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Glaze problem pinholes or blisters

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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.ceramicindustry.com/articles/diagnosing-glaze-blisters-part-2-kiln-firing-conditions. Does anybody have an idea on what should be done?

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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.

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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.

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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

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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?

post-5926-135254900637_thumb.jpg

post-5926-135254900637_thumb.jpg

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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

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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...

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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.

 

post-5926-135254956678_thumb.jpg

post-5926-135254956678_thumb.jpg

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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.

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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.

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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.

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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

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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

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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

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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.

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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!

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I have been getting a bit of blistering, usually at the bottom of the kiln.

The thermocouple is brand new (~6 firings), and I have just replaced the elements.

Also, I re calibrated the controller.

Can't think what else to try.

Possibly related: it appears that my three center elements only begin to show a little color at around 1000 degrees. That is, the top and bottom elements get nice and red almost right away, but the center elements lag pretty far behind. According to a rep I spoke with at Evenheat, this is normal, but I don't remember it from previous kilns...

One last piece of data in case it helps: despite the new thermocouple and elements, the kiln is firing too hot, probably to 7 or a little higher.

Whew.

Why would I get blistering only in the bottom of the kiln? Suggestions?

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Your kiln has graded elements. The top and bottom elements run hotter to account for heat loss out the floor and lid. Glaze problems at the bottom could be from the bottom running hotter or colder. More likely colder. The thermocouple is only reading temp in the middle, so if the bottom is packed too tight, it can run cold. Put in cones bottom, middle and top to verify evenness.

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I just recently had a pinhole problem with a glaze that has been very stable.   But....I had two boxes of Bee Mix that had gravel in it.   I am thinking that there are some organic things that are unable to burn out all the way.  I do bisque to 04 and it is a glaze I mixed myself.   and have used successfully.  I only find the gravel chunks when I am trimming.  It's rather frustrating but I have been able to smooth over the holes left by the removal of the gravel, sooooo  I am wondering if my little pinhole is a result of the rough stuff in the clay rather than my glaze?

 

Roberta

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I am applying the glaze to the crucibles in the form of spray with spray gun. The glaze maturing temperature is around 1350 deg. celsius. I have glazed around 100 crucibles with the same technique. We preheat the crucible before glazing and the fire at the maturing temperature with a cycle of 4 hours and 20 minutes of holding time at maturing temperature. I also use three different fuel kilns (lets assume them as K1, K2 and K3). In K1 the cycle time is 2.5 hours.  One thing that I've observed is small crucibles in K1 are subjected to pinholes while large crucibles in K2 and K3 don't show any signs of pinholes. I have to fire small crucibles in K1 only. So, how can I solve this problem in K1 to eliminate pinholes?

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I am applying the glaze to the crucibles in the form of spray with spray gun. The glaze maturing temperature is around 1350 deg. celsius. I have glazed around 100 crucibles with the same technique. We preheat the crucible before glazing and the fire at the maturing temperature with a cycle of 4 hours and 20 minutes of holding time at maturing temperature. I also use three different fuel kilns (lets assume them as K1, K2 and K3). In K1 the cycle time is 2.5 hours.  One thing that I've observed is small crucibles in K1 are subjected to pinholes while large crucibles in K2 and K3 don't show any signs of pinholes. I have to fire small crucibles in K1 only. So, how can I solve this problem in K1 to eliminate pinholes?

 

Slow down the firing or increase the hold.

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