Hairline 'crack'

[Bonnie Rubinstein]

I have a 24 x 30 piece in kiln, double thick clear base with with some layered glass throughout. I can see on a clear area (no layers) a hairline crack. Still too warm to take out, but it is there for sure. I have made many pieces before, no problems.

It is System 96. I blocked the sides with bricks, but not the top and bottom. The hairline stated by a brick and goes halfway across the piece.

I ramped at 120 to 350, hold 10, then 200 to 1250 hold 30, then 270 to 1405. Maybe this was a bit fast.

It is such a very subtle line, that I think it may have ‘healed’; not sure. Should I refire and ramp very slowly? Does a hairline crack mend? Could it already be mended?

Thanks!

Bonnie

[DonMcClennen]

At what temp did you clearly see the crack? It is not going to be "Healed" (refused) if it was less than about 1350 on the downside of the firing ramp. If it started by a brick was it stuck to brick?

[Bonnie Rubinstein]

I noticed it ramping up- so it did anneal after. Maybe that is hopeful. But I may need to ‘disguise’ the line with some colored glass and that means re-firing. I wondered if that ensures success for healing, as well.

[Ed Cantarella]

Finger crossed for you Bonnie, gee that stinks. Sending you healing vibes. I had one a few weeks ago that was 3 layers of 3 mm and the bottom layer cracked and separated about 3/16-1/4". I'll be if it didn't fill itself, heal and level in one darn fuse(1470-1480). And all was good. Break a leg! (good luck in theatre lingo). Let us know how it goes. Peace

[Marty]

I noticed it ramping up- so it did anneal after. Maybe that is hopeful. But I may need to ‘disguise’ the line with some colored glass and that means re-firing. I wondered if that ensures success for healing, as well.

Why are you looking in the kiln on the way up?

[Bonnie Rubinstein]

I was looking at it through the peep hole at 1400, because I wanted to determine when to stop the fuse based on the art. I do not open the kiln.
I have now taken the glass out; it has 3 faint hairlines. I have not seen this before in my work,. This glass base is 5.5 mg. So, as it is very thick -maybe my ramp up was too aggressive (?).

[Marty]

I have issues with "120 to 350, hold 10, then 200 to 1250 hold 30"- I think it's mostly voodoo- but for glass less than 6mm thick you should be ok with that ramp.
Did the glass get bonked (technical term) in any way before firing?
I've healed cracks that hadn't separated by halving the ramp up to make sure the crack didn't go further.

[Ed Cantarella]

Lots of people, pros included, look in somewhere over 1000-1050 by just cracking the lid as that is a very safe zone, so they can decide if they want to abort the firing and save design elements and uncracked pieces. Even the cracked pieces - they might be easier to use and cut if they aren't fused to something else.

Totally agree on the voodoo aspect of some ramp schedules. There is nothing conservative about a super slow ramp schedule in a stress free zone. Just wasting time and electricity.

[Bonnie Rubinstein]

Marty- some of the glass, pre-firing, was 11.5 mm. The base was 5.5mm.. some areas had another 2 layers of 3 mm, so another 6mm. It was also 24 x 30. So, I think big enough to ramp slowly. I am thinking, even tho the line seems solid, of firing again for insurance.

[Ed Cantarella]

Bonnie: Just to clarify - I didn't think your schedule was unreasonably slow, for a piece that size & configuration it was reasonable. Personally I might not have exceeded 150 dph on a piece like. I was thinking more of folks slumping a basic 6 mm piece or such and cruising at like 50 dph from 1000-1200. In those cases I guess it makes a difference on how often you hope to fire.

[Marty]

Bonnie- I hadn't understood how much difference in height you had. I'd do it at 100dph to process temp, no holds or changes on the way.

[Bonnie Rubinstein]

Marty, thanks. Why would I not want to have holds or changes? For instance, if I ramp up at 50 to 350, then 75, and later 100, is that an issue? What is holding not advised?

I am wondering….Do you think there is any chance that the 3 lines (former cracks) were created because I have bricks on those 2 sides?(the lines start at the bricks). And the bricks could have caused a temperature differential?

[Marty]

Bonnie- In my opinion those holds are completely unnecessary, as is any change in ramp up to 1000 (or so). You want to go slow enough to avoid thermal shock but that can occur anywhere along that ramp so going slower to 350 and then speeding up does nothing for you. What do the holds do?
I'd like to know where those schedules got their start (and if in fact there is a reason for them). Perhaps the original intent was to speed the process but the best way to do that is to buy another kiln.
Thermal mass can give you enough differential to shock glass but if you're going up conservatively, then the bricks should be heating at the same rate, no?

[Bonnie Rubinstein]

probably they should. But the bricks are so dense and a different material; plus I was wondering if they also 'block' the heat at the edge.

[Bonnie Rubinstein]

Update-
I did re-fuse with very slow ramp and anneal. I believe the piece is solidly healed. Yes, the line remains, as predicted.

I am about to fuse a duplicate piece, same width and dimensions. I am wondering about those bricks on the 2 sides.. I would have them there to ensure I get an over 1/4" thickness to the glass, as I using different layers in different parts. If I use the bricks, does that present an increase risk of those cracks/lines again? I will ramp up very slowly. I could fuse without them, but prefer to use.

It might be Marty that is correct; it may not have impact, but it just may be that the bricks did block some evenness of heating.

[Marty]

Bonnie- "bricks" sounds ominous, large masses of heat sink. Can't you use something smaller, like cut up shelves or other kiln furniture to dam your work?

[Bonnie Rubinstein]

yes, I have some long steel rods I can hold in place with some small brick fragments and cover with thin fire paper to prevent spalling issues. That would prevent the heat sink. thanks!

[Marty]

I wouldn't rely on thinfire for that. Do you need serious weight to dam your work?

[Bonnie Rubinstein]

yes, I need weight- the steel rods lie next to the art and would be held in place by the brick fragments. (paper covering the rods.)