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Frosty back on glass of open faced molds

For discussion of processes related to using Bullseye glass, including kilnforming and kilncasting, torchwork, blowing and stained glass.

Frosty back on glass of open faced molds

Postby library » Wed Jun 06, 2018 12:56 pm

Project Information:

1) A description of the problem:

Cloudy back of work

I fired two similar pieces in open-faced investment molds with different firing schedules and one of them has a frosted back with noticeable strain lines. Both of the molds were a regular investment mix. They were both placed in the middle (both vertically and horizontally) within a Skutt Gm-1014 kiln. The firing schedule for this first piece that didn’t have any cloudiness or strained corners was:

1. Rate-100, Temp-200, Hold-6:00
2. Rate-100, Temp-1250, Hold 2:00
3. Rate-600, Temp-1525, Hold-1:00
4. Rate-9999(AFAP), Temp 900, Hold-6:00
5. Rate-12, Temp-800, Hold-0:00
6. Rate-22, Temp-700, Hold-0:00
7. Rate 72, Temp-70, Hold-0:00

The Firing Schedule for the second, which had a cloudy back and strained corners, was:

1. Rate-300, Temp-300, Hold-5:00
2. Rate-150, Temp-1250, Hold-5:40
3. Rate-150, Temp-1525, Hold-3:00
4. Rate-999(AFAP), Temp-900, Hold-4:00
5. Rate-27, Temp-800, Hold-0:00
6. Rate-49, Temp-700, Hold-0:00
7. Rate-162, Temp-70, Hold-0:00

Please help me figure out what this happened.

2) If possible, a digital image of the piece is and kiln is often helpful.

Attached and listed above

3) Style codes (i.e. 1101-30), production dates, and sheet numbers for all of the glasses in the project in question.

Don’t have this information, but it was Bullseye green billet.

4) Design of the piece (dimensions, arrangement)

6” x 11” x .5”

Kiln and firing info:

1) Description of the kiln (type, interior dimensions, controller, etc.). Describe location of elements.

Skutt gm-1014 (26” x 26” x 12”)

2) Firing cycle for this project (including the annealing portion of the cycle)

Mentioned above

Kiln Furniture and supplies

1) What kind of kiln shelf are you using?

One that came with the Skutt gm-1014, the investments were placed on sand.

2) What are you using for a shelf or mold separator? (Thinfire, fiber paper, shelf primer, etc.)

Kiln wash, sand

3) Where is it situated in the kiln during firings?

Middle, middle.

4) Describe any additional kiln furniture used (dams, molds) and the separators used.

Do you have a hunch about what could be done differently to avoid the problem?

I opened the kiln at 98 degrees with the problem piece and room temperature for the good one. Perhaps this was the issue?
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