Is this a stress crack?

[welcombe8]

This is my first time at sending a query. I have been sitting on the side-lines for many months, learning such a lot from the questions sent in by others on this forum. It is fantastic. I suppose I am fairly new to fusing having worked with stained glass for about 3 years prior to moving to the more creative and exciting aspect of glass working in fusing over the last year. My latest piece of work 'curvesblackstringers is
http://community.webshots.com/user/suejoyce1
(hopefully this works)
I live in the UK and have had to convert my fusing schedule from deg C to deg F but here goes: 111deg/hr to 955 soak 20min, Full to 1535 soak 5mins,Full to 955 soak 40mins, 145 deg/hr to room temp. I left the platter in the kiln overnight to cool. Unfortunately as soon as I started to wash it in warm soapy water I heard a ping and there is a small curved crack about 1" at the lower edge on the right of the blue join near the first black stringer. All glass that I use is Bullseye 90. The top glass was fused on a blank of clear with frit between the curves and black stringer on top. The size is 8" diameter. I am going to fuse it again in the hope that the crack disappears - the crack does not appear to be in the base clear layer.

[Peg]

I left the platter in the kiln overnight to anneal. .

I don't understand - BE anneals at about 516 deg C - how long did you hold at that temperature?

Peg
Also in the UK

[welcombe8]

I held it at 516 deg C for 40 mins ( sorry that was my conversion) for annealing.

[Peg]

Full to 1535 soak 5mins,Full to 955 soak 40mins, 145 deg/hr to room temp. I left the platter in the kiln overnight to cool. .

I'm no expert - but I wouldn't drop from 1535 to 955 at full rate. I take it down from top (820 full fuse, 760 ish slump) (sorry, can't think in farenheit) to 560 or so at full rate, hold it a while to stabilise, then take it slow to 516 (100 dph), anneal, then slow to 300, then turn off the kiln to cool at it's own speed.
I tend to anneal for longer too - at least an hour if not 2 for a plate up to 30cm diam. (12 inches in old money).

As a matter of interest, where do you get your BE from?
Peg

[welcombe8]

I am learning all the time. Thanks for your firing schedule - I will adapt mine and see how it goes.

I have purchased my BE from various sources but mainly from Creative Glass , Rochester, Kent. Where do you obtain yours?

[Bob]

Hi Sue,

Interesting problem ... which is the short answer for "I'm not really sure". But here are some suggestions. First of all your firing schedule appears OK... conservative if anything. The initial heating at 115deg F/hr to 955 is very cautious. You could easdily go to 300 deg/hr and(if you are gutsy) 600deg/hr. The rapid cool to 955 (actually I would choose 960/ 516deg C) is OK. And the gradual cooling at 145deg/hr appears OK (you really only need to go to 740F/399C... and turn the kiln off.... it should cool slowlty enough)

WHy did it crack? I have two suggestions:

1) was the piece still warm or hot when you washed it in water? If so then the piece broke due to thermal shock. Glass is a poor conducor of heat and even though the piece might have been warm to touch it might have been still relatively "hot " inside. When you washed the plate the rapid cooling of the exterior of the glass caused thermal shock. I would suggest that you let the plate cool completely before washing it.

2) The design of the plate with the black stringers meeting near the joins between several other colours, and the slight indentation along the edge of the piece (where the pink and blue join) might have caused local stresses. Normally this wouldn't present a problem but the washing probably added a bit of thermal stress that was enough to crack the piece.

Go ahead and refuse it ... the crack should disappear.

Oh by the way... the cracking during washing... I had the same thing happen. It is hard to wait for pieces to cool. But you really should.

I don't know if this is correct but it is a reasonable guess. Hope it helps.


Cheers,

Bob

[welcombe8]

Thanks Bob. I will re-evaluate my fusing schedule and change it accordingly. Possible reasons for the crack - I do not think that it was due to your number 1 suggestion as the plate was quite cool, but maybe it was due to number 2 reason and there is a design weakness there. I am now fusing again and will let you know tomorrow what happens.

[Bob]

Hi again Sue,

In reality I think that the "design problem" I mentioned should not have been that significant a factor. In theory there can be residual stress around pointy shaped design elements, and black sometimes can cause a problem because it is such a "soft" glass. But the shapes near your crack aren't that severe, nor do you have enough black in the piece to be a major factor. Guess I just don't know.

Option #3: Have you been kind and friendly to your kiln goddess lately? Anything to upset her delicate disposition?

Cheers,

Bob

[welcombe8]

Hi Bob, Great results with second fusing - no crack. I must have been good to the kiln goddess last night.

[Bert Weiss]

Sue

You can learn to see where the crack starts and continues by looking at it's shape and at the edge markings once it seperates. Annealing cracks tend to be S shaped. Compatibility cracks tend to happen around the incompatibility. Cracks from a blow will eminate from that blow. The same with heat or cold shock. Sometimes it is difficult to be sure but often it becomes perfectly clear.