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Thursday, March 18, 2010
Unpredictable ash glaze!
Ok clubbers. Here's a query for you. I have been using the alkaline ash glaze pictured above for about two years now. Roughly a year ago, I started struggling with TWO of my ash glazes. They were both, all of a sudden, bubbling/boiling, pin holing and blistering. This mostly affected my plates and the interior of bowls. (For reference on the severity of the issue, I once lost all 22 plates I had in a firing to it.) I have tried just about everything to fix it! However, the really really perplexing thing is how absolutely RANDOM this blemish is. It does not happen strictly in the hot or cold spot, it's not strictly in oxidized or reduced spot, it's not strictly on the top or bottom of the kiln, it's not strictly on the left or right. HOWEVER, it can affect one plate on one side of a shelf, and not another plate on the other side of the same shelf! This is actually a frequent occurrence. All three plates on the shelf pictured above are the same glaze and as you can see, the one on the right really took one for the team. These are boiling clusters of bubbles. I thought this shelf of pots was a great example because the one in the middle has just a few on it and the one on the left was spotless.
So, I thought it was the clay and bisqued hotter and slower. Nothin. I thought it was a thickness of application. Nope. I thought it would help to refire. Uh uh. I thought it was an over firing problem and fired cooler. (www.sadtrombone.com). Also, for a while I really thought it was happening in the cooling because it seemed as though the plates with the shelves directly above them (meaning having a kiln post the height of the plate) got less bubbles. But, as you can see, the shelf with this affected plate has a bunch of space above the plate, but so does the shelf a few rows below it... and all three of those were great.
Now, I am pleased to say that the firing I unloaded yesterday (that these pics were from) was a huge success. Out of 70 plates in there (I was back ordered!) I had only 4 fail. What did I do, you ask? I washed my ash. Also, thanks to John Brits help, I have been lengthening the end of my firing each time and, this firing took 7 hours between cone 6 and cone 10 (11s were down in the front, as usual). In previous firings this lengthening tactic seemed to be a promising avenue to explore. Now, there is a chance that this firing was a fluke. It may have been packed tighter with all shelves and plates (therefore cooling slower?? Maybe??). But maybe not, only my next firing will tell.
However, the question still remains... what the heck happened to the pot on the right side of that shelf?! did it do some thing wrong in a former life? The other pots in the firing that bit it were in the back two stacks, at random heights in the kiln.
So, if you want, chew on that for a while and let me know what you think!