bulbous protrusions on slumped platter

[melanie feerst]

Seeking help / advice for an enthusiastic novice who purchased a kiln to do lost wax investment casting but wanted to play around with slumping and fusing. This project is less marketable than I planned (but actually looks great as sculpture on the wall!). However, I would like to assume control and find out why this "mistake" happened. (in case I WANT to reduplicate the effect!)

Upon the first slumping (firing schedules below), the two layer opal System 96 fused blank (all black bottom layer, top layer pattern with stripes - opal green, yellow, blue and black) had a couple of bumps where the glass didnt settle into the plate mold (Bullseye ceramic square plate). The two layers were still fused (i.e. there was no separation or air BETWEEN the layers).

So, being NEW to this, I tried to refire the slump, hoping the glass would settle in to the mold. What came out wasthe plate with large bulbous protrusions (one section even folded over on itself).

The kiln is a paragon fusion 10 xtra wide and xtra deep (custom) and the plate mold was on kiln shelves about 10-12 " from top elements (side elements were exposed, the pyrometer was above the kiln shelf). I used thin fyre paper on the kiln shelf for the fusing, and kiln washed the plate mold each slumping. (my first double layer blank of cathedral system 96 slumped beautifully, the firing schedules were the same for fusing and slumping)

FUSING
600 dph to 1000 degrees, hold 0 (prop kiln lid to 500 degrees, then close lid)
2000 dph to 1440 degrees, hold 5 min.
crash kiln to 1000 (open lid)
600 dph to 955, hold 10 min
400 dph to 800 hold 0
800 dph to 120

SLUMP #1
400 dph to 1000, hold 0
900 dph to 1298, hold 10 min
crash kiln to 1050, hold 0
150 dph to 950, hold 45 min
100 dph to 100

SLUMP #2 (took it a little hotter)
400 dph to 1000, hold 0
900 dph to 1360, hold 10 min
crash kiln to 1050, hold 0
150 dph to 950, hold 45 min
100 dph to 100

THANKS VERY MUCH FOR YOUR COMMENTS

[melanie feerst]

Just read the advice given for Aimee a few postings back. I will try to drill a few more holes in the bullseye mold and try a new blank - however any other advice is certainly appreciated (especially about my firing schedule!)

[charlie]

slow down between 1100 and 1250 during fusing. that will tend to eliminate bubble formations between the layers, especially if you're using a complete piece of glass as any layers except the bottom.

no need to crash. just program in an afap (9999 on most controllers) decrease to your annealing temp.

[Cynthia]

...Upon the first slumping (firing schedules below), the two layer opal System 96 fused blank (all black bottom layer, top layer pattern with stripes - opal green, yellow, blue and black) had a couple of bumps where the glass didnt settle into the plate mold (Bullseye ceramic square plate). The two layers were still fused (i.e. there was no separation or air BETWEEN the layers).

Are there vent holes in the mold? I would expect that you need to drill holes that are 1/16" or smaller. One in the center with three surrounding that central hole about a couple inches out. Also, put your mold up on kiln furniture so it's not sitting directly on the shelf. This way air can be pushed out from under the glass rather than being trapped between the glass and the mold. YOur primer should be dried on the mold prior to firing. Although the moisture should be driven out before the glass bends, better safe than sorry, so cure those molds first.

So, being NEW to this, I tried to refire the slump, hoping the glass would settle in to the mold. What came out wasthe plate with large bulbous protrusions (one section even folded over on itself).

The kiln is a paragon fusion 10 xtra wide and xtra deep (custom) and the plate mold was on kiln shelves about 10-12 " from top elements (side elements were exposed, the pyrometer was above the kiln shelf). I used thin fyre paper on the kiln shelf for the fusing, and kiln washed the plate mold each slumping. (my first double layer blank of cathedral system 96 slumped beautifully, the firing schedules were the same for fusing and slumping)

With side elements, and the fact that your piece is so far from your top elements it's likely that your piece heated and softened to moving on the sides more rapidly than the center. The edges seated against the mold before the center dropped, trapping air. When you tried again at a higher temp, the air expanded even more, the glass was even softer and that is why you got even larger domes with one even collapsing on itself when the air could escape. You might want to slow your ramp rates down overall because of the distance from the top elements, or baffle the side elements from the work with fiber board.

FUSING
600 dph to 1000 degrees, hold 0 (prop kiln lid to 500 degrees, then close lid)
2000 dph to 1440 degrees, hold 5 min.
crash kiln to 1000 (open lid)
600 dph to 955, hold 10 min
400 dph to 800 hold 0
800 dph to 120

SLUMP #1
400 dph to 1000, hold 0
900 dph to 1298, hold 10 min
crash kiln to 1050, hold 0
150 dph to 950, hold 45 min
100 dph to 100

SLUMP #2 (took it a little hotter)
400 dph to 1000, hold 0
900 dph to 1360, hold 10 min
crash kiln to 1050, hold 0
150 dph to 950, hold 45 min
100 dph to 100

THANKS VERY MUCH FOR YOUR COMMENTS

Check out this thread. It's pertinent to your question.
http://www.warmglass.com/phpBB/viewtopic.php?t=1106

Also...your firing schedules could be altered a bit. I would fire up at 100 dph from 1000 to slumping, which should actually occur at around 1100 to 1200 F. Your best success will come from firing with lower temps and allowing the process to go a bit slower. When you fire AFAP from 1000 up to your process temps, you invite trouble. Bubbles under and within the glass are the most common. Venting or crashing is hard on your kiln brick. I only crash when I must arrest a process. If you fire lower and slower, there is no need to crash your kiln. Use an overglaze like Superspray to prevent devit if this is why you crash your kiln.

You only need to prime your mold once. Unless you scrape off the kiln wash somehow, the temps that you use for bending don't alter the properties of the wash, so once your mold is primed and cured (washed and fired), it's good to go from then on. Only need to reprime it if it gets ginked up or scraped off from use.

[Barbara Muth]

the other thing Melanie is that the sucessful firing was with cathedral glass and the unsuccessful with opals. My guess is that if you had fired slower, soaked longer on the slump with the opals, you would have filled the mold out the first time. Opals are harder than cathedrals, so it would probably necessitate a differnt firing schedule. Everything takes practice till you know your glass and your kiln! Have fun with it.

Barbara

[melanie feerst]

Thanks for the help. The bullseye mold was raised up from the shelf with kiln furniture, and it does have three small vent holes in it. I put on three layers of primer and dried in kiln at 200 degrees for one hour, then put the blank on and continued the cycle.

I have the ability to turn down the side elements, so i might try that.

thanks also for the help with firing schedules - I'm used to pouring metal - one temp it melts and get it in the mold as soon as possible!

I am concerned about drilling additional holes in the mold - I just saw another posting where Aimee had a disaster (crack) after she drilled more holes (see LARGE BOWL WOES CONTINUED)

I do somehow think that it might be cathedral vs. opal and one is stiffer than the other as part of the problem.

I will continue to have fun and experiment.