Combing problem

[jjeakle]

I have attempted 2 combings now and both have cracked. Both pieces were 3 layers thick dammed with an 1/8 inch fiber paper boarder in a12 inch ss circle. First one had all larger pieces with a couple wedges of glass pointing to the center, these were the areas that I combed, used oranges and various greens. I took it to 1700 deg F. Broke after I took it out of kiln. Second piece had layer of clear on bottom, second layer - strips of clear (using up scrap), third layer - strips vanilla, blue and purple. Max temp this time was 1650. Glass moved easily at 1650 maybe could have used less. Kiln shelf was supported on 2 bricks, Bullseye kiln wash (8 or so layers), glass was about 1 1/2" - 2" from heat elements. Hot Shot top loading kiln.

Clearly both pieces cracked on cooling. I have read coe can get funky when glass is hot (>1600 I assume?) and the whole thing is somewhat sketchy and unpredictable. But I know some people must have success. Anyone see any glaring errors or have ideas on how to at least tip the scale in my direction? Shelf on bricks, I don’t normally do that but to comb I had to get the glass higher in kiln. I wonder if the contact with the bricks cooled the self in a non uniform matter. Too close to elements ? I have no idea. Not sure what to change on the next attempt. Your help is greatly appreciated. Could post pics, not sure it would help. - Jim
Schedule first piece 200/1000/10
600/1250/60
600/1700/30 took to 1700 again for second comb. Tried to comb just top layer of glass
9999/900/1
100/800/1
180/700/1
600/70/1
Schedule second piece 200/1000/10
600/1250/60
600/1650/30 combed once
9999/900/1
100/800/1
180/700/1
600/70/1

[Brock]

Where is the annealing?

[jjeakle]

Sorry 2 hours at 900 - Jim
Schedule first piece 200/1000/10
600/1250/60
600/1700/30 took to 1700 again for second comb. Tried to comb just top layer of glass
9999/900/120
100/800/1
180/700/1
600/70/1
Schedule second piece 200/1000/10
600/1250/60
600/1650/30 combed once
9999/900/120
100/800/1
180/700/1
600/70/1

[Kevin Midgley]

That was annealing?!?!?!
You'd better learn about the nature of glass before wasting more materials.

[jjeakle]

Wait - what???
2 hours at 900 degrees then from 900 to 800 at 100 degrees/h then 800 - 700 at 180 degrees/h ?
Right from Bullseye technical and product information - Jim

[Kevin Midgley]

Ahh but that is for normal firing of glass.
As soon as you have exceeded the amount of heat work that the glass was originally designed for ...... fusing not combing, all bets are off as to the annealing schedule you have used. It is totally inadequate.
Lesson over. Start studying.

[jjeakle]

Well thank you for "the lesson"
I have read some on the subject but as this is a hobby, my time is somewhat limited - I guess I appologize for that.
Any more tips, like an example of a proper annealing schedule would be great. - Jim

[Kevin Midgley]

Graham Stone's Firing schedules for Glass. Read it at least 3 times cover to cover and you'll learn how to fire glass. Read it fewer times at your own risk. Do not go directly to the schedules without reading cover to cover.

[Warren Weiss]

Jim,
I would add 30 min. at 900F and go down from 900 to 700 at about 80 F / hour.
Warren

[jjeakle]

Thanks Warren - Jim

[Tony Smith]

The 900 degree annealing hold that Bullseye has changed to has me concerned. Especially with glass that was so hot.

Here's my concern: objects that are cooling lose their heat in three ways: conduction (through the kiln shelf or furniture); convection (through air circulating over the hot surface); and through radiation (infrared coupling between hot and cold surfaces that can "see" each other). The most efficient cooling method is conduction. So the cooling process starts with cool room air surrounding the walls of your kiln. the bricks are cool on one side (relatively speaking) and hot on the other, so there is conductive heat flow from the inside of the kiln to the outside where heat is lost through convection to the air in the room. Inside the kiln, your kiln shelf is also cooling, radiating heat to the roof, floor and walls, and convecting heat to the air, and the edges are going to cool faster than the center where there is a large mass of hot glass. Finally, there is a hot mass of glass that is radiating heat to the roof of the kiln, convecting heat to the air and conducting heat to the shelf. All this time, the temperature controller is monitoring the air temperature inside the kiln and not the actual glass temperature, so consider that the controller is looking at an average temperature (not a true average, but something between the hot glass temperature and the cooler outside wall temperature).

At some point during the cooling process, the controller reads 900 deg and the elements heat up again holding the kiln air at that temperature. We already know that the controller is reading the air temperature which is a mix of hot air coming off the glass and cooler air coming off the kiln walls. In the case of glass coming down from 1650 deg, the center of the mass of glass could be at 950 deg and the edges at 850 or less which could be below the strain point where stresses get frozen into the glass. These numbers are illustrative only. There are many more variables that need to be considered to really know the temperature distribution in a kiln. But the idea I'm trying to get across is that by having your first hold at 900, you may be freezing stresses into the edge of your glass, and that's what's causing your stress fracture.

I would recommend that you try holding at a hotter temperature like 960 or 1000 for two or three hours, then go to 900 and anneal per the BE schedule. That should reduce the temperature between hotter and cooler parts of your glass before you get to the strain point and lock in the stresses.

Patty Gray has been combing for years, and she holds 4-6 hours at 960 followed by two hours at 850. Her schedule is based on her experience, and I think is supported by everything I said above. http://pattygray.com/demo/instructions.html

Tony

[Brad Walker]

Interesting thought, Tony. I do know that the only annealing problem my wife has ever had was on a fairly complicated piece held at 900. She now anneals everything at 960 and the problem has never happened again, even for pieces virtually identical to the one that stressed at 900. There's no doubt that 960 has worked well for decades and the main argument for switching to 900 is to save the cooling time from 960 to 900. That time may be significant for thicker pieces, but for two layer combed pieces it's pretty close to insignificant.

By the way, Graham Stone recommends 950. And in any case the annealing temperature only matters if you don't have a compatibility problem from the high temp.

[Tony Smith]

Brad,

It doesn't surprise me that Jody has had issues with 900. Faster isn't always better.
As far as CTE shifts in some glasses, a more conservative annealing schedule will likely mitigate some of the problems. Annealing theory has been evolving in the art glass world for decades and the proof is in the practice. Stick with what works.

Tony

[Havi]

This discussion is very interesting to me.

I have posted few days ago a question about a piece of mine that cracked - after having been at that high temp. 1700F for 45 minutes. I worked on a mesh melt, but the process temp. was the same....................

Somehow I got the impression that the cracks were the result of compatability change because of the 1700 degrees.

NOW I understand that the ANNEALING temp., might have been wrong, as I annealed at 900F [our 482], while you are talking about 960F which is probabley our 516C - the past BE schedule.

I am planning to continue experimenting with mesh melts, and would appreciate your comment on the apropriate ANNEALING temp.

Please view my post at

http://www.warmglass.com/phpBB/posting. ... =2&t=39779

I'd be more than grateful, as always

Havi

[Tony Smith]

Havi,

It wouldn't hurt to try the same approach that I suggested here. Hold at some temperature above 900 for two or three hours to get everything in the kiln to a common temperature that is well above the strain point of the glass, then anneal per the Bullseye schedule for the appropriate thickness of your glass.

Tony

[Havi]

Good!

Thanks, Tony

Only I am wondering why nobody mentioned this on my thread, I feel lucky to have read it here.


Thanks again,



Havi

[Morganica]

Havi, I think we are talking about two different problems. I haven't had an issue with the drop from 960 to 900 in Bullseye glass anneal soaks, but my anneals are probably conservative enough that it's not an issue in the first place.

The problem you're describing in the other thread is very different. If poor annealing caused those cracks, a refiring could fix it, assuming the glass was then annealed appropriately.

In your cracking problem, however, there is a good chance that the glass has chemically changed due to prolonged, high temperature firings. I can't know if that is absolutely the case, but if it is, changing the annealing schedule won't change the glass chemistry. No matter how slowly you anneal, the glass will still expand, contract and move at a different rate than the unchanged glasses, and so it will eventually crack again.

[Havi]

Cynthia,
I am honestly very grateful for your response.
I believe my anneaniling in the first place was good, I am 'old' enough' now, to know not to play with this. I am even more conservative.
The fact that the piece cracked TWICE - is really against the idea that the annealing was poor.

However,
In my next firing, I plan
a. Not to go as high as 1700, I have a pip hole and I'll watch the kiln thru the process temp.
b. I SHALL do the annealing at 960, just to be on the safe side.

And thank you so much for your comments.
I appreciate it


Havi
I am very excited about working with Melts , and I have a strong urge to it the best possible way. [with maximum knowledge]

[Tony Smith]

Cynthia makes a good point. What a different annealing approach will do is rule out annealing as a potential problem.

Tony