Bubbles, tiny bubbles in my glaze! Let’s talk about blistering.

Too bad he didn’t really have lumps on his head! Then it would have worked!

bumpy ted bumpy ted2Some of my pieces have been coming out of the kiln covered with tiny bubbles and bumps! Very aggravating! What could be causing it? The clay, the temperature, the glaze. Not everything comes out with them and sometimes the whole thing is covered and sometimes only half?

The clay- Porcelain Cone 6?

The glaze- speedball clear and Peacock?

the temperature- cone 6 ?

My kiln ?

Some answers by other people:

Blistering is a glaze defect similar to pinholing and pitting. As the glaze melts, gases vaporize out of the clay and glaze. In essence, blisters are the result of the glaze being frozen in the midst of boiling.

In most instances, this out-gassing is completed and the glaze flows back together to form a smooth surface by the time the kiln has reached temperature. However, too much gas or too viscous a glaze may interfere with all the gas being released in time for the glaze to smooth back out. In such cases, blistering is likely.

Reasons for Blistering

The most common reason for blistering is an overly thick application of glaze, especially if the glaze has pooled at the bottom of the pot. Firing too hot or too quickly at the end of the cycle can also contribute to glazes blistering.

Certain ceramic materials are also prone toward blistering. Lead is especially sensitive to atmosphere and any lack of oxygen in the kiln will cause it to gray out the colors, blacken, and blister. Borax, potassium carbonate (and carbonates in general), magnesium sulfate, soda ash, and fluorine can produce large amounts of out-gassing.

Solving Blistering


  • Begin by making certain greenware is properly bisqued at a high enough temperature (up to cone 04), eliminating those gases. Do not apply glaze too thickly. For one-coat dip glazing, bisqueware should be in the glaze for no longer than a three-count.
  • Make certain your firing schedule is not too fast. You may also want to extend the time of the soak (keeping the kiln at the maturation temperature for a time before shutting down). The soak can be extended up to a half hour in total.
  • Use frits instead of raw ingredients in your glaze, or try substituting other fluxes.
  • As a last option, try slightly increasing the glaze’s flux. However, if the problem is that resistant to correction your best bet may be to find a different glaze.

From Sandy:

This happened to me once with peacock,I think you need to get some cones.We have the same brand kiln, mine fires hot.  To get a perfect cone 6 I fire to cone 5 with a 5 minute hold, I only know this was necessary by using cones.You need cones 5, 6, 7 to do the test. To test cone 10 you need cones 9, 10, 11 . I have not had problems with the cone 6 porcelain and I have fired a lot of it recently. I think it over-fired. Might be worth a try of re-glazing & re-firing. Slow glazing may have upped your temperature a full cone! I think you do need to add more glaze… you need that stuff I have in the little bottle (something porcelain & stoneware enhancer) to make it stick. I would do a fast glaze cycle

Keep in mind that this may not help

 From Robert:

Can you crush the bumps and they break?  when you said raised bumps, I wondered if the problem is in the clay body….years ago I had some pieces that were in a community kiln and they were bisqued too low

the result showed up in the in the glaze firing-there were hundreds of bumps all over the pot– under the glaze, like air bubbles in the clay that raised up

the studio  manager raised the bisque temp from 09 to 06 and it never happened again

From Tory:

My experience.  Blisters are from overfiring the glaze.  Pinholes, its the clay.  I have using Highwaters Ellen buff and all of a sudden I started getting pinholes. Only with some glazes.  Usually the glossy ones.  Tried everything for two years fighting it, destroyed alot of pots.  Changed to Red Stone and no more pinholes.  I threw out about 300lbs of Ellen Buff.  Tried to call Highwater, but they were no help.  I tried all the suggestions and it did not work.

My answer:

I believe it was the clay I used. It is a cone 6 porcelain and the problem started when I started using this clay. I’m not sure yet but since only pieces that were made from this clay bubbled, I believe it to be the problem. The only thing that stumps me is that  not everything made with this clay bubbled. More than half did so I will remake everything and see what happens. Wish me luck! I’ll let you know how it turned out.



  1. Good luck! It is very disheartening to lose a whole kiln load of work. Let us know if you figure this one out! I have some cones if you need to borrow until you can get to Philly

  2. Hi there! I just completed short pottery course and all my clear glazed mugs have blisters and bubbles on them. Can I use these mugs for drinking from or do the blisters damage them? Should I just keep them as decorative pieces? Was so hoping I could use them!!

  3. Sara, I would be hesitate to use them because the bubbles could hold bacteria. That’s an official answer. But I don’t really worry that much about that so I personally would use them.

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