Blistering on Stoneware

I have been struggling with this problem for about a year now. Can
anyone please help with any suggestions or comments? I am getting a
variety of blister sizes. The areas affected seem to be verticle
surfaces, the underside of plate rims, edges of plates, but
interestingly, not the rims of taller vessels.I am using a clear bright
glaze recipe which melts totally. This is what I have found so far...

1. I may have started experiencing this problem when I reduced the
whiting content in my glaze by around 3-5%. I don't have enough
documention on this; it's just one possibilty I need to test. I was
glaze firing to ^6, bisque to ^04.

2. I discovered in one glaze fire that the cooler shelves had
significantly less blisters, so I started doing all the glaze fire to
^5. This reduced the incidents significantly. Still firing bisque to
^04.

3. My large witness cones placed in the center of each shelf typically
read one cone less than the junior cone in the kiln sitter. Bottom-most
shelves maybe yet another half cone cooler.

4. Bubbles in the glaze application don't seem to factor at all. The
water soaking and deflocculating of the glaze related to this seems not
a factor either. This is because these techniques just seem to result
in a thinner glaze application, which does, in fact reduce the problem.
However, this doesn't really help me because I am going for a very
thick application.


5. I have not been able to reproduce this problem on test tiles except
in one special case. I suspected that I was contaminating the clay body
with plaster from my wedging board. I wedged bits of this plaster into
a large test tile, bisqued, then glazed, and voila, got beautful
blisters. Until I find a better surface, I am doing all my wedging on
canvas. Sadly, I am still experienceing this problem on accasion from
plaster-free wares.

6. I am also suspecting contamination from kiln shelves; kiln-wash
powder dropping onto the wares. Another possibilty I need to thorougly
test.

7. I don't have a digital controller, so I can't really do a soak which
is another possible solution.

8. I am just beginning to venture into bisq at higher temps, such as
^03. Can't say anything about this.

9. Try looking at your pinholes with a magnifying glass. See if there
are any foreign particles in the center.

If you have any ideas or comments, I would very much appreciate it.

I'm still learning, but I solved my blistering problems with a 20 minute
soak at ^6 and then a 10 minute soak at 100 degrees lower on the way down.
I guess it gives the gases time to get out. No blisters since then. I do
have an electronic control, thank heavens.

Tom


"Davis Salks" wrote in message
oups.com...
I have been struggling with this problem for about a year now. Can
anyone please help with any suggestions or comments? I am getting a
variety of blister sizes. The areas affected seem to be verticle
surfaces, the underside of plate rims, edges of plates, but
interestingly, not the rims of taller vessels.I am using a clear bright
glaze recipe which melts totally. This is what I have found so far...

1. I may have started experiencing this problem when I reduced the
whiting content in my glaze by around 3-5%. I don't have enough
documention on this; it's just one possibilty I need to test. I was
glaze firing to ^6, bisque to ^04.

2. I discovered in one glaze fire that the cooler shelves had
significantly less blisters, so I started doing all the glaze fire to
^5. This reduced the incidents significantly. Still firing bisque to
^04.

3. My large witness cones placed in the center of each shelf typically
read one cone less than the junior cone in the kiln sitter. Bottom-most
shelves maybe yet another half cone cooler.

4. Bubbles in the glaze application don't seem to factor at all. The
water soaking and deflocculating of the glaze related to this seems not
a factor either. This is because these techniques just seem to result
in a thinner glaze application, which does, in fact reduce the problem.
However, this doesn't really help me because I am going for a very
thick application.


5. I have not been able to reproduce this problem on test tiles except
in one special case. I suspected that I was contaminating the clay body
with plaster from my wedging board. I wedged bits of this plaster into
a large test tile, bisqued, then glazed, and voila, got beautful
blisters. Until I find a better surface, I am doing all my wedging on
canvas. Sadly, I am still experienceing this problem on accasion from
plaster-free wares.

6. I am also suspecting contamination from kiln shelves; kiln-wash
powder dropping onto the wares. Another possibilty I need to thorougly
test.

7. I don't have a digital controller, so I can't really do a soak which
is another possible solution.

8. I am just beginning to venture into bisq at higher temps, such as
^03. Can't say anything about this.

9. Try looking at your pinholes with a magnifying glass. See if there
are any foreign particles in the center.

If you have any ideas or comments, I would very much appreciate it.

Tom wrote:
I'm still learning, but I solved my blistering problems with a 20 minute
soak at ^6 and then a 10 minute soak at 100 degrees lower on the way down.
I guess it gives the gases time to get out. No blisters since then. I do
have an electronic control, thank heavens.


I'm still learning, too, after 30 years, and am having a similar problem
with a gloss glaze at cone 10--blistering on undersides of plates. What
I think is happening is that the shelves retain heat longer, and heat
the pot bottoms more. In some instances what Tom says might apply.

Firing cooler would help prevent the glaze from boiling, or try
reducing the whiting or other flux.
I've found it helpful to brush on a bit of more refractory glaze in the
areas likely to be affected before dipping the pot in the glaze which
causes it.
I've also found that it's likelier to happen when the glaze gets used up
closer to the bottom of the bucket (at which point the formula may be
changed from being more of the heavier constituents). In this case
mixing up a fresh batch can help.
Also, unless this glaze you're using has some special properties, you
might do better to change to a different recipe with some different
materials--some glazes can be too fussy to keep working with...
I think I may be facing that myself.
Raw materials change from shipment to shipment--so a once reliable glaze
can go belly up.
Brad Sondahl

--
For my pottery how-to videos, original art, music, pottery, and
literature, visit my homepage
http://sondahl.com

I've found it helpful to brush on a bit of more refractory glaze in the
areas likely to be affected before dipping the pot in the glaze which causes it.

rafractory glaze: Are you suggesting the same glaze but with the
addition of refractory materials?

...you might do better to change to a different recipe with some different materials...

This is worth considering. It would certainly be easier than trying to
analyse the effects of different firing temps. Thanks.

am having a similar problem with a gloss glaze at cone 10--blistering
on undersides of plates

I'm thinking that it may be a combination of glaze thickiness and
proximity to radiant heat source (such as the elements or, as you
suggest, reflection or radiation from the shelves).

When glazing, I dip pots upside-down and allow them to drain this way.
There is always a heavy layer around the rim. Although the rims tend to
drain off pretty well in the fire, the are beneath them tend to have
the heaviest layer if glaze.

Althought his may also be true of the lining, I don't get blistering in
here, and that may be because the lining doesn't get any direct radiant
heat.

but I solved my blistering problems with a 20 minute soak at ^6
and then a 10 minute soak at 100 degrees lower on the way down

I am tending more and more toward this solution. My concern with this
is: Soaking would allow formed bubbles to break and heal. However, what
if the glaze continues to boil at the soaking temp?

Sorry. I just realized that you are soaking at 100 degrees lower in
your cooling cycle. This would answer the concern about boiling during
the first soaking,

Sorry. I just realized that you are soaking at 100 degrees lower in
your cooling cycle. This would answer the concern about boiling during
the first soaking,

Then try to soak at a lower temp. That's just a guess, of course. It's all
a learning process and experimentation. I'm just now getting into mixing my
own glazes. So far so good.


"Davis Salks" wrote in message
ups.com...
but I solved my blistering problems with a 20 minute soak at ^6
and then a 10 minute soak at 100 degrees lower on the way down

I am tending more and more toward this solution. My concern with this
is: Soaking would allow formed bubbles to break and heal. However, what
if the glaze continues to boil at the soaking temp?

Davis Salks wrote:
I've found it helpful to brush on a bit of more refractory glaze in the
areas likely to be affected before dipping the pot in the glaze which causes it.


rafractory glaze: Are you suggesting the same glaze but with the
addition of refractory materials?

No, I actually meant a glaze you have which doesn't exhibit the problems
cited--such as a white or clear glaze.
Brad

--
For my pottery how-to videos, original art, music, pottery, and
literature, visit my homepage
http://sondahl.com