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Hi Must be beginners luck ....... Second unsuccessful firing .These are thin porcelaine slabs .That I coated with three layers of underglaze. And fired to 1240 degs .Some came out okay and others had these blister effects Any thoughts would be appreciated 

Thank you Nicky

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Have you tried this with other colors of underglaze??  I ask, only because I have discovered that certain colors can lead to some problems.  A problem I had repeatedly with a gray underglaze was really bad crawling.  I contacted a rep for the underglaze company and they said that particular gray had been a problem for some people.  Another potter said some potters mix their own gray with black and white.  

Roberta

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Nicky, did you dilute the underglaze or use 3 coats as it comes straight out of the bottle? Each coat dry before putting on subsequent coat(s)? Have you tried a test tile with just 2 coats to see if that is enough to get opaque coverage (if that's what you are after)?

6 hours ago, Nicky S said:

About cone 6 ( from what I understand ) . But why would some work and others not ? Could this then mean different heat on top bottom shelves ? 

Depends on how fast you fire the kiln during the last 100C. Could be different thicknesses of underglaze and how dry each coat was. Do you use witness cones to check the heat differences in your kiln? Manual or single zone electronic kilns can definitely run hotter in the middle (usually) than the top or bottom. 

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7 hours ago, Roberta12 said:

Have you tried this with other colors of underglaze??  I ask, only because I have discovered that certain colors can lead to some problems.  A problem I had repeatedly with a gray underglaze was really bad crawling.  I contacted a rep for the underglaze company and they said that particular gray had been a problem for some people.  Another potter said some potters mix their own gray with black and white.  

Roberta

@Roberta I only use blacks on these .I’ve tried using stains ( on greenware ) .But was not happy with the results either 

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6 hours ago, Min said:

Nicky, did you dilute the underglaze or use 3 coats as it comes straight out of the bottle? Each coat dry before putting on subsequent coat(s)? Have you tried a test tile with just 2 coats to see if that is enough to get opaque coverage (if that's what you are after)?

Depends on how fast you fire the kiln during the last 100C. Could be different thicknesses of underglaze and how dry each coat was. Do you use witness cones to check the heat differences in your kiln? Manual or single zone electronic kilns can definitely run hotter in the middle (usually) than the top or bottom. 

@MinHi Min ........ Did not dilute underglaze (good shaking ) Waited as each coat dried . Found that two coats doesn’t give me the opaqueness I’m looking for . Don’t use cones

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