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My white gloss glaze has recently started pinholing I believe. We have been using it for years with no issue.

i initially started with the kiln it is firing to temp. We fire slow bisque and slow glaze programs to 06 and 6. I have tried bisquing to 04 with a hold on the end but the defect came back... I have tried a hold on the end of a glaze firing (20 minutes) with little improvement.  

We have tried the ware with this glaze in 3 different kilns all have the same problem especially on larger ware. 

We have also tried wiping down the ware lightly with a sponge on both sides did not help. 

I am thinking it must be the glaze. But I have no idea where to start. Help. 


Edited by Liz C
More info needed
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What is the glaze recipe, and what has changed? 

What materials are new? 

What clay?  


The minerals we use are fairly variable because they're mined and can be more or less pure each time a batch is milled.  Best to keep track of new materials so you can diagnose issues like this more precisely.

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So the clay is a commercial standard clay body... light stoneware.

the glaze is comprised of

epk, minspar, gerstley borate, silica, calcium carb., zircopax


nothing is crazy new it’s been in 50 lb bags for about a year? I haven’t recorded each new bag which is a good idea moving forward... 
is it possible that materials can be effected from sitting around for periods of time? Can older materials cause issues?



Edited by Liz C
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So two of those ingredients are fairly gassy, the gerstley borate and calcium carbonate.  You can definitely reformulate to have very little LOI by switching to a boron frit and wollastonite.

For the frit I'd try recalculating with 3124.

If you post the recipe here I can try rejiggering it to have less LOI, if you don't feel comfortable doing it yourself.


Things sitting around can take on water and affect the recipe measurements, but other than that they're mostly stable.

Edited by liambesaw
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@Liz C, did the pinhole problem coincide with a new batch number of clay? Witness cones in your firings to verify cone reached? There is a lot of boron in the recipe, durability could be compromised because of this. There is nothing really to cause concern about leaching in this recipe but I would test it for that if colourants are used with it. Excess boron can make a "soft" glaze and also can actually increase crazing. 

That being said, another version of your glaze below. I dropped the LOI to 4.1 so that might help with the pinholes but I think the issue could be from more than that. Yours on the left, mine on the right. Flux ratios, expansion etc the same in both.





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Hey guys !

@liambesawand @Min

So I ended up trying liam’s version it didn’t really solve the problem .... still got weird pinholing. Min I definitely will try your recipe next.

So with the kilns... they are reaching cone 6. I have done 15 minute holds as well that have even hit a hotter 6 more like 7. The pinholing still happened. To be fair not as severely. I currently am out of 5 and 7 witness cones so my last firing was in the dark but that kiln has consistently hit cone 6. So I don’t feel like that’s it.. maybe I don’t know maybe do longer holds?? Increase by half a cone like previously mentioned?


Where we are located is extremely humid right now... is it possible for bisque ware sitting around to absorb atmospheric water and be able to take on less glaze? The forms we are working on are fairly thin for their size. And I’ve noticed I’ve needed to apply glaze thinner especially with the Liam version because those pesky small bubbles appeared when a surface is over-saturated . Is this a thing ? Humidity affecting bisque ware?

thank you for your time and responses 




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1 hour ago, Liz C said:

Increase by half a cone like previously mentioned?

Often increasing the temp only makes this problem worse. A drop and hold of one or two cones at the end of firing can be a much more successful method. I couldn’t help notice but there is enough boron in this recipe to melt at cone 04. You might want to fire this once on a test tile to 04 and note the results. It likely will be fully melted. Might have found a favorite lowfire glaze for yourself.

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Something has changed. If this glaze worked for years and now it isn't you need to figure out what that is. If you have ruled out new glaze ingredients then look at your cones or your claybody. Pinholes and blisters can be a real pain to figure out the cause of. Like Bill said overfiring can cause them as well as underfiring, as can the bisque schedule, the claybody, firing schedule for glaze, glaze viscosity, LOI etc. Instead of trying to fix this glaze I'ld be looking for another recipe without the issues this one has.

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  • 2 months later...

Hello .. I'm  with Liz C and we are still struggling with this issue  and at a total loss for what to try next. 


Thank you to everyone who has posted on this !

It would seem that it was a glaze issue but testing commercial glazes in the same load leads us a bit  dumfounded. 

We have now tested commercial glazes (cone 6 )on different clay bodies and we are still having the pinhole issue in the firings .

We have used witness cones and everything seems to be correct. 

The bisque schedule we have been recently trying is a new vary fire program holding the ware at 1500 for 1 hour and again at 1800 for 30 min before going to end of 1924.  This schedule was suggested in an effort to burn off more completely. Thinking the gases are still releasing from beneath the surface of the glaze. This makes the surface a little less able to accept the glaze and changes the color a bit so is not ideal.  Previously we were bisquing to 06 where this was not an issue.  The results were better on wares that had less glaze applied. Having said that,  the " normal " glaze application is not thick and we have been using this method for a long time. What is super strange is that some wares that are flat like plates and tiles are coming out with less issues the wares that are tall.  The last firing I added a tile with the glaze overly thick and there were no pinholes.  

I do notice a bit of an " orange peel " effect happening when the glaze is thicker. 

Wares are all wiped down with a damp cloth for dust prior to glazing. We have tried full loads and loads with less things to eliminate the variable of an oddly stacked kiln. 

We added a vent to the kilns . Have fired in 3 different kilns. Replaced elements and thermocouples. 

We are now testing other clay bodies with the same glaze and the same pinholing result is occurring . 

We tried moving to a temperature controlled room to build work . 

We tried the drop and hold. 

We tried firing to cone 5 instead of 6 . We fired with holds on the end 5, 10 , 15 min .. The 15 min hold seemed a little better but still not correct. 

Our next step is just to scrap everything and try all over with a new glaze and a new clay body .. but if anyone has any thoughts that we have yet to try please let us know. 

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Hi Nat!

You've tried other clays with same glaze, also firing to a lower (and higher) cone, longer/targeted bisque schedule, and holds. My guess was try firing at the lower end of the clay's maturation range...

Have you tried an extended hold, e.g., an hour, after drop - perhaps 100F - from peak, then controlled cool (down through 1800F or so)?

Wait, you're also getting pin holes using other glazes and clays? Wow, kudos for trying just about everything; hope the culprit, else an alternative, is found soon!

Any chance the glaze in question is being used on the outside only? If so, what are you using for a liner? If the glaze in question is used inside and out, are the pin hole problems same on both sides?

From way out, past left field:

What's your elevation?

What are your tools made of?

Anything weird with your water?

Edited by Hulk
out o' th' box, step away, keeeeep walkin'...
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It doesn't sound like you are using a very "dirty" clay but if it is, a bisque firing schedule should be slow (ie around 150 F - 200F / hour) between 300F - 600F for the organic carbons to burn out and again between 1280F - 1650F for the inorganic carbons and sulphur to burn out. Adding a hold like you did at 1500F would have helped, I don't think a 1 hour hold at 1800F is necessary though.

Are the pinholes/blisters worse over trimmed clay and is there grog or sand in the bodies you've tried, any correlation?

Did you get a chance to try my recipe above?

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36 minutes ago, Hulk said:


Hello ! and thank you !

Hey Hulk : We did try a slow cool firing . It did not help . 

The glaze brushed and applied on the inside and outside. It's fairly simple .. or so it should be.

I don't know about the elevation but I have tried to do this firing in 2 different studios . I am 

We store the glaze in plastic bucket . I mix them usually with a stainless steel paddle/ spoon or a mixer . Sometimes we use a wooden one also ? 

I have not used the wooden spoon in  the smaller commercially made pints  I have tried. 

Water ~ this has occurred to me as there was a filter installed in the building .. which should make things better .  There is certainly water in the glaze we mix in our studio 

but with the commercially mixed pints there is no added water. In this case , the only water used would be in wiping down the wares before glazing.  


Hello Min : We will try your glaze recipe in the line up of new glaze tests . I am so desperate I want to test at least 3-5 new recipes .. Overkill, I know but this feels out of hand. 

Great information about the bisque firing.  Thank you . Agreed , the clay is not that dirty .  It is a standard clay body.  Curious to know if how long you would recommend holding at those intervals for future reference ? Ultimately , I would like to not have to use these vary fire programs which hold for so long. I don't want to wear out the elements if possible. I would also prefer to conserve the energy.

The pinholes are really on smooth surfaces . Not much forgiveness there in that . There is some grog as these are hand built forms. The grog really comes through when I bisque to 04 . 



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Hulk : inside and outside glaze is pinholing not much difference . Maybe .. worse on the inside at times but that may be only application. The thicker the glaze application , the worse it does seem to be. 

I'm so grateful to have people to ask .. thank you 


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41 minutes ago, Nat P said:

Curious to know if how long you would recommend holding at those intervals for future reference ? Ultimately , I would like to not have to use these vary fire programs which hold for so long. I don't want to wear out the elements if possible. I would also prefer to conserve the energy.

I wouldn't be overly concerned with the extra cost of electricity and element wear versus the cost of failed pots at this point. Get it sorted out then look at energy / element cost of longer firings. pm sent with firing chart for problem clays but I'm not so sure this is it.

One thing at a time, if you can rule out the bisque firing then move on to other possibilities.

Post the recipes of other glazes you have tried or are trying. 

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fwiw, I'm seeing improved results (my problems weren't pin holes, however) from: extended bisque fire; a bit cooler glaze fire; and from there the aforementioned drop, hold and slow cool.

...re-reading this thread, not seeing any discussion of kiln atmosphere - are you venting during both bisque and glaze fires (assuming electric)? Oh, wait, you did add a vent - is it pulling from the kiln; can you smell wax and/or feel warmth at the exhaust end? That air movement through the kiln can make a big difference. The kiln space/room would need a path for "make up" air to enter the space, else the kiln vent's effectiveness would be compromised.

Any road, I was slowing through three temperatures for bisque, 752, 1063, 1500F - slowing more seems to be helping!

Post back on your progress please!

Edited by Hulk
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