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Kath K

Underglaze Under Clear Glaze

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I have recently fired some earthenware items decorated with 3 coats underglaze and clear glazed over. The items have pinholing particularly where the underglaze pattern is. Fired to 1080 - 1100degC.

 

Is it possible to re-fire these items with the hope the pinholing will heal?

 

The glaze is:

 

Borax frit 90

 

china clay 10

 

bentonite 3

 

Cesco underglazes were used.

 

Any suggestions?

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I use Amaco underglazes under Hesselberth and Roy "Clear Liner" glaze, and occasionally get pinholes; I have had good luck refiring them. The only thing you could do would be to try it, I bet it would work.

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ok here is what I have found from having this happen to me as well and how I have come to fix it. I first had this problen on a peace of slip cast that I had done and then fired with a cone 10 clear in a reduction atmosphere gas kiln. The problem I found was that the clay body was not porous enough to allow the glaze to soak in and dry like it did in my stoneware body. This in turn caused the clear to cake up and not fire in an even manner. The solution that I came up with was to brush my clear on rather than to dip it on. This has rendered the pin-holing a thing of the past. I can still get a nice clear glaze on my pieces without over glazing it to the point that it causes pin-holes and runs.

 

Hope this helps

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I use Amaco underglazes under Hesselberth and Roy "Clear Liner" glaze, and occasionally get pinholes; I have had good luck refiring them. The only thing you could do would be to try it, I bet it would work.

 

 

 

Thanks, I think I will give it a go. Just another question...did you refire to the same temp and schedule as your original glost firing? I have also been told soaking helps at top temp, although I did soak for 30mins originally, maybe it should be longer. My gas kiln tends to hover above and below top temp when soaking, that might be half the problem too!

 

Kath

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ok here is what I have found from having this happen to me as well and how I have come to fix it. I first had this problen on a peace of slip cast that I had done and then fired with a cone 10 clear in a reduction atmosphere gas kiln. The problem I found was that the clay body was not porous enough to allow the glaze to soak in and dry like it did in my stoneware body. This in turn caused the clear to cake up and not fire in an even manner. The solution that I came up with was to brush my clear on rather than to dip it on. This has rendered the pin-holing a thing of the past. I can still get a nice clear glaze on my pieces without over glazing it to the point that it causes pin-holes and runs.

 

Hope this helps

 

 

 

Possibly my glaze was too thick. I will try a thinner application in future. I did brush the glaze on a couple of the smaller pieces, there were still pinholes, but minimal. Brushing this glaze was quite gluggy on application so I thinned it with water, but had to do extra coats anyway! Maybe I just need to find that perfect consistency for brushing. Thanks for your help!

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Pinholing is usually caused from gases escaping from the clay which did not burn off in the bisque. You could try bisque firing a little higher or slower or try soaking for an hour at the bisque cone temperature.

Marcia

 

 

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Pinholing is usually caused from gases escaping from the clay which did not burn off in the bisque. You could try bisque firing a little higher or slower or try soaking for an hour at the bisque cone temperature.

Marcia

 

 

 

 

We have done extensive study on that here at Alfred and that is a common myth. Because of the high viscosity of glass and the low pressure of gases, this is impossible. Pinholes are caused by poor packing of the glaze particles. If you want the background, you can check out Dave Finkelnburg's NCECA talk from 2007.

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Dear Matt,

I have followed Ron Roy's advice on pin holing which is fire slower and maybe higher. He has been giving this advice for years on Clayart. I have always found Ron's advice helpful and accurate. I know there are always many variables in ceramics. Please post Dave's research so we can understand some various points of view. I think it is very helpful to know several opinions as there never seems to be only one real answer in ceramics.

 

Thanks,

Marcia

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I am not aware of a online version of the article, but it is in the 2007 NCECA journal.

 

 

I have read several people's comments on clayart citing success using Ron's advice to fire higher and longer. It eliminated their pinholing problems. The expression of "poor packing" of glaze particles makes me wonder if lower bisque temperatures suck in the glaze because of higher porosity and create the pinholes. When I have that problem I use a dry finger to rub them smooth. Could you elaborate on glaze packing?

 

Marcia

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Materials pack themselves based on a variety of factors. Each of the materials has a different size and shape and when mixed into a suspension they distribute themselves (hopefully homogeneously). We refer to it as the Room full of Balls; Fill a room with Beach balls and you can fit soft balls in the empty space between the beach balls. Golf balls in between the space in the soft balls, etc. , Different materials (ie excessive clay content) cause materials to pack inefficiently, leaving large void spacing between the materials. As the glaze melts, void space air is trapped by glass. Which become pinholes and the bubbles we often see inside glazes.

 

 

The simple proof that bubbles cannot be a result of "Gas release" is ask yourself the question. "Do you see pinholes on the bottom of plates and bowls?" Of course the answer is yes, with the problem being that gases (at least the ones generated in a firing) rise. And this is also accentuation with the viscosity of glass pressure of bubbles problem mentioned earlier.

 

 

Biqsue temperature should have no effect. The bubbles you are indicating are much larger then the void spacing that I am referring to. They need attention but are not the same problem.

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I know this is an old post, but I'm hoping someone is still looking.

Is the needed attention you are referring to just mixing the glaze better before applying? I have the same problem, frequently. Is the problem just that I need to mix mix mix and mix again before applying? How do you get the "balls in the room" closer together!???

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I refired to the same time and temp as the first firing. It is going to work!

 

 

 

Well I refired my plates, but unfortunately the pinholing remained:(

 

I also fired some other plates with the glaze application thinner and rubbed over the decorated area with my finger, but...same problem resulted, even where the underglaze was applied thinly.

 

The glaze by itself is fine (where there is no underglaze) so I don't think it's a mixing problem. I think the two just don't get along! I think I need to try another leadless clear for earthenware (if Ican find a reliable one).

 

It's a learnng experience, I'll get on top of it eventually!

 

Cheers

 

Kathy

 

 

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pinholling normally occurs when a glaze is too think and or the clay body is releasing gas maybe try a longer hold on your kiln at the glazes maturation point. then for the cooling i would cool it slow.

 

 

not true, read my previous posts in this thread.

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Guest Herb Norris

Nice of you to point out erroneous information, but the question I have for you is:

 

What is the take-home lesson for the average person...?

 

What is the average potter supposed to do regarding "material packing"? Where does the average potter get ahold of "The 2007 NCECA journal"? And finally, what does the average person DO about their pinholing problem?

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pinholling normally occurs when a glaze is too think and or the clay body is releasing gas maybe try a longer hold on your kiln at the glazes maturation point. then for the cooling i would cool it slow.

 

 

not true, read my previous posts in this thread.

 

 

yes, i did. but are there not many reasons for why pinholing may occur?

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pinholling normally occurs when a glaze is too think and or the clay body is releasing gas maybe try a longer hold on your kiln at the glazes maturation point. then for the cooling i would cool it slow.

 

 

not true, read my previous posts in this thread.

 

 

yes, i did. but are there not many reasons for why pinholing may occur?

 

 

there are a few, but gas escape is only possible in once fire, never in bisque ware.

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Just to let you all know, I had a small win (not a solution) with my pinholing problem. I re-applied the clear glaze to the pinholes, brushed off the excess and refired... and voila! pinholes gone! (some slightly visible but nothing to worry about). What a nice surprise!

 

Still working on a solution to the problem though! The commercial clear earthenware glaze works fine over the underglazes, but this turns out to be expensive. I'm trying to be economical and make my own clear glaze that is food safe.

 

Thanks for the past and future comments and advice!

 

Maybe my small win will help some others.

 

Kath biggrin.gif

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