limitations).  Review this document

http://www.bullseyeglass.com/pdf/technotes_tipsheets/TechNotes_04.pdf

and you will see that going slow in this region is going to give a problem of devitrification.

“I also read that a bubble squeeze could be held as long as 2-3 hrs for projects that pose problems. So I believe that means I could hold at 1000 for a much longer time.”

Again, 1000F is not a bubble squeeze, it is the strain point.  The bubble squeeze is at 1150F.  The long soaks are for very complicated and multilayered pieces.  30 mins should be sufficient.   A soak at too low a temperature does not do anything as the glass cannot fully slump, so when you speed up and go to higher temperatures, the piece seals the air that is left within.  Make sure your bubble squeeze is at a high enough temperature.  Look again at the S96 site and you will see 1150 as the temperature.

“Please tell me what you suggest and again thanks for sharing your knowledge and time. Jan”

Best wishes

Stephen Richard

blogs at: http://www.verrier-glass.blogspot.com/ and  http://www.glasstips.blogspot.com/

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