glass splitting in a bisque bowl mold

[RobinH]

I am getting a split (approx. 1" wide 6" long split) on both sides of the bowl while slumping. It is a bisque mold from one of those painting ceramic stores. 16" diameter 4" deep. Kilnwashed.
I'm using System 96. Fused two layers then a layer of glass bubbles laid in a pattern. 16" width...the glass was about ⅛" larger than the bowl on one side and it did seem to catch on the edge while starting it's slump.
Older ceramic kiln with side elements. The shelf was raised 8" off the floor. I do not have a controller, except myself I keep the temp at 200fph to 300, then increase to 300fph to 700f. I increased the heat to 400fph to 1250 and noticed the glass had split on the sides.
This is the second time that this has happened with this mold.
I don't have any photos...when I noticed this had happened again I elected to ramp the temp up to a bit past full fuse and let the bowl settle until the splits were closed. I figured it would be better to have a thicker section in the bottom of the bowl and later melt it to form a new base piece rather than scrapping it for future pot melts.

Am I using a ramp speed which is causing this splitting, or is it using these bisque type of molds for glass? They are quite different than glass molds and would tend to get quite a bit hotter than a glass mold.

[Marian]

You must have a very deep kiln to raise the mold 8 inches from the floor. Manual control is difficult to keep ramping consistently , only side elements means you probably have uneven heat on your piece, plus the glass you are fusing has added texture which means more uneven reaction to the heat. The overhang isn't being nice to you either. I think your mold is blameless, go way slower on the ramp up, keep the mold closer to the bottom of the kiln so it will be more like a top fired kiln. Holding your temperature probably means you are hovering and adjusting a lot, but can you hold a fairly even temperature at all? Folks with side element ceramic kilns probably have more experience and swear it is possible to fuse successfully. I gave up trying to fuse with mine, and got a kiln for glass with electronic controls. How is the annealing coming out? You may even be fighting poor stress control from the first fuse cycles. Run a search on this forum and see what other ceramic kiln users have for a program. What are the choices in your control? Low up to 1000,likely 5 hours or more then med to 1150 or slump, off to 960 or recommended anneal, try to hold for an hour and a half then low to 700, off to room temp. Does your unit have a pyrometer? If you have to open the kiln to check, it contributes to thermal stress, too.

[Valerie Adams]

I suspect your glass is under annealed. You say you fire a two layer piece, then add a layer of glass bubbles; how thick are these 'glass bubbles'? And what is your firing schedule when you add them? They're adding significant volume and thickness to your piece and I'm betting you're not annealing for that extra thickness. Posting your schedule would help diagnose your problem.

[Brad Walker]

If this is a split that's almost like a knife cut in the glass, then it's thermal shock caused by the difference between the temperature at the top of the glass and the bottom of the glass. The deep mold aggravates the problem because the air in the mold (under the glass) is not the same temperature as the air above the glass. The solution is to slow down, especially at the top end of the range. 400 dph is way too fast.

[RobinH]

The kiln has two sections and I usually will set the piece while slumping as low as possible while keeping a good air flow underneath the shelf...6-8" off the floor. Then I try to use just the top sections elements, but they heat at a much higher temperature then the lower elements.
I try to keep within 50-100f of my target.

Brad, yes it is like a knife cut right in the middle of my side wall of the bowl on both sides. I hadn't thought of the temperature under the glass as being the culprit.
Of course both of the fused pieces in the mold were fabulous and would have made great larger bowls.
Back out to give this one another shot, lower, slower.

Val...my error - blame it on late night brain fog - I don't have bubbles, it was frit balls and rolled cane murini I had used on top of a black base covered with a periwinkle blue...16" circles. I've never had bubbles in my glass...yet

Annealing I don't seem to have problems keeping my temperature steady...it is only on the way up. I move the digital pyrometer close the glass, whether it's up high for raking or pot melts...or low for fusing / slumping.

[Morganica]

Hmmm...were those frit balls fully melted into the base, or standing proud of it? Did your splits go all the way through the glass and separate it into two pieces, or did they seem to start in the middle underside of the piece and not get quite to the edges? What is the shape of the split(s)?

I think I'd agree that you're probably going too fast and may have had annealing issues going into your slump firing...but suspect there are probably other factors, too.

If I understand correctly, you're turning off the elements in the bottom half of the kiln and using only the elements in the top half? That means that you're going to be setting up a rather large coldspot at the bottom of the kiln (since heat rises anyway). The comment about keeping with 50-100f of your target is a bit disturbing; that's a pretty wide differential, especially since conventional wisdom says that there shouldn't be more than about 10F degrees difference (the deltaT stuff) between any part of the glass during a firing.

[RobinH]

"were those frit balls fully melted into the base, or standing proud of it?" The frit balls were more than tack fused but still holding their shape.

Did your splits go all the way through the glass and separate it into two pieces, or did they seem to start in the middle underside of the piece and not get quite to the edges? What is the shape of the split(s)? The split was like a knife cut.... The split ran horizontal about 6", with the middle of the gap opening 1"....there were two of them, one on each side. They weren't like a simple cracking of glass...it was like a ripping open. It happened once before while using this mold.

If I understand correctly, you're turning off the elements in the bottom half of the kiln and using only the elements in the top half? The reason I was doing this was to create a heat from the top instead of from the sides of the area where the mold sat.

Instead of scrapping the glass, I took it to a higher temp and let it settle in to the bottom of the mold. Then tonight I draped it over a stainless bowl. It is going to take some coldworking but I may salvage the glass. The entire piece is now a flat fuse though...it will be interesting to see what it does.

I really need a controller! I sat out in the shop for hours ramping up the temp at 200f/h and then most of this evening while annealing at 940 for 60 min, then 200f/h to 750 for 60 min, then letting it cool on it's own at the same 200f/h.

I've been holding out on purchasing a controller while I decide if I want to build my own fibreboard kiln...but that's a discussion for another thread.

Learning from mistakes. The worse that can happen is that I end up with pot melt glass

[Marian]

My sympathy, for the watching and worrying sitting by the kiln. You will love your controller even more. On another note, though, please consider extending your anneal, an hour for any thickness never seemed to work for me because it will break on the slump more often than not. Study more posts here, there are lots of program notes.

[Bert Weiss]

Uneven heating during the heatup can cause these lesions to form. The stress is enough to split the glass, but not strong enough to break it in 2. When the glass gets hot enough to slump, it does, which tends to separate the split more. If the glass were to crack from a thermal shock after the anneal, it would just be a crack with no slumped separation. Edges would be sharp.

So the answer is to either change which elements are on, or slow down. Or maybe baffle the elements such that the glass does not "see" a hot element towards it's edges.

[RobinH]

Thanks Bert for the responses, they helped explain the problem.

I was going to start building a new fiber kiln...but went out today and bought a new Paragon glass kiln. While I would have preferred a nice square fiber unit, I don't really have the time to weld together and then set up the electrics in one myself or to get my husband to build me one. The new one bought today was a compromise since it's the
same circumference as my old ceramic kiln, but not as deep since it's only one section.
The elements are in the lid and 3 rows at the bottom.
The bonus.. it has a controller!!!
Now I have to relearn the schedules for this new kiln.

I'm looking forward to using my old kiln for just pot melts, bars and raking, where the temp on the way up is not as important, and my new kiln for regular fusing and slumping.
I guess these glass messes were bound to happen. I was getting pretty cocky that I had never had a bubble, or crack...then all the sudden in one week it was a waste of glass.