full fuse disaster

[marybc]

I recently fired an 8"x12" single layer of spectrum 96 glass using the tack fuse preset on my kiln. I have used this setting many times without a problem. When I removed it from the kiln it had broken into 3 pieces. I then placed the three pieces onto a 96 clear sheet, filled in the cracks with frit and full fused it using the pre sets on my kiln. It fused fine. I then slumped it in a mold, and when I removed it, it had a "C" shaped crack in it. I filled in the crack with frit and full fused it in the mold. When I opened the Kiln at 1400 degrees, it was a mess of large bubble,s holes and melted misshaped glass. I will try to upload a photo Some of my many questions are
Should I not have full fused it in the mold?
When I repair with frit, do I need to full fuse it, or is there another schedule I should use?

I have used the presets for a year with no problems.
There is no question of glass compatability, I only use 96.
Any comments would be appreciated.
Thanks
mary

[DonMcClennen]

It looks like you were trying to fuse on a slumping mold?? Fuse flat, then slump at much lower temps. (eg. 1200) Need to see schedule for more helpful comments.

[marybc]

I was fusing on the slumping mold. It was already slumped, so I just put the frit in the crack and refused on the mold. Didn't realize that it would make a difference.

[Tom Fuhrman]

It sounds like this was the 4th firing for this piece and it had gone thru a lot of stress and development thru all this. In my experience, I've found that the more times you fire a piece the more likely you are to have problems. Glass changes each time you melt it. Some glasses will even change their COE drastically each time you reheat it. Other glasses will change their molecular structure and change their viscosity characteristics each time you melt them. It appears as though this piece had lots of different colors in it and it is not out of the question that some of the glass changed a lot each time it was fired and fused. Even though these were all 96 doesn't mean that they won't react in very diverse ways when placed near some others and all put on clear.
Some reds will change their COE as much as 8 points just by being reheated in the glory hole over a period of 1/2 hour. Other colors will do similar weird things. I think you were asking a lot of the material to be able to salvage what were some severe challenges that it had gone thru.
Sometimes it works and sometimes it doesn't. Luck isn't always with you.

[marybc]

Thanks Tom, for your advice I have never fired anything that many times, but I liked the plate and thought I would give it a try. Could it also have been the fact that I full fused it on a mold?

[The Hobbyist]

In my experience, if you put a slumped piece back in the mold and refire you are likely to get bubbles. I had it happen once by accident and then again, intentionally, to see if my theory was correct.

I believe the bubbles happen because air is introduced and has no avenue of escape. Normally, as the glass slumps into the mold the air is pushed out through the holes provided at the lowest points of the mold. Also, as the glass slumps it conforms exactly to the shape and texture of the mold. When you reintroduce an already slumped piece to that same mold you (a) may not orient it the same (squares and circles) and (b) the surfaces will not conform exactly. Thus there are many air pockets formed at the start of the firing with no escape routes. If done at slump temp these small pockets form dimples in the underside of the finished product. If done at higher temps the bubbles can grow very large. I believe that is what occurred in your case.

Here is a picture of the underside of the first piece I did. Those shiny spots are dimples (bubbles). I have sandblasted around the dimples to accentuate them. The piece was fired, slumped and then reslumped to about 1350 F to tack fuse some elements to the top side.

Adding frit and having cracks also contributed greatly to introducing trapped air before you started the firing.

Jim

[Marty]

Thanks Tom, for your advice I have never fired anything that many times, but I liked the plate and thought I would give it a try. Could it also have been the fact that I full fused it on a mold?

I think the problem is more basic. Relying on pre-programmed schedules is folly- once you exceed the very basic parameters of those schedules, you're flying blind. You need to know what happens to the glass when you apply this amount of heat over that length of time. Get Brad's book, study it and then experiment. Merely throwing stuff in a mold and hoping for the best can get expensive (and frustrating).

[marybc]

I have programed other schedules into my controller. I just have never had any problems with the preprogramed ones. I will have to experiment more, thanks for your advice.

[JestersBaubles]

The problem is trying to full fuse in a slump mold. I had the same thing happen a month or so ago, when I was being lazy/wanting to test if you really *could* fire polish in a mold after the slump, if the bend to the mold was fairly subtle (as I had read).

As you can see from the above, and from my pic, the answer is a resounding NO

[url]http://www.flickr.com/photos/48008910@N06/9421969107/[\url] OK, how about just a URL. I will NEVER figure this image thing out.

Dana W.

[JestersBaubles]

The problem is trying to full fuse in a slump mold. I had the same thing happen a month or so ago, when I was being lazy/wanting to test if you really *could* fire polish in a mold after the slump, if the bend to the mold was fairly subtle (as I had read).

As you can see from the above, and from my pic, the answer is a resounding NO

http://www.flickr.com/photos/48008910@N ... 421969107/

[marybc]

Thanks so much for the photo, glad to hear I am not the only one making these mistakes. Although I am sorry for your disaster. It looks like it would have been beautiful. Thanks for sharing.

[Lauri Levanto]

I recently fired an 8"x12" single layer of spectrum 96 glass using the tack fuse preset on my kiln. I have used this setting many times without a problem. When I removed it from the kiln [1] it had broken into 3 pieces. [2]I then placed the three pieces onto a 96 clear sheet, filled in the cracks with frit and full fused it using the pre sets on my kiln. It fused fine.

I then slumped it in a mold, and when I removed it, [3] it had a "C" shaped crack in it. I filled in the crack with frit and full fused it in the mold. When I opened the Kiln at 1400 degrees, [4]it was a mess of large bubble,s holes and melted misshaped glass.

I have numbered the stages foe easier reference.
[1] Have you any idea of the reason of this initial crack. The picture is later and makes me confused. It has plenty of colors and you say it was single layer. You fused it, so it was assembled from pieces? Did they overlap? That makes thick and thin areas, a great way to introduce stress. Also different colors absorb and emit heat differently. Cynthia Morgan's advice is that a tack fuse needs at least double cooling time. Were the first cracks rounded (=way up) or sharp (=way down). My uneducated guess is it was a thermal shock on the way down.

[2] The second firing was fine. all was about 6 mm thick. That's how to do it right at the fist place.

[3] The C-shaped crack when slumping is usually a sign of induced stress. The final picture tells that the corners were hanging outside of the mold. When the glass contracts while cooling, the edges are trapped and it must 'give' in the middle forming a crack.

[4] The final disaster was fusing in the mold as The Hobbyist described. Fusing in the mold means that the glass is flowing down the sides adding irregular thickness. I am surprised it did not crack again. Cynthia can do it. I can't.

my 2 cents
-lauri

[Morganica]

So first, that was a heckuva firepolishing schedule--the glass is literally boiled. Way too hot--tone it down a bit.

You absolutely can firepolish in SOME molds, as long as you're careful with the schedule (it takes a trial or two to figure out how the glass will behave in a particular mold). The sides of the mold can't be too steep, because the glass will be soft and moving before it firepolishes, and it WILL move down the sides of the mold if gravity takes a hand.

The trick to getting it to work is to pick a mold that's relatively shallow (Bullseye's square slumpers are a good example), and make sure that your piece is at least 6mm thick throughout (the more even, the better), and be very careful with your heat.

If the piece has fully slumped into the shape, there's already a mass of perfectly stable glass in the center of the mold. The soft, moving glass that's trying to slide down the edges to the center of the mold either has to push the mass already there out of the way, or add to it, i.e., get thicker. It takes a fair amount of heatwork to do that--remember the "glass wants to be 6mm" mantra, and your glass is already 6mm or more--and your glass firepolishes before you get to that point.

But, as I said, you've got to test it, watch very carefully to see how the glass will behave, and stop the process temp when the glass is nearly polished. When you've figured out a schedule for that particular mold, record it and use it again...but you still must watch carefully.

You can also fuse directly in the mold, but again, the trick is to construct the piece so that gravity and the 6mm rule are your friends. If the glass will slide down the mold, plan for it, and design the layup so that when the pieces slide down there's at least 6mm at the join. I often use a "support structure" of glass and then weave other pieces of glass in and out--as everything slides down the mold they tend to run into each other and lock the structure into place. This is an example I posted a long time ago:

You can see how it was done on the old post at http://www.morganica.com/bloggery/2007/ ... er-basket/

Typically you can't do a full fuse this way, there's too much heatwork and the pieces are too large.

[JestersBaubles]

Thanks so much for the photo, glad to hear I am not the only one making these mistakes. Although I am sorry for your disaster. It looks like it would have been beautiful. Thanks for sharing.

Yes, it was a nice piece. My impatience taught me a lesson... again!

It actually is kind of cool. I'm trying to figure out a way to salvage it .

Dana W

[marybc]

[1]Thanks for responding so quickly, Unfortunately my job gets in the way of my glass. The initial piece was a single layer of spectrum 96 and bits of broken glass and frit on the edges, so there were thick and thin areas. I will double cool next time The edges were sharp on the three pieces.
[2] thanks

[3] The piece may have slightly been bigger than the mold.
[4] it probably would have cracked had I not shut down the kiln when I saw the massive bubbles.
I appreciate all of your help. Cynthia, I checked out your site and blog, your work is amazing, and your comments helpful.

[Morganica]

Well, thanks, ma'am. The trick with figuring out an anneal with a tack-fuse is, well, tricky. You start by finding the thickest part of the shortest dimension of the piece (LxWxH), which for most fused pieces is usually the height, i.e., the dimensions measured from the kilnshelf up to the very top of the highest part of the fired blank. If you have two layers of 3mm glass, that's 6mm. If you have 4 layers, that's 12mm, etc.

Then you double that dimension and look up the annealing schedule for a piece of that thickness. So if you have 4 layers or 12mm, you want to pick an annealing schedule for 24mm of glass thickness. That's your STARTING point. If you have radically different colors and/or opacities of glass (the worst is black against white, for example), you edge up the table to something for, say 30mm. If the shapes are extreme, such as right angles joining to each other, you go up again.

It frequently takes a bit of experimenting to get it right. If I'm working on a really extreme tackfuse, such as the one in that picture, I may start at 3X and make the piece up in all clear glass, so that it's easy to see in a polarizer. It doesn't solve the problem of different colors of glass but if I get minimal/no stress with clear I know I'm at least on the right track.

Also, you really can't mend a broken piece by sticking frit in the crack, putting the piece back in the mold and refiring until it's smooth. The glass needs to be hot enough to flow together and smooth over, and if it can do that, it's more than likely going to wind up in a puddle in the bottom of the mold.