thermal shock issues - WarmGlass.com

thermal shock issues

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Michael Stevens
Posts: 81
Joined: Thu Dec 26, 2019 2:52 pm

thermal shock issues

Postby Michael Stevens » Wed Apr 21, 2021 7:05 pm

I've been using the same firing schedules for similar projects all year, but lately things are cracking in. half.

I had a 12 inch 6mm piece I was slumping. I made same thing last week and it slumped fine. lastnight I did another one but when opened kiln this morning it was brokej in half.

my firing schedule is basically 5 hours to get to 520 degrees c. then rest then slowly ramp until 680.
. not near my computer to write the exact schedule.

but I'm. wondering why same project cracked today but didn't last week.

could the kiln need recalibrate.

also I noticed during cool downs it reduces temperature from 800 degrees to 300 degrees in about 4 hours but takes 10 more hours to get to 50 degrees. is there a reason it's not cooling more consistent.

it used to hold temperatures more evenly. I didn't need to anneal before because it stayed at 500 degrees range for 2 hours on its own. but now if I don't anneal around 510- 520 it cracks also.

I'm. not sure if there is an insulation issue, a heating calibration issue, if it's the heating element, therkal couple, the programmer. just seems strange that the same schedules I've used for 2 years don't work anymore

Kevin Midgley
Posts: 721
Joined: Mon Mar 10, 2003 11:36 am
Location: Tofino, British Columbia, Canada

Re: thermal shock issues

Postby Kevin Midgley » Fri Apr 23, 2021 11:27 am

you have heat loss issues. Somewhere in that kiln you have a draft of cool air getting inside is my thought.
Check your kiln. I may suggest the following may help.
Get a really bright battery operated light. Put it inside your cold non running kiln in a blacked out room and look for light escaping anywhere.

Michael Stevens
Posts: 81
Joined: Thu Dec 26, 2019 2:52 pm

Re: thermal shock issues

Postby Michael Stevens » Fri Apr 23, 2021 6:35 pm

Because it also has problems on heat up. I thought could be the coils, or thermocouple might be off if annealing at wrong temperature. also noticed some cracking in the refractory mud holding firebricks together

Michael Stevens
Posts: 81
Joined: Thu Dec 26, 2019 2:52 pm

Re: thermal shock issues

Postby Michael Stevens » Fri Apr 23, 2021 7:05 pm

Kevin thanks for that. I don't see any external light but there might be cracks in. the insulation and the metal exterior covers the night but heat distribution might be affected. I appreciate your insights


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