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Low fire commercial glaze blistering frustrations

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Hello all,

My first time posting here, but I am at wit's end! Problem should not be this complicated. I'd appreciate any advice: Blistering of  Amaco LG-10 clear glaze used on Amaco Sedona Red Clay body  on interior of 5 1/2" diam bowls.  Exterior worked  fine using layered Amaco Artist's choice Exotic Blue, Seafoam Green, a band of clear. 

Will try to summarize scenario and conditions :

1. Electric Kiln in the damp basement. No vent.

2.. Bisque fired (tightly packed kiln!) ^04

3. Wiped interior and exterior with damp sponge, holding base of bowl. 

3. Glaze poured into interior, brushed on exterior. 

4. 4 bowls on each shelf on  three levels of shelves.  Bottom shelf was about 1.5" above tops of bowls,

5. Subsequent shelves less than 1" above bowls.

6. Fired to ^05 at preprogrammed medium speed (8.5 hours)

6. Blistering occurred at all shelf levels and equally on insides of bowls.

7. touched up glaze, Re fired at ^05 on Slow speed with a 10 min hold (13.5 hours)

8. STILL BLISTERING - Maybe  fewer blisters than originally, and some dark spots and indents where maybe a blister burst and settled out. 


Please advise at your earliest convenience!!!5a73540b77d99_Refiredbowl.jpg.30b1edcf9f5653301ca10a1dd53f5edb.jpg

Edited by joanbrady

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Couple questions about your bisque firing. How long was your bisque firing and did you leave the peep holes open? Pinholes and blisters are sometimes caused by gassing out of the body during a glaze firing what should have gassed out during a bisque. Having a tightly packed unvented kiln would contribute to this, as would bowls that are nested together or bisque fired rim to rim. 

Do you have good ventilation in the room where your kiln is firing? Not healthy to fire indoors without venting.

Welcome to the forums!

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I have run into similar issues, in the past.  I never fully investigated the issue, as my knowledge and experience were far less than they are now.

If I had to guess, my problem was an issue with glaze thickness, and the kiln firing unevenly.  It was a computer controlled kiln, but the firing was "iffy".  With my current kiln, I have no issues with the same clay body and glazes/ glaze combinations.

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Thanks for the quick replies!

To answer your questions: Min- Bisque fire was 8.5 hours. Nerd, Clay body is a nice red earthenware Sedona Red, sold by Dick Blick.

I left two peep holes open. Propped lid to 1000degrees. Then  closed lid for remainder. I did run a box fan on a chair blowing away from the kiln while lid was propped. An old house but fairly tight to outside air. At one point I opened the door about 4" (It is FREEZING cold up here on an island in Maine!) for a while. 

I think you may be onto some things. The kiln might have been just too full to allow gas to escape especially in light of not having ventilation.  (I used to love the downdraft I had installed at the high school where I used to teach). I think I packed 36 bowls  into the bisque kiln- nested those bowls three deep.

Another possibility  you might have hit on is too thick glaze application. Normally I brush it on. Label says it's pourable, but it's very thick and in retrospect maybe it should have been thinned in order to pour. 

Finally,, Benzine, I don't think a new kiln is an option for me right now!  But thanks-  I'm lucky to have one out here on this island! It was l"oaned"  to me by a wonderful, generous potter-turned-painter friend. I did replace the computer panel about 8 months ago, Maybe I could replace the thermocoupler? 

Your replies have actually caused a feeling of hope when I thought there was none!

Appreciate it...


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One of the things that is off gassed from firing a kiln is carbon monoxide, as I’m sure you know it’s both odourless and deadly. Updraft, downdraft or if you have to as a last resort, a cross draft with lots of air exchange in the room, one way or another I would address your ventilation. I would also suggest installing a carbon monoxide detector and another in the living area of your home.

Are you using witness cones? If they are correlating with your controller end point then there is no need to change the thermocouple. You usually get some drift in the thermocouple before they really go. 

Commercial brush on glazes can be poured or used for dipping but they need to be adjusted. Amaco sells a product called SuspendAid which is added with water to brushing glazes to convert them to brushing/dipping glazes. Might be able to just water it down though.

Pinholes and blisters can be super frustrating to figure out but my go to is to look at the bisque firing. Glaze firings can generally go pretty quickly but don’t rush the bisque. I would leave all the peeps open up to 1200, leave the top one open for the entire bisque and close it up when firing is done. Since you aren't venting the kiln try stacking it looser. As you are using commercial glaze we can't look at the formula for that but if you have a white clay I would make up some test tiles of both that,  and the Sedona Red and apply 1 coat of glaze over the entire tile, 2 coats over 2/3rds of it and 3 coats over the last 1/3 of it and include them in your next glaze firing. I would use horizontal flat tiles as pinholes and blisters on those don't have the benefit of gravity to help heal them over. (worst case scenario) This test will tell you a couple things, effect of glaze thickness in regards to developing blisters or if the clay is the problem.

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Need to start wearing my glasses when reading the forum. You mentioned Sedona Red clay, with clear glaze in your original post. After enlarging it,  I am even more persuaded that the off- gassing is sulfide related. Red bodied clays are high in iron5-7% typically; and that comes in the form of iron sulfide. You have large blisters that collapsed, and others that the surrounding glaze is swelling upward around the hole itself. Off-gassing sulfur trioxide will produce that kind of pressure while escaping. Try min's recommendations first, but you may have to approach this from a different angle. It is not the carbon in red clay that creates the big problem, but rather sulfides.


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