My slumps keep cracking

[BadExampleMan]

Hi,

I'm trying to make some very small cups (for an oil lamp). I'm draping over some small forms (about the size and shape of a mini-Reese's, an inch wide and an inch high). I'm using a pretty slow profile, annealing for an hour. Yet the glass keeps shattering at some point during the cooldown.

Any suggestions for changes to the firing profile or otherwise to avoid this failure?

Thanks!

[Brad Walker]

What's your firing schedule? What kind of glass? What do the pieces look like? What kind of kiln wash /paper/ whatever are you using?

[Glass Fever]

What is the form you're draping over made of?

[BadExampleMan]

The firing schedule is a simple 4-phase: 250/300/10, 400/1225/15, 9999/950/60, 100/800/15

This is bullseye 90 COE; 8 roughly pie-wedge-shaped pieces full-fused together into a just-under-3" diameter rough circle.

The kiln wash is whatever came with this Skutt kiln.

The form is some kind of rough pottery (intended for this purpose).

[Brad Walker]

The firing schedule is a simple 4-phase: 250/300/10, 400/1225/15, 9999/950/60, 100/800/15

This is bullseye 90 COE; 8 roughly pie-wedge-shaped pieces full-fused together into a just-under-3" diameter rough circle.

The kiln wash is whatever came with this Skutt kiln.

The form is some kind of rough pottery (intended for this purpose).

Your piece is cracking because you're slumping over pottery. Although some people sell them and make that claim, there's no such thing as pottery made for slumping over. The differences in expansion will cause the glass to crack as it cools.

You need to either use stainless steel or use fiber paper between the glass and the pottery.

[Morganica]

I don't know as I agree with that in all situations, Brad. As long as the mold is drafted properly, so that the contracting glass slides up the mold easily, slumping over isn't a problem.

You *do* need to use a schedule slow enough to allow for that adjustment (or, as you say, provide a fiber paper cushion which does the same). 100dph from 900 to 800, hold 15, and then off (i.e., freefall to room temp) is probably too fast.

[Brad Walker]

I don't know as I agree with that in all situations, Brad. As long as the mold is drafted properly, so that the contracting glass slides up the mold easily, slumping over isn't a problem.

You're certainly right, IF the mold is angled properly, but most aren't. And it's a pretty good bet that the one in this situation isn't, else the cracks wouldn't be happening repeatedly.

Still, why wouldn't you just make a metal mold and avoid the issue in the first place.

[JestersBaubles]

The firing schedule is a simple 4-phase: 250/300/10, 400/1225/15, 9999/950/60, 100/800/15

Just as a side note, I don't think that hold at 300 degrees is doing you any good. I typically ramp to 1100, hold, and then continue on up to my process temp.

2 cents,

Dana W.

[JestersBaubles]

The firing schedule is a simple 4-phase: 250/300/10, 400/1225/15, 9999/950/60, 100/800/15

And another note -- Bullseye's recommended annealing temp is 900 deg F. 950 is for system 96.

Dana W.

[Brad Walker]

The firing schedule is a simple 4-phase: 250/300/10, 400/1225/15, 9999/950/60, 100/800/15

And another note -- Bullseye's recommended annealing temp is 900 deg F. 950 is for system 96.

Nothing wrong with annealing Bullseye at 950. For over a decade they recommended 960. 900 saves you time when casting larger pieces, but doesn't make much difference vs. 950 or 960 on most typical firings.

If you go back 15 years or so, Bullseye actually had three different annealing temperatures, one for opaques, one for transparents, and one for glasses that contained gold. And back then Uroboros recommended just cooling slowly through the annealing zone and not holding anywhere at all. More than one way to skin a cat.

[BadExampleMan]

Wow, thanks to everyone. I'll try wrapping the mold in paper, and a longer cooldown, and report back. For anyone who cares.

[Morganica]

I don't know as I agree with that in all situations, Brad. As long as the mold is drafted properly, so that the contracting glass slides up the mold easily, slumping over isn't a problem.

You're certainly right, IF the mold is angled properly, but most aren't. And it's a pretty good bet that the one in this situation isn't, else the cracks wouldn't be happening repeatedly.

Still, why wouldn't you just make a metal mold and avoid the issue in the first place.

If I were making the mold, yep, I'd do it in metal. But I frequent those paint-your-plate places for unusual bowls and such, and have had pretty good luck slumping into and over.

The big thing to look for is a good negative draft and (probably more important) nowhere for the glass to be trapped. As long as the bowl is symmetrical and smoothly, continually flaring out, it'll probably work. Danger signs: Angled cuts/curves (like spirals), undercuts, straight sides, rims and feet, etc.

[Bert Weiss]

All good advice. The thing to picture and think about is COE. The glass and the mold are shrinking at different rates. In general, glass shrinks more than ceramic and less than stainless steel. So, if the glass has somewhere to go when it shrinks, problems can be averted. If the glass has nowhere to go, it simply cracks. Another solution is to make plaster refractory molds that shrink more than the glass.

96 and Bullseye compatible anneal about the same. So, it is interesting to note how many different approaches people make in their efforts to anneal them. There is not just one way to do it. When the entire mass of glass is within 5ºC inside the annealing range for about 15 seconds, stresses will relax. The key phrase is, "the entire mass of glass". The time we spend soaking and ramping is designed to get to the point where stresses will relax. If for any reason, the glass never achieves evenivity, it can't anneal.

[BadExampleMan]

In general, glass shrinks more than ceramic and less than stainless steel. So, if the glass has somewhere to go when it shrinks, problems can be averted. If the glass has nowhere to go, it simply cracks.

Thank you for this extremely simple insight. I confess I never thought about it that way before but that makes the problem crystal clear.

Shorter me: Duh.

[BadExampleMan]

Paper wrapped over the drape + cup placed atop small block to make sure the inside air is not sealed + ramp down to a lower end temperature = success.

Thanks to all who offered advice.

[Bert Weiss]

Everybody fusing glass has heard the term COE, referring to glasses. What it takes a while to learn is that every material has a COE, and most behave quite differently. Compatibility is simply, what works. The trick is to be able to observe what doesn't work and figure out why, and more important, how to redesign the process so it works.

The more experimental you are, the more likely you will run in to breakage. If all you do is take a class and do what you were taught, you should be successful, assuming your teacher has a clue.

The really neat part of participating in this community is that somebody may well have a handle on what happened to you. I once tried putting down a base of glass and laying a mound of sand/alumina hydrate on it, then draping another piece of glass over top, and fusing the glasses together. The top glass cracked. It turned out that I used olivine sand for this. My olivine sand mix had a smaller COE than the glass. I posted about this and somebody happened to know that silica sand has a greater COE than the glass. So, I tried it using silica sand/alumina hydrate, and it worked.