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kenthenricksen

Pitting and pinholing

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Hi Everyone.  I'm having major problems with pinholing and pitting.  I am getting hundreds on pin holes on only the outside of small pots.  I am using tucker's mid smooth stone and bisqueing to 04 with a 16 minute hold.  I am dip glazing using amaco white with a 16 minute hold.  I have noticed after I dip glaze that lots of popping bubbles appear on the outside of the pots in the glaze.  More hold time?  Is my kiln broken? 

  Any help would be good .  Thank you.  

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Guest JBaymore

Does this happen on particular types of forms?  Does you kiln have a local pickup (downdraft" vent system?  How tightly do you stack bisques?

best,

.............john

 

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Guest JBaymore

On bisques, do you nest pieces inside pieces and stack things as densely as possible trying to cram as much as you can stuff into the kiln?

Are your bisque loads pretty uniform in the amount of stuff that gets loaded into each single firing, or are some bisque loads having more work than others?

best,

.......................john

 

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2 hours ago, kenthenricksen said:

I have noticed after I dip glaze that lots of popping bubbles appear on the outside of the pots in the glaze

This is normal. If you are rubbing the bubbles out before firing, it shouldn't affect the final outcome. However, if you are not rubbing them smooth, lots of glazes do not flow enough to smooth themselves out. Some will, some won't. 

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I do not stack pieces on top of each other when bisque firing.  

I do not rub out the pin holes.  This glaze does not move at all at 16 mins should I hold longer?  How long?  

The glaze is HF-11.  Bisque temp 1945.  

I use a skutt kiln 818. Bisque firing slow 12.5 hours and glaze medium speed 8.5 hours.

This is what I make- www.coldspringceramiccompany.com- for reference.  

The problem is I cant seem to control when the pinholes come and when they dont.  For instance, i make pie dishes some glazes will come out perfect other will be covered in pinholes, but only on the outside.  

 

Thank you

 

 

Edited by kenthenricksen

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The fact that Amaco is recommending a firing schedule as precise as a 16 minute hold tells us that they know this glaze is problematic, especially since it's not a specialty glaze that needs a hold time to develop color or some other special effect. It's just white. I would find a different glaze if I were you.

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Quote

The problem is I cant seem to control when the pinholes come and when they dont.  For instance, i make pie dishes some glazes will come out perfect other will be covered in pinholes, but only on the outside.  

It is interesting they are only on the outside. 

We had a discussion a few months ago with brandy lee.

She was having pinhole problems, she adjusted her hold from 20 to 16 to 10 minutes and got rid of the pinholes,but she also added a downdraft vent. Maybe try decreasing your hold since only the outside stuff is pinholed. Isn't usually the inside of pots cooler because of the radiant heat? Maybe the outside is getting overfired and is causing miniblisters?

I am not real sure just going from others experience. 

Edited by Joseph F

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@kenthenricksen I would start by always rubbing out the bubbles that form after dipping. I taught my students to do this for every glaze no matter what. It's a good habit. 

I would also get a hydrometer or start measuring specific gravity with a graduated cylinder and scale (search this forum for more info). Make sure you are always using the glaze at the same consistency. Pinholes are more likely if the glaze it applied too thick. 

Dipping technique also affects how thickly the glaze is applied. I'm wondering if you glaze the insides of your pots with a different technique than the outside, which results in the glaze being thicker on the outside? The longer the pot is submerged, the thicker the glaze layer. For some glazes the difference is very sensitive. One second too long and the glaze might be too thick. Pay attention to technique and be consistent. 

I also agree with Neil that a 16 minute hold is an oddly specific number. It might indicate that they know the glaze is fussy. 

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Did you mix this from dry? It almost looks like it needs to be sieved.

I checked out the pieces on your web site, and on those the glaze appears to be thicker and glossier. I'm voting for this one being underfired, and the glaze applied too thin.

Edited by neilestrick

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Just to add to what Neil and Pres suggested, are you loading your kiln to ensure evening firing throughout the kiln? Are you inadvertently making cool zones in your kiln?

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Is this on the inside as well? Could be from trimming too. I've noticed pinholes on pots that only appeared where it was trimmed (bottom 2/3 of a pot). It seems throwing seals the clay off in the interior and elsewhere. I would try sponging the whole pot down next time and see if it makes a difference. 

Edited by BlackDogPottery

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I get some pin-holing but only on the side nearest the electric elements in my kiln.  I'm sure that is telling me something but I dont know what.  Too hot?   Suggestions for avoiding this?

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@kenthenricksen

@Rick Wise

I’ll give you something to try and since it is a commercial glaze we do not know what is in it but your comment of air bubbles during dipping leads me to wonder if it is a packed bed porosity issue.

Some glazes do not fill the voids in your bisqueware well and trap air. If the glaze is not one that flows much then some of these will not heal over in the firing.  Often lots of little bits of trapped air coalesce forming a pinhole too large for the glaze to heal based on its melted surface tension.

Some cures for this are to bisque to 05, or glaze one side of a pot at a time and let that side dry. The best inferential test I have read about is adding Darvan at the time of glazing. 1,2,3,4,5 drops and bam, you might find the perfect amount just for use when you use this glaze.

Obviously the Darvan is easiest to try and if you get a result then you can decide how to handle it from there as you now know it is a packed bed porosity issue for that glaze and that clay body.

At least ruling this out will be helpful. It is common, and folks often simply carefully rub out all these pinholes while glazing and that works most of the time.

one question, the run in your glaze, was that applied that way or did it run like that upon firing?

 

Edited by Bill Kielb

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2 hours ago, Bill Kielb said:

Some glazes do not fill the voids in your bisqueware well and trap air. If the glaze is not one that flows much then some of these will not heal over in the firing.  Often lots of little bits of trapped air coalesce forming a pinhole too large for the glaze to heal based on its melted surface tension.

Some cures for this are to bisque to 05, or glaze one side of a pot at a time and let that side dry. The best inferential test I have read about is adding Darvan at the time of glazing. 1,2,3,4,5 drops and bam, you might find the perfect amount just for use when you use this glaze

Wetting the bisqued surface with water prior to applying the glaze will fill  the pores and often corrects the problem of air bubbles Bill describes.

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