Category Archives: Pottery

Discovering Clay

PotteryPotters have been digging and processing their own clay for millennia. It has only been since the Industrial Revolution, which began in the 1800s, that clay started being sold by suppliers on the market. Before that time potters situated themselves near a good source of clay and always the trade was passed down from generation to generation. In many places such as China, Korea and Great Britain whole families of potters would build small towns near a clay source and the local economy centered on pottery making.

Clay is a smooth soft rock made up of mineral particles as fine as dust. Clay particles are all that remain of rocks such as feldspar after these have weathered and decomposed. Most clay remains at the site where it formed thus making a clay deposit. In its undeveloped state, it is one of the few natural resources that has no perceptible value of its own yet can be transformed into some of the most valued works of art.

Today, many potters caution that it is hardly worth the effort and time to dig your own native clay while others strongly encourage those that are able to take advantage of this abundant resource. A potter can also gain a great deal of practical experience and vastly broaden his or her knowledge by going out and digging one’s own clay and feeling the fulfillment of actually making a pot from the ground up.

Here at our shop quite a number of people ask if we use clay from the Brazos River, which borders our farm. We have been unable to utilize Brazos River clay because of a major lime contamination. A good part of this is due to the high limestone cliffs just above the river and every time it rains more limestone washes down the banks contaminating the very absorbent clay. James Chappell, author of, The Potter’s Complete Book of Clay and Glazes, says, “ While the presence of alkalies can be tolerated, the presence of lime cannot; when such clay is fired, the lime turns into calcium oxide, which will absorb water, expand inside the pot and cause it to crack, flake or chip.”

There are two basic clay bodies, earthenware and stoneware. Earthenware can only be fired up to the temperature range between 1700 degrees (F) and 2000 degrees (F). Because of this it is not waterproof and the finished product can be chipped or scratched easily. Stoneware is much less common than earthenware, yet it is highly sought after for it’s durability and lasting strength. This type of clay can be fired up to 2400 degrees (F) to become vitreous [meaning ‘like a rock’] making it water proof even when left unglazed, thus the name, stoneware.

About four miles from the Northern branch of our Ploughshare school, in Idaho, is a large deposit of kaolin called Helmer kaolin. The mineral kaolin is an extremely refractory clay with a melting point at 3200 degrees (F). It cannot be used alone as a clay body due to its highly nonplastic texture. Because of this it must be combined with other clays to increase its plasticity and lower its maturing temperature. However, this clay is very durable and has a low rate of shrinkage thus making it one of the most sought after ingredients for making pottery found in the United States. We have yet to work with it ourselves but we are looking forward to the possibility of finding a native clay source with which we could supply the needs of our school and craft shop.

The First Firing of Our New Wood-Fired Pottery Kiln

Inside the kiln, a view through the peep-hole

Inside the kiln, a view through the peep-hole

We finished the construction of our new wood-fired kiln on June 16, 2012 and fired it the same day. It was a great success!

Inspecting a vase after the first firing

Inspecting a pot after the first firing

We started the fires at about 5:20AM and fed wood into the kiln for the next 12 hours. The temperature climbed steadily until it reached about 2100°F, then it began to stall. Toward the end of the firing, it is not uncommon to have difficulty getting the temperature all the way up to the required 2400° F.

Feeding the kiln

Fueling the fire

One of the men from the woodworking shop next door came by and gave us a few suggestions on how to stoke the fire, which helped a lot! Once we made some adjustments to how we were managing the fire, the temperature resumed it’s climb and reached 2400° F in roughly 45 minutes. Then we stopped feeding the fire and closed the fire box doors and the damper.

Unbricking the Kiln Door

Unbricking the kiln’s door

On Monday morning, when we unbricked the door (removed the bricks that form the kiln door) and began taking pots out, the results were beautiful! The colors were more vibrant than they had been in our gas-fired kiln, due to the influence of the ash on the glazes.

Inside the Kiln

A look inside the kiln after the first firing

Overall, we are very pleased with the results of the new kiln and look forward to gaining more experience with it in the weeks ahead.

Pottery at the end of our first firing

Pots fired in the first firing

Related articles

To read more about the kiln and how and why we built it, read the articles listed below:

 

Building a Wood-Fired Kiln — A Photo Essay

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.

Base of the fireboxes

Mortaring and constructing the fireboxes.

Mortaring the fireboxes

Setting the hard bricks for the fireboxes.

More setting of hard bricks for the fireboxes:

Setting hard bricks for the fireboxes

The completed fireboxes with the firebox roof in place.

Completed firebox

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.

Firerod

 

The inside of the fire box.

 

Inside the firebox

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

The flue

The first course of bricks for the firing chamber.

The first course of bricks for the firing chamber

Setting the firebrick in place.

Setting the firebrick in place

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

Top view of firing chamber

The firing chamber at about one-half its height.

Firing chamber

Laying bricks from the inside and out.

Laying bricks from inside and outside

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.

Firing chamber at full-height

Bolting together the frame.

The completed chamber and framework — ready for the roof.

The completed chamber and framework

The free-standing arched roof from the inside.

Inside view of arch

The arched roof from the outside.

Top view of roof

The kiln with roof in place.

Kiln with arched 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.

 

Update on Our Wood-Fired Kiln

Wood fired kiln

The new kiln is nearly completed.

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.

Inside view of the arched roof

Inside view of the arched roof

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!

Wood fired kiln

Pottery students helping lay bricks.

Our goal for completing the project is June 1. We hope to fire the kiln for the first time shortly after that.

Update

See also our photo essay on building the kiln.

Our Plans to Build a Wood Fired Kiln

 

This article was also updated and featured in the Winter 2012 issue of the SustainLife journal.

Pottery from Our Existing Gas-Fired 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.

Our existing gas fired 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.

“Red” pottery fired in our existing gas kiln

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.

Visit SustainLife journal for more information about the journal.