Trouble with pinging and Amaco potters choice glazes

[Bailey_rae_clay]

I have quite the pinging and crazing dilemma. Let me start by saying I use a cone 5/6 brown clay (Sheffield 4D). Bisque to 06 and glaze fire to 6 (with a 45 min hold).  Use only Amaco potters choice glazes(cone 5/6). Each and every single piece is either pinging or showing visible crazing. I’m certain they are vitrified.  But now I believe my clay body and glaze aren’t the right fit... Or could this be caused by over firing? The kiln is manual and we are thinking it may have reached closer to a cone 7 in some spots. Any opinions and advice are much appreciated. 

[Hulk]

Hi Bailey!

Ouch, sorry about the crazing. My experience with brown and red clays - they'll fizz (more bubbles) and turn colour when fired a half+ cone over target. The red went dark and purplish, the brown got darker and weeped tiny droplets of somewhat (which felt like sand, but with magnification, clearly droplets, which melt into the glaze, if the glaze is thick enough).

Any road, for sure place cones on each shelf, for quite some time - until your process is so solid and familiar you can tell the cone by looking at the ware.

Probably a misfit. How close the pattern of craze lines - quarter inch, eighth inch? Crazed inside and out?

[Bill Kielb]

1 hour ago, Bailey_rae_clay said:

I have quite the pinging and crazing dilemma. Let me start by saying I use a cone 5/6 brown clay (Sheffield 4D). Bisque to 06 and glaze fire to 6 (with a 45 min hold).  Use only Amaco potters choice glazes(cone 5/6). Each and every single piece is either pinging or showing visible crazing. I’m certain they are vitrified.  But now I believe my clay body and glaze aren’t the right fit... Or could this be caused by over firing? The kiln is manual and we are thinking it may have reached closer to a cone 7 in some spots. Any opinions and advice are much appreciated. 

Well the glaze definitely doesn’t fit but if the clay was misfired then it would not be fair to expect it has the fired characteristics it was designed for. Has pottters choice ever fit this clay? If yes, then I would say the firing affected its properties. Since you have a manual kiln and bisque to 06, how long do you bisque to remove all the impurities in the clay? Dark clays often require a nice slow bisque for effective removal. Often 10-12 hours. This may not be your issue but it’s something you should be aware of.

@Hulk is spot on, without cones, it’s really hard to tell if it’s a misfit or a misfire causing a fit issue that would not occur if the clay was properly fired.

[Bailey_rae_clay]

12 hours ago, Hulk said:

Hi Bailey!

Ouch, sorry about the crazing. My experience with brown and red clays - they'll fizz (more bubbles) and turn colour when fired a half+ cone over target. The red went dark and purplish, the brown got darker and weeped tiny droplets of somewhat (which felt like sand, but with magnification, clearly droplets, which melt into the glaze, if the glaze is thick enough).

Any road, for sure place cones on each shelf, for quite some time - until your process is so solid and familiar you can tell the cone by looking at the ware.

Probably a misfit. How close the pattern of craze lines - quarter inch, eighth inch? Crazed inside and out?

Thank you! This was our first time placing cones, so we are also learning how to “read” the results. The cone 7 was bent about halfway I’d say.  So I think it was over 6 but not quite 7. I had bubbles in my clear. And fizzing in my glazes as you mentioned! So I’m really thinking this was a combo of over firing and choice of clay. The craze lines are about 1/4 to 1/2 inch apart. Really hard to see them on some pieces but I KNOW they are there. Your information was very helpful, thank you! With darker clays would you recommend a slow bisque to 04?

[Hulk]

The large craze pattern may indicate closer to fit than a smaller pattern

For red, buff, brown, black, I'm holding at 1500F on the way up, half hour, and for a short time on the way down as well, kiln vent running (gotta have oxygen in there); seems to help.

My guess would be that a thorough bisque, lower glaze fire peak, and drop&hold after peak (I'm doing -100F from peak, hold sixty minutes) could (could) help with bubbles, but may not help the glaze fit; test, test, test!

Glazenerd had posted a schedule for dark clay bisque ...ah

http://community.ceramicartsdaily.org/topic/17903-critical-firing-temperatures/ 

see also

http://community.ceramicartsdaily.org/topic/17882-blistering-bloating-coring/

I'm finding real-time feedback o' Pyrometer very helpful as well.

See the APR-22-18 post, page 2

Cone 6 Firing Schedule- Nerds - Page 2 - Clay and Glaze Chemistry - Ceramic Arts Daily Community

[Bill Kielb]

51 minutes ago, Bailey_rae_clay said:

 So I think it was over 6 but not quite 7.

Others have found this useful

 

 

[Bailey_rae_clay]

On 4/15/2021 at 12:07 PM, Hulk said:

The large craze pattern may indicate closer to fit than a smaller pattern

For red, buff, brown, black, I'm holding at 1500F on the way up, half hour, and for a short time on the way down as well, kiln vent running (gotta have oxygen in there); seems to help.

My guess would be that a thorough bisque, lower glaze fire peak, and drop&hold after peak (I'm doing -100F from peak, hold sixty minutes) could (could) help with bubbles, but may not help the glaze fit; test, test, test!

Glazenerd had posted a schedule for dark clay bisque ...ah

http://community.ceramicartsdaily.org/topic/17903-critical-firing-temperatures/ 

see also

http://community.ceramicartsdaily.org/topic/17882-blistering-bloating-coring/

I'm finding real-time feedback o' Pyrometer very helpful as well.

See the APR-22-18 post, page 2

Cone 6 Firing Schedule- Nerds - Page 2 - Clay and Glaze Chemistry - Ceramic Arts Daily Community

Thank you for all of this info! I’ve got lots of learning to do. As far as my craze issue, I’m definitely leaning towards bad fit after further inspection. My clay body has an absorption rate of .39% at ^6.  I’m quite certain we fired to a 6.5 or possibly a little higher. Could a piece that is too vitreous also cause a fit issue?? 

[Hulk]

Unsure on that!

Each clay has (should have) a point where it is as vitrified as it will get, and additional heat work = overfired. Will over firing cause crazing? Good question.

[Min]

1 hour ago, Bailey_rae_clay said:

 I’m quite certain we fired to a 6.5 or possibly a little higher. Could a piece that is too vitreous also cause a fit issue?? 

Doesn't seem logical. As densification increases with raised firing heatwork from an extra 1/2 cone or so the body isn't going to expand. Crazing is from the glaze being too "small" for the body, (too low a coefficient of expansion) an underfired claybody is physically larger than a body fired to maturity therefore crazing would be less likely on a mature or slightly overfired body.