WarmGlass.com

My plate broke while slumping. I think I understand why it borke, it went over the edge of the mold. I don't understand why it did as it was the same size as the mold. In the photo you will see how much it ran over. The plate was two layers of tetka with powder frit on a leaf. The fuse was great. Why would it run over the edge?

Firing schedule for slump
300 to 1210
full to 900
100 to 700
off

It is obvious to me it did not break in the ramp up because the edges of the break are sharp not rounded.

Good photo !
The parts have moved far apart.
I understand it so that the blank exploded quite early, before it started to slump.
It was the explosion tha moved the pieces over the mold rim.
Why did it do that?
Maybe too fast ramp up
or
the leaf pattern has absorbed heat faster than the clear part,
thus getting hotter and causing a thermal shock.
The shape of your mold exaggerates that as it serves as concave mirror focusing heat back.

On a second thought - were the leafs real? If so did they contain moisture.
If there is any water somewhere, that part cannot get hotter than boiling point.
I am sorry for you, it was a nice design.
Double or triple the ramp length next time.
-lauri

As Lauri said...Thermal shock breaks are usually long, lazy "s" shapes, generally with a hook at once end.The idea that sharp edges mean it couldn't have broken on the way up in a slump is actually a myth. Most slumping schedules don't give the glass enough heatwork to round over the edges of the glass very much (although if you look closely you may see a very slight rounding).

Sooooo...it's probably a thermal shock, i.e., the glass was either not well-annealed going into the kiln, or the schedule was too fast, or both. It went over the edge of the mold because thermal shock can be kinda explosive and it probably pushed the pieces apart. If it was exactly the same size as the mold, there wouldn't have been much room for it to travel, so it went over the edge.

Thanks to both of you for explaining everything so well, I guess I thought the bullseye firing schedule was fine and have used it several times. This is a new mold, and maybe it just heated faster than the previous mold which was stoneware. I will go slower next time.

The leaf is a real leaf, I used Paul Tarlow's fossil Vitra tutorial to make the plate. It had been full fused so there shouldn't have been any moisture. Thanks again Morganica and Lauri.

It's hard to tell from the picture but you might want to raise your kiln shelf up a few inches. It looks as though your plate mold is at the same height as your element, which can cause the edge of the glass to heat up faster than the center. This will help with air flow inside the kiln and a more even heating chamber.

In that same thought process,

I sometimes raise my molds slightly on pieces of kiln furniture (checking for level afterwards), in order for the air to move more freely around the molds. It looks like your mold is sitting on the kiln floor. Proximity to elements and restricted air flow beneath the glass/mold can be issues singly, but are double-trouble when combined. Check to make sure the holes in your mold are open (not blocked by kilnwash, etc.).

(my two pennies...)

warm regards,

Eric

Eric Baker wrote:I sometimes raise my molds slightly on pieces of kiln furniture (checking for level afterwards), in order for the air to move more freely around the molds.

I always elevate my molds. Dana W.

Either raise the shelf or baffle the sides. Glass wants to be the same temp all the way through and at all times. You must think like the glass and think like the kiln to make that happen.

gemhuyck wrote:My plate broke while slumping. I think I understand why it borke, it went over the edge of the mold. I don't understand why it did as it was the same size as the mold. In the photo you will see how much it ran over. The plate was two layers of tetka with powder frit on a leaf. The fuse was great. Why would it run over the edge?

Firing schedule for slump
300 to 1210
full to 900
100 to 700
off

are there any vent holes in that mold?