Blisters/Pinholes in Glaze

[Salt.Forest]

Hi all,

I'm hoping for assistance with solving a persistent pinhole/blister problem. I am firing in a new L&L E23T-3 Kiln. In my prior kiln, I had experienced this same issue, but had a kiln sitter and much less control.

Details:

-Using Seattle Pottery Supply Alpine White

-Using Spectrum Commercial Glaze

-Bisque Fired to Cone 05 using the "slow bisque" program on first fire -- checked witness cones, it was more like a cone 06; adjusted thermocouple offset slightly, paranoid bisque fired a second time, this time to cone 04. Witness cones showed a 05ish. Figured that was good enough (obviously need to adjust my thermocouple offset further)

-Glaze fired to Cone 6 using the below program with a slow cool with no holds--actual witness cones showed slightly underfired;  (I only had cone 6 and 7 on hand to use, see photo below). However, these glaze are rated for Cone 5 to Cone 6 so it should have been fine.

• 100F per hour to 220F
• 350F per hour to 2000
• 120F per hour to Cone 6 (you can set a cone instead of a temp)
• free fall to 1900F
• 150F per hour to 1400

I've experienced this with other commercial glazes I've been using, and I'm starting to suspect it's the clay body. Am I missing something or should I just give up on this clay body--I have quite a bit of work made and ready to fire, but it's a bit discouraging to see this problem over and over.

 

[Sorcery]

I recently became convinced that without a controller, you can't best control this.

So at least it's fixable!

There is a popular drop and hold schedule for PC glazes, I'd use that.

Firing Schedule all day. Not Clay.

 

Sorce

[Min]

12 hours ago, Salt.Forest said:

I've experienced this with other commercial glazes I've been using,

Are the other glazes rutile glazes too? Glazes high in rutile can be problematic for pinholes and blisters.

I'ld suggest trying a soak just under your top temperature then dropping 100F and doing  a second soak for 20 minutes. There isn't a one size fits all top temperature and soak to get to cone 6. What temperature did your programmed ^6 take you to? What works for my kilns is going at 108 from 2000 to peak temperature of 2185 then doing a 15 minute soak then dropping to 2085F and doing a second soak. The second soak allows the pinholes and blisters time to heal over without overfiring the clay and glaze. The temperature at which the second soak takes place will probably need tweaking; you are trying to catch the glaze when it's still fluid enough to heal over but 100F under peak temperature will give you an idea if this method would work, see if there are any less pinholes/blisters. If not then need to look further. 

Re Alpine White from SPS, I don't know if they have changed the body since I used it about 20 years ago but I didn't have the pinhole and blister issue you are having. I was firing to cone 6. My hunch is it's a glaze / firing schedule issue.

[Salt.Forest]

2 hours ago, Min said:

Are the other glazes rutile glazes too? Glazes high in rutile can be problematic for pinholes and blisters.

I'ld suggest trying a soak just under your top temperature then dropping 100F and doing  a second soak for 20 minutes. There isn't a one size fits all top temperature and soak to get to cone 6. What temperature did your programmed ^6 take you to? What works for my kilns is going at 108 from 2000 to peak temperature of 2185 then doing a 15 minute soak then dropping to 2085F and doing a second soak. The second soak allows the pinholes and blisters time to heal over without overfiring the clay and glaze. The temperature at which the second soak takes place will probably need tweaking; you are trying to catch the glaze when it's still fluid enough to heal over but 100F under peak temperature will give you an idea if this method would work, see if there are any less pinholes/blisters. If not then need to look further. 

Re Alpine White from SPS, I don't know if they have changed the body since I used it about 20 years ago but I didn't have the pinhole and blister issue you are having. I was firing to cone 6. My hunch is it's a glaze / firing schedule issue.

Thanks, Min! I'll give that a try. How long do you typically do the second soak for?

I may have cleared out the final temp so I'll need to go check and see. 

In your opinion, this seems like a glaze schedule problem, not a bisque schedule problem, right? 

Amanda

[Mark C.]

ah the elusive Rutile glaze

(Glazes high in rutile can be problematic for pinholes and blisters.)

well thats almost an understatement .

 

[Min]

2 hours ago, Salt.Forest said:

How long do you typically do the second soak for?

 

Try 15 minutes for the second soak. It's difficult to dial it in specifically with the first try but this should give you some info. ie if the blisters are worse/better/same. I'ld also include one of the blistered pots in the next firing, see if the blisters get worse. (put the pot on a waster slab in case of glaze runs)

"this seems like a glaze schedule problem, not a bisque schedule problem, right?" Glaze blisters can be one of the trickier faults to figure out. It could very well be a combination things, glaze itself and some part of the glaze firing schedule.  Have you had successful firings with other glazes on this body? Alpine White isn't a very "dirty" clay so I think you bisque is probably okay. Do you run a vent during the bisque or leave a peep open or ?

[glazenerd]

Salt:

Looking at your cone pack- you hit perhaps cone 5+ or so. Clay body fluxes are still off-gassing at this cone temp. In your first picture; upper right hand is a blister with a raised rim and exposed clay in the center. Anytime you get a raised rim: that means off gassing spars in the clay have created enough pressure to raise the glaze where it pushes through. Raised rims also occur when firing over red body clays; when inorganics have not been properly burned off- not applicable in this case. Potassium body fluxes create blisters, with less population and sodium body fluxes create pinholes: but more numerous. I would recommend a -20 degree thermocouple offset. Look in your kiln manual for TC or thermocouple offset and follow the steps to program that. If your current controller is reading 2000F, after you make the offset it will still read 2000F, but will actually be 20 degrees hotter. Judging by your cone pack: that should get you pretty close to a true cone 6 bend. 

The red glaze has potassium body flux and the green has sodium body flux. It will help you judge what you are dealing with in the future.  Tom

[Mark C.]

Tom

(a blister with a raised rim)

I call them innies and outies

the innies sometimes you can live with  as they fall back on themselves-the outies never as they are sharp

Since I use about 400-500#s a year of rutile dry glaze I see my share of this stuff

[Roberta12]

@Salt.Forest this is probably a silly question, but have you simply used the L&L slow glaze program (love my L&L)  The other question I had was have you tried other brands besides Spectrum for glazes?  

Roberta

[Salt.Forest]

Okay! Thank you all for your suggestions and help -- I did another fire with a couple of test pieces and have some more answers and more questions.

Answers to various questions asked by others:

• During bisque I leave the top peephole open
• I do think that this is largely a rutile glaze problem (though one of my non-rutile glazes had one tiny, annoying pinhole after firing).
• Refiring one of the blistered pieces--I would say maybe a slight improvement, but hard to tell. The outside definitely improved, the inside just changed blister patterns (see photo below)
• The new pieces fired with rutile glazes definitely had a less overall volume of blisters but still present (see photos below)
• I did adjust the thermocouple offsets, but not enough. Getting closer, but still need more dramatic tweaking
• I haven't used the L&L slow glaze, but at this point my programmed schedule is much slower than the pre-set program:
  • 100/hr to 220
  • 350/hr to 2000
  • 108/hr to 2185 (hold 15 min)
  • 100/hr to 2085 (hold 15 min)
  • 150/hr to 1400

Questions:

• What now? I'm going to keep dialing in the Thermocouple offsets for accurate temps. In terms of blisters -- longer soaks? Soaks at different temperatures?

Out of three pieces glazed without rutile glazes, one small pinhole total:

New pieces with pinholes:

 

Refired blistered piece, better on the outside, about the same on the inside:

[Mark C.]

The glaze is  looking to thick  especially in bottoms which will worsen the issue. 

Rutile gazes never are easy or that dependable . The best you can do is make sure you hit all the points .

[glazenerd]

[Bill Kielb]

1 hour ago, Salt.Forest said:

• What now? I'm going to keep dialing in the Thermocouple offsets for accurate temps. In terms of blisters -- longer soaks? Soaks at different temperatures?

Out of three pieces glazed without rutile glazes, one small pinhole total:

 

It’s my belief that these pinholes are the result of the fired surface tension which keeps these areas from healing. My suggestion, try the tried and true built in  slow glaze firing schedule, no holds and at the conclusion of the firing drop 100 degrees from normal peak and hold for 15 - 20 minutes. From there nothing else, let it cool on its own.

The reasoning: often glazes that have less than desirable fired surface tension to self heal get even worse as the temperature gets higher. In general if it’s a cone 5/6 glaze,, firing near cone five often works better than than six or six and a half or six with a hold at top temp. A final segment of 108 degree per hour should uniformly heat most normal wares so no need for a hold unless your stuff is extremely thick.

Dropping 100 degrees from peak hopefully allows the glaze to heal. Obviously at some point continuing to drop has no effect because the glaze is too cool to move.

I think @Minmentioned above that you will be searching for the sweet spot where this glaze will heal, so drop and time at that drop are the two items to vary when testing.

Personally when I get glazes that demand too much attention to work, I replace them. My time is far more important than nursing a glaze through every firing unless it’s spectacular.

[Min]

5 hours ago, Salt.Forest said:

• 100/hr to 220
• 350/hr to 2000
• 108/hr to 2185 (hold 15 min)
• 100/hr to 2085 (hold 15 min)
• 150/hr to 1400

Agree that the glaze looks too thick in the middle of the bowl. Looks like the drop and hold helped somewhat but not enough. Since we don’t know the makeup of the glaze it’s hard to say if it’s an early melter. Early melters can need to go lower for the second soak as they are fluid  at a lower temperature than glazes that are late melters. 
 

if I was testing this clay and glaze i’ld change your second rate to 400/hr to 2000. Second to last ramp is too slow, needs to drop quickly @9999  down to  second hold temperature. What the temperature for the second hold needs to be will be trial and error.  Does Spectrum recommend a slow cool for these glazes? If not then I”ld omit the last ramp. Did you have witness cones in this latest firing and if so what did they show?

 

[Roberta12]

I agree with what @Min said about thickness.  I also noticed you have some large crazing lines on the first cup.  Maybe there is a fit issue along with everything else?  

[C.Banks]

9 hours ago, Bill Kielb said:

when I get glazes that demand too much attention to work, I replace them. My time is far more important than nursing a glaze through every firing unless it’s spectacular

This option is guaranteed to work.

[Hulk]

Hi Salt!

Are there more craters where the wares have been trimmed, and/or where the clay is thicker?

Looks like gasses from the clay, a bit cooler top/peak temp might help, quick drop from peak to -100F hold (I'll suggest a bit longer at -100F, then slow cool to 1850F) - per all aforementioned. A more thorough bisque might help as well, for perhaps there are large(r) bits of gassing stuff in thar?

[Min]

@Salt.Forest, any chance you have another white cone 6 clay that you can try these glazes on at the same time? 

[Mark C.]

Yes brown clay will make any rutile glaze have even more issues with outgassing bubbles-good call Min

[Salt.Forest]

Hi all--

 

4 hours ago, Hulk said:

Are there more craters where the wares have been trimmed, and/or where the clay is thicker?

Not particularly--they seem to be pretty evenly distributed.

 

12 hours ago, Min said:

 Did you have witness cones in this latest firing and if so what did they show?

I did! Still not quite there but closer (top shelf on the left, then middle, then bottom)-- I'm going to adjust the thermocouple offsets a bit more dramatically. I was being a bit cautious because it seemed weird to need to adjust them so much with a new kiln. I don't currently have another white clay body I can try, but I was using a porcelain previously with some of these glazes and had significantly fewer problems --out of four bowls i had one tiny blister total, in an older kiln with a kiln sitter.

For the glaze being too thick in the middle of the bowl--this is the second time I fired that particular piece, to see if the blisters improved with a different schedule. The glaze pooled quite a bit more in the middle of the bowl during the second firing. Still too thick?

Given all the above I'm considering another test fire with similar temps but a adjusted thermocouple offsets (15 degrees for the middle and bottom shelf, 5 for the top shelf) and the below schedule:

• 100/hr to 220
• 400/hr to 2000
• 108/hr to 2185 (hold 15 min)
• Freefall to 2085 (hold 15 min)

Any suggestions or tweaks?

 

[Bill Kielb]

2 hours ago, Salt.Forest said:

ny suggestions or tweaks?

I would try it on fresh test materials. Nice uniform application on new bisque, same clay, same glaze.  I think a consistent baseline would allow you to solve it quicker. Re-fires rarely work out (my experience) and rarely represent what a new piece would do. Claybody has already fluxed and so has the glaze actually.  Sorry, not a fan of refires.

[Min]

44 minutes ago, Bill Kielb said:

Re-fires rarely work out (my experience) and rarely represent what a new piece would do. Claybody has already fluxed and so has the glaze actually.  Sorry, not a fan of refires.

Refiring a blistered piece will give some info about whether the glaze has high fluidity plus high surface tension in addition to what firing higher than the recommended cone is doing to the glaze. If the blisters get worse it supports the theory of the glaze being highly fluid with a high surface tension. Agree re-fires are a crap shoot but the reason to try it in this circumstance is to determine the glaze quality, not necessarily to fix a blistered pot but that would be a nice bonus if it happened. Since the refired pot didn't get worse it's probably safe to go a bit hotter than the last firing.

 

2 hours ago, Salt.Forest said:

Given all the above I'm considering another test fire with similar temps but a adjusted thermocouple offsets (15 degrees for the middle and bottom shelf, 5 for the top shelf)

Sounds about right. Once the tip of the cone reaches 3:00 there isn't a huge amount of heatwork needed to bring the tip to touching shelf.

[Mark C.]

This is your easy out answer

(but I was using a porcelain previously with some of these glazes and had significantly fewer problems )

[Salt.Forest]

21 hours ago, Mark C. said:

This is your easy out answer

(but I was using a porcelain previously with some of these glazes and had significantly fewer problems )

I know  but there's a stubborn part of me that just thinks it should work with commercial glazes and a relatively basic commercial clay. I've probably got one more try in me, then I'll start moving to another clay body.

[Roberta12]

18 hours ago, Salt.Forest said:

I know  but there's a stubborn part of me that just thinks it should work with commercial glazes and a relatively basic commercial clay. I've probably got one more try in me, then I'll start moving to another clay body.

I do understand that thinking.  Why on earth would those materials not work well together????  But, I have discovered, that is not always the case.   Sometimes even trying a different glaze on the clay body.  or a different clay body or a different firing schedule or different application method or when the moon is in the 7th house and Jupiter aligns with Mars......