Failure

[Jennifer Polver]

Third failure with this configuration

12" diameter using a 11" diameter bowl mold. I am using a smaller mold to create overhangs that create a handle effect.

The bottom is french vanilla, top is steel blue with nougat rods around the edge and white vitrigraph

Fusing Schedule:

300 to 1050
50 to 1250
250 to 1450 hold 10 minutes
800 to 960 hold 3 hours (went back to Bullseye original temp)
100 to 700 off
Waited until room temp to remove

Slump Schedule:

100 to 1150 hold 3 hours - Slowed down 50 degrees this time use to do 150
800 to 960 hold 3 hours
100 to 700 off
Waited until room temp to remove

Have what I describe as a tear in the piece. Edges are smooth to the touch at the tear. Should I just forget this idea or should I slow way down. The tear happens on the slump

Should also mention that I have a shallow kiln and the elements are in the top and the piece sits close to the elements.

Is this configuration doomed?

[twin vision glass]

Slow down to 50 deg. per hour. Perhaps this will help OR fire you pieces then bring somewhere else to slump that is not close to the elements. You are for sure thermal shocking you piece. Can you slump in shallower vessel form. Les

[Valerie Adams]

Also, the T-grade rods aren't recommended for kiln forming.

[Jennifer Polver]

Les - thank you for the suggestion. I was leaning towards the closeness to the elements being the problem.

Valerie - I am using these rods http://shop.bullseyeglass.com/nougat-op ... -each.html, which are fusible and I have used them successfully in many other projects.

[twin vision glass]

Hi again, it sure looks as if it is a thermal shock because how it has healed almost and softened edges. It broke going up. I toooo use the almond rods and they give a lovely reactive quality with the dense white (which must be fired lower!) but I always go very slow when slumping, and especially if close to elements. Slow right down to 50 deg. per hour, right up until desired slump probably at 1100 to 1150 F.
Sincerely, Les (just my suggestion but has worked for me for many years now)
P.S. I also might suggest to slow down from Anneal to 700 F. as well as 100 is to quick . What do others think.

[Bob]

Hi Jennifer,

Spectacular tear in the glass. I agree with the comments that it is thermal shock and that it happened on the way up.

I'd like to speculate on several reasons why it shocked. The bottom is french vanilla which is a very stiff glass. The top is steel blue which , because of the dark colour, is probably much softer than the FV. The steel blue will absorb heat much more quickly than the FV because dark colours absorb heat more quickly than light colours .... kind of like how dark clothing absorbs heat more than light clothing which reflects it. Plus the steel blue is directly exposed to the lid elements which are quite close according to your description. The glass is lying on the rim of a mould and the air trapped inside the mould is cooler than the air in the rest of the kiln . So the FV underside is not "seeing" much of the heat. The steel blue really wants to expand whereas the FV is lagging behind. That led to the thermal shock.

Interestingly, when I looked at other pieces on your web site ( very nice work btw) you have successfully slumped steel blue bowls and I am assuming the backs are FV ("No. 1" seems to have a very light back). So why are the pieces in your current design more prone to thermal shock? I wonder if it has something to do with the light coloured rods on the rim. The tear in the glass is only in the steel blue. It ends at the light coloured rim. I am assuming that they will be stiffer than the steel and will be reflecting heat while the steel blue is absorbing heat. This may add stress in addition to the stress from having a colour/ viscosity difference between the top and bottom of the piece, and being close to the elements.

So ... is the variation in viscosity between the rim and the interior resulting from the design creating too much stress within the piece? I have made pieces with a FV rim and steel blue interior and had them work. The three differences in my set up were that the glass was only 1/8 inch thick (easier to get heat into the interior), the FV was only around the rim (not on the back), and I had good separation between the glass and the elements.

Your ramp rate of 100f/hr is very conservative. The only suggestions I can make is, as Les suggested, slow down the ramp and maybe add a few soaks while heating through the brittle zone to increase the likelihood of getting heat even throughout the piece... not only from top to bottom but from edge to center. Soak it for 30 minutes say at 400F, 600F and maybe even 750F. I would still use a very slow ramp between soaks.

It is unfortunate to have had problems with the piece but what a great learning piece. Hope this helps.

Cheers,

Bob

[Stephen Richard]

My experience is given here:
http://glasstips.blogspot.co.uk/2011/11 ... racks.html

[Bob]

Interesting interpretation Stephen,

I agree that the differential heating between the top of the piece and the bottom is the likely culprit and we both are suggesting a slower ramp rate to get heat into the glass.

But I still think that viscosity plays a role. In both your example and Jennifer's the break seems to occur within the area where the viscosity contrast between the top and base is greatest. Jennifer has the rim of light coloured rods and the excellent example on your blog has the break within the dark glass with minor extension into the clear (?) rim. I have seen one other example of this style of crack developed during slumping. In this case the crack was on the top surface and the crack was limited to a small patch of white and black glass that formed a small part of the surface. Heat conducts more quickly through dark colours than light colours. So within the interior part of the glass where the dark colour is adjacent to the light colour the dark colour expands more quickly resulting in the stress differential.

Small point. Whatever the cause the solution is to allow more time for the heat to work into the glass.

Cheers,

Bob

[Jennifer Polver]

I am going to attempt this piece again

Les I will slow down to 50 and I will slow after my anneal

Bob - thank you for the comment on my work it is much appreciated. I actually work predominately in Steel Blue / French Vanilla. I have more work that isn't on my site. I also sometimes use Steel Blue and White. I have more successes than failures (hopefully that didn't just totally jinx me - knock on wood). I definitely see your point about viscosity and the french vanilla being a very stiff glass. I will also adjust my schedule for the soaks you recommend.

I'll post once I have fired it again. Hope to have better news this time around.

Thank you all for the suggestions

[Jennifer Polver]

Took all the advice and still failure. Fuse schedule remained the same and adjusted slump to the following:

50 to 400 hold 30 minutes
50 to 600 hold 30 minutes
50 to 750 hold 30 minutes
50 to 1150 hold 2 hours
800 to 960 hold 3 hours
50 to 700 off

waited until the kiln was room temp and this time it split in two. The only change in set up I made was to use thinner posts to set the mold on so it wasn't so close to the elements.

[twin vision glass]

Oh my!! because you have many things happening, (thick to thin around edge to middle, close elements , and well I truly think you could try this in another kiln BUT ) like Bob said earlier, you have a WHITE base, dark middle with light around the edges and the Canes are making it quite thick so you can see the variance in thickness as well.
I usually will do a slump with thick to thin at 50 degrees per hour up to 1200 F. BUT it usually slumps at 1180 . BUT I have many different kilns and even if I put it in different kilns I still have success. Do try not so many stops and starts for sure. It should just heat up nice and even and slump so nicely. To me you are tooo fast coming down still. If you measure the outer rim and the lower section, you will see quite a difference in thickness for sure. Don't give up. ! Leslie
P.S. what are the WHITE canes . Are the cream ones Nougat . Did you make sure the canes are for fusing. Just a thought. Hopefully not dense white or Opaque white

[Bob]

Hi Jennifer,

The kiln gods are not in a good mood. I would have thought the schedule you used should work. Did the piece break after it came out of the kiln or during cool down? It looks like the two halves would fit together to make a properly shaped bowl.
This is a tough one!

Cheers,

Bob

[Jennifer Polver]

Les - the rods this time are nougat and french vanilla. I didn't have enough nougat.

Bob - The piece came apart as I removed it from the mold and does fit together perfectly. The edges were it broke are sharp but smooth if you know what I mean.

I am putting this aside for now, I am out of nougat rods, I'll try again down the road

Thank you all for your advice. I really appreciate it.

[Bert Weiss]

If the 2 pieces fit perfectly together, it broke on the cool down cycle. If it had broken on the heatup, the 2 sides would have slumped differently. The first picture was a heatup break.

Your anneal schedule looks all wrong to me. 960 is above the anneal range. 3 hours is too long to spend there. An anneal schedule that starts at 960 is designed to stabilize there with a short soak, and then anneal on the next segment. By the time 3 hours is up, if the temp inside the kiln is uneven, that gradient is now inside the glass and could make anneal impossible.