Tuesday, November 6, 2012

Wood Firing Questions

 The kiln firing went well, but there are definitely some noticeable kinks I need to work out. Anyone with wood-firing experience, I would greatly appreciate your input. I'm thinking to switch up my stacks - put the larger set of shelves in the back where there seems to get more heat and ash and the shorter stack in the front on the side. I'm afraid to put glazes on the pieces in the front corner since this has been my coolest area because the ash will cling to the glaze and ruin the pots.
Brown ash run that was a little matte
The last time I fired the heavily salted mugs had a green surface. This time around the ash runs had a brown color that was a little matte. The salt I used this time was cattle salt from the feed store, but is supposed to be pure salt. So I wonder whether my using sea salt in the last couple of firings made any difference. Has anyone else noticed a difference in their salt choices? Table salt vs. cattle salt? I opted for the cattle salt because it was cheaper and came in large amounts. That, and it doesn't have scary additives like prussiate of soda which turns into cyanide. 
Coating on the ash on the shelf

Ash settled in the bottom of an ash-glazed dish that did not flux out or melt

Dry ash on the edge of a decorated dish
The ash is still piling up even though I brought the shelves several inches away from the door and the wall with both sets of shelves. The biggest problem I have been having is that the ash doesn't melt down in all places, and it is settling as dry ash on some of my glazed surfaces. I'm afraid that crash cooling the kiln has been having this effect on the ash as it is not giving it enough time to heat up and soak. This most recent firing cooled as slowly as the other firings (about 2 days).
Spotting in the glaze

Glaze separation/split in the bottom of a dish
The other issue is that my ash glaze liner has been appearing to be thin (but I have skimmed water and it goes on thick) and in this firing showed spots from reduction or cooling and the glaze split in the base of a few pieces - not the clay, just the glaze. If I'm only firing to cone 10/11 should I not use an ash glaze as a liner? Do ash glazes inherently need a higher temperature in order to truly flux out?

So here are my questions:
Should I salt earlier in the firing and then soak the kiln to cone 11 in order to flux out the ash?
Should I not crash cool the kiln?
Should I insulate the kiln even more in order for it to take longer to cool (more than 2 days)?
Why is the ash not melting out on the glazes or remaining in dry patches on pots?
Should I use a different liner glaze? If so, does anyone have any recommendations that isn't an ash base for single-firing?


Michael Mahan said...

Hey Brenda. I find a good liner glaze is 100 percent Alberta slip. My last firing was the most even firing I've had, no dry ash on pots. I had a lot of dry ash on pots at the back of the kiln for the first few firings, until I learned to get the temperature up in the back. For me, a longer soak at high temperature helps with melting all the ash. Also, moving the first shelves farther away from the back of the kiln (chimney) seemed to help with bringing the back temperature up.

Dan Finnegan said...

Hi Brenda,
Your 'cattle salt' might not be as pure as you think if it's a 3rd of the price. I use kosher salt or any other salt without iodine, but I've never seen the kind of salt make a difference. Salt is cheap, you can't be saving THAT much money buying cattle salt.
I use shinos as my liner liner glazes, but ash glazes are also fine. Sometimes I've had trouble with ash from the fire collecting in a thin ash glaze and not melting well before.What is the recipe that you are using?
I salt little bits over a long period, with a short stoking time after salting. The unmelted ash on the shelf is exactly that...more heat and it will melt nicely to the shelf!
2 days is plenty for cooling... I think that after a good soak at top temperature it should be fine if followed by a crash cool. Just don't stir the ashpit near the end.
You can always refire in an electric kiln. I sometimes refire to cone 6 with amazing results!

Liberty Stoneware said...

Thanks Michael and Dan, greatly appreciate the input. I talked to Joseph Sand for a little while about it yesterday as well. I think I am going to salt a little earlier (cone 8) and then soak through cone 10/11 and try to get the whole kiln even. I don't have the ash liner glaze recipe on hand, but I am going to modify it a bit with some additional lime, feldspar, and ash to help it mature at a lower temp. Michael, is Alberta slip still mined? I thought like Albany it is done. I have been using the A&K Clay Company's Ohio slip which has been really nice in my other pots. I think for the Christmas mixing bowls they are just going to have to have a brown liner. I just picked up 150 pounds of Morton salt with no iodine or anti-caking agents yesterday. The cattle salt was 80 pounds for $11, compared to the $6.95 I just paid each for 25 pound bags, perhaps it's not a huge difference, but I thought I would give it a try since it was a little better. Here's to hoping #4 works out well! Thanks again!