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glaze cratering problem


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#1 Bob Coyle

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Posted 28 June 2013 - 01:57 PM

This is a modification of a glaze from Glazes Cone 6 by M Bailey
It gives a real nice matt blue break where thick and matt green where thin.
My problem is that it always looks fine on the test tiles I fire but some of the
pots I fire show craters and fish eyes, while others, from the same kiln load,
look fine. A second firing sometimes solves the problem but not always.
Anybody else use a glaze similar to this?
Any thoughts?

Neph Sy 69
GB 9
Ball Clay 8
Whiting 5
Silica 5
Rutile 2
Cobalt Carb 1
Titanium O2 1

#2 Wyndham

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Posted 28 June 2013 - 03:36 PM

Bob if you are talking about glaze blisters, I'd check if your kiln might be firing longer than what you expect and ending up hotter.

If one element is not getting hot enough the other zone might be way hotter to makeup the difference
You might check with cones on each shelf

When my elements were getting old i noticed this happening to one of my glazes that had a lot of nep sys and gb.
You might try adding a bit more silica , maybe 5% and see if this helps.

Wyndham

#3 Bob Coyle

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Posted 28 June 2013 - 04:04 PM

Bob if you are talking about glaze blisters, I'd check if your kiln might be firing longer than what you expect and ending up hotter.

If one element is not getting hot enough the other zone might be way hotter to makeup the difference
You might check with cones on each shelf

When my elements were getting old i noticed this happening to one of my glazes that had a lot of nep sys and gb.
You might try adding a bit more silica , maybe 5% and see if this helps.

Wyndham


They are really not blisters, they are kind of what you would call fish eyes in spray painting. My best guess is that they are bubbles that broke but did not flow together. I have tried modification of the oxide ratios, and firing at a higher temp, but when I do this, the green breaking blue disappears or the glaze goes shiny. This also happens with a soak at the top.

The heck of it is, when I try a new formulation, the test tiles look good but some of pots don't . So I am kind of stuck on the ragged edge of an unstable glaze. I tried going up a cone and added Magnesium to get a magnesium matte, which worked but I lost the break. I use MR-5 clay and I think the break come because of the iron in the clay, but I am not sure.

#4 jrgpots

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Posted 28 June 2013 - 07:21 PM


Bob if you are talking about glaze blisters, I'd check if your kiln might be firing longer than what you expect and ending up hotter.

If one element is not getting hot enough the other zone might be way hotter to makeup the difference
You might check with cones on each shelf

When my elements were getting old i noticed this happening to one of my glazes that had a lot of nep sys and gb.
You might try adding a bit more silica , maybe 5% and see if this helps.

Wyndham


They are really not blisters, they are kind of what you would call fish eyes in spray painting. My best guess is that they are bubbles that broke but did not flow together. I have tried modification of the oxide ratios, and firing at a higher temp, but when I do this, the green breaking blue disappears or the glaze goes shiny. This also happens with a soak at the top.

The heck of it is, when I try a new formulation, the test tiles look good but some of pots don't . So I am kind of stuck on the ragged edge of an unstable glaze. I tried going up a cone and added Magnesium to get a magnesium matte, which worked but I lost the break. I use MR-5 clay and I think the break come because of the iron in the clay, but I am not sure.



Have you tried slowing down the rate of temp increase as it approaches cone 4 to 5. You may be have some trapping occurring at the surface of the glaze. A slower ramp may allow whatever is trapped under the top layer to come to the surface without producing the large fish eye effect.

It's just a guess. You may want to try it.

Jed

#5 Bob Coyle

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Posted 28 June 2013 - 08:00 PM



Bob if you are talking about glaze blisters, I'd check if your kiln might be firing longer than what you expect and ending up hotter.

If one element is not getting hot enough the other zone might be way hotter to makeup the difference
You might check with cones on each shelf

When my elements were getting old i noticed this happening to one of my glazes that had a lot of nep sys and gb.
You might try adding a bit more silica , maybe 5% and see if this helps.

Wyndham


They are really not blisters, they are kind of what you would call fish eyes in spray painting. My best guess is that they are bubbles that broke but did not flow together. I have tried modification of the oxide ratios, and firing at a higher temp, but when I do this, the green breaking blue disappears or the glaze goes shiny. This also happens with a soak at the top.

The heck of it is, when I try a new formulation, the test tiles look good but some of pots don't . So I am kind of stuck on the ragged edge of an unstable glaze. I tried going up a cone and added Magnesium to get a magnesium matte, which worked but I lost the break. I use MR-5 clay and I think the break come because of the iron in the clay, but I am not sure.



Have you tried slowing down the rate of temp increase as it approaches cone 4 to 5. You may be have some trapping occurring at the surface of the glaze. A slower ramp may allow whatever is trapped under the top layer to come to the surface without producing the large fish eye effect.

It's just a guess. You may want to try it.

Jed


The ramp starts to go non linear at about 1900 F my guess is that the average is about 100- 150F/hr. Maybe I'll try 75F/hr next time and see how it goes.

#6 Wyndham

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Posted 29 June 2013 - 10:29 AM

Bob you might be right on the underlying clay. You could try a slip over the clay to test your theory. If it's the clay, a longer or higher bisk might burn out the problem.
Wyndham

#7 Bob Coyle

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Posted 29 June 2013 - 01:23 PM

Boy! Lots more things to try... you know, the hell of it is, for about three years this was a stable glaze. It did what I wanted and was not too temp sensitive. The materials I am using are the same. Even the G.B.

I am a retired chemist...you would think that might help...it doesn't.

Glaze formulation is NOT chemistry it's alchemy.

#8 Diane Puckett

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Posted 29 June 2013 - 04:50 PM

I was having a similar problem, and it went away when I switched clays. Nothing else i tried worked, and i was having the problem with almost all my glazes. Maybe something has changed in the clay you have been using.
Diane Puckett
Dry Ridge Pottery

#9 Bob Coyle

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Posted 29 June 2013 - 05:12 PM

I was having a similar problem, and it went away when I switched clays. Nothing else i tried worked, and i was having the problem with almost all my glazes. Maybe something has changed in the clay you have been using.


That could be. I have been using Laguna MR5 for years. The clay body may have changed some. It's probably not the best for what I am doing, but my choices are limited by what the one and only ceramic supply in town will stock. I would have to special order other clays, and that is too expensive.

#10 Wyndham

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Posted 30 June 2013 - 09:33 AM

Laguna as well as other clay mfg's will change ingredients sometime for cost and availability. The ball clays sometime change because of different strata and some things like coal pockets or seams that get mixed in.
That's why the longer bisk or higher bisk to help burn out the organics.

One of my clays I no longer use had grog from old fire bricks that were from furnaces that had some slag and other stuff that caused the same problem.
Wyndham

#11 Bob Coyle

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Posted 30 June 2013 - 10:27 AM

Laguna as well as other clay mfg's will change ingredients sometime for cost and availability. The ball clays sometime change because of different strata and some things like coal pockets or seams that get mixed in.
That's why the longer bisk or higher bisk to help burn out the organics.

One of my clays I no longer use had grog from old fire bricks that were from furnaces that had some slag and other stuff that caused the same problem.
Wyndham


I'll give a longer bisk a try... thanks for the info.




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