Pesky Pinholes

[Skip]

I am slow bisque firing Standard 553 clay to cone 06, painting on 4 coats of Amaco glaze and then slow glazing to Cone 6 with no soak. Now that I am aware what a pinhole is I am concerned. I have fired 79 bowls and 27 of them have pinholes. I have fired 112 plates and none of them have pinholes. A few questions... is the pinhole pottery foodsafe? Why are bowls getting pinholes but not plates? My application thickness is the same. Would bisque firing to cone 04 help? I am firing in an L&L e23t kiln with a bottom vent running the whole time through firing and cool down. The peephole plugs are in the whole time. Any insight would be greatly appreciated. My dipped glaze bowls of which there are 22 are not part of the 79 referenced above) have no pinholes. If I can’t figure this out I am going to stop painting on glaze and only dip, but the colors of the painted bowls are awesome. Thanks in advance for any help!

[xinyao]

我认为可以在瓷器表面用湿海绵擦拭一下然后再进行上釉可能可以解决这个问题。你可以尝试一下。并告知实验结论

 

 

Edited to add translation:

I think this problem can be solved by wiping the porcelain surface with a wet sponge and then applying the glaze. You can have a try. And inform the conclusion of the experiment.

[Pres]

@xinyao When posting here, please do not place in outside links, and please use english. . . .to make it easier for the majority of the forum population to understand. We can all google for a translator, as I have with your post, but would prefer things posted in english.

 

Welcome to the forum!

 

best,

Pres

[Skip]

Thank you xinyao. I will try what you suggest, using a damp sponge to wipe down the bowls before I glaze them.

[Pres]

@Skip, A few answers to some questions may help the community with some helpful suggestions: Where does the pinholing occur, where glazes overlap, on the inside of the bowl, outside of the bowl, in the bottom of the inside of the bowl? What is your prep for this brushing on, have you washed the pottery with a damp sponge, or do you dip the bowl in a sink of water as some do? What is the consistency of the glaze that you are brushing on? A picture would also be helpful.

As to the food safeness of the bowls, I would not sell bowls with pinholes on the inside if prevalent.

 

best,

Pres

[Skip]

The pinholing occurs primarily on the inside of the bowl but the location varies with no real consistency. Many of the bowls only have a single pinhole which sometimes is not very visible. I have been brushing the bowl with a dry cloth before glazing, but I will try using a damp sponge or even the dunk method you mention. The glaze is the consistency of heavy cream, so perhaps adding water could help? I will add some pictures tomorrow. Thank you for your suggestions, Pres.

[Mark C.]

Bowl forms pinhole more than than other forms -or insided forms pinhole mnotre than outside forms. I would clean your bowls 1st and let dry before glazing. A short soak may help smooth them out as well.Stoine wares pinhole more than porcelains in most glaze applications. Thickrer glazes will also pinhole more. Try the  short soak at temp.

[Skip]

[Skip]

[Skip]

The two pictures above are typical of the pinholing I am getting.

[Skip]

Mark, thank you for your suggestions. I will try what you have suggested.

[Skip]

After trying lots of things that did not work to eliminate pinholes (bisque to cone 04, applying glaze thinner, applying glaze thinner, etc) I finally have solved the pinhole problem. I am using a ramp down temperature program after firing, decreasing the temp 150 degrees per hour until the kiln reaches 1500 degrees. The kiln load that came out yesterday had no pinholes. Yay!!

[Bill Kielb]

2 hours ago, Skip said:

I am using a ramp down temperature program after firing, decreasing the temp 150 degrees per hour until the kiln reaches 1500 degrees. The kiln load that came out yesterday had no pinholes. Yay!!

This is usually an indication that the fired surface tension does not allow a composition to heal. Firing down with success would infer that this was the issue with this glaze. Often soaking at peak temperature counterintuitively makes this problem worse. Congratulations, nice work!

[Skip]

As with some things in pottery solving one issue has raised another. The pinholes are gone, but the slower cooling has affected the look of the glaze quite a bit in an unpleasant way. I would like some advice. Should I try to fire to Cone 5 and hold for 20 minutes with no slow cooling program, or should I do new test tiles with the Cone 6 temp and the cool down program? My witness cones show that the kiln may be slightly over firing at Cone 6 since the Cone 7 cone dipped to horizontal on all 4 shelves in the most recent firing. 

[Skip]

On 9/2/2021 at 6:11 PM, Bill Kielb said:

This is usually an indication that the fired surface tension does not allow a composition to heal. Firing down with success would infer that this was the issue with this glaze. Often soaking at peak temperature counterintuitively makes this problem worse. Congratulations, nice work!

Thank you for your encouragement Bill. 

[Mark C.]

I have been fighting a few pinhole issues of late as well.Its rutile glazes that do not play well with others. Reduction also plays a part in this for me in the gas kiln. Long slow fires help.

[Skip]

10 minutes ago, Mark C. said:

I have been fighting a few pinhole issues of late as well.Its rutile glazes that do not play well with others. Reduction also plays a part in this for me in the gas kiln. Long slow fires help.

Maybe I can try a slower climb to Cone 6. There are so many variables. The good news is I only have to figure out what works one time, and then the issue should be solved.

[Bill Kielb]

4 hours ago, Skip said:

Maybe I can try a slower climb to Cone 6. There are so many variables. The good news is I only have to figure out what works one time, and then the issue should be solved.

It might work, but often when a glaze does not heal from fired surface tension issues it’s the peak temperature that is the culprit so until the glaze cools and the surface tension changes enough it won’t heal itself. My observation:  some glazes perform better at cone 5 than 6 so cone 5 with a fifteen minute hold keeps the peak temperature down and still fires to cone 6 heatwork. Or for stubborn glazes a drop and hold is easiest to get this done. It’s so counterintuitive that it often takes a bunch of frustrating testing. You are ahead of the curve as you have already discovered a successful method.

The last 200f of the firing is where most of the melting  or maturity happens (eutectic: where there is finally enough energy to get the flux, silica and alumina to melt) and if following the Orton cone chart middle column 108 degrees per hour should get cone 6 to drop at 2232 f.. rates over  the last 200 degrees of the firing are very important to achieve predictable heatwork.

[Hulk]

Hey Skip, just curious, can you describe "...slower cooling has affected the look of the glaze quite a bit in an unpleasant way." ?

My guess is there's some matt-ness imparted? Less clear/translucent, more frosty, crystal-y?

[Morgan]

You could try the drop and soak firing schedule…