TO RE-FIRE OR NOT TO RE-FIRE

[Havi]

This piece broke on me twice.
On the second time, I thought it might have been bad annealing. Broke I mean BRoKE the sheet became 4 pieces.

So I fired again, not only a conservative going up, also sitting at process temp 830C 1526F, for 20 minutes [had also some pattern bars that I wanted to add]
Then, went down to 515C - 959F, stayed there for 2 hours, and went down C15 dph 27F
The piece came almost absolutely clean. Only small scars, that I wanted to correct.

So I fired again. This time it really cracked on me. It is not broken yet, but It is a deep long crack, and I am afraid that in few days it WILL break.

I wonder if I should try and fix it - or leave it, let it break to pieces eventually and continue to the next project. I am afraid the the viscosity had changed by now, and that it will keep on breaking, for ever.

[b]It became for me a technical PROBLEM, as well as philosophical DILEMA....
[/b]I shall try attach a photo of it as one piece. Note below the signs of the break - but it is not broken. I tried to cover some of the marks of the break - but they remained seen in spite of the added powder...

I'd be grateful [as always] for your advise

HaVi

YOU can see marks of 2 breaks, which I had been trying to fix. the new crack is in the same 'neighborhood' - therefore I suspect that something is wrong with the glass. I need to add that the sheet is composed of BE 1101, AND Be irid, both 3 millimeters, that were fused together. The irid is kind of old - could this be a problem???

[Brad Walker]

So I fired again. This time it really cracked on me. It is not broken yet, but It is a deep long crack, and I am afraid that in few days it WILL break.

What schedule on the one that broke?

[Havi]

Brad,
I am posting 2 schedules
All the temp. and rates are translated from the Centigrade which I use - therefore the 'strange' numbers

1. the one that came almost complete
it was on vermiculite 1", board + thinfire

90 - 1112 - 15min.
599.4 - 1526 - 20 min.
skip - 959 - 2:30 hrs.
54 - 752 - 10 min.
99 - 482 - 0
end.
This firing was almost absolutely perfect, except a small 'scratch' in 2 places.

2. THe one where I got a deep crack [the latest firing]
On Mullite board 1/2" with 3 m'm fiber under the glass

The same beginning up to 1490F 10 min.
skip - 959 - 2:30
27 - 800.6 - 10min.
63 - 698 - 5 min.
99 - 370 - end

I reaLise from writing that the vermiculite was better shelf for this project, I see that the annealing is probably better in the first firing.
My question is
1. can I do the annealing from 482C [your 899.6F] but still go as down as 400C [your 752F].
2. Being that I do not have to melt pattern bars in to the piece - can I go as high as 1490 only?

Please correct anything you consider a mistake, I'll be so very grateful. Including if you think there is a point at all to try and fix this thing. Is there ANY chance that it will come out one piece without scratches or cracks?


Many thanks, again,
HaVi
ps the piece is less than 1/4" thick now.

[Brad Walker]

Well, for starters if there's a compatibility issue no amount of re-firing will solve the problem. Have you tried to view the piece through polarized film, to see if there are areas of stress? At any rate, the crack does not look like a compatibility crack, it looks closer to an annealing issue.

Those schedules are incredibly conservative for a two layer (6mm) piece. They should be sufficient for a half inch (12mm) thick piece. So I'm not sure that it's an issue with the firing schedule.

I probably have more questions than answers:

1. Are you really close to the elements anywhere during the firing? Or is there a reason to think the kiln may be firing really uneven? Or that some areas of the piece are close enough to the sides of the kiln to cause trouble?

2. What do you mean by "slight scratch"? Was that a crack? Or something else? Was it in the same place as the crack that showed up on the second firing?

3. Have all the cracks occurred on the way down (cooling), not on the way up? That's generally either annealing or thermal shock. Neither is obvious from the schedules you gave us.

As you can see, I don't have an answer, only more questions. If you're thinking of abandoning the piece, then it certainly wouldn't hurt to fire again. No need to go higher than 1490F and the cooling/annealing schedule you've used should be conservative enough.
'
'I'd be interested in hearing anyone else's thoughts.

[Havi]

Well, for starters if there's a compatibility issue no amount of re-firing will solve the problem. Have you tried to view the piece through polarized film, to see if there are areas of stress? At any rate, the crack does not look like a compatibility crack, it looks closer to an annealing issue.

Those schedules are incredibly conservative for a two layer (6mm) piece. They should be sufficient for a half inch (12mm) thick piece. So I'm not sure that it's an issue with the firing schedule.
I am looking for my polaroid sheet, even though I doubt that it is because of incompatibility. The break begins at the edge of the glass, where is it clear transparent. Hopefully by tomorrow I shall have a knowledgeable answer to this.

I probably have more questions than answers:

1. Are you really close to the elements anywhere during the firing? Or is there a reason to think the kiln may be firing really uneven? Or that some areas of the piece are close enough to the sides of the kiln to cause trouble? I am not close to the elements, I shall take a photo and post it, later. At least no closer than any other pieces of the same size which did not break,

2. What do you mean by "slight scratch"? Was that a crack? Or something else? Was it in the same place as the crack that showed up on the second firing? By crack I mean that the glass is cracked to its whole thickness. Slight crack I mean when it is only on the upper surface.

3. Have all the cracks occurred on the way down (cooling), not on the way up? That's generally either annealing or thermal shock. Neither is obvious from the schedules you gave us.
Definitely yes. the edges are sharp
As you can see, I don't have an answer, only more questions. If you're thinking of abandoning the piece, then it certainly wouldn't hurt to fire again. No need to go higher than 1490F and the cooling/annealing schedule you've used should be conservative enough.
'
'I'd be interested in hearing anyone else's thoughts.

Thanks a lot Brad, I trust you and your knowledge and experience, yet confirmation , or rejection of what you say, by someone else, would be interesting for me too. Tomorrow, If I can't find my polaroid, I'll buy a new one and perform the test. It is important for me too to know where I am standing.

This forum is invaluable for me!!!

HaVi

[Brad Walker]

I'm almost certain it isn't incompatibility. At least not glass incompatibility.

[Havi]

I'm almost certain it isn't incompatibility. At least not glass incompatibility.

Yet it is better to check.
BTW, the piece is a bit thicker - due to the wafers on it - this is another reason for my being conservative.


H.

[Havi]

I am refiring now.
Due to the very conservative schedule, I shall be done tomorrow night, or the day after tomorrow morning.

I found my polaroid sheets. While transferring the glass - it broke where the crack was [!... ] not too surprising. Still checked for incompatibility which was not . No incompatibility.

I photographed with my camera, but can't unload the photos. My computer man did something that disables me from downloading photos. I'll have to find someone else for now.

I'll keep you posted when there will be news


Thanks for everything,

H.

1. Here one can see how far the piece is from the kiln elements....
2. I have marked In black where the piece had broken....

[Havi]

Hello,
Please see the photo I put on my previous post.

- - - - - - -

After 32:07 hours I took out of the kiln ONE piece.... [quite an exaggeration in conservative schedule]
Of course I am happy, or shall we say satisfied, but still quite suspicious, I really do not know what will happen in a week time, or longer time or less time.
I also checked for stress , found none.

Thanks for your help and co-operation.
Now got to go to fix another one - whose design I do not like, but I feel I can not continue, unless this one will satisfy me.

Havi