A somewhat long missive. If you get the opportunity to take a workshop with Akira, I highly recommend it. His workshops are hands-on and he is very generous with his knowledge of technique and craftsmanship.
First, here is Akira's slip recipe:
Goldart, 6 lbs. (30%)
Kaolin - EPK, 10 lbs. (50%) [Akira also uses Grolleg or Tile 6 for a whiter slip and Helmar for woodfired items]
Custer Feldspar, 2 lbs. (10%)
Silica, 2 lbs. (10%)
His recipe makes a five-gallon bucket of slip; I usually half the quantity and make a smaller 2 1/2 gallon bucket. For the 2 1/2 gallon bucket, I add between 4 and 4 1/2 quarts of water; for a five-gallon bucket, add 8 to 9 quarts. I generally hold off on the last quart, adding a bit at a time to get the right consistency. This will be a thicker slip than you are probably used to making -- almost yogurt consistency. I've found that leaving it a bit on the thicker side and then adding some water as needed is better than making it too thin and watery. After mixing, it is sieve time (30 or 40 mesh should be fine); this is somewhat labor intensive but the creamy slip you get as a result is well worth the effort. I let the bucket stand overnight and remove any excess water that rises to the top of the bucket.
Recently, I tried using Grolleg instead of EPK and it worked fine; fired a little whiter, which was expected.
For clay bodies, I've used Standard 153 (^10), Laguna Dark Brown (which fires to black at ^10), and Highwater Hestia (^10) that is fired in reduction. I've played with a couple items at ^6 using Standard 266 and Highwater's Red Rock that is fired in my electric kiln.
So, start with a slab . . . either from a slab roller or made by hand. I use a slab roller because I vary the thickness of the slab just a bit depending on what I plan to make. For things like boxes and ikebana vases, I prefer a thicker slab to start as it allows for a wider edge seam for joining (I use a 45 degree bevel cutter). Also, as described below, when you stretch the slabs after the slip firms up, the slab compresses and becomes thinner. So, you want to allow for that compression at the outset. I generally set the slab roller for a generous 1/4 inch slab for general items and between 1/4 and 3/8 inches for boxes. My slab sizes generally run 12"x15" (my slab roller is small); I've found that to be a good size for doing the stretch part described below.
I place a couple sheets of newspaper on a table and then set the slab(s) on top of the newspaper. On top of your slab, add about 1/3 to 1/2 cup slip. Glop it in the middle and spread it around evenly -- I use a three-inch spackling knife (you can also use a wide hake brush); don't worry about going over the ends of the slab. At this point, just try to even out the slip, don't worry about the surface. (If you want to do some very precise patterns, like the seashell Akira uses on his teapots, use less slip or a thinner slip.) After allowing the slip some time to settle -- maybe 30 minutes, I go back with a second brush to make my final pattern. Lately, I've been use a stiff brush . . . a basting brush from the kitchen. Using that brush, I make my patterns . . . generally just a fluid arm (not wrist) movement from right to left, left to right, starting at the top and working toward the bottom. You can criss cross or just make parallel lines. If there is too much slip building up on the brush, remove it and continue. At this point, the brush lines will begin to develop; work (but don't overwork) until you see a pattern that you like. If you work it too long, the slip will become less thick and flatten.
At this point, you need to let the slab set . . . I can leave mine out overnight in the studio; how long this takes will depend on temperature and humidity (during the DC summer, it can set in a few hours; during the winter, I've let them set out a couple days in the garage studio). You want the slab to get about medium leather hard. Too soft and the slip will not create the breaks you want when the slab is stretched; too hard, the slab will crack and tear. If the slab does dry faster than you thought, you can use a water spritzer to rehydrate the slab (spritz both front and back to restore even moisture). If you can bend the slab and the slip does nothing, its too soft; if you bend the slab and it breaks, too hard.
Once the slab is set up, the next step is to stretch it. This is how Akira creates those beautiful patterns. For stretching, gently toss the slab across a plain piece of unfinished plywood, 24"x24"x3/8". First, I drop the slab on the plywood from a height of about 12 inches (just a plain pancake drop). . . this wakes up the clay. Then I turn the clay over so the slip is face down on a piece of thin foam. I take a cardboard tube . . . mine are the tubes from rolls of shrink-wrap, about 4" or so in diameter (you could substitute a piece of PVC wrapped in newspaper so the clay won't stick) and if roll the slab around the tube. This helps break the slip along its texture lines and also begins to give the slab memory of being a curved surface and not flat slab. [if you are not doing round work, no need for this step.] Remove the slab from the tube and place it slip-side up on the plywood. The next step is stretch the slab. To do this, pick up the slab by the sides and gently toss it -- at an angle -- against the plywood slab so that the slab stretches as it strikes the surface of the wood. I generally hold the slab in my outstretched arms and toss it at an angle towards me. Rotate the slab 180 degrees, and repeat. With each strike against the plywood, you will begin to see the slip stretch and pull away from the underlying clay slab. Repeat until you get the look you like but don't get greedy . . . too much stretching and the slab will tear or it will compress unevenly. This is the hardest part of the process and it takes some time to get a feel for doing. I've noticed a tendency for slabs to thin more in the middle and tear along the edges if you stretch it too many times.
Once you've gotten this far, your slab is ready and you can make whatever form you want. If the slab is very pliable, you can let it set up for a while. For joining edges, Akira uses an overlap technique where he takes a rasp (Sherrill's Mud Tool rasp) and shaves down opposite sides about 1 inch, then scores and adds slip to join. Basically, you want the thickness of the join to be the same as the thickness of the slab wall . . . if it is thicker, it can distort while drying.
Bisque firing is your usual bisque firing. Once bisqued, Akira applies a red iron oxide wash to the vessels and sponges off the wash from the high areas of the surface, leaving the RIO to penetrate into the exposed clay body. I've used straight RIO and water, as well as a combination of RIO, Frit 3124, and water. Insides are glazed -- he seems to favor shino or iron red, but its really up to you.
Akira fires in reduction to cone 10 in either his diesel fueled kiln. I fire my kohiki work in reduction in a natural gas kiln. I've been trying to get a similar look in an electric kiln by applying a soda ash wash to the outside to get a similar sheen on the slip; I'm not quite there yet.
The two keys seem to be catching the slab at the right time to begin stretching, and then stretching itself to get the slip to break and expose the underlying clay body. When I do kohiki, I'll make 6 or 8 slabs at a time, fills up a 6' folding table top. Once stretched, I'll store them in a plastic bin, with sheets of plastic separating each slab, to keep them moist while building various boxes, bottles, vases, etc. The process is a bit time consuming, but well worth the effort, in my opinion. And, I've adapted and added to what Akira taught in the workshop. Right now, I'm trying thicker slip applications so the surface is more 3-D feeling.