Slumping Multiple Identical Pieces

Jackie Braitman · Post by **Jackie Braitman** » Tue Sep 09, 2003 5:25 pm

From all the threads I've read (this board and the archives) I expected that when I put 4 identical 9" bowl Molds into the kiln I could use the same firing schedule I used for a single mold.

It didn't work. The bowls (2 1-2" deep) had barely started to slump and were slumping very uneven. [I wasn't able to check the kiln during the process so I just saw the end result.]

The schedule was 300/hr to 1260 hold 5 minutes
AFAP to 960 etc. This schedule worked perfectly several times for a single mold.

I did a Bullseye "Knowning Your Kiln" test at the low end expecting to find that my kiln didn't fire evenly but all 13 strips slumped the same so apparently the kiln is firing evenly. It's a Paragon GL24

Is my problem too high, too fast? My next test was going to be to ramp to maybe 1200 and let it sit 30 min. Any suggestions, help? Did I just mis-read the earlier threads?

charlie · Post by **charlie** » Tue Sep 09, 2003 5:37 pm

i'll hazard a guess: when you did one, you put it in the middle of the kiln. when you did 4, you put them into the kiln. however, each was a different distance from the sides, where your elements are. the center and the edges of each blank were at different disance from the elements, so each bowl slumped weirdly (maybe offcenter).

if you had let it run for a longer time, i would bet that would have solved the problem. you should also perhaps get in the habit to peek in and look at the glass when it's slumping. you'd have a better idea what goes on in there at what temps/times then depending upon your controller.

Lisa Allen · Post by **Lisa Allen** » Tue Sep 09, 2003 5:53 pm

I would try going slower up to your final slump temp too. You have got alot more in there to heat up evenly. Maybe 200dph or even 150dph to 1200, then hold and you can peek to see when they are down.

Lisa

Jackie Braitman · Post by **Jackie Braitman** » Tue Sep 09, 2003 7:04 pm

Thanks very much for the advice. Charlie, I guess what you're saying makes sense but isn't the "knowing your kiln" testing whether the kiln is heating evenly. Why did the 13 strips slump evenly? And not the 4 molds?

Lisa, I certainly understand that the heat was being absorbed differently because of the extra mass of the molds and glass. Or Charlie, is this what you're saying also?

It definitely make sense to go slower, longer. I will try your schedule next time Lisa.

I usually do peek but just couldn't this time. Isn't is always the way. I should have made time.

Thanks, again, for the help. I've learned so much and solved so many problems reading the threads.

Jackie

Steve Immerman · Post by **Steve Immerman** » Tue Sep 09, 2003 7:13 pm

Jackie,

It should have worked. Was the glass that you slumped in the group of four the same color and thickness as your single piece firing?

300 dph is a good rate of heating. I usually go to 1000 and hold for about 15 min, then heat to slumping temp at a faster rate (like 600 dph) and then hold at the slumping temp (eg 1200 degrees) for about 10-20 min.

It is a good idea to peek when you reach slumping temp to make sure it is doing what you expected.

Steve

Jackie Braitman · Post by **Jackie Braitman** » Wed Sep 10, 2003 7:13 am

Steve,

The glass slumped in the group of four was fairly equivalent to the single slumps I had done. All pieces were an even 1/4". All were Bullseye transparents with some Opal powders. While the original single slumps were clear plus 1137 plus opal powder or clear plus clear plus opal powder; the group of four also included clear plus 1147 and clear plus 1116. Do you think this could have made the difference? I know that opals heat up very differently than transparents but I work almost exclusively with transparents and haven't noticed too much difference between the colors.

The earlier ones had slumped just as predicted so I hoped I was safe being out when the group of 4 reached slump temps. I know now never to assume.

Jackie

rosanna gusler · Post by **rosanna gusler** » Wed Sep 10, 2003 7:31 am

i think that it was a factor of more mass in the kinl to heat. i fuse from 1 to 5 shelves in my kiln. there is a huge difference in process/ time with the addition/subtraction of each shelf. same in my test kiln. more mass = more time. i also find that when slumping off center to the elements that slow makes alot of difference in getting an even slump. i like to hold at 1100f+- for 20 min or so in the interest of evenivity. (i use float) rosanna

Jackie Braitman · Post by **Jackie Braitman** » Wed Sep 10, 2003 7:48 am

Rosanna - I was building up to using multiple shelves. When you use multiple shelves, are they all the same size or progressively smaller? I think I read somewhere in the old archives that when you use multiple shelves you should use side elements only. Is this what you do? Do you have an algorithm for how much additional time for each shelf added?

Thanks for any insight.
Jackie

charlie · Post by **charlie** » Wed Sep 10, 2003 10:40 am

Jackie in Washington DC wrote:Thanks very much for the advice. Charlie, I guess what you're saying makes sense but isn't the "knowing your kiln" testing whether the kiln is heating evenly. Why did the 13 strips slump evenly? And not the 4 molds? Jackie

remember, the sides of the molds shield the direct radiant heat, but not he conductive heat. the strips don't have any shielding, so are heated on all sides directly.

rosanna gusler · Post by **rosanna gusler** » Wed Sep 10, 2003 12:26 pm

i only have side elements. i use all the same sized shelves. i would think that top elements should be off for multiple shelf firing, but i do not know that. i have an orton kiln vent and zone controll on my kiln. that helps with evenivity. even with that i have to elevate the bottom shelf to above the bottom element for it to fire the same. a friend has an ancient beat and battered ceramic kiln same size as mine. no controller,kiln sitter wired shut, not enough juice to run all elements on hi at once........ she fires 4-6 shelves using a timer and pyrometer. she uses the uneven firing pattrern in her kiln to advantage. rosanna