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pinholes on glazed bowls

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Hi all. I am new to this forum.  I just bought a Cone Art electric kiln and did my first firing. Using a Bartlett control panel and vent system.  Fired to cone 6 with Amaco  glazes. fired slow glaze Pieces on the top section were fine, middle and some of bottom section full of pinholes.  Cones on middle shelf were slightly under fired.  Would this have caused pinholing?  If yes, should I program in a hold ?  I don't know the cause of pinholes so could use some advice.  thanks!

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If the pinholes were in the parts of the kiln that you know were  underfired, then chances are good that a bit more heat work was required to smooth them out.  Pinholes in this instance are the result of different gasses from the clay and/or glaze being released as part of the chemical reactions that happen at high temperatures. Because glaze is pretty viscous, it needs time to properly release all those gasses and have the resulting bubbles heal over.  

What was your kiln pack like? if there's only a small amount of difference in the temperatures and heat work between the top and bottom of the kiln (say half a cone or so) the pack may be to blame. Heat rises, even in a small-seeming kiln. To counteract this, you want to have more thermal mass at the top of the kiln than at the bottom. It's a bit counterintuitive, but you want to have your larger objects in the bottom of the kiln, because they tend to be less dense to pack than say, a shelf full of small cups that are all clustered pretty closely.

 

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First you need to verify what cone each shelf is being fired to.  The  best way is to use self standing cones.  If you decide to use a hold than you also need to change the top temp you fire to. For example if the final ramp to temp. is 120 F than a 10 min hold would subtract 13 F from top temp., a 108 ramp  -16 F for 10 min hold.  Unfortunately this means writing your own program.

David

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On Tuesday, February 06, 2018 at 11:22 PM, BlackDogPottery said:

Out of curiousity what kind of Amaco glazes? I use the Amaco celadons sometimes and they will pinhole on areas where you've trimmed. I've fixed this with sponging down the whole piece before glazing. 

Thanks for replying. I am using Amaco Potters Choice glazes. Pinholes happened with afew different glaze combos.

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On Wednesday, February 07, 2018 at 4:56 PM, Callie Beller Diesel said:

If the pinholes were in the parts of the kiln that you know were  underfired, then chances are good that a bit more heat work was required to smooth them out.  Pinholes in this instance are the result of different gasses from the clay and/or glaze being released as part of the chemical reactions that happen at high temperatures. Because glaze is pretty viscous, it needs time to properly release all those gasses and have the resulting bubbles heal over.  

What was your kiln pack like? if there's only a small amount of difference in the temperatures and heat work between the top and bottom of the kiln (say half a cone or so) the pack may be to blame. Heat rises, even in a small-seeming kiln. To counteract this, you want to have more thermal mass at the top of the kiln than at the bottom. It's a bit counterintuitive, but you want to have your larger objects in the bottom of the kiln, because they tend to be less dense to pack than say, a shelf full of small cups that are all clustered pretty closely.

 

Thanks for the feedback Callie. My kiln pack was pretty balanced, not too dense, not too loose, but I did pack taller pieces with a little more space on top,  and bowls and cups on middle and bottom. Sounds like you are suggesting packing the reverse. I can give that a shot in this next firing. 

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On Friday, February 09, 2018 at 12:37 PM, David Woodin said:

First you need to verify what cone each shelf is being fired to.  The  best way is to use self standing cones.  If you decide to use a hold than you also need to change the top temp you fire to. For example if the final ramp to temp. is 120 F than a 10 min hold would subtract 13 F from top temp., a 108 ramp  -16 F for 10 min hold.  Unfortunately this means writing your own program.

David

Ok thanks gonna do another bisque firing. Will make sure I put witness cones on each shelf to verify how each shelf is firing. 

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