As described in the update on our wood-fired kiln, our new kiln project is nearly complete.  We thought you might enjoy seeing a photo essay of how it was built.

The base of the fireboxes are laid out, along with the first one-and-a-half courses of fire brick.

Mortaring and constructing the fireboxes.

Setting the hard bricks for the fireboxes.

More setting of hard bricks for the fireboxes:

The completed fireboxes with the firebox roof in place.

One of the fire rods. These are made from a ceramic refractory material that last a lot longer than steel pipes. The wood will be fed into the fire on top of these rods.

 

The inside of the fire box.

 

The flue, where smoke and fumes will exit the kiln. This channel connects the fireboxes to the chimney.

The first course of bricks for the firing chamber.

Setting the firebrick in place.

A view of the firing chamber after several courses of brick have been laid.

The firing chamber at about one-half its height.

Laying bricks from the inside and out.

Some of our helpers.

The firing chamber at its full height, before the roof has been added. The iron frame is being put in place. The bulk of the kiln is constructed using a method called dry-stacking. The bricks are laid so that they interlock, and the iron frame holds it together.

Bolting together the frame.

The completed chamber and framework — ready for the roof.

The free-standing arched roof from the inside.

The arched roof from the outside.

The kiln with roof in place.

 

Related Articles

For more information about the kiln see Update on our Wood-Fired Kiln and Our Plans to Build a Wood-Fired Kiln.

 

As mentioned in the spring issue of the SustainLife journal, we recently began building a wood fired kiln. We have been using a gas-fired kiln for the past twelve years, but due to some design flaws and repair needs, we decided to build a new one.  This new wood-fired kiln will be a big step toward sustainability for us, as potters. The only cost in firing the new kiln will be our labor, as we will be able to heat it by burning scraps of wood.

We started laying bricks in March, 2012, and as of May 21, we have completed the construction of the fire boxes, the walls of the firing chamber and the arched roof. The next thing we will build is the chimney.

Most of the kiln is built from dry-stacked, high-temperature, insulating fire bricks, in a double-walled construction. The fire boxes are an exception, as they are mortared together and constructed of very hard, dense higher-temperature brick.  These high-duty fire bricks can handle the intense heat of the fire boxes better than the insulating bricks used throughout the rest of the kiln.

The arched roof is also made of dry-stacked brick. To lay those bricks in place, we built an arch former of wood. It’s top surface matches the shape of the kiln’s arch. We braced the arch former from inside the kiln, then laid the bricks in position on top of it.  Once the arch bricks were all in place, we removed the arch former, allowing the roof to stand on its own, held in place by the weight of the bricks, the side walls and the design and shape of the arch.

We have done most of the work with volunteer labor, including help from some of our eager students who wanted to place a couple of bricks on the walls!

Our goal for completing the project is June 1. We hope to fire the kiln for the first time shortly after that. Check our blog or our email newsletter for further updates and for the announcement of the first firing!

Update

See also our photo essay on building the kiln.

 

We currently use a gas-fired kiln that we built about 10 years ago. We have fired a lot of pottery in this kiln over the years, and in many ways it has worked well for us, but it has a fundamental design flaw that makes the kiln very difficult to use, especially with certain types of glazes.

Types of Kilns

There are two basic types of fuel fired kilns: natural draft kilns, and forced air kilns. A natural draft kiln uses the draft produced by the chimney to pull air into and through the kiln, and dampers are used to regulate the atmosphere within the kiln. A forced air kiln, such as ours, uses fans to blow the flame from the kiln’s burners into the firing compartment where the pottery is located. This type of kiln doesn’t need any secondary draft, such as that produced by the chimney, and in fact any secondary draft makes the atmosphere inside the kiln difficult to regulate.

The Problem with our Existing Kiln

The chimney on our kiln is too large in proportion to the rest of the kiln. Because of this, it produces some secondary draft, which causes turbulence. The draft also makes it hard (or impossible, depending on weather conditions) to regulate the atmosphere inside the kiln.

When firing pottery, one very important stage in the process is the reduction stage, in which firing temperatures are in the 1,600 to 1,900 degree range (Fahrenheit), and the air intake is adjusted to reduce the amount of oxygen in the firing chamber. This “oxygen poor” environment draws out oxygen that is stored in the chemical makeup of the glazes. This  causes the glazes to change to the proper colors.

If there is too much oxygen in the air, the glazes will not change color properly, and if there is too little oxygen, the fire will become sooty, producing a lot of carbon, which also interferes with the glazes. It is important to get just the right oxygen content, and that is what we are unable to do with this kiln.

Red Glazes Turn White

This has been most apparent when we have used red glazes. If the oxygen level is incorrect during the reduction phase, the red pieces turn white. Sometimes the whole piece turns white. Other times, part of the piece will turn white.  Once the piece has been fired, certain minerals in the glazes have been vaporized, and re-firing the piece will not correct the problem. We no longer fire red pieces in this kiln because it is impossible to fire them reliably. In the photo below, both pieces of pottery were glazed with a red glaze, but due to the incorrect atmosphere in the kiln, the pot on the left turned white. The piece on the right turned red in most places, but turned white near the rim.  Although the glaze color does not affect the usefulness of a piece of pottery, it does make the piece unsalable when part of a custom order.

Our signature Homestead Green pottery also is difficult to fire in this kiln. The effect is less pronounced than for the red pieces, but still very noticeable. Normally, the glazes produce a beautiful translucent, opalescent finish, but when the kiln’s atmosphere is incorrect, the finish becomes metallic green.

A Sustainable Wood-Fired Kiln

Because of these problems, we plan to begin building a new wood-fired kiln very soon. It will be a double fast fired kiln and will use only renewable resources (wood, which is plentiful here). It will require no electricity to operate. We are working toward sustainability in our community, and this kiln will take us another step toward developing a sustainable pottery shop and teaching facility.

We’ve discussed the kiln design with a number of other experienced potters, and we feel that this particular design is the best suited to our needs. There are several types of wood kilns, but this particular type will let us produce pottery that is very similar to the pottery that we currently fire in our gas kiln.

Once we have built the new kiln, we plan to operate it alongside our existing gas kiln until we have gained enough experience with it. Then we plan to dismantle the gas kiln and build a second wood kiln using its materials.

The wood fired kiln will cost about $5,000 to build and will contain more than 1,500 fire bricks.

Update 5/21/2012

We have nearly completed the kiln.  See this update on the kiln for more information.