Can anyone tell me what causes bubbles in my glaze?

Portland, OR

I am a ceramics newbie and my first glaze left lots of bubbles on some of the pieces.
I fired 25 identical (for the most part) pieces and the pieces varied from zero bubbles to one piece with a dozen or so.
Could it be that I didn't fire the bisque hot enough? Or is the temperature for the glaze all wrong. The bottle said to fire the glaze to cone 06 but all I had was 07 cones. I also fired the bisque to 07 (but I do have 03 cones for that)
Using a kiln sitter so the cones are sitter cones.
I don't have any witness cones.

P.S. This was my very first time with clay and glaze so I'm not too surprised that I am having problems.

Also, is there a more active ceramics forum out there somewhere?

Thanks

Minneapolis, MN(Zone 4b)

Welcome to the group. If there is a more active group I haven't found it. We are getting new members slowly, but since the forum is relative new it takes a while to build up.

Bubbles- there may be a contaminate in the glaze, there may have been dust on the pieces (make sure to rub the work with a damp sponge to get any dust off.), the glaze may not have been well enough mixed, I have experienced the problem with too high heat but not to low. You should get some 06 cones and dust the next pieces off to check it again. Try some other glazes at the same time to see what results you get.

Did you find the the pieces in one area of the kiln had more bubbles and in another? I wouldn't think the firing of the bisque would be the problem. I usually bisque at 06.

Delray Beach, FL(Zone 10a)

Here is my theory. I could very well be wrong and I will live with it if I am. I'm just trying to help here.

Did you sieve your glaze before applying it to the pot? Whenever I used a glaze that had been sitting around for a while, I would run it through a 200 or 400-mesh sieve, working the lumps through the sieve with a 4-inch paint brush. If the flux was not properly and evenly distributed within the glaze itself, it would cause certain areas of the glaze to become overly vitrified which may have caused the bubbling. The bubbles can be ground down using an electrical grinder. It's then a matter of reglazing the piece and refiring it. I hope this helps.

Best of luck with tracking down your culprit.
Sylvain.

Delray Beach, FL(Zone 10a)

Although I fired stoneware at cone 6 and porcelain at cone 9, I have never had bubble problems. Zenpotter's observation correlating the location of the pieces within the kiln with the amount of bubbling is interesting. I also thought about the temperature having something to do with those bubbles.

Another possible problem could have something to do with the glaze itself. I gather you purchase your glaze. There is nothing wrong with that. I used to mix my own, 5 gallons at a time. I remember using a very fine sieve (200 or 400 mesh, I forget) to sieve the glaze before applying it to my pots. One day, a friend of mine needed a clear glaze and I let her use my 2-gallon bucket. It turns out she only stirred the glaze with a paint stirrer and applied it, without sieving although I had warned her about not sieving. The result was horrendous. Gobs of white stuff showed up under the vitrified transparent glaze. She lost her production and I lost a bucket of glaze.

I hope this helps.
Sylvain.

Stafford, VA(Zone 7a)

I was going to ask....are your glazes pre mixed or did you make and mix your own?

Deb

Gilroy (Sunset Z14), CA(Zone 9a)

We've had a lively discussion about glaze bubbling in another thread, which unfortunately I can't find at the moment. I asked my instructor about it, and she said she typically "soaks" a load for at least 10 minutes when it comes up to temperature, which she says helps to eliminate bubbling in glazes that are prone to it (not all are).

"Soaking" means the kiln sits with the power on and remains at the same temperature for a period of time. This will help compensate for cool spots in the load, which can occur due to placement in the kiln, size of pieces, etc.

I have a few questions:

What kind of clay are you working with?
What cone did you fire your bisque to?
If low-fire glazes are applied too thickly, I have also seen them bubble. Are you dipping or brushing?
How fast the kiln temperature rises up can also impact the glazes. What was your firing cycle like?

As for other forums about ceramics, there are many available. Clayart is a good one, but it's often a challenge to find specifics. Many of the sites I use are quite technical and the terminology might be confusing to beginners. Although we all came to DG initially because we're gardeners, we found that a substantial number of us were also interested in clay. We requested our own forum about 6 months ago so we could share with other mud lovers. Some of us have worked with clay for over 30 years, while others are taking their first steps. It's been a great exchange for all of us, and we hope you'll stay with us!!

Churubusco, IN(Zone 5b)

Ya, I was the other unfortunate one with bubbles / blisters. I have found several causes and also found that if I only have a few I can usually refire the piece and get rid of them.
#1 Brought the temp up too fast
#2 Didn't rinse the pieces prior to glazing
#3 Used a cone 05 glaze at a cone 6 temp :(

Portland, OR

Thanks for all the replies.

Looking at the suggestions above I think I have figured out the problem(s).
I am using a store-bought glaze. It's a Duncan Envision glaze (IN 1036 to be exact). I prefired the bisque to 07 cone and the glaze says to fire at 06 (07 should be close enough). I'm sure that the temp isn't rated that exact and even 100 degrees of leeway would be acceptable to expect.
The clay is "play clay" from Georgies Ceramics ( http://georgies.com/ ). It's most likely a low grade of clay but does seem to be an excellent quality.
I set the kiln at medium for an hour to ramp up and then set to high for the actual firing. I don't think, at this point, the firing is the problem unless it's because the bisque was still curing since I hadn't fired it to 04 cone.
What i think is the problem is that I didn't mix the glaze well enough prior to painting and/or I put it on too thick. It seems like the pieces with the thinner glaze did much better than the ones with the thicker glaze. Refiring the same glaze seems to have knocked a good deal of the bubbles down.
I noticed a small swirl on the surface of the glaze before painting it on which makes me think it isn't mixed well enough. I really didn't think all that much about mixing the glaze. I just shook the jar a few times and opened it.

This is just a silly little project that doesn't even have to be done at all. I can redo it easy enough and there are about half of the pieces that can be used since it's a giveaway, non-promo, item. I might just have to experiment a few more times.

Thanks again.

Stafford, VA(Zone 7a)

I have used quite a few of the Duncan Invision glazes with good success. The INvision GLazes like to be fired to 06.
Was your clay bisque fired before you glazed it? Some people will only fire it once anr fire it with glaze on it. IT can cause the piece to crack, explode or sometimes the glaze to bubble or peel off.
How many coats did you use?

deb

Portland, OR

Oh, I had my explosions. Those came when I fired from clay to bisque. I tried to rush it and learned.
I only used one thick coat of glaze which was probably the problem. It was really thick.

I did a test to see if I could fire from clay to glaze and it didn't turn out too bad but the definition of the stamp was affected.


Here is what I was making.

Thumbnail by bittsen
No. San Diego Co., CA(Zone 10b)

"Do not meddle in the affairs of dragons, for you are crunchy and taste good with ketchup."

Beautiful color.

Gilroy (Sunset Z14), CA(Zone 9a)

Beautiful stamp, too! My DH loves dragons....

With low-fire clay bodies, it's best to fire the bisque at least one cone hotter than the glaze. (That's different from high-firing.) Firing a low-fire clay at a lower bisque temperature than glaze might have left residual gases in your clay body that bubbled through the glaze during firing. I usually do all my bisque firing at 04, my low-fire glaze loads at 06, my mid-range glazes at 6 and my high-fire gas loads at 10.

Also, in the firing cycle it's best to start off with a low-temperature soak with the lid cracked about an inch and the spyholes open, to make sure all the moisture is baked out. Depending on the thickness of the pieces and the size of the load, that can take anywhere from an hour to maybe 3 hours. I don't close the lid completely until the internal temp has reached 400. I don't close the spyholes until an hour after that. Then I slowly bring the temperature up----I increase it slightly every hour until my dials are maxed out (usually around 1000 degrees).

Portland, OR

I'm going to retry it with a higher fired bisque and a thinner layer of glaze. I will also try ramping up the temp instead of going for the gusto.

I'm also going to dump the wax paper that I used to put the glazed pieces on before firing.

Thanks for the help and the compliments.

Gilroy (Sunset Z14), CA(Zone 9a)

The waxed paper burns up long before it would impact the firing, but it also doesn't really buy you anything. If you have glaze on the bottoms of your pieces, it will still be there when the paper is gone and your piece will stick to the shelf unless you have it stilted.

When using low-fire clay and glazes, it's best to glaze the bottom of the piece and stilt it, as low-fire clays are not blended to be fully vitrified at the cones they're typically fired to. The glaze will seal the piece and make it less likely to absorb moisture, and it will last longer.

After you shake up your glaze and make sure it's well-blended, check the thickness by dipping your finger in it. You should be able to just see the outline of your cuticle. I've found that glazes directly out of the bottle are almost always way too thick---but you get your money's-worth that way! After it's the right thickness, brush on three coats in opposite directions (letting the glaze dry to the touch between coats), or dip the piece once. I was also taught to rinse my piece and let it dry for 15-30 minutes before glazing. It gets the dust off, and also the piece doesn't absorb as much glaze so it's less likely to be too thick. But not everyone teaches that, so zen's recommendation of sponging may be a better solution.

Good luck, and let us know how it goes!

No. San Diego Co., CA(Zone 10b)

Lots of glazes we stir up from the bottom - and then shake well. (Those with crystals we shake first, put one coat on, then stir crystals up from the bottom for the next coat(s).)

I'm still a little confused about the hi-fire/lo-fire clay thing. We use a cone 10 or cone 6 clay (hi/med), but fire to 04 and 06 (bisque and glaze). So, it's hi-fire clay being lo-fired. What happens to it with the differential in temp?

And if we bisque a cone 10 clay, then glaze it at cone 6, what is the difference? I'm sure our teacher has gone over this, but it doesn't stick in my head.

Churubusco, IN(Zone 5b)

I know that when you bisque fire at a cone 10 the clay doesn't absorb the glaze near as much. Glaze kind of sits on the surface...if your brushing it on it takes longer because the glaze doesn't dry as fast between coats. Not sure what the long term effects are

No. San Diego Co., CA(Zone 10b)

Sorry, I wasn't clear, Shelley. I was wondering what happens if you bisque a cone 10 clay at 04, then glaze at 6 instead of 06, if there is a benefit (besides different glaze colors).

I guess what I'm getting at is why use cone 10 clay if you're only going to fire at 04/06? Does the clay fully develop? What are the benefits versus the disadvantages?

I'll probably not go above cone 5/6 at home to save power costs, but I'd like to understand what the differences are and it hasn't quite 'clicked' yet.

Portland, OR

Quote from KaperC :
Sorry, I wasn't clear, Shelley. I was wondering what happens if you bisque a cone 10 clay at 04, then glaze at 6 instead of 06, if there is a benefit (besides different glaze colors).

I guess what I'm getting at is why use cone 10 clay if you're only going to fire at 04/06? Does the clay fully develop? What are the benefits versus the disadvantages?

I'll probably not go above cone 5/6 at home to save power costs, but I'd like to understand what the differences are and it hasn't quite 'clicked' yet.


This one I could probably answer, just using logic ~LOL~

If you fire clay at 06 to turn it to bisque and then glaze with a much higher temperature, you risk any contaminates in the original clay vaporizing if it does so at a higher temp than 04. Firing bisque higher means there are no elementary changes to the clay (bisque) when glaze fired at a lower temperature. This would insure that any problems with the glaze comes from the glaze application, composition, etc than from extra gassing of the elements of the bisque.

Hope that made sense.

Portland, OR

Quote from imapigeon :
The waxed paper burns up long before it would impact the firing, but it also doesn't really buy you anything. If you have glaze on the bottoms of your pieces, it will still be there when the paper is gone and your piece will stick to the shelf unless you have it stilted.

When using low-fire clay and glazes, it's best to glaze the bottom of the piece and stilt it, as low-fire clays are not blended to be fully vitrified at the cones they're typically fired to. The glaze will seal the piece and make it less likely to absorb moisture, and it will last longer.

After you shake up your glaze and make sure it's well-blended, check the thickness by dipping your finger in it. You should be able to just see the outline of your cuticle. I've found that glazes directly out of the bottle are almost always way too thick---but you get your money's-worth that way! After it's the right thickness, brush on three coats in opposite directions (letting the glaze dry to the touch between coats), or dip the piece once. I was also taught to rinse my piece and let it dry for 15-30 minutes before glazing. It gets the dust off, and also the piece doesn't absorb as much glaze so it's less likely to be too thick. But not everyone teaches that, so zen's recommendation of sponging may be a better solution.

Good luck, and let us know how it goes!


Oh, I don't think I was clear on the use of the wax paper. I wasn't firing on the wax paper, I was just using it to protect my dining table (my studio) from anything and to keep any wet glaze from sticking to the table and peeling (chunking) when I picked the piece back up.
When I fired the pieces I put them on stilts that I made. Making 50 stilts took a little while.
I do wish there was a way to prevent stilt holes though.

Lots of advice here. I like the idea of removing contaminants before glazing. I was just brushing them and then blowing on them before applying glaze.

Thanks

Gilroy (Sunset Z14), CA(Zone 9a)

We cross-posted, bittsen! I can't even imagine making my own stilts---and yes, it sure would be nice if they didn't leave those obnoxious little sharp holes!

KaperC:

The key concept is to understand vitrification:
"vitrify: to convert into glass or a glassy substance by heat and fusion"

Most low-fire clay bodies (cone 04/02) are high in talc and low in silica. If you use a low-fire clay, you have to glaze all the surfaces or it will absorb water/moisture from the atmosphere because the clay will remain porous. This is an important consideration when making dishes or garden ware because any crack or chip in the glaze will allow moisture and bacteria to get into the piece.

Stoneware or porcelain bodies are higher in silica and will vitrify. However, there are "mid-range" (cone 5-6) and "high-fire" (cone 9/10) stoneware and porcelain clays.

If you fire a mid-range clay body to cone 5/6 it will vitrify completely and no longer absorb moisture. You can leave it unglazed if you want. If you fire that same clay body to cone 9/10, it may melt or sag because its molecular structure is being pushed beyond its formulated limits.

If you fire a high-fire cone 9/10 clay body to cone 5/6, it will not reach its designed vitrification point. It will remain somewhat porous (though considerably less than a low-fire body.) If you're going to do this, it's probably best to completely glaze functional ware on all surfaces to reduce its potential to absorb moisture. You can glaze the bottoms and stilt them, though---I've done it. You may encounter problems with the glaze not "fitting" the clay body as well as if you use a cone 6 clay body with a cone 6 glaze.

Does that help at all?


This message was edited Jan 1, 2010 3:12 PM

No. San Diego Co., CA(Zone 10b)

Quoting:
If you fire a mid-range clay body to cone 5/6 it will vitrify completely and no longer absorb moisture. You can leave it unglazed if you want.


This helps.

Quoting:
You may encounter problems with the glaze not "fitting" the clay body as well as if you use a cone 6 clay body with a cone 6 glaze.


??? Don't quite understand. Why would there be a problem with matching temps?

I don't think I've hit upon the right question yet, but I found some ref material from my supplier about shrinkage and absorption rates that will help, plus an article on choosing the right clay from Ceramic Arts Daily.

Thanks. I don't draw conclusions from data very well - 'ergo' is not in my vocabulary! LOL

Gilroy (Sunset Z14), CA(Zone 9a)

Quoting:
You may encounter problems with the glaze not "fitting" the clay body as well as if you use a cone 6 clay body with a cone 6 glaze.

Let me reword this. I didn't do a very good job before.

If you use a cone 6 clay body with a cone 6 glaze, you have a better chance of them "fitting" than if you use a cone 10 clay body with a cone 6 glaze.

There are never any guarantees, but typically all the glaze tests to declare a glaze "cone 16" or "cone 10" will be done on a clay body rated for the same temperature.

That being said, my instructor said she used a cone 6 oxidation glaze on a cone 10 clay body in a reduction firing and it came out gorgeous....

No. San Diego Co., CA(Zone 10b)

The other thing I just read was that the rating on the clay (e.g. cone 10) is the MAXimum temp for the clay - above that it will distort. Pays to read again. :-)

Thanks Ima.

Minneapolis, MN(Zone 4b)

I use a clay that matures between come 04 and 10. It really makes glazing easier.

Stafford, VA(Zone 7a)

If anyone gets CERAMICS MONTHLY Magazine. There is a long article on GLAZING.

eb

No. San Diego Co., CA(Zone 10b)

Mayco has some good reference material on their site. One booklet I found is "Ceramics 101," which has some good tips on glazing problems and what might cause them. http://www.maycocolors.com/catalogs.cfm

I also d/l each spec sheet on the different lines of glazes they have(Series 2000, Stroke & Coat, etc.) - there are lots of tips in the fine print that you won't find elsewhere, such as which glazes can go under or over another. I'm sure other brands have similar things available - the Orton site, for instance, has some very good info at http://www.ortonceramic.com/resources/

Stafford, VA(Zone 7a)

i agree Kaper! Mayco has grat info and I have printed them out and keep it handy.

deb

Portland, OR

Next batch is in the kiln.

Using the advice above I did the following.

Slow fired bisque to cone 03.
Washed all pieces before glazing.
Baked bisque in oven to remove any moisture from washing.
Used 3 thin coats of glaze instead of one heavy coat.
Glazed pieces putting them on aluminum foil instead of wax paper between coats.
Slow firing glaze to cone 7.

I will let you all know tomorrow what I find waiting in the kiln for me.

Thanks again for the help.

Portland, OR

I couldn't wait to post the results.

The new pieces look absolutely awesome!! They are gorgeous!!

Churubusco, IN(Zone 5b)

Pictures? :)

Minneapolis, MN(Zone 4b)

Great, awaiting pictures.

Stafford, VA(Zone 7a)

PICTURES!!!!!!!!!

Portland, OR

This is exactly the density, fill and finish I was hoping for.

Thumbnail by bittsen
Portland, OR

I put 50 "tokens" in the kiln and got 50 back out.
Much better than the 50% loss rate the first time I tried.

Thumbnail by bittsen
Minneapolis, MN(Zone 4b)

Great!!!!

Stafford, VA(Zone 7a)

Bittsen!
I LOVE THEM!!!!!! YOu did great
Which Glaze did you use again and what will you do with your Tokens?

deb

Portland, OR

Thanks
I used Duncan Envision glaze IN1036 (bluegrass)
It's a translucent (semi opaque) glaze that was picked to fill in the indentations and thin on the flats to get the two tone look that I did. I wouldn't change a thing. As I said, they turned out exactly like I had hoped.
I have a hobby that gives me a chance to leave the signature items all over the place. That's what I will do with them. Some people love signature items more than the actual hobby (geocaching).

Gilroy (Sunset Z14), CA(Zone 9a)

Those turned out just beautiful----perfect glaze choice! And I'm so glad our little team was able to help you with the bubbling problem. There are a lot of terrific folks here on DG!

No. San Diego Co., CA(Zone 10b)

Beautiful dragons! Folks will love finding those in the cache!

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