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Black growth on glaze (Thermocouple covers and offsets)


Ceramic Human

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Wanted to ask you guys about your experience with this defect we've been having. It's not glaze specific and I highly doubt these growths are just bubbling up from the clay during a glaze firing (but maybe). This defect does not occur only on mugs that are close to the thermocouple, but I don't want to rule out the possibility that it is thermocouple related.  Ring any bells?

IMG_3970.jpg

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11 minutes ago, JohnnyK said:

Sounds like you're operating a production facility...How big are the kilns you are using? Are you finding any kind of oxidation flakes on the kiln shelves in the areas where you have the problem on your mugs, or is it just the mugs?

The kiln shelves sometimes get flakes but it is more rare, because most of the surface of the shelf is covered by mugs. 

 

6 minutes ago, neilestrick said:

Is it coming through from the body, buried in the glaze, or on top of the glaze?

looks to be on top of the glaze. 

 

I did go through with pliers and clean the ends of the thermocouples and they were VERY flaky, so I'm pursuing that as the most likely culprit. Potentially adding in routine thermocouple maintenance, or purchasing caps  to slide over the thermocouples. 

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2 minutes ago, neilestrick said:

Thermocouple protection tubes will keep that from happening, and your thermocouples will last longer. You'll need to adjust the offset in the controller for the insulating factor of the tubes, though, somewhere around 15-20 degrees F. L&L currently uses an 18 degree offset.

So if we fire to 2167, we'd change the firing to 2185 ish? And all of the target temps by +15-20?

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7 minutes ago, neilestrick said:

Thermocouple protection tubes will keep that from happening, and your thermocouples will last longer. You'll need to adjust the offset in the controller for the insulating factor of the tubes, though, somewhere around 15-20 degrees F. L&L currently uses an 18 degree offset.

Sorry. I'm reading about offsets now and what you said makes more sense.

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12 minutes ago, Ceramic Human said:

I did go through with pliers and clean the ends of the thermocouples and they were VERY flaky

Sounds like it's time to change the thermocouples. If they are spalling that badly they're probably close to the end of their life. Can get temperature reading fluctuations before they fail or they can just fail abruptly. Are you seeing any fine cracks in the weld across the tip? 

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Just now, Min said:

Sounds like it's time to change the thermocouples. If they are spalling that badly they're probably close to the end of their life. Can get temperature fluctuations before they fail or they can just fail abruptly. Are you seeing any fine cracks in the weld across the tip? 

Yes. They are quite thin in some parts and have minor cracking. Seems like we may need to increase routine maintenance until we can get those covers put on.

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22 minutes ago, neilestrick said:

Thermocouple protection tubes will keep that from happening, and your thermocouples will last longer. You'll need to adjust the offset in the controller for the insulating factor of the tubes, though, somewhere around 15-20 degrees F. L&L currently uses an 18 degree offset.

Looks like you guys covered this topic 4 years ago. I think I just need to do more browsing before I ask. Lots of good stuff here. 

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You'll want to change the thermocouple offset rather than your target temp, so the thermocouple thinks the kiln is 18 degrees hotter than it actually is. So at room temp instead of reading 70 it'll read 88. Because the protection tube insulates the TC, if you didn't change it, when the TC thinks it's 100 degrees it's actually 1018 degrees, and it over-fires a bit.

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Makes sense. Looks like we have to drill 3/4" holes in order to use the protective case. That's what the bailey kiln person told me on the phone.  Looks like I have an exciting afternoon of MANUAL READING ahead of me! To learn about the joys of "offsetting your bailey electric kiln." But hey, totally worth it if we can add 50 firings to the lifespan of a thermocouple and prevent defects in the process.  

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