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Glaze Defect - pinholes/pitting/airbubbles


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Hello, I am a new member (and new to pottery in general) and have struggled to find the answer to the glaze defects I have been experiencing on previous posts in this forum. I am having trouble with pinholing/pitting/airbubbles (not sure exactly what to refer to it as). Most of my glazed pottery either has loads of little holes (this is especially bad with black underglaze topped with clear) or lots of tiny bubbles trapped beneath the surface. 

To provide a background, the clay I have been using is Scarva Earthstone ES5 Original White Stoneware Clay, which has a firing temperature of 1200-1290°C and some grog in. I have primarily been using Amaco velvet underglaze, which I apply on cone 04 bisqueware and top with Amaco clear gloss (HF-10), before firing to cone 6 on slow. I only use the standard programmes on my little Skutt Firebox as I have not figured out setting up my own yet (i.e. with soaks).

Using tips from previous posts, I have tried wet-sanding bisque fired pieces before glazing, and making sure they are washed and dust free. I have also experimented with slow firing vs medium. The sanding helped a lot and I have fewer pinholes now, but am still getting loads of tiny little pits on (specifically) the black underglaze. I am not sure why this is different to the other colours or clear gloss over bare stoneware (but this still has lots of airbubbles underneath, they just aren't breaking the surface). 

I have attached some close-up photos of the issues, which hopefully are good enough quality to be visible. I am not sure what is causing this and would be really grateful if anyone has any ideas?

Thanks,

Ellie :)

 

Screenshot 2020-05-27 at 20.10.51.png

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Have a look at the Amaco Field Guide that shows underglazes fired at ^04,5 and 10. For all 3 of the blacks in the Velvet series they use HF-9 Zinc Free Clear on their test pieces. Could be the Amaco HF-10 (contains zinc) isn't as compatible with the black underglaze as the HF-9 is. Adding a soak at the end will help smooth out pinholes and blisters but I don't think it would be enough to clear the bubbles I think I'm seeing in the glaze.

Are you using this clay for functional work or just decorative? Reason I ask is it's likely to still be fairly porous at ^6 as it is rated to go to ^10. It would be fine for decorative only work but I'ld test the absorption if using for functional ware as over approx 2% can lead to leaky pots.

Welcome to the Forum. :)

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3 hours ago, ellievsclay said:

Hello, I am a new member (and new to pottery in general) and have struggled to find the answer to the glaze defects I have been experiencing on previous posts in this forum. I am having trouble with pinholing/pitting/airbubbles (not sure exactly what to refer to it as). Most of my glazed pottery either has loads of little holes (this is especially bad with black underglaze topped with clear) or lots of tiny bubbles trapped beneath the surface. 

To provide a background, the clay I have been using is Scarva Earthstone ES5 Original White Stoneware Clay, which has a firing temperature of 1200-1290°C and some grog in. I have primarily been using Amaco velvet underglaze, which I apply on cone 04 bisqueware and top with Amaco clear gloss (HF-10), before firing to cone 6 on slow. I only use the standard programmes on my little Skutt Firebox as I have not figured out setting up my own yet (i.e. with soaks).

Using tips from previous posts, I have tried wet-sanding bisque fired pieces before glazing, and making sure they are washed and dust free. I have also experimented with slow firing vs medium. The sanding helped a lot and I have fewer pinholes now, but am still getting loads of tiny little pits on (specifically) the black underglaze. I am not sure why this is different to the other colours or clear gloss over bare stoneware (but this still has lots of airbubbles underneath, they just aren't breaking the surface). 

I have attached some close-up photos of the issues, which hopefully are good enough quality to be visible. I am not sure what is causing this and would be really grateful if anyone has any ideas?

Thanks,

Ellie :)

 

Screenshot 2020-05-27 at 20.10.51.png

Not sure this will help but we had the same issue, especially with certain underglaze  colors and Amaco clear glaze. As a result I fired many colors and test titles only to find out that many bubbled and basically underfired as my underglaze got thicker. So after many many tests we settled on making a clear glaze that began to melt closer to cone 4  which alleviated the majority of issues for our wares that use heavy underglaze. We created a matte and glossy clear for our use and studio use and it has solved most of those issue going on two years now I think.

Some folks mix their known most refractory underglazes with a little high boron low alumina Fritt and this does the trick for them. So I think our conclusion in general, from many tests, some underglaze colors, especially applied heavily are refractory.

I posted the matte recipe on glazy years ago,  just never got around to posting the gloss. I’ll try and get to it.

https://glazy.org/recipes/19734

 

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8 hours ago, Min said:

Have a look at the Amaco Field Guide that shows underglazes fired at ^04,5 and 10. For all 3 of the blacks in the Velvet series they use HF-9 Zinc Free Clear on their test pieces. Could be the Amaco HF-10 (contains zinc) isn't as compatible with the black underglaze as the HF-9 is. Adding a soak at the end will help smooth out pinholes and blisters but I don't think it would be enough to clear the bubbles I think I'm seeing in the glaze.

Are you using this clay for functional work or just decorative? Reason I ask is it's likely to still be fairly porous at ^6 as it is rated to go to ^10. It would be fine for decorative only work but I'ld test the absorption if using for functional ware as over approx 2% can lead to leaky pots.

Welcome to the Forum. :)

Ah ok - that's really helpful, thank you! I'll get some of the HF-9 and see if that makes a difference. I also have some F-10 that I haven't used yet, as I bought it mistakenly. I'll get a lower firing clay and do some test tiles to see if these make a difference!

I took it to mean that a clay that fires between 1200-1290°C would be ok as long as it was fired within those temperatures (i.e. 1200), but I suppose that was wishful thinking? I am not making functional wares at the moment (the two mugs I have made were gifts to family, who are just keeping them as decorative items rather than using them). Otherwise it has just been little jewellery dishes and plant pots. I would like to be able to make functional wares though, so will try some lower firing clay for this (my little kiln only goes to cone 6). 

Thanks again :) 

Edited by ellievsclay
clarifying jewellery dishes and not food dishes
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7 hours ago, Bill Kielb said:

Not sure this will help but we had the same issue, especially with certain underglaze  colors and Amaco clear glaze. As a result I fired many colors and test titles only to find out that many bubbled and basically underfired as my underglaze got thicker. So after many many tests we settled on making a clear glaze that began to melt closer to cone 4  which alleviated the majority of issues for our wares that use heavy underglaze. We created a matte and glossy clear for our use and studio use and it has solved most of those issue going on two years now I think.

Some folks mix their known most refractory underglazes with a little high boron low alumina Fritt and this does the trick for them. So I think our conclusion in general, from many tests, some underglaze colors, especially applied heavily are refractory.

I posted the matte recipe on glazy years ago,  just never got around to posting the gloss. I’ll try and get to it.

https://glazy.org/recipes/19734

 

Ah it is such a relief to know I am not the only one, haha. Thank you very much for replying! Narrowing it down to the clear glaze is really helpful,  as there seem to be so many different things to troubleshoot with pottery. I actually have a lower fire clear gloss that I previously bought mistakenly and haven't used yet, so I will try doing some test tiles. I am also planning to get some lower firing clay to play around with and see if that helps. I probably have been quite heavy handed in glaze application too, so will test some different thicknesses.

Thank you for a link to the matte glaze recipe - I have saved the link for when I am brave enough to try mixing my own glaze and will keep an eye out in case you post the gloss. 

Thanks very much :) 

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  • 9 months later...

@Bidpil, ellievsclay hasn't been back to the forum since June of last year. If you hover your mouse over a persons onscreen name or avatar you can see when they were here last.

What clear glaze are you using? A commercial one or one you mixed yourself? If it's the latter could you post the recipe. A picture of your problem might be helpful too.

Welcome to the forum. :)

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