Two 11" Round Plates Broke During Slump

[Suzan]

For the 1st time I tried to cram in 4 shelves for slumping in my 17.5" wide x 13" high kiln. Could that be the reason? The two top items, which were both gold powder with a clear cap, were fine. The two bottom plates had two full layers of glass, plus cut glass design elements and frit. Both broke into about 10 pieces each, largely wedge shaped. The kiln posts separating the shelves were 3" high. I normally slump up to 3 shelves using the 3" posts, with no problem.

Also, a piece of glass is stuck to the element. Should I heat up the kiln to release it? If so, to what temperature?

Here is the firing schedule I used for the slump:

250F dph to 1225F, hold 45
AFAP to 960F, hold 30
200F dph to 750F, hold 30

Cheers,
Suzan

[Marty]

It sounds like you found the limit for multiple shelves in your kiln. Cut your ramp up by at least half to 100dph, hold at 1000F for an hour to let the heat equalize. Put the thinner pieces on the bottom. Hold at 960 for longer and go a little slower to 700, not 750. You don't say how long to cool from 750 but it should be at least 5 hours to room temp.

As far as glass on the elements, I've never had that problem but remember possible solutions posted here- check the archives.

[Suzan]

This time, I only had two 11" plate molds. The bottom one exploded into 2 pieces, which slumped over the rim of the mold. (In the previous mishap, the pieces all slumped over the rim as well). This was 2 full layers of Spectrum, clear and white opal, with a frit painted design. A very pretty bird, sigh!

I don't think annealing is the issue, because the break occurred when the glass was still hot enough to slump. And besides, I have previously slumped 3 plates with the above mentioned firing schedule successfully several times.

Breaks occur when thermal shock occurs, i.e. cooling too fast, but in this case the breaks occured in the higher range, at around 1225F, because they continued to slump. What are other causes of breaks?

Also, I managed to release the glass from the element by heating to 1200F for a few minutes, and gently pulling it off.

Cheers,
Suzan

[Phil Hoppes]

Breaks can occur on the way up or the way down and are not restricted to annealing issues. Are the edges of the breaks sharp or smooth and rounded? If they are clean the break most likely occured on the way down. If they are smooth then the break probably happened on the way up. I agree with Marty's schedule. The addition or subtraction of shelves to a kiln is not necessarily a "linear" event meaning that things scale in a linear porportion to the amount of shelving and glass added to your kiln. In your case I suspect you pushed it way over the limit with the 4th shelf such that heating became very uneven within your kiln. If you feel you really need to use 4 shelves for slumping I would suggest either doing some type of calibration test (ala Bullseye application notes) or getting some type of dataloging temperature recorder

http://www.omega.com/ppt/pptsc.asp?ref= ... Nav=teml06

and characterize your kiln under these conditions.

Phil

[Carol]

[quote="Phil Hoppes"] The addition or subtraction of shelves to a kiln is not necessarily a "linear" event meaning that things scale in a linear porportion to the amount of shelving and glass added to your kiln. In your case I suspect you pushed it way over the limit with the 4th shelf such that heating became very uneven within your kiln.


I have experienced a huge shift in kiln behavior even just varying the height between the shelves in my old ceramics kiln. It might as well have been a different kiln, so I second Phil's thoughts.

[charlie]

This time, I only had two 11" plate molds. The bottom one exploded into 2 pieces, which slumped over the rim of the mold. (In the previous mishap, the pieces all slumped over the rim as well). This was 2 full layers of Spectrum, clear and white opal, with a frit painted design. A very pretty bird, sigh!

here is your problem. clay or ceramic molds, right?

when the glass slumps over the edge, it is on both the inside and outside of the mold. when cooling, the glass shrinks more than the clay. since it's on both sides, and the clay isn't movable, the only thing left is for the glass to crack.

you can't have overhangs (more than 1/8" or so) using a mold.

[Tom White]

The fact that the glass is breaking and moving outward until it slumps over the edge of your molds and even contacts the kiln elements clearly indicates that the breakage is from thermal shock in the heatup stage. If you were close to the kiln you should be able to hear the glass break and move. With 3 inch shelf spacing the edges of your glass are heating much faster than the center thus the breakage. Anealing of your fused blanks is critical! consider anealing at 75 dph from 960 to 700 in your fusing cycle. Then, heat up your slumping cycle at no more than 100 dph, might even consider 50 dph with 4 shelves. You have to determine if 4 shelves at slow ramp is more practical than 3 shelves at slightly faster ramp.

Best wishes,
Tom in Texas

[Suzan]

Charlie

As mentioned, I had used these molds and firing schedule successfully a number of times before. The molds measure 10.75" in diameter, edge to edge, and the edges are well rounded. I checked my log book and saw that previously, I was cutting circles to 10.25" to account for stretch, but the three that broke all started out as 10.75" circles, and yes, they all slightly extended the mold. Because the very edge of the mold slopes downward, even the 1/8" extension was too much.

In spite of losing 3 nice plates, I'm very happy to at least know why. Thanks again Charlie, and of course others who responded with suggestions.

Cheers,
Suzan