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Blisters on Pottery - Any suggestions to eliminate?


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I am using a combination of glazes, Olive Green Celadon and Waterfall Brown, to get the effect I like on some of my work.   Unfortunately I am running into a blistering issue more times than not which ruins the piece.  Check out the image below. 

These are Cone 6 glazes and I have tried changing the temperature for the bisque firing as well as making changes to the glaze firing.   The most recent changes to the glaze firing is to fire to Cone 6  then drop about 100 degrees (Farenheit) and hold the temperature for 30 minutes and then slow cool to 1,400 degrees.    I have listed the recipes for the two glazes below for information purposes (approx 10,000 g batches).    Glazes were sieved using a 100 mesh screen. 

If anyone has had a similar problem and come up with a solution I would appreciate hearing from you... Thanks.

IMG_7856.jpg.4d9ea892da049db5e8ae60e0666da547.jpg

Olive Green Celadon

Custer Feldspar          2000

Gerstely Borate         2000

Nepheline Syenite   2000

Silica                                 1500

Wollastonite                1000

Dolomite                          200

EPK                                      500

Red Iron Oxide             200

Titanium Dioxide        100

Cobalt Carbonate         50

Copper Carbonate    300

 

Waterfall Brown

Frit 3134                     3350

Frit 3195                     2600

OM4 Ball Clay            1700

Silica                                 2350

Red Iron Oxide           1250

Rutile                                   100      

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Aye, curious what others might offer on the topic, thought more info might help.

My guess is gas(ses) being expelled by the clay, perhaps a chunk of something to generate a persistent flow? Any chance the problem coincides with opening of a new box/bag/batch of clay?

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I don't think it relates to any one batch of clay.  I have used many boxes of mid smooth stone  with the same glazes over the last 6 months with some pieces having the blisters and others with no blisters.  I have mixed new batches of glaze numerous times in case it is related to the glaze components.  Going to try another glaze firing test today to see how it turns out. 

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Try a slower bisque, especially through the critical temps, supply ample oxygen; see Nerd's post

https://community.ceramicartsdaily.org/topic/22538-bubbling-glaze-re-firing-and-now-more-bubbles

Where the blisters form, is the clay thicker? Any chance you're running the glaze fire a bit hot?

Good luck on finding the culprit(s) - likely you'll get more input from other forum members as the day wears on - I'm heading out!

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When I see big open blister like that, it's because the glazes are over-fluxed and not stable, and in need of more silica or alumina or both. It's hard to say which one is the real culprit here, because you're overlapping them. The brown is really high in boron, both are really high in calcium, one is on the lower end for alumina, the other on the lower end for silica. Put those together  and you've got an unstable glaze. Some glaze just don't play nice together. If you wanted to try to fix the problem, I would take one or both and increase the silica and clay in the glaze and increase them in equal amounts. Test them in 2% increments and see what happens. The problem will be that they may stiffen up as the problem is fixed and not give you as runny a surface as you're used to.

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@Sea Winds, try refiring one of the badly blistered pots (on a waster slab to catch the glaze runs) and see if the blisters get better or worse. If they get worse then it's likely from high surface tension of the glazes, high boron which doesn't help with this plus the titania. Are you using witness cones? Any chance you are overfiring? What is the rate at which you are slow cooling? I believe for the Waterfall Brown 125/hour from 1900-1500F is about the rate that works for that glaze.

Edited by Min
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I have some pieces in the current firing that blistered so I will see how they turn out.

I use witness cones 5, 6 and 7 and they are obviously in that order in the below photos

The kiln is a Skutt 1027, 7 cubic feet.  It fires hottest in the middle and coolest on the top.  The cones below are all from the same firing. 

Based on the cones the Olive Green Celadon doesn't appear to be overfired if it is a true cone 6 glaze.  (I know that for Floating Blue that it will turn out more greenish if I do a true Cone 6 firing in my kiln so I fire it to about 5 1/2 to get the blue I want)

I cool from 2095 to 1400 at 150 per hour and seem to get the flow and colour I like for Waterfall Brown. 

 

 

Top Shelf Cones .jpg

Middle shelf cones.jpg

Bottom Shelf Cones.jpg

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8 hours ago, Sea Winds said:

I don't think it relates to any one batch of clay.  I have used many boxes of mid smooth stone  with the same glazes over the last 6 months with some pieces having the blisters and others with no blisters.  I have mixed new batches of glaze numerous times in case it is related to the glaze components.  Going to try another glaze firing test today to see how it turns out. 

I had the same problem with floating blue.  On most any clay body.  I refired a few pieces which gave mixed results.  So, I gave up on it.  I love your combo.  I hope you can get it worked out.  

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18 hours ago, Sorcery said:

Is that unbroke blister mug only Celadon or both?

Is it true that it's never on the brown alone?

Sorce

The blister is on the Celadon alone on that mug, so I am thinking if it is glaze related that it is the Celadon.  The entire mug is glazed with the Celadon first.  Once it dries the top one inch is dipped in the Waterfall Brown and as it is a runny glaze if flows down the mug.   As a result there is no place on the mug where it is just the brown glaze.

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11 minutes ago, Sea Winds said:

glaze related that it is the Celadon

If I'm not mistaken, this is a common problem with Celadon. Or known anyway, maybe not common. 

I'm about keeping variables minimal. So adjusting only that glaze seems necessary.

Yes pretty!

Sorce

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On 8/31/2020 at 8:59 AM, Sea Winds said:

I am using a combination of glazes, Olive Green Celadon and Waterfall Brown, to get the effect I like on some of my work.   Unfortunately I am running into a blistering issue more times than not which ruins the piece. 

a beautiful glaze when it works!  was a favorite before  I switched to cone 10. 

... used to use a glaze similar to what you are using; some times beautiful results, other times not so beautiful with pits, bubbles, and dark spots.  

the flaws in the pictures you posted look exactly like the failures I was getting.  After some effort, it was determined that the root cause was due to poor application and was not related to firing temperature, clay body composition, where it set in the kiln, or what I had for breakfast.  

The bubbles and burst bubbles were due to trapped air during application.  The dark spots were due to lumps of iron (and other ingredients) particles in the glaze slurry. 

The glaze slurry must be well stirred and the ingredients dispersed uniformly; the mix should feel smooth, not gritty, to your fingers in the bucket.  The bisque ware must be clean and dust free.  I learned to pre-moisturized the surface with a damp sponge (being careful to remove any sponge crumbs after wiping).  I was using various red, white, recycle clay bodies and there was no correlation of flaws with clay bodies. 

Thickness was also a variable. Thin layers seldom had problems (except for  bare spots from wiping all the glaze away; that was pilot error!). The burst bubbles were in thick application areas. 

The studio prof NEVER had problems with the glaze, only us students had failures; we all used the same suite of glazes and clay bodies; all fired in the same kiln at same time.  

conclusion: 
All the flaws and failures were due to poor application of the glaze on to the pot.  

quite using the glaze when I switched to cone 10 firings. 


LT

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Glazes loaded with colorants especially rutile can cause issues. I longer end point soaks can help but one must be carefull as the glaze will run off the pot if its to long.I like a long slow climb at the end of fire as this helps. I think MMud R said it best application and what you are eating that day affects it-meaning its a cr-p shoot

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Using information people have suggested I just completed another test firing.  This firing went to about Cone 5 (instead of Cone 6)  and then temperature was dropped to 1950 degrees with a 30 minute soak, then cooled slowly at 150 degrees to 1400 to get the blueish colour from the Waterfall Brown.

The photo shows two mugs.  The one on the left was fired for the first time and no indication of blisters.  The one on the right was a refire.  Originally it had only one blister and after being fired a second time has many more.

It seems that Cone 6 is too hot a temperature for this glaze/clay combination.  I had 8 pieces in the kiln to test the top, middle, and bottom of the kiln.  All the pieces that were being fired for the first time turned out fine.  Only the refired piece came out with blisters. 

Will run another test with the same firing schedule to ensure I can get consistent results.

Mugs Comparison.jpg

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I had the exact same problem and solved it with some modifications. 

First i programmed a hold at 650°C of 20mins for the bisquefire. 

I changed the curve for the glaze firing to a really slow firing at peak. 4h until 500°C

500-1200 with 150°C per hour. 

1200—1250 333°C/h

1250 10min hold

Drop to 1200 15min hold

Drop to 830°C  50mins hold

Natural cooldown 

It solved it for me, of course it depends on the kiln, clay, glaze etc and also the glaze applicatiom like mentioned before really does a huge difference. Don't let glaze high in boron pool up on the bottom. 

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