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Firing Log

ancient kiln | 21st century logbook

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July 21, 2006

Rockware Yakishime

Filed under: Clay Bodies,Pieces — odin @ 7:47 pm

The word “yakishime” refers to pottery that is fired without any applied glaze. Of course, natural ash glaze builds up on the pottery in different ways during a wood firing and thus, “yakishime” is a broad term covering many types of wood-fired pottery effects. One type of yakishime I’m quite interested in learning is hi-iro (“fire-color”) … or so I believe. The truth is, I’m beginning to doubt my desire to some degree because I keep doing things that are completely contrary to production of hi-iro. Perhaps I’m simply not ready for hi-iro yet.

Musings aside, before the last anagama firing I mixed up a batch of “clay” which I had hoped would develop colorful hi-rio (I say “clay” in quotes because it had very little actual clay in it). I must have been drunk or distracted because what I made was something destined to be glassy — and I knew that as I was dumping everything together — I just didn’t really think about it. Secondly, I did a two hour period of quick cooling at the end of the firing in which I used this clay. Quick cooling an anagama brings out the gloss and wipes out orange and red. As I mentioned — my actions say I want glassy shizenyu, not hi-iro.

CORRECTION — I mixed up my measurements here and after making this clay the other day, I wondered why it was so darn short and unthrowable compared to the first batch — looking back at my notes, I see halved the clay measurement and doubled the feldspar. That should make an interesting test!

Here’s the recipe. Take note however, all measures are volumetric — no scales were harmed in the making of this “clay”:

  • 40% 20% Custar Feldspar, crushed 10 mesh (not powder)
  • 40% silica
  • 20% 40% Lincoln Fireclay

As one might expect with all that glass making stuff, I didn’t get bright orange fire-color. Regardless, this clay is yummy. The anagama gave me a shizenyu piece that ranges from steel-blue to aquamarine. The glaze surface is exceptionally deep and three-dimensional. Some of the feldspar granules were too tough to melt and the white speckles give it a coarsely polished-rock sensibility — sometimes sharp, sometimes smooth. I love its obsidian rock-like qualities.

On the downside, throwing this clay is like throwing knives — not in the circus sense of course, but imagine a clay body full tiny blades. That’s what this one is like. If you make it, I suggest you sacrifice some sponges instead of your fingers.

Enough chatter though, pictures are worth more. Do note, the chunk taken out of the rim is my fault. In my exhaustion at the end of loading the kiln, I grabbed this jar roughly:

rockware yakishime -- custom clay fired in an anagama enlarge

Color in protected areas isn’t bad:
rockware yakishime -- custom clay fired in an anagama enlarge

Oily streams over clear glaze. In person, this glaze is deep — it’s possible to see under the streams which float over clear layers:
rockware yakishime -- custom clay fired in an anagama enlarge

rockware yakishime -- custom clay fired in an anagama enlarge

The sort of fuzzy white looking stuff on the foot is bonded but unmelted sand. I didn’t wad this piece — I just set it on the floor sand. But what a nice blob of multi-dimensional glass in the center this shot:
rockware yakishime -- custom clay fired in an anagama enlarge

The following pictures show something very interesting about this clay. First, this piece is simply a small cup. In the shelf failure that ensued in the 6th firing, it was pushed up against something at the place you see the blobby brown scar. When it was detached, a chunk of the piece was pulled away below the brown patch.

rockware yakishime -- custom clay fired in an anagama enlarge

Here is a detail of that area. Note the peach colored network surrounded by cells of white body — that peach colored material is the fireclay. Everything else is glass from the silica and feldspar. This “clay” is mostly glass with some fireclay glue to make it stick together and hold its shape.

rockware yakishime -- custom clay fired in an anagama enlarge

I’m curious about what this clay would do in a regular gas firing. If you should conduct such an experiment, please send me a picture.

July 17, 2006

Bamboo Trimming Knives

Filed under: Studio — odin @ 12:01 am

Hands and mind are a potter’s most essential tools. Despite this truism, I feel that my most interesting pieces have been those I touched least; those which grow from vague direction rather than wilt under intense control. In contrast, pieces I dote on — picking and poking and prodding — those typically become uglier with each stroke.

Trimming is a task which lends itself to excessive manipulation and indeed, I have committed much potricide in my day. It would be best to not trim at all, but I have yet to discover whatever dark art is needed is to accomplish such making. Till then, I must settle for restraint of my tendency to keep working a piece over. Restraint is one of the most difficult skills to master (I’m barely an acolyte), though I have found some tools help me touch less, in particular: wooden knives. While I still sometimes use loop tools in hard to reach areas, I find that with knives I can touch less and thus allow more of the clay’s personality to show through. Sometimes I get a better result. Mostly, it’s still simply a different result.

Wooden knives have a couple disadvantages. They dull quickly. They wear down and change shape. They’re breakable. But they have some distinct advantages. Clay doesn’t stick to them as readily as it does metal. They are easy to make into custom shapes and there is no waiting — simply grab something from the woodpile, chop, cut, and carve. No ordering, driving, or shopping. Just instant gratification.

As a subset of wooden tools, bamboo knives are quite interesting. A nearly razor sharp cutting edge can be made simply by splitting a bamboo tube with a hatchet. The edge is surprisingly long lasting, and the semi-tube shape of the bamboo slat is very rigid thus resisting chattering. Long metal knives chatter far too easily.

In the picture below, note the thin shavings from the base of this coiled and thrown tsubo despite the fact that the clay body is full of large chunkies (clicking pictures leads to medium sized shots for better detail):

tsubo trimmed by bamboo knife

I usually hold the bamboo stick a little differently than shown in the picture above — more like a knife (either politely or Henry VIII style) but I couldn’t really manage that and operate the camera at the same time. I also sometimes hold both ends.

In the picture below, note that the bamboo knife is actually more akin to a chisel:

end of a bamboo pottery trimming knife

Bamboo knives are easily made and when they get dull, easily refurbished with a metal knife (a small block plane works best when renewing the edge). As for expense, an 8 ft (2.4 m) length of bamboo is about $3 around here and each section yields many knives:

splitting bamboo for pottery trimming knives

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