Sporadic blistering on some kiln loads?

[katkat1000]

I've been having some issues with blistering and I can't seem to figure out why. For background, all of my work uses the same Kristy Lombard clay (by Georgies), Amaco Velvet underglazes, and a gloss glaze that I mix myself. Even with every variable seemingly being the exact same, in about half of my firings all of my pots have small blisters all over. Could it be caused by the kiln? My community studio fires my work, so I don't know how they do it or have any control over it. 

Do any of you know what might be causing this? Or how do I fix it? 

I've attached some pictures of the blistering and my gloss glaze recipe if that's helpful. Thanks!

[Bill Kielb]

While the glaze recipe looks decent, and you have successfully fired with it, not sure there is a need for the bentonite as you have 15% clay. First question that comes to mind is do we know their bisque schedule, how long they fire and anything about their glaze firing schedule?

[Min]

First off I would ask if you have tested this glaze for fit and absorption. Your glaze recipe expansion is a bit on the high side, what this means is there is a possibility of crazing, if not straight from the kiln then delayed crazing. Some people don't mind crazing, a lot do. If the studio where you get your work fired is firing on the cool side of cone 6 then the likelihood of crazing and or the absorption of the clay would increase versus if the clay is fired to a full cone 6. Ideally you want the absorption under approx 2% for functional pots so the pots don't weep/leak/absorb moisture.

Link here to test for absorption and crazing here if you need it. 

If you are happy with the clay after doing your own tests on it then I would be looking at the whiting (calcium carbonate) in the recipe which is supplying the calcium. If the studio is firing to a cool ^6 then it might be worth a try to reduce the amount the glaze gasses off during firing, this is called loss on ignition (LOI). Easiest way to do this with your glaze is to supply the calcium with wollastonite instead of whiting. Wollastonite is basically a frit made by nature of calcium + silica but it has a far lower LOI than whiting. In addition to this the same formula made with wollastonite vs whiting is going to melt earlier in the firing than the whiting version. 

I redid your recipe subbing wollastonite for the whiting and balanced all oxides. If you land up trying it just round off the numbers to the nearest tenth for a small test batch. If you have a look at the "Ratios" column you can see the LOI went down from 9.0 to 2.6  I left the bentonite out of both recipes but I think it's fine to add 2% to help the epk keep the frit an nepheline syenite suspended.

edit: As both under firing and over firing can cause glaze blisters I would also ask if the studio uses cones in different areas of the kiln to verify the firing. 

 

[neilestrick]

Are you covering the piece with white underglaze? Is it only happening on the rims?

[Roberta12]

Is it the grog coming through??  I couldn't tell from the pic.  The description says it is a 200 mesh grog.   Just a thought.  

[oldlady]

thanks, min, for the info on wollastonite and whiting relationship.  

[Babs]

How are you glazing?

Are the rims double dipped by chance? Looks like a few defects have occurred where blobs are.

Has it been sieved properly? 

Glaze looks thick