[cambriapottery]

I have some issues with pinholing or blisters on some bowls I have fired. I refired them trying to follow advice from Robin Hopper who suggests holding the glaze at mature temp for up to 2 hours. However I read contradictory advice from Jeff Zamek here http://www.ceramicin...ing-conditions. Does anybody have an idea on what should be done?

[neilestrick]

Holding for 2 hours would generally result in an over-fired glaze. That's a lot of heatwork to pile on at the end of the firing. Do you have photos? Pinholes and blisters can be quite different from one another, and may require different solutions. More info about the glaze, cone, etc would help, too.

[bciskepottery]

Harry Fraser, in Ceramic Faults and their Remedies, states blistering (craters and bubbles) are caused by overfiring which causes the molten glaze to boil and generate the same type of bubbling that is seen in pots of boiling water. Possible causes include wares placed too close to elements, resulting in blisters in the area close to the element, very rapid firing as the glaze and body approach maturity -- especially in low thermal mass kilns that tend to cool very rapidly that results in bubbles and craters being frozen before the glaze has had time to flow and clear the defects in its surface, presence of some oxides (manganese especially) and carbonates in the glaze that alter their state during oxidation and release gases which may cause pinholing (more often) or blistering (less often). Fraser recommends a longer soaking period or a slower firing cycle over the last 122 degrees F of the glaze cycle.

Is the problem with one glaze, or all glazes in the load? What is your firing cycle -- especially the last portion where you reach glaze and clay maturity? Could you provide a picture -- helps with trouble-shooting. Also, pinholing and blistering are two different faults and may require different solutions.

Every potter should have a copy of either Frazer's book or Hamer & Hamer's The Potters Dictionary of Materials and Techniques to trouble-shoot problems (or copies of both). Both have great pictures of faults and are full of information and solutions to frequent (and not so frequent) problems.

[Marcia Selsor]

there are contradictory suggestions on this also:
some people believe firing to a slightly higher bisque and hold for a 1/2 an hour will eliminate gases that cause pin holing. If you go back and look at the topic pin holing in past posts you'll find a huge discussion about this.
Ron Roy, a technical consultant for Digitalfire and co-author of Mastering Cone 6 Glazes suggests the higher bisque.
Marcia

[cambriapottery]

bciskepottery, on 09 November 2012 - 04:22 PM, said:

Harry Fraser, in Ceramic Faults and their Remedies, states blistering (craters and bubbles) are caused by overfiring which causes the molten glaze to boil and generate the same type of bubbling that is seen in pots of boiling water. Possible causes include wares placed too close to elements, resulting in blisters in the area close to the element, very rapid firing as the glaze and body approach maturity -- especially in low thermal mass kilns that tend to cool very rapidly that results in bubbles and craters being frozen before the glaze has had time to flow and clear the defects in its surface, presence of some oxides (manganese especially) and carbonates in the glaze that alter their state during oxidation and release gases which may cause pinholing (more often) or blistering (less often). Fraser recommends a longer soaking period or a slower firing cycle over the last 122 degrees F of the glaze cycle.

Is the problem with one glaze, or all glazes in the load? What is your firing cycle -- especially the last portion where you reach glaze and clay maturity? Could you provide a picture -- helps with trouble-shooting. Also, pinholing and blistering are two different faults and may require different solutions.

Every potter should have a copy of either Frazer's book or Hamer & Hamer's The Potters Dictionary of Materials and Techniques to trouble-shoot problems (or copies of both). Both have great pictures of faults and are full of information and solutions to frequent (and not so frequent) problems.

The firing is a slow glaze cycle that is programmed into the Coneart kiln. The temperature is Orton cone 7. I have no pictures but I have used the same glazes on teapots with no problems. It could be that the clay body is a little different. I appreciate the book recommendation, As I am a member of a group I will get it for our library. In general if I refire ware should it be to a lower temperature?

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[cambriapottery]

Marcia Selsor, on 09 November 2012 - 06:54 PM, said:

there are contradictory suggestions on this also:
some people believe firing to a slightly higher bisque and hold for a 1/2 an hour will eliminate gases that cause pin holing. If you go back and look at the topic pin holing in past posts you'll find a huge discussion about this.
Ron Roy, a technical consultant for Digitalfire and co-author of Mastering Cone 6 Glazes suggests the higher bisque.
Marcia

Very helpful thank you
Joan

[cambriapottery]

neilestrick, on 09 November 2012 - 04:07 PM, said:

Holding for 2 hours would generally result in an over-fired glaze. That's a lot of heatwork to pile on at the end of the firing. Do you have photos? Pinholes and blisters can be quite different from one another, and may require different solutions. More info about the glaze, cone, etc would help, too.

Do you think that refired ware should be fired at a lower temperature than the first firing? I was taking it to a Orton cone 7 originally. I know many potters will work with multiple firings to achieve what they want. In this case I just want to eliminate the pinholes if that is what they are. It is a matt glaze (has tin oxide) with a glossy glaze over sometimes using brush. I have other work with same glazes that didn't have this problem. Puzzling...

[cambriapottery]

neilestrick, on 09 November 2012 - 04:07 PM, said:

Holding for 2 hours would generally result in an over-fired glaze. That's a lot of heatwork to pile on at the end of the firing. Do you have photos? Pinholes and blisters can be quite different from one another, and may require different solutions. More info about the glaze, cone, etc would help, too.

Attached File(s)

•  136.JPG (3.57MB)
  Number of downloads: 24

[neilestrick]

So are you getting pinholes or blisters? Is this a commercial glaze or do you mix it?

[Diane Puckett]

I am having the same problem with Opulence Eggshell glaze which I got as a premixed dry glaze. It blistered on Desert Buff clay but not on Little Loafers, which is a cleaner clay body. I had tried bisquing to ^08, which may not be hot enough for Desert Buff, so I am going to try bisquing to ^04. For what it's worth, Opulence Denim Blue fired perfectly on Desert buff in the same firing, but another glaze was full of pinholes.

[neilestrick]

In general, I would bisque everything to 04. Pinholes typically need a soak or higher firing. Blisters typically need a lower firing or the glaze needs to be stabilized with more clay and/or silica.

[cambriapottery]

neilestrick, on 10 November 2012 - 12:26 PM, said:

In general, I would bisque everything to 04. Pinholes typically need a soak or higher firing. Blisters typically need a lower firing or the glaze needs to be stabilized with more clay and/or silica.

How can I identify pinholes or blisters don't they somewhat appear the same? I really appreciate the information on the method used to resolve the two different situations.

[cambriapottery]

cambriapottery, on 10 November 2012 - 08:16 PM, said:

neilestrick, on 10 November 2012 - 12:26 PM, said:

In general, I would bisque everything to 04. Pinholes typically need a soak or higher firing. Blisters typically need a lower firing or the glaze needs to be stabilized with more clay and/or silica.

How can I identify pinholes or blisters don't they somewhat appear the same? I really appreciate the information on the method used to resolve the two different situations.

If I assume they are pinholes would you refire at a lower cone with a soak? And would it be a cone lower if I go that route and how long a soak would you try? They are glazes made from scratch.

[bciskepottery]

Your pick . . .

Pinhole or pitting


Blistering

[OffCenter]

I've been testing a dug clay that is really nice except that every glaze I've tried on it pinholes even with soaks and slow cool downs. I had to bisque up to cone 02 and do a soak to get it to stop pinholeing.

Jim

[cambriapottery]

Slurrious, on 10 November 2012 - 11:50 PM, said:

blisters/pinholes are sometimes difficult to diagnose/remedy.

there are numerous causes. hopefully, in your case, it's an isolated incident with a glaze you have been using without issue. *if it's a new glaze and not the result of things like dust on bisque ware you need to re-evaluate every ingredient. there are some usual suspects and searches on the digitalfire website will identify most of the culprits.*

we fire to a 04 bisque which helps to alleviate some issues but in the end we've had to abandon a few glazes because of blistering/pinhole issues.

it's fun trying to figure a glaze out but after you've exhausted every internet resource and you are tired of making excuses for that one blister that always shows up - it's time to let the glaze go. for a couple of firings anyway.

re-firing, in my experience, does not work. every glaze has it own personality so hopefully you have some success. we've only ever tried re-firing to the same temperature as the initial firing but we were only trying to fix the most severe cases.

long story short - don't get too attached to a dysfunctional glaze unless you really enjoy testing glazes.

p.s
i learned the hard way that a failing thermocouple will lead to over-firing will lead to blistering so careful your temperature isn't creeping up on you.

*p.p.s.
i really dislike blisters.

**please excuse my failure at reading comprehension. i'm guilty of not reading and understanding the entire thread before braving a reply. it was late - i was tired.

some glazes just refuse to get along.

/lurk

[cambriapottery]

Slurrious, on 10 November 2012 - 11:50 PM, said:

blisters/pinholes are sometimes difficult to diagnose/remedy.

there are numerous causes. hopefully, in your case, it's an isolated incident with a glaze you have been using without issue. *if it's a new glaze and not the result of things like dust on bisque ware you need to re-evaluate every ingredient. there are some usual suspects and searches on the digitalfire website will identify most of the culprits.*

we fire to a 04 bisque which helps to alleviate some issues but in the end we've had to abandon a few glazes because of blistering/pinhole issues.

it's fun trying to figure a glaze out but after you've exhausted every internet resource and you are tired of making excuses for that one blister that always shows up - it's time to let the glaze go. for a couple of firings anyway.

re-firing, in my experience, does not work. every glaze has it own personality so hopefully you have some success. we've only ever tried re-firing to the same temperature as the initial firing but we were only trying to fix the most severe cases.

long story short - don't get too attached to a dysfunctional glaze unless you really enjoy testing glazes.

p.s
i learned the hard way that a failing thermocouple will lead to over-firing will lead to blistering so careful your temperature isn't creeping up on you.

*p.p.s.
i really dislike blisters.

**please excuse my failure at reading comprehension. i'm guilty of not reading and understanding the entire thread before braving a reply. it was late - i was tired.

some glazes just refuse to get along.

/lurk

Interesting info and I could be having an issue with a thermocouple. T

[cambriapottery]

OffCenter, on 11 November 2012 - 09:50 AM, said:

I've been testing a dug clay that is really nice except that every glaze I've tried on it pinholes even with soaks and slow cool downs. I had to bisque up to cone 02 and do a soak to get it to stop pinholeing.

Jim

I think I should bisque fire to the suggestion of cone 04 at least.

[koreyej]

I recently had problems with blistering/pinholing issues. My issue was in the bisque. I was using recycled clay which was heavy in organics, tumblestacked the kiln pretty tight, and fired too fast for everything to gas out. The thinner pieces, like the mugs and bowls, did fine. However, the plates were thicker and I had troubles with them. Refiring could possibly be helpful. It depends on the glaze. One glaze that I use is thin and very fluid. Refiring worked for those pieces, and they all recovered very nicely. The plates fired with the thicker glaze did not work out. Also, the lower in the kiln the work was, the fewer glaze defects (manual kiln with switches, more heat on the bottom, slower cooling).

So, it could be the thickness of the work, the bisque fire they were in, or their placement in the glaze kiln. Or some other unknown factor. BTW I fire to cone 6, I refired to cone 5 (no hold). One of the glazes I was having trouble with was Amaco's Blue Rutile, and it does not like to be refired hot. It turns pea brown. As I was told, refiring is a crap shoot. But I was willing to pay the electric bill for one refire. Why not? If you get one piece that melts out right and gets along, it will more than pay for the refire. If you do refire though, put those pieces on the bottom of the kiln for sure. Best of luck!