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Rick Wise

Blistering / What Causes it?

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Can anyone tell me why I am getting blistering at the bottom (only) of the piece when using this homemade glaze?  Its called Westcountry Water and is:

s_15400.5a6a6c135b2e7.jpg Silica 36
Potclays/BPS High Alkaline Frit 30.5
Devon China Clay 20
s_15253.5a6ba39ed314a.jpg Gillespie Borate 12
s_15457.5a6a6d4411ad0.jpg Whiting 1.5
Total Base Recipe 100
s_15469.5a6a6d4b393c5.jpg  Zircopax 4
s_15072.5a6a6a3c01481.jpg  Bentonite 3
 Copper Carbonate 2
Total 109

 

Otherwise, a very satisfactory glaze.

IMG_0725.JPG

IMG_0726.JPG

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I see holes spread across most of the white glaze.  Looks like it's doing the same thing all over - but it's thicker at the bottom, so the bubbles don't pop - and turn into blisters instead of tiny craters.

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@Rick Wise What cone are you firing? It's difficult say say exactly what's going on without knowing the makeup of that frit. Running the formula with a few different US frits, it's possible that it's high in silica, high in boron, or high in calcium. Any chance you have the formula for that frit? The Potclays web site doesn't have any information on it.

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6 hours ago, neilestrick said:

Any chance you have the formula for that frit?

It's this one right here (BPS = Bath Potters; also identical to Potclays High Alkaline Frit, which is Potclays 2275):

High Alkaline Frit

% ANALYSIS

K2O = 17.35

Na2O = 17.46

Al2O3 = 5.69

CaO = 5.32

SiO2 = 50.30

B2O3 = 3.88

Edited by Sputty

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@Rick Wise Turns out I have that frit in my materials index! With the correct frit, I'm not seeing anything particularly worrisome about that glaze. It's at the high end for silica, and the sodium is a little high, but nothing that would point to what's happening in your photos. Are you sure it wasn't under fired? Did you sieve the glaze well before applying?

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19 hours ago, neilestrick said:

@Rick Wise Turns out I have that frit in my materials index! With the correct frit, I'm not seeing anything particularly worrisome about that glaze. It's at the high end for silica, and the sodium is a little high, but nothing that would point to what's happening in your photos. Are you sure it wasn't under fired? Did you sieve the glaze well before applying?

Any time I've run into similar issues, it was because of underfiring/ cooling relatively quickly.

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On 2/27/2018 at 8:36 PM, neilestrick said:

@Rick Wise What cone are you firing? It's difficult say say exactly what's going on without knowing the makeup of that frit. Running the formula with a few different US frits, it's possible that it's high in silica, high in boron, or high in calcium. Any chance you have the formula for that frit? The Potclays web site doesn't have any information on it.

Sorry -- no idea on the formula.  I fire to cone 6.  BTW, the picture is deceptive as to color.  It is a light green with a hint of blue in it.

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A recent set of posts in another thread reminded me that clay bodies can contribute to glaze blistering. 
 
The specific reason for the blisters is that gas is trying to migrate through a very stiff melt.  The root cause will be found when you chase all the possible sources of gas formation, not just the sources in the glaze. 
 
The mug appears to have a thicker wall at the bottom than at the top. Also there seems that the number of  blisters per unit area increases from the top to the bottom.  This pattern is consistent with the clay body not being fully free of gas producing 'stuff'. 
 
Try the glaze on a different clay body using the same glaze. 
 
Critique the bisque firing protocol, especially for   the  time at burnout temperatures.   Thicker walls require longer to burnout times  than thin walled items.   Also a full kiln takes longer to burnout than a not-so-full kiln. 
 
From the glaze side, look carefully at the initial melt temperature of the glaze based on the specific ingredients.  Frits often begin to fuse together at lower temperatures than non-fritted components.
 
LT

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5 hours ago, Rick Wise said:

... I fire to cone 6.  ...

What clay are you using? I guess you're in the UK using those ingredients - there has been debate on here about how many UK commercial stoneware clays are actually suitable for ^6. Quite a few seem to be marketed with wide temperature ranges which may be misleading.

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On 3/21/2018 at 9:12 AM, Rick Wise said:

I fire to cone 6.  BTW, the picture is deceptive as to color.  It is a light green with a hint of blue in it.

I agree with what Magnolia Mud Research has said about looking at your firing schedules, both for bisque and glaze firings. Do you use witness cones to verify the firings? I found this image of the same glaze, looks vastly different from yours. I know you said your picture is deceptive but there is a huge difference between your glaze surface and the one from the link. Given the silica:alumina ratio I would expect it to be a high gloss when fired to maturity. Leaves me to think that you have two issues going on here, firstly the bisque firing wasn't thorough enough to burn out all the gas producing materials and secondly the glaze firing didn't get hot enough. Clean and slow bisque firing with as much ventilation as you can get, (vent the kiln or peeps out for most of the firing if you don't have a vent) to cone 04 plus verify your glaze firing with cones, add a soak just below your top temperature to allow time for the pinholes and blisters to heal and smooth over. 

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I also wonder if after stirring bubbles are forming on top of glaze in bucket.

I did have this prob with one glaze and when fired the bubles deposited on ware did not go away on firing unless fettled, or as Min? suggested at time, sprayed glaze in bucket with hairspray to dissipate bubbles before the dip...

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On 3/21/2018 at 4:49 PM, Joe_L said:

What clay are you using? I guess you're in the UK using those ingredients - there has been debate on here about how many UK commercial stoneware clays are actually suitable for ^6. Quite a few seem to be marketed with wide temperature ranges which may be misleading.

Standard 112 and 306.  US

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