Friday, December 30, 2005

We Live in Interesting Times

We fired the raku kiln today.


After a raku session, there is a smokey odor that you carry with you. Its almost a sweet, carmelized smell, which may come from the partial combustion of newspaper.

We rented the use of the raku kiln at The Ceramics Shop, down in Philadelphia. We have to drive across the Betsy Ross Bridge to get there.

Their raku kiln is behind their building, which is an old, four or five story red brick building, with foundations made of cut stones covered in concrete. The kiln sits in a brick courtyard, surrounded by tall buildings, and is accessable by a flight of wood stairs out the back of the building. The courtyard is lower than street level, and also plays host to a rusty swingset, several wooden lawn chairs, and a planter made by cutting up an old automobile tire, and painting it white. The back of the building is panes of glass, held in wooden frames. Most of the windowns have sheets of plastic screwed over them, but some are left uncovered, and, in some places, the wooden frames have rotted, and the panes fall out.

My first task of the day was to sweep up some broken glass, a pane that had fallen out of a window, and broken at the base of the stairs. I borrowed a broom, dustpan, and a cardboard box from Mark, the shop owner.

The kiln sits under a sort of awning, built out of two by fours, with an old camper shell for a roof. The raku kiln is a converted electric kiln, and sits on the same red brick that the courtyard is paved with. The first course of kiln above the kiln floor is made from soft kiln brick, which has been cut to match the octagonal base of the kiln, with a hole cut for the gas burner to fit in. There are two steel cased rings of kiln, the remains of the electric kiln, including the heating elements inside. The lid is fashioned from ceramic fiber, a matted, white material, with a sheet of rusted metal mesh on top, with a hole in the center to act as a vent. Inside the kiln, two kiln shelves are supported by a set of kiln bricks. One of the kiln shelves has cracked into two pieces, and there is a heavy residue of glass in a few spots from the previous firings of the kiln. We remove the kiln shelves, and paint over the glaze spots with fresh kiln wash. We then put the shelves back in the kiln to dry.

Off to the side of the kiln, but still under cover, are there metal trash cans, covered with their lids. One lid is missing the handle. These are the primary reduction chambers. Once fired, the hot pieces of pottery will be transferred from the kiln to the reduction chambers. We have brought sawdust, newspaper, and shredded documents (aka, 'shredded secrets') from our home office to place in the chambers, to ignite when the ware is placed amongst them. A fourth can sits off to the side, and has a large square of plywood on top. Julie and I use it for a table. A large bucket sits to the side, full of cold water.

On the table, I place two shiny new gallon paint cans. When I rakued in California, Jan Babcock developed a technique for reduction using paint cans and corregated cardboard. It works well for smaller pieces and heavy reduction. I have brought them for my rockets. I have two with me. Julie has two dragon pots, and we also have three test pieces.

We turn on the gas, and light the kiln. The kiln is run from a large propane tank, maybe three times as tall as one found on a home grill, but the same diameter. We place a digital pyrometer through a small peep in the wall of the kiln to monitor temperature. We fire the kiln to about two-fify degrees fahrenheit, then kill the gas. This is to dry out the kiln wash on the kiln shelf, and the fiber in the lid of the kiln.

We reopen the kiln, and place in one rocket, one dragon vase, and three test pieces. The kiln had room for more, but we spread things out, and didn't want to be too rushed when the kiln is opened. I relight the kiln to start the firing. Julie and I then start to prepare the 'nests' in the reduction chambers.

I show Julie how to take a double sheet of newspaper, wet them, and drape them around the rim of an open trashcan. This makes a tighter seal when the lid is slapped on. It takes four doubled sheets to go around each rim, and we prepare two of the larger chambers. We take several sheets of newspaper, and rip them the long way to make long strips of paper to place in each chamber, shaking them into the chamber to spread the strips out. Once a pile of strips is in the chamber, they are spread out to make a birdnest shape. Shreaded secrets are sprinkled on top, to aid in quick combustion.

We watch the pyrometer. Whenever it slows to the point where it takes several seconds to increase a degree, we boost the gas pressure a little. Eventually, so much gas is entering the kiln that flames start to lick out of the vent hole, the cracks in the kiln brick, and back through the opening where the burner fits in. There isn't enough oxygen in the kiln to burn all of the gas, so some of it is burning after it leaves the kiln. The interior of the kiln is in a reducing atmosphere. Taking with Mark afterwards, we could have prevented this by moving the lid of the kiln over a little, to allow a larger vent hold to form at the edge of the kiln. The exterior flames would have died out.

We can peek slantwise through the hole in the lid of the kiln to inspect our pieces. At first, everything is fairly dark in the kiln, with a glow coming from the combustion taking place below the kiln selves. As the temperature rises, the interior of the kiln, and the pieces it contain, start to glow orange.

Julie and Steph make a run to Wendy's to get burgers. By the time they return, we have enough time to start eating before the firing is complete.

We have the best visibility on Julie's dragon. We can see reflections in some of the glaze on the vase.

The kiln reaches our target temperature. We had chosen 1828 degrees F, which corresponds to cone O6. Julie puts on one of our gloves, and turns off the gas valve. We wait a few seconds for the flames of combustion to die out.

We use two pairs of long handled tongs to remove the lid of the kiln. Julie has a three foot long pair of simple tongs, with a single pivot. Mine are articulated scissoring tongs, and are a little harder to use. We place the lid on top of a hard lid, the former lid to the electric kiln that had been removed.

The pieces are glowing orange hot; some of the glazes look shiny. There sounds of cracks and pings as they start to cool. We have to reach into the kiln with our tongs to pull the work out, but we encounter problems from the excessive heat. I give Julie the other glove, and she the pulls her dragon vase out. I was going to help spritz water onto the piece to promote cracking, and got off the first shot from my spray bottle, when Julie asks 'How come the glaze has turned green?'

My brain shuts down. Her piece is supposed to be a deep red, but it looks minty green. I'm clueless. She places her piece into its nest, the paper burst into flames, then she puts the lid on, and hands me the gloves. I fish out my rocket, and move it to its can. After fiddling with the lid a bit, I get it sealed. Greasy gray smoke curls around the lid of the paint can.

I give Julie back the gloves, and she places two more test pieces, drafted rocket chucks, into another of the larger reduction chambers, and seals the lid. I move the last test piece, a little vase, into the second paint can, and slap on the lid.

First firing is done.

We allow the kiln to three hundred degreeds before reloading. One more dragon, one rocket, and two test pieces. We fire off the kiln, then wait to inspect the cooling pieces. Julie's dragon vase is mysteriously minty green instead of deep crimson. The other glazes made with Mason Stains have held their color, but not the Deep Crimson. She had used a glaze called alligator matte for the skin of her dragon, and it had lived up to its name, breaking into cells and rectangular shapes. I had moved my rocket so fast to the reduction chamber that I had forgotten to allow it to cool and heat fracture the glaze. My white crackle was a seamless white.

We wondered if we overfired the kiln.

The second firing was taken to 1730 degrees F. We had more strange results. The following two issues need to be resolved before the next firing:

1) We mixed up a base glaze to use with Mason Stains. The base glaze was 80% gerstley borate, and 20% cornwall stone (by weight). I mixed in 25%, by weight, of Deep Crimson Mason stain. We fired twice, once taking the kiln to 1828 degrees F (cone O6), and the second time taking it to 1730 degrees F. In both cases, the glaze came out mint green instead of red (or deep crimson). We had success using Lobster and Tangerine Mason Stains on test pieces, as well as the Florentine Green in the same manner (25% by weight). The one caveat that we have is that
the kiln might have been under heavy reduction for the entire firing cycle for both firings.

2) In the same two firings, we had samples of Seth's Luster that I had had mixed up for us. At the higher temp, Seth's came out almost black, with a really rough surface. At the lower temp, we have two samples, one where Seth's was thick, and the surface is really rough, and the collor looks like bad corrosion, with one or two drops of bright copper color. The other is where it is rough, very matte, and a deep blue color. I seem to recall Seth's needing to 'age' a bit after mixing the dry glaze chemicals with water, but that might be a recollection of Floating Blue.

We have sent an email off to Vicki, my former instructor in Georgia, to ask for her advice.

I've posted pictures of two rockets and two dragons on the flickr pottery photo site. You can access it in the links column on the right side of this page, right above the flikr badge that shows preview of our pictures.


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