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ancient kiln | 21st century logbook

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July 26, 2006

Budget Clay Mixing

Filed under: Clay Bodies,Studio — odin @ 9:52 pm

It would be sweet to have a clay mixer and a de-airing pug mill. Of course, even setting aside the significant expense, I’d have to build a building to house the equipment and worse, upgrade the wiring at the anagama site. I have enough electricity to run some lights, a toaster oven, a tiny fridge, and a radio (I have a kick wheel). While somewhat minimal, I consider even that much a luxury compared to how things started out.

Given certain limitations on money and power, I nevertheless have a strong desire to experiment with clay. Anyone who becomes consumed by woodfire develops such urges because the claybody is a critical component of the pottery’s final appearance. For example, take two pieces, one porcelain and one stoneware, fired near each other in almost identical conditions — they will exhibit vastly divergent glaze texture, color, and coverage despite the similarity in firing conditions. For yakishime, clay experimentation takes the place of glaze testing. So how does one experiment when “necessary” equipment is out of reach?

Hint one: nothing is indispensable — alternatives always exist.

I made my clay mixer for about $75 (not including drill). Here’s the shopping list:

  • 5 gal (19 l) plastic bucket.
  • Mud Mixer (essentially a shaft with blades for mixing drywall mud — attaches to a drill).
  • Plywood and boards.
  • Plaster of Paris (yes, I bought the cheap stuff at a home store rather than the expensive stuff at an art store — it was about $10 per 25 lb (11.4 kg) sack). I used slightly less than three full sacks to make a wedging table 37x18x2.5 inches (94 x 45.7 x 6.4 cm).

Now, if you plan on making a ton of clay at a single sitting, my method won’t scale that well. But it’s a snap to make a 35-50 pound batch — although at 50 pounds, it works better to use two buckets.

Hint two: just dive in, the world won’t end if you mess it up.

First an aside. I’ve been very interested in coarse materials in my clays. One thing I’ve decided has potential is Lane Mountain Sand, a white sand sold in 100 lb (45.4 kg) sacks for about $6 at my local home store. As another aside, I’m very hip to cheap and convenient. For example, a trip to the pottery store requires either a 200 mile (322 km) round trip to Seattle, or a shorter trip to Canada coupled with a search delay on returning to the US. Apparently, the Post Office doesn’t communicate with the Border Patrol so I get tagged for using fake documents when they can’t find my “new” zip code in their database — that’s “new” as in 3 or 4 years old. Borders are stupid. Anyway, back to the digression at hand: What convinced me to use this sand in some clay is how beautifully it melts. Part of the anagama floor has no bricks — just a layer of this sand for a floor. During firing, a crust forms on the sand and in places that get a blizzard of fluxing agents, the sand melts into beautiful blue-green blobs of glass. I want some of that on my pots!

Finally, having accepted that for now, I’m looking for drippy shizenyu rather than hi iro, I mixed up this batch (do not weigh, these measurements are volumetric):

  • 2 parts Lincoln Fireclay
  • 1 part ball clay: SPG
  • 1 part Georgia Kaolin
  • 1.5 parts Lane Mountain white sand
  • 1 part Custar Feldspar (powder)

First, fill the bucket up a bit less than half full with water. Here you can see the bucket and the mud mixer I mentioned earlier:

low budget clay mixer for making stoneware enlarge

Next, add all the clay ingredients. Don’t add grog, sand, or gravel yet — just mix the clay into a nice frosting like consistency:

mixing clay the cheap way -- bucket, mud mixer, drill enlarge

After the clay is thoroughly mixed, add the sand, grog, and/or gravel bit by bit, mixing between additions and adding water if necessary. Drills aren’t as powerful as industrial mixers so the mix should be kept somewhat on the soft side — if the clay is too hard, the ingredients won’t get mixed thoroughly and it will increase the amount of wedging work. Besides, you risk burning out your drill on a stiff mix.

Once all of the ingredients are added, dump it out on the plaster wedging table and wait for it to get firm enough to wedge up. In the spirit of not following my own advice about mixing everything together wetly, I decided after I dumped out the clay that I wanted some more sand in it. Pouring sand on top of the freshly mixed clay compounded my work during wedging:

Pouring out freshly mixed clay on the wedging table enlarge

When all finished, this batch made about 35 lbs (16 kg) of clay — note the clay in the bag is 25 lbs (11.3 kg) of commercially prepared clay. The column of clay on the right is wider, deeper, and taller than the bag of clay, thus my 35 lb guesstimate:

Homemade clay all done enlarge

In terms of time: I mixed the clay around noon and then let it sit in the bucket under the hot sun all day. My purpose was to let it get warm and help incubate whatever it is that’s growing in my uber-stinky recycle clay (I tossed in a handful of recycle slip). It doesn’t hurt to let the clay mixture sit around in the bucket for a few weeks or so, particularly after adding a bit of compost starter (if you are looking for a more alive clay). Anyway, that evening, I poured the slip out on the table, added sand, and covered it with plastic (it has been hot here recently). I went home, returned the next day about mid-afternoon and it was ready to wedge. With mixing, pouring, and wedging, I probably spent about one hour making the clay … and got some free exercise to boot.

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.

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