Blistering clay? Or glaze problem…

[Fromphyl]

Is this bloating or the glaze?

I used b mix with speckles. Typically when I fire to cone 6 I have so bloating but not really bad. I recently learned it’s better to fire cone 5 with this clay so I did this time and I have these tiny bumps but only with this glaze. And the glaze looks kinda underfired? Other pieces with different glazes, same body, came out fine. Can I refire at cone 6? Or what can I do to selvage these pieces? I added pictures - 3 from the same firing. And one I did previously where I fired at cone 6 where the glaze is more mature and smoother. 

[Hulk]

Hi Fromphyl!

Welcome!
While waiting on more replies, some questions:

Looks like the glaze on the handle portion of "Cone 5 Bmix Speckles Alabaster 3 coats" is smoother; is it?
Is the glaze on the inside of the same piece also smoother (or different) than the bumpy and pin holed outside?

Are you doing any holds and/or controlled cool?
Looks like that Alabaster needs a bit more heat work and then perhaps a drop and hold*.

Are you bisque firing, then glazing and glaze firing?
If so, what cone are you bisque firing to, and is your kiln fitted with a kiln fan setup, else, are you leaving the top peep out?

One more question - how big is the kiln (in other words, how quickly does it cool off)?

*Drop-and-Soak Firing (digitalfire.com)

[Min]

Looks like brush strokes on the last image, is this a brush on glaze? Looks quite underfired. Is it just this BMix that you get bloats with when fired to cone 6? Could you post an image of the bloats? Are you using witness cones?

Yes, I would refire those underfired pieces, nothing to loose. 

[Fromphyl]

@MinYes! It does look like brush stroke. I was confused because I read the little bumps or blistering is when its overfired, but the glaze looks underfired? And yes it's typically the Bmix with speckles. But it usually looks fine at cone 6, but this time I did cone 5 and it looks underfired and bumpy. Would you suggest refiring at cone 6? 

[Fromphyl]

@Hulk The top of the handle has those little bumps. I'm not doing any holds or controlled cool. I fired this batch at cone 5 (typically I do cone 6) , at medium speed in a small studio kiln. Dont have exact dimensions but take about 8 hrs to reach temp then around 10 hrs to cool. I leave all the peep plugs plugged in, in both bisque and glaze firing. I do bisque fire to cone 04. Don't have a fan set up. When glaze firing, when it gets arond 500F I take out the peep plugs. I'm not expert, so if I'm wrong, please correct me!  Also just want to add as an reminder, the other items in this firing, with the same clay but different glaze, turned out fine. 

Edited October 3, 2022 by Fromphyl

[Kelly in AK]

I think it’s a stage the glaze goes through before it fully melts and lays down.

Based on the the fact that other glazes in that firing came out like they normally do, this one seems to be just on the cusp of melting at cone 6.

I humbly suggest doing some simple at-home tests before using it on the insides of functional ware.

Some basics (Set one sample aside as control.):

1. Put vinegar in a cup with that glaze and check against the control after 24 hours for texture or color change.

2. Put a piece in the dishwasher and leave it there a month or two or six. Check it against the control.

3. Vigorously scrape a butter knife across the surface of the glaze.

Basically, if the glaze is unchanged that’s awesome. If not, you get to think it over. 

[Roberta12]

@Fromphyl I had picture #2 experience the first time I fired bmix with speckles.  I fired to cone 6.  Which turns out was probably hotter.  Closer to 7.  But...I called Laguna and they said that clay is definitely a cone 5 clay.  I told him it looked like chicken pox or pimples.  And he said it was overfired.  My bumps/blisters were with 2 different glazes.  I now fire to cone 5 with that clay, and so far have only used clear glaze on it and no blisters.  I even did a refire and it was fine.  Were all the cups in the same place in the kiln?  I ask because I now know that my top shelf is a lot cooler than the rest of the kiln.  

Roberta

[Hulk]

Your first pic, nicely melted, smooth. The other Alabaster ones don't look melted - underfired - gives a good idea how much smoothing out there is to do from there. That other glaze must melt sooner?

Was curious about oxygen supply - especially for bisque fire - and how hot a bisque, as the more out-gassing/burn-off is done, the less fizzing there is to pass through the glaze. Also, the more thorough a bisque fire, if not hotter, than a significant hold above say fifteen hundred F, maybe help with some bloating, if there's some bits that could still be more burned out.

Still, that first pic looks on target, nice work!
Looks like more heat work required.

I started with the drop and hold looking to clear fine bubbles and get a smoother surface, especially buff and red, less the white clays, all rated cone five or five/six.
It does seem to help.
Can't quite tell if the Birch one has some tiny pin holes?

Would like to know where the bubbling is coming from, how much from the glaze, how much the clay.

[Callie Beller Diesel]

+1 for Alabaster being underfired at cone 5, confirming your firing temperatures with cones, and doing a drop and soak cycle to fix it. 

[Fromphyl]

@@Hulk@Callie Beller Diesel this is my first time doing drop and soak. Is this an accurate program to use ? https://digitalfire.com/schedule/plc6ds

[Callie Beller Diesel]

@Fromphylit is a most excellent place to start. There’s always going to be a bit of fine tuning of your firing cycle when you’re sorting out how to use new glazes in your reporitoire, but the info on Digitalfire is good. It’s one of the best free reference s the pottery world has. 

[Kelly in AK]

I have to laugh here, after writing a long treatise on how blisters on the clay surface and blisters (and pinholes) in glaze are related but not exactly the same thing…

I realized the picture I use for my profile is a great example an over fired, blistered clay surface. No glaze on the exterior of that piece.

The greatly shortened message is I use a clay that off gasses like crazy at maturity and a drop and hold firing schedule is the only way I can get a good strong well fitting glaze without pinholes. After trying so many glaze tests to solve the “pinhole problem,” it was like a miracle. 

Edited October 6, 2022 by Kelly in AK

[Callie Beller Diesel]

I had a colossal misfire a few years ago, and severely bloated an entire load of pots. I suspect I hit cone 10 because the cones were liquefied. I did post what that looks like on my IG stories so that others know what the cross section looks like. 

I do use my IG for commerce so the photo isn’t a link, but there is one in my signature below if you want to check it out. Click or tap on the Meltdown playlist at the top. 

[Fromphyl]

@Callie Beller Diesel oh wow! I would have been so devastated:( did you try a drop soak to fix it? 

[Callie Beller Diesel]

It was indeed a big blow, not going to lie! I was in a bit of shock and trying to salvage some sort of good when I took that video. It’s important to note that that was a very extreme example of bloating, not something you see a lot of. It’s also a good reminder that even us experienced folks can have giant mishaps sometimes! There was nothing salvageable in that load.

Bloating in a clay body is really only different from a glaze in the initial stages of melting in that they’re located in different places on the pot, but that difference is a significant one. You can rescue an underfired glaze by refiring, as long as the clay body underneath isn’t going to cause issues. And many times they don’t! But adding more heatwork to those pieces would have turned those pots into slag. 

The pots in those videos are doing essentially what your underfired  glaze is doing: partly melting, being stopped part way through, and trapping gas bubbles.

 

[Fromphyl]

Thanks so much guys!!!!! This has been extremely helpful and a learning experience for me! I will update after the firing! 

[viperguynaz]

On 10/3/2022 at 2:18 PM, Roberta12 said:

@Fromphyl I had picture #2 experience the first time I fired bmix with speckles.  I fired to cone 6.  Which turns out was probably hotter.  Closer to 7.  But...I called Laguna and they said that clay is definitely a cone 5 clay.  I told him it looked like chicken pox or pimples.  And he said it was overfired.  My bumps/blisters were with 2 different glazes.  I now fire to cone 5 with that clay, and so far have only used clear glaze on it and no blisters.  I even did a refire and it was fine.  Were all the cups in the same place in the kiln?  I ask because I now know that my top shelf is a lot cooler than the rest of the kiln.  

Roberta

I just had the same problem, but there were unglazed portions that had the same blistering/pimples, Pieces were bisqued to Δ04. I run a manual updraft gas kiln and did a fairly fast fire, ~5 hours to 1000°C which required ~1 hour @ 1200°C for Δ6 to mature. I figure I need to run a much slower ramp and then hold at a lower temperature for this clay.