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Marge

I'm Getting Pinholes In My Glaze

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One of my favorite cone 7 glazes is consistently getting pinholes.  I hold for 17 min at the highest temp.  I tried a little longer and shorter hold, but that doesn't make a difference.  I've tried to apply a very thin coat, but still pinholes.  Also, I turn off the vent after it hits the final temp... to slow down cooling.  Does that do anything, or am I just wasting time waiting to turn off the vent?  I use a Skutt Kilnmaster without any special schedule other than the the hold.  I would LOVE and words of wisdom!!  

 

Thanks a bunch!!------- Margie

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Could your work be dusty? I sand mine after bisquing, then dunk it in water to wash off the dust. If pieces have sat around in your studio for awahile they will get dusty and pinhole.

TJR.

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Thanks to everyone for your help on my pinholes in my pottery.  I've vacuumed my kiln and rinsed by bisqueware.  I have a new load being fired tomorrow, we will see if that makes a difference.  I bisque to cone 04, and fire to cone 6.  With my 17 min hold, my cone 7 bends halfway down.  Its the same batch of glaze that I've been using all along.  It used to only pinhole sometimes, now that specific glaze pinholes consistently.  Perhaps I should make alot more... so I can dip quickly vs. pour over my pots.  Perhaps I will get a thinner application that way.  Thanks!!

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Thanks Norm, Its a majolica Glaze that I fettle and because I decorate I want to remove any lines or drips from my glazing efforts , but I also notice small holes as if air has escaped as the glaze dried and so I fettle them as well.. prob bit thick application though.

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Probably 80% of pinholing issues are related to the bisque firing, not the glaze firing.

 

Most of those deal with a lack of sufficient oxygen able to circulate through the densly packed wares to allow some body reactions to go to completion.

 

Secondarlly, thermal lag in a tightly packed bisque can mean that a lot of the wares are not fired to the witness cone level )or the sitter cone level or the computerized controllers idea of what the kiln did).... but to some level of heat work below that. So there are reactions that are not happening in the bisque that should.

 

Does this problem happen on certain types of forms more than others? Bottoms of bowls that have been heavily "nested" in bisques is a typical indicator of this issue. Ditto for anything that is contained "within" other forms. And also for stuff toward the middle of the stacking of wares... the furthest away from the electric elements.

 

Changes to the bisque firing profile or stacking techniques (slower firing, better air circulation, less nesting and density) or slowing down the front end of the glaze firing can alleviate this......assuming that you are not using glazes on the wares that have a very low start to the sintering/melting process... and thereby make the surface gas impermeable before oxygen transfer and the outgassing can take place.

 

best,

 

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

 

PS:  I fettle often.  I use a lot of natural matreials glazes. 

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