two cracks bowl

[msch]

G’day people of the forum

There are two cracks in my latest stringer bowl and I need to determine if they are the result of thermal shock or pressure applied by the cooling stainless steel or other reasons. The two cracks are different from each other and so may have different causes. Could the techoes here or anyone else listening help me determine the cause of the cracks as I need to make this again( a commission that they still want it).

4 layers of stringers 1mm clear and dichroic, each layer perpendicular to the previous layer were tack fused resulting in a 500mm x 500mm square with fringes just 2 layers of stringers(2mm) like a frayed cloth

tack fuse degrees celcius as follows
250 538 10
400 720 10
afap 482 45
100 427 10
100 371 off no problems

3 days later the resulting fused piece was slumped into 2 stainless steel bowls, a smaller bowl upside down and inside the larger bowl. Both bowls were air brushed with many layers of kiln wash many days previous
250 538 0
100 650 20
afap 482 45
100 371 off

The kiln is an evenheat 2451-13 bath shaped one with side and top elements.
the set up was in the middle of the kiln on top of kiln posts layed flat on the floor avoiding the holes in the bowl. The top elements were approx. 5” from the glass

The slump and cooling was closely watched thru the peep hole
I decided to skip step at 620 degrees celcius instead of going to 650 as programmed because the glass had slumped far enough i.e down the sides of the large bowl over the small bowl somewhat but not down too far into the acute angle where the two bowls met. I stayed near the kiln during the cooling and annealing, peeped occasionally till 300 o c . and neither heard nor saw anything to worry about. So the cracks may have appeared after this temp on the way down, they were certainly there the next morning at 19 o c
There are 2 cracks of different shapes. One is in the fringe of one side and S shaped(see photos). The other is rough to touch, straight except that it skips a stringer or two then continues in a line.
The resulting bowl was very difficult to remove from the mold. The bowl wasn’t sticking to the steel but seemed to be caught in it, wedged perhaps. I had to hold it upside down and shake vigorously. It came away in one piece and has been moved around a lot and seems quite sturdy. I can send more photos if it will help.
Thanx in anticipation
michelle

[Twin Vision Glass]

I know this will not help you at all now BUT I once got my vessel form stuck in a metal mold and wound up putting it back in the kiln upside down and brought up VERY slowly (25 deg. per hour) up to (I was going to go to 450 deg. F. BUT it dropped out at 400 deg. F. ) I had layed out 2 layers of 1 1/2 inch fiber underneath for it to fall onto and it all was SUPER. I let it cool down slowly toooo! , but not sure of the correct cooling but I went 50 deg. Per hour down to 80 Deg. F.
I NEVER peek below 1000 Deg. F. and never open until it has reached room temp. (totally eliminating the thought that I thermal shocked it.) I have also had EXCELLENT results slowing the rise in temp to 50 deg. per hour. but once again my pieces are usually 3/4 of an inch thick.
When tack fusing , I even slow down to 50 deg. per hour when slumping as it is thick meet thin meets thick again and well, I would rather it turn out than go tooooo fast.
You are QUITE close to the elements tooo , so again I would slow it down. BUT you did mention that it did not break on the way up because you did monitor it and look at top temp. Is this correct.
So not sure if I addressed many questions , but just how I am having success of late.
Leslie
http://www.twinvision.fusedglassartists ... s2009.html

[Stephen Richard]

I think you are inadequately annealing the piece.
I never anneal between 482 and 371C at more than 80C/hr
Also tack fused items need more care in annealing so I would slow down to something like 40 or 50C/hr on annealing.
Although you did not see any cracking on the way up, I think the firing is too fast on the slumping. I suggest no more than 150C/hr advance all the way up to top temperature.
You might want to use some thinfire or other thin ceramic fibre over the stainless steel to give a bit of cushion between the glass and steel.
The pictures did not come through so I am only guessing without visual evidence.
steve

[msch]

oops forgot to attach these

[Mary Kay Nitchie]

Hi Michelle,

I essentially agree with Stephen Richard's advice, and confirmed with Ted Sawyer, Bullseye's Director of Research and Education. The annealing is insufficient.

Ted wrote,

"...Because the piece is tack fused and set up in a multiple component slumping situation I would suggest treating it as if it were at least two-three times as thick as it actually is. Since it’s 4mm, treat it as if it were 12mm. 2:00 at 482°, 55 dph to 427°, 99 to 371°, 330 to 21°C.

It is possible that there is some other strain resulting from design induced stress. Tack fused pieces are notoriously more difficult to execute because the texture can concentrate strain into the weakest areas of the piece and cause them to fail."

I hope this helps.

Mary Kay

PS: I am curious--do you usually use dichroic stringers in this design? Paul Tarlow recently posted a comment in another thread that made me think that mixing dichroic glass and plain glass may (in some designs) require a more conservative schedule. http://www.bullseyeglass.com/forum/search.php?author_id=397&sr=posts

[msch]

thanx very muchly to leslie, stephen and mary kay and ted
as a result of your feedback i'm feeling like i can give this one another go using a less complex slump shape, thin fibre blanket and 40 degrees celcius for annealing and heating up, cooling down as if it were 12mm thick
as u all suggested. i'll let u know how it goes. thanx again 4 your input
michelle
ps mary, i've done this once before. i included dichroic 1mm stringers then too with great results but it was only 2 layers thick(2mm) and a much more simple large bowl shape. i don't think the dichroic caused any stress, it survived a 800km to its new owners

[Twin Vision Glass]

Hi again, at some point I would LOVE to see the slumping designs you are using too. I wish I had of asked for a more wide photo before but !!!! It would help to understand more what happened as well.
Leslie