Raku Pots and Cones Blistering

[Stu Thompson]

I have been firing raku for approx. 45 yrs. and have encountered a problem never before experienced. Most of the pots retrieved from the latest firing were blistered. The kiln was brought to temp. fairly slowly--2+ hrs.--and to cone 05-04. Even more surprising the 05 and 04 cones themselves, while fully deformed, were also both blistered and bloated. At no time during the firing did there seem to be evidence of excessive reduction. Clearly the kiln atmosphere became quite funky. The glazes I used were a combination of commercial low-fire and my own mix of 3% copper carbonate copper glaze, the same stuff I've used quite successfully in the past. The only variable from previous firings was that I had just repaired some Fiberfrax in the firing chamber (mice problem), though I am hard pressed to believe that the binder in the QF-180 coating cement could have negatively affected the atmosphere. Suggestions?

[Pres]

What about the clay body? Recycled, new clay, old clay? Sometimes the clay body had been changed without your knowing, and crazy things happen.

 

 

bst,

Pres

[Stu Thompson]

Two clay bodies were used, both commercial and new: an off-white, coarse grog raku body, and a light brown, fine grog earthenware body. The blistering seemed to occur more or less equally on both bodies. Oddly, a slab, 16 x 20," fired on edge, was not negatively affected. Both the commercial glazes and my copper glaze worked perfectly.

[liambesaw]

On the right side of this cone chart it mentions firing too fast will cause cones to bloat or blister.

[Babs]

Are  you using cones to tell when to shut of kiln or a thermcouple pyrometer?

Could be fast overfired pot

Slow towards end of fire and a soak to settle glazes

[Stu Thompson]

Thanks to all who have responded to my little tragedy. And in particular to Babs and liambesaw. As I look at things it makes sense that we had a case of a too fast firing. (The down side, I suppose, to a very efficient firing chamber.) My millions of previous firings came to temp in the 3 to 3 1/2 hr. range. This, as I said was approx. 2 1/4 hrs. to temp. First firing of the summer; guess I was a bit too anxious. That additional hr. is obviously quite important. I'll just use that extra time to have another beer.

Mud rules.

[Babs]

Yeh glazes need temp and heat work to mature nicely.

Good luck with your next firing.

This is a slow down time mate.

Think about that beer for a while before action.....taste better.