Thursday, May 6, 2010

Gearing up for another firing

First off thanks to everybody who made it out to our gallery during the day, and everyone who made it for the music and s'mores afterwards. We were quite happy that our first annual Clay in May was such a success.
We've decided to have another firing in early June to fully stock up our gallery after Clay in May. The firing is only a month away. Christy is working full time for the next couple of weeks (as she has been for the last couple of months) before she can begin making pots, and I'm working part time helping with the construction of a friends workshop/studio. So needless to say this won't be a full load. I'm figuring we'll just fire the first two chambers at most. Which is good as there will be a lot of new variables for this next firing and it's nice not to have too much invested in it. What are the new variables you ask?
Well new clay for starters:

We've already discussed this one else where, and I'm pretty confident this new batch of clay will be good. We are trying a new clay drying system as well. Before we had all our drying troughs outdoors. We've decided to bring them all in our barn and stack them up to promote slower drying. Before we would have to pull clay up at least twice a day or risk letting the clay getting too dry to work with. This new system is drying quite a bit slower and should dry more evenly as a result.

We're also hoping to make some minor adjustment to the kiln:
The spy holes in the first chamber have never been as tight as we would like. This is due mainly to the fact that such thick mortar joints were used in building the first chamber that a regular 2.5" brick ends up with quite a gap above it when it is cut up for a plug. So we've decided to make our own spy hole plugs and receptacles inspired by Michael Kline's spy holes and our own stoke hole covers. The receptacles will be embedded in the cob surrounding the spy hole, with the hopes that the ring around the receptacle will keep in from pulling out of the cob. We've also had some trouble with kiln junk falling onto the pots below when the top spy hole was opened in the first chamber. Hopefully this will address that problem as well.
The biggest change though to our kiln though will come from those small clay squares. We're going to use those to replace the brick spacers that are in between the grates now. There by tightening up our grate system . I would like to keep more of the embers up on top of the grates where they will be able to contribute more to the ash that lands on the pots and the charring effects that we like so much on the pots in the front stack. I tried to be scientific about how much I could close the grates without effecting the air flow too much. I read an excerpt of Gary Hatcher's article "Chariots of Fire: Principles of a Bourry Box Kiln" (I should really track down the whole article sometime). Gary mentions that at top temperature in a wood kiln gas expands to approximately 13 times it's size. So I figured out from our regular one brick air inlet, the size of opening that would be needed to allow all the air from the main air inlet through the grate system after expansion at cone 11. Long story short: for proper expansion the grates were just right the way they were before. So I just guessed at a good size for spacing. I doubt that it gets that hot below the grates, certainly not cone 11-12. The grates were spaced 1.25" apart. I rolled out the slab for the spacers at .875" (7/8"). I don't think our "kiln" clay which is epk, kyanite, and grog will have much shrinkage. We'll see how it all works out. Certainly we'll have more embers above the grates. Hopefully it will work out well.

Last variable:
Glazes. After our last round of intensive testing we surprised to find out how much different some of our glazes were out of the wood kiln than they were in our little test kiln. What we thought was going to be a nice Tenmoku/oil spot glazes came out like this in cooler areas of our kiln:

and like this in areas that got more ash and heat:

Quite a bit different than we expected. Well I've done some research and it looks like the most likely culprit to the changes is the magnesium crystallizing. Apparently without alumina the magnesium will start to crystallize during slower cooling cycles. A slow cooling cycle is something that is difficult to replicate in our little soft brick test kiln. In the making of this new glaze we used local red clay instead of the ball clay that we would have used otherwise. We'll try adding some kaolin to increase our levels of alumina.
We also tried to achieve a matte yellow glaze with little success. We got a nice yellow color but it would always be to runny. In removing some of the calcium to reduce the runniness we would loose the yellow as the calcium tends to bleach iron glazes from green to yellow. We ran our selves in circles trying to get a glaze that was both matte and yellow to no avail. With a little distance and time to think about it I've come to the conclusion that what we should have done when the glaze was yellow but too runny is to add more calcium and magnesium. As a glaze that is saturated with calcium and magnesium it should become more matte right? Time and testing will tell.
hmm... hopefully people have found my technical pondering/rambling mildly interesting. If anybody has any insights or suggestions I'm all ears.

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