Plate slump with vertical lines on back

[Carol D]

I slumped transparent glass ..half green, half purple with the colored glass on the back side, clear on top with frit in the center. After the slump the back (color side) show vertical lines around the plate radiating outward. Center slumped ok. I suspect I may have held it at temperature too long but I have a real problem seeing into the kiln if it has slumped adequately. Top looks ok after firing, just the edges. Could you give me feedback on this schedule....Anything I can do to refuse? Then reslump....Thank you.

400 to 1000 0 hold
1200 to 1225 17 min hold
Full to 1000 5
300 to 950 20
200 to 800 5
400 to 120 0

I have a Multi Purpose Paragon Kiln. Could being too close to the top elements cause this.

[Stephen Richard]

I slumped transparent glass ..half green, half purple with the colored glass on the back side, clear on top with frit in the center. After the slump the back (color side) show vertical lines around the plate radiating outward. Center slumped ok. I suspect I may have held it at temperature too long but I have a real problem seeing into the kiln if it has slumped adequately. Top looks ok after firing, just the edges. Could you give me feedback on this schedule....Anything I can do to refuse? Then reslump....Thank you.

400 to 1000 0 hold
1200 to 1225 17 min hold
Far too fast here. There is no reason to change the rate of advance at 538ºC. Leave it at 222ºC until top temperature.
Your top temperature will be related to the nature of the mould. Simple curves - slumpers, ball moulds - will not require such high temperatures as those moulds with complex curves and angles. I call these lines stretch marks. I think they come from causing the glass to move too quickly at too high a temperature. You have part of the answer in the description of the problem. The centre has no stretch marks. It has changed shape - from the flat start - less than the glass nearer the edges. The glass there is in contact with mould longer as it changes shape and (at the speed and temperature you are using) slides along the surface of the mould.

I think you should be trying to allow the glass to gently change shape with the minimum of stretching. This means low temperatures with long soaks. For example, I can slump a 300mm dia. sheet of 6mm into a ball mould at 620ºC in 30 mins. I could probably do it at 600 with 90 minutes, but don't see the need. This 620C is 50ºC less than you are using presently.

If you can't observe the progress of the slump, you may need to do a series of firings. E.g., 150C/hr to 620 for 30 mins, anneal, view the results. If not enough of a slump, decide on whether more time or more heat is needed and try that. Generally, I would try more time on the principle of trying to get the work done at the lowest practical temperature. But you will want to try a compromise between time and temperature. When you have found a suitable time and temperature combination record it for that mould and glass configuration. It will change for other thicknesses and sometimes colours.

I will make some comments on the remainder of your schedule, but they are not so important.
Full to 1000 5
I don't see the need for a soak here nor the slow fall to the annealing soak. The object is to get the glass from top temperature to the annealing soak relatively rapidly (reduces firing time and devitrification possibilities). At the annealing soak, you are trying to get the glass the same temperature throughout so a significant soak relative to the thickness of the glass is required.
300 to 950 20
So for this piece a soak of at least 30mins is needed.
200 to 800 5
I think your annealing cool is too fast; 80C/hr would be safer, especially if you went on down to 370ºC. Then you can turn the kiln off and let it cool by itself.
400 to 120 0

I have a Multi Purpose Paragon Kiln. Could being too close to the top elements cause this.

So the short answer to your question is - no. The long answer is above.
Of course, a lot of this answer is predicated on several things that I don't know:
glass dimensions
mould size and nature - round, square, simple or complex curve, angles, amount of detail
how close to the elements the glass is at the start
A photo would help.

[JestersBaubles]

I find if I ramp at a level appropriate for the piece to 1100 and hold for 30, then ramp to 1225, my piece will usually always be "finished" by the time I hit 1225, or it requires only a few minutes hold to finish it off. AFAP to 950 and anneal, then a slow ramp down to 750, then cool naturally (I have fire brick kilns, so they cool VERY slowly).

This is with a fairly gently-sloped mold -- not deep drops.

Dana W.

[David Jenkins]

I, too, thought you were going way faster than I would have. As an example, here's the schedule I use for a gently sloping 2-layer dish I often make:

150/1100/10
50/1150/30
FAP/900/30
100/800/0
180/700/0
FAP/125/0
OFF

For me, with the layout used, in a Paragon Fusion 10, this results in a perfect slump. YMMV, of course.

[DonMcClennen]

Mold shape and size....glass size and thickness all play a part...Go lower and slower for better results!

[Stephen Richard]

Dana,
Have you tried 630ºC for 30 mins to see if the lower temperature with a soak will do the job as well or better?

[Carol D]

Thank you all for the good information. Stephen I appreciated your detailed info. and since I should have put fahrenheit after my schedule.. I did trans. to it. You all gave me several schedule changes but the consensus is "low and slow" and that is where I failed. Do you suppose I can re-fuse again and then slump properly and be successful with this plate? This is a great site!! I would have sent a picture if I was smart enough to figure out how to do it......

[JestersBaubles]

Dana,
Have you tried 630ºC for 30 mins to see if the lower temperature with a soak will do the job as well or better?

I haven't... maybe I should .

I've been quite successful with the schedule though, so I hate to go changing things!

Dana