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Beardbangs

Tons Of Little Bumps/bubbles Showing Up After Glaze Firing

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I hand-mix a transparent glaze, which I've used for the last year continuously with great success. The last two times I've glaze-fired, TONS of little bumps appeared, a few of them up to a cm wide (those ones looks a little more like an air bubble in the clay. The clay is white and the glaze is clear, but the bubbles that are appearing are white (so it looks like the glaze has fused to the clay body and pulled it out into bumps/bubbles. Please see the attached photo- this is what all my pieces came out like. I thought my glaze might have been contaminated so I mixed a whole new batch but the same thing happened. The only other thing I can think of that might be different is that it's more humid in the studio because it's the middle of summer.

 

Please help! I'm getting so delayed on my orders because I keep having to re-make everything and I'm freaking out!

Should I try re-firing the pieces and doing something differently, like holding it a bit?

 

I fire my bisque to cone 06, and the glaze to cone 6. The kiln sometimes runs a little on the hot side, but I haven't found that to cause this particular problem before. I use PSH cone 6 white stoneware #519.

 

My glaze recipe is: 25% epk, 25% silica, 10% wollastonite, 25% frit 3134, 15% feldspar minspar.

 

Thanks in advance!

 

 

 

post-68010-0-25620500-1438012666_thumb.jpg

post-68010-0-25620500-1438012666_thumb.jpg

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these  bumps match the problem i had and asked about here.  the suggestion was that the glaze layer was too thin.  i was able to reglaze some pieces but not all of them worked.  hope this helps.

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I don't use any cones because it's a digital kiln and when I set it to cone 6 it's usually fine. So do you think it might be worth re-firing to cone 5? From reading a little more about bloating, I'm wondering if that's the issue, related to your second idea- I still don't really understand how to better burn out my organics. I'm a bit nervous to do anything more than a pre-programmed firing on my bisque as I've never done it. Could I just fire to cone 04 instead of 06? would that burn off the organics or do you need to soak?

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these  bumps match the problem i had and asked about here.  the suggestion was that the glaze layer was too thin.  i was able to reglaze some pieces but not all of them worked.  hope this helps.

 

Hmm, ok, I'll try re-glazing a few of them. I'd be surprised if that's the problem I'm having, as I don't think it's thinner than I usually make the glaze.

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Being that it is a white stoneware, a bisque to cone 06 is probably fine. If you have always bisque to cone 06, then I don't think that's probably the issue unless this is a new clay batch from your supplier. 

 

That being said when these problems pop up you should put in some cones to see if its a kiln overfiring issue or a glaze issue. It will narrow it down really fast. Also another way to check for this without cones is to fire a pot without glaze in the same load and see if the actual clay body bloats without glaze, however this wont tell you if the glaze is getting too hot or not.  You should get some cones!

 

Since you are in a hurry and getting behind the quick thing would be to buy a set of 567 self supporting cones at your supplier, glaze a few pots with different thicknesses (mark them in some way so you will know, Red Iron Oxide w/ water works fine), then compare just in case it is a thickness thing.

 

My bet is still that your kiln is probably reaching cone 7 and your glaze is blistering/bloating. Don't think that just because its digital doesn't mean it can't overfire. As your Thermocouples(TCs) and elements get used they can change in the way they heat and read heat. I had this issue with my kiln. I was glazing away fine to cone 6 every time. Then each load started getting darker and darker, I put in some cones and my cone 6 firing preprogrammed was firing to cone 7+. I had to adjust my cone offsets, but then I started using my own programming schedules and I ended up adjusting my TCs instead of the cone offset. Now I always fire with cones in my kiln. They are well worth the extra cost, because it lets you know right away about any future problems that might occur.

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Ok, yeah, that makes sense. I do have cones, but I just stopped bothering to put them in after a while because it was just always working fine. I think I'll try bisquing to cone 04, just to see if that helps, and then put in my cones for the glaze firing and keep a close eye on them.

thanks so much for your help.

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Only do one thing at a time if you make changes.

 

Your glaze is going to apply differently at cone 04 than 06. So be aware that it might change everything again. 

 

I would start with making sure your kiln wasn't over firing. If it isn't then go to bisque. But if you have been bisqueing that same clay to 06 for all this time, I highly doubt that is the issue. If it is a new batch of clay, then you could start there. Either way just make sure you only change one thing at a time.

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Those look like bloats to me, which are bubbles in the clay, usually due to over firing. You can see that there a very small, smooth bumps that haven't come through to the surface quite yet, as well as the big bumps. Could also be a bad batch of clay, which is what I'm betting on. Use some cones to rule out the over firing possibility, though. In my experience, digital kilns tend to under fire as the thermocouple ages, not over fire, but it's definitely a possibility. However if it was truly over firing enough to cause bloating, I would expect your glaze would also show some extra fluidity, which you haven't mentioned. If you've still got the boxes that the clay came in, check to see if they're all from the same batch. Also try a couple of pots with another glaze to see if it still happens. If the kiln is not over firing, call your clay supplier and let them know what's going on. They'll want to know the batch numbers.

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Those look like bloats to me, which are bubbles in the clay, usually due to over firing. You can see that there a very small, smooth bumps that haven't come through to the surface quite yet, as well as the big bumps. Could also be a bad batch of clay, which is what I'm betting on. Use some cones to rule out the over firing possibility, though. In my experience, digital kilns tend to under fire as the thermocouple ages, not over fire, but it's definitely a possibility. However if it was truly over firing enough to cause bloating, I would expect your glaze would also show some extra fluidity, which you haven't mentioned. If you've still got the boxes that the clay came in, check to see if they're all from the same batch. Also try a couple of pots with another glaze to see if it still happens. If the kiln is not over firing, call your clay supplier and let them know what's going on. They'll want to know the batch numbers.

Does anyone else remember the huge batch of Laguna BMix that bloated? I had a half ton, but some big schools had much more.

A mention to the clay supplier is not out of line. I think a lot of small-time potters like me thought it was our fault!

I still have one of those pots as a reminder.

 

ps Laguna replaced my clay

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(ps Laguna replaced my clay )

you must live close to them and picked it up as they never ship clay in replacement.

I recall many bloating bodies from them over the years.

I am not a bix mix user but my friends all had issues then.

Mark

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Would bet the ranch that is bloating due to insufficient organic burnout.

 

This has been discussed several times in the last few months. Search "small air bubbles" and you will see a pot just like yours. The fix is to have a soak for an hour or so at around 800 degrees Celsius.

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beardbangs, i may not have made myself clear.  i did not mean that there is too much water in the glaze, but that i did not apply enough of it to make a thick enough layer on the pot.  

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(ps Laguna replaced my clay )you must live close to them and picked it up as they never ship clay in replacement.I recall many blaoting bodies from them over the years.I am not a bix mix user but my friends all had issues then.Mark

They did. Early 1980s? Maybe?

Never had a problem with it since.

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I had this *exact* same issue recently, and I think it might be due to a bit of "mystery clay" (which is likely low-fire, ^04) contaminating my ^6 stuff. Have you been doing any clay mixing lately?

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I have recently had the same 'bumps' problem with clear glaze over white stoneware, your photograph shows precisely what happened to me.  So did you solve your problem?  I need to put cones in to work out what's going on before I make any decisions, but I'd love to know if you got things sorted.

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6 hours ago, Mary Cousins said:

I have recently had the same 'bumps' problem with clear glaze over white stoneware, your photograph shows precisely what happened to me.  So did you solve your problem?  I need to put cones in to work out what's going on before I make any decisions, but I'd love to know if you got things sorted.

The member who wrote the original post hasn't posted since Nov 2016 so they probably won't see your question. You could pm them and perhaps get an answer, there is a little envelope on the upper right side of the screen to do this. Yes, definitely put some cones in to see if you are overfiring your clay. 

Welcome to the forum!

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