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I am using onglaze paints or decals on glazed ceramic mugs. I have never had any problems until just recently, when every mug I take out the kiln has impurities in the ceramic. 

I’m not sure what is the cause of this? Could it be that the kiln is firing too high, not high enough? Or just a bad batch of mugs? 

I have taken one of the peep holes out to make allow for more air flow but this hasn’t made any difference. 

Would love some feedback as it’s very frustrating having to redo my work. See attached images

AF9D2831-BA0B-474A-BE14-80370646AF62.jpeg

8B839720-0098-40D4-BA02-A6D39E6DD65D.jpeg

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Just to clarify, the mugs are already glazed and no chance of contamination from the onglaze? No change in mug supplier / onglaze / decals?

Is there any pattern to where the mugs are in the kiln in relation to the problem mugs? I'm wondering if it's corroded bits from the lid band. Do you have thermocouples in the kiln and do they have protection tubes? 

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Yes the mugs are already glazed and there have been no change in the decals, supplier or onglaze paints. 

The mugs are usually sitting on the very bottom of the kiln or middle shelf. No I don’t have thermocouples, have no idea what they are?

I can maybe try vacuuming out the kiln again to see if that works? Do you think it could be small bits of metal or something?

Thanks so much for taking the time in replying

I really appreciate the help

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Could you post a picture of the band around the lid? Yes, I would vacuum the kiln, use a soft brush attachment for going over the element grooves. Thermocouples are used to measure the temperature in the kiln, they often have protection tubes over them but not always. When corroded they spall little black bits. (image below of one with a protection tube beside it)

https://hotkilns.com/sites/default/files/feature-thermocouple-and-protection-tube2-950.jpg

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Lid band looks good so I think you can rule out that. 

8 minutes ago, Rowemade said:

I generally place the mugs rim down on kiln sitters.

You mean you are firing them on stilts? The metal pin ones absolutely will corrode over time and will leave black marks.

 

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2 hours ago, Rowemade said:

Do you think I even need to use stilts? The shelves have been kiln washed 

Wouldn't fire a glazed surface on kiln shelves, washed or not. Might just need to replace your stilts.  How come you fire the mugs upside down?

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They sit on the stilts better. It’s never affected the glaze on the lip of the mug. 

Ive just done another kiln load,after a good clean and no stilts, just sat the mugs on the shelf, bottom on the shelf, rim up. Two out of the six mugs have come out with impurities. I think it’s the mugs?! 

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Just to be clear, are these are finished commercially made mugs that you're applying the decals to? How hot are you firing? I'd try firing a batch of the mugs without any decals, just plain, and see if it still happens. Make sure your hands are clean. I'm also not sure why you need the stilts at all. Isn't the foot unglazed? Even on commercial pots, there's just enough unglazed area on the foot to make stilts unnecessary.

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Most likely a bad batch of mugs,  maybe a problem in the casting process?  

Judging by its location and intensity, that contamination in the pictures looks like impurities in the slip casting clay or the molds.  It is too  concentrated and deep and localised to be contamination you are introducing in handling at your end, or from your kiln environment.    They are specks, not smudges, so unlikely to be handling contamination.  

Also, the slight fuzziness around the spots and the way they are breaking (at least on the mug handles) through the glaze surface suggests to me that they are a significant contamination (some kind of metallic based crud) coming from the surface of the clay itself, which is poking up through the glaze.  Or possibly, but less likely in my view, contamination in the glaze itself (but the pattern of specking is too particular to be from the glaze in my opinion.)

are you glazing them yourself of do they come pre-glazed?  If unglazed, I might try washing the mugs carefully and examining with a magnifying glass around the rims and handles.   Also maybe if unglazed try firing one without glaze and see if it happens.

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