When considering the size of your new kiln you should ask yourself 4 questions:

What will fit in the space I have designated for the kiln?

Kiln manufactures generally recommend you have 18” of clearance from any combustible wall and that the kiln be placed on a non-combustible surface such as concrete or ceramic tile. Make sure the space you have designated meets these requirements and is large enough to fit the kiln you choose.

What is the largest piece I want to fire?

Kiln chambers vary greatly in size. Consider the length width and height of the pieces you wish to fire when selecting a chamber size. Take into account that most manufacturers recommend the following clearances:

1.        1” from the floor.
2.        1 to 2 “ from the sidewalls.
3.        2” from the thermocouple.
4.        2” from the lid. 

How many pieces do I want to fire at once?

If you are teaching or involved in production you will want to consider how many items you can fit in your kiln to meet your production requirements. The shape of the chamber can also be important to avoid wasted space. For example, square chambers are more conducive to efficiently loading tile.

What will my needs be in the next 5 years

What are your plans for the future? Kilns can last a long time if you take care of them. Make sure you do not pick a chamber size you will grow out of in 5 years.

Purchasing a kiln is a decision that will most likely live with you for a while so it makes sense to spend some time figuring out the perfect fit for you. There are a number of considerations you need to ponder and they all play a part in determining the best model for how you are going to use the kiln. 

Do you want to fire ceramics? Fuse glass? Make metal clay jewelry? Or … do you want to do a little of each? There are certain kilns that are specifically designed for a particular discipline but can be used by all three. You should carefully consider the following topics

Controls

In the early 90’s kilns made a huge leap in leap in technological advancement with the introduction of low cost automatic controllers. Prior to their introduction artists had to manually turn up switches to try and achieve the proper temperature rates for the items they were firing.

Switches and Pyrometers

The most basic controls on the market today involve simple infinite or 3 heat switches that are used in conjunction with a pyrometer and or pyrometric cones. These require the user to have an intimate knowledge of how their kiln heats at different switch settings, with size different loads, and at different temperature ratings.

Models equipped with these types of controls are still available but usually limited to very small, low cost, introductory kilns. You must monitor these units throughout the firing and it is absolutely necessary to be there to shutoff the kiln otherwise it will continue to fire until it burns out the elements.

KilnSitter

The KilnSitter was designed in the late 50’s by a man who was tired of being the “Kiln Sitter” for his wife’s kiln. It utilizes a mechanical device that senses when a pyrometric cone bends to shut-off the kiln. The operator still needs to manually turn up the switches to control the firing rate of the kiln. Most kiln manufactures still offer models equipped with these devices however most consumers choose to go with the digital controllers since the cost difference is minimal.

Digital Controllers

Most kilns today are equipped with digital controllers. There are many different brands and styles but most of them offer similar types of programming options. Most all controllers will give you the option of writing your own programs and have some form of built in factory programs for specific uses like firing ceramics, glass or metal clay.

Most kiln manufacturers also offer full sized controllers with numbered key pads and single use buttons and smaller more economical controllers with 3 or 4 buttons that require the operator to scroll to various programming options.

Temperature

The critical factor is generally going to be temperature. Ceramics generally need to be fired to a minimum temperature of 1828 F, whereas the maximum temperature for glass fusing techniques are around 1700 F, and only 1650 F for metal clay.

Ceramics are generally divided into two camps; high fire clay and low fire clay. Low fire clay such as earthenware is generally fired to a maximum temperature of 1950 F. Earthenware, if fired properly, is food safe but cannot be placed in the oven or dishwasher. Stoneware and porcelain fire to much higher temperatures which seal the clay body and make it safe to place in the oven or dishwasher without worry that it will craze (glaze cracks) and allow bacteria into the clay.

If you choose a kiln that is not rated to the temperature of a technique you may want to experiment with in the future, you will need a new kiln so think ahead. 

Physical Design

There are many different styles of kilns. In ceramics you have front loaders, top loaders, bell kilns, raku kilns…the list goes on. The most widely used kilns in the home are top loaders and small front loaders.

Unique kiln designs are usually only necessary if you need access to the clay, glass or metal clay while the kiln is firing. For example, you may want to consider a clamshell design if you want to rake a fused glass tile to mix the glass while it is fluid, or, you may want a kiln with a bead door if you or a doing glasswork using a torch. Visit the various kiln manufacturers to see their different designs.

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Hand-building

Handbuilding is exactly what it sounds like; using your hands to form an object out of clay. It encompasses some other, more specific, forming methods as well, like coil building and slab building, and is often used in conjunction with other forming techniques. Ceramic sculpture would fall into the hand-building category since it incorporates many different techniques.  

Slab Building

A process whereby slabs of clay are rolled or pounded out, either by hand, with a slab roller or rolling pin, and then used to construct objects or vessels. Depending on the aesthetic or design requirements, clay slabs can be used from the wet stage up through leatherhard. Many slab builders embellish the slabs with surface designs or textures prior to construction because it is easer to create surfaces when flat rather than after a piece is made.

Check out these short videos on slab building provided by Ceramic Arts Daily.
Slab Vase Project
Slab Tray Project

Coiling

This is perhaps the most simply understood technique for making clay vessels and sculptures. Coils of clay are rolled out, and are built up in a spiral fashion, with the coil being added joined to the coil below it layer after layer until the desired wall height and profile is achieved. This is a great entry-level technique, and will teach you a lot about what clay will and won’t do when wet.

Throwing

Wheel throwing is probably what most people think of when they think of making pottery. A potter will use a manual or electric potters wheel to center a ball of spinning clay, open it into a vessel, and lift and shape the walls while the wheel is spinning, thereby producing a symmetrical vessel. These vessels can then be textured, decorated, reformed into alternative shapes, or adorned with handles for cups spouts for tea pots or anything your imagination can conjure up.
Check out this video provided by Ceramic Arts Daily on throwing.
Faceted Bowl 

Extruding

This technique was borrowed from industry, and has been adapted by clay artists to fit the studio scale. An extruder is a tool with a plunger that forces wet clay through a tube and out through a die at the end of the tube, effectively squeezing the clay into a shape that is determined by the die. Dies can be purchased or made, and the possibilities are many. They are often used to form shapes that are attached to items that have been constructed through other techniques (like handles for mugs).

Slip Casting

Slip Casting involves using specially designed molds that you pour liquified clay into to form the internal shape of the mold. The clay is poured into the mold and allowed to sit for a while so the mold can wick some of the water from the clay, causing a thin layer of more rigid clay to take the shape of the mold. The excess slip is then poured out of the mold and set aside to dry. When it has sat for enough time, the mold is opened and the hollow clay form is removed. Once removed the seams from the mold are scraped smooth, and the piece is allowed to dry further until it is ready to be bisque fired.

Check out this video supplied by Ceramic Art Daily on Slip Casting
One Piece Slip Mold

While there are thousands of clay bodies available for purchase, the 3 basics types are porcelain, stoneware and earthenware. The maturity temperature, workability, and color of these 3 categories can vary based on what is added. Before we talk about the specific clay bodies it would be helpful to talk about the properties that define them.

Properties

Temperature

Different clay bodies “mature” at different temperatures. By “mature” we mean fired to the point for which they were formulated. In general terms, the higher a clay body is designed to fire, the more vitreous it is.

When a clay body is vitreous, it means that water cannot be absorbed into the clay when it is fired to maturity. This is important because it can affect how susceptible the clay is to freezing. 

For example let’s say you lived in an area where it often froze at night and you wanted to make a fountain with ceramic tile. If you chose a clay body that was non-vitreous, water could enter the pores of the clay body during the day when it is warm. At night, when the temperature drops, the water would freeze and turn to ice. Since water expands when it turns to ice, the clay body would be more susceptible to cracking.
When we talk about clay body firing temperatures we refer to low fire, mid fire and high fire. These are the temperatures you fire the clay to during your bisque fire. You will notice that only the common cone ranges are listed. Here are the temperature ranges:
Low Fire – Cone 06 – Cone 04 (1828 F – 1945 F)
Mid Fire – Cone 5 – Cone 6 (2167 F – 2232 F)
High Fire – Cone 8 – Cone 10 (2280 F -2345 F)

Workability

The workability of a clay generally refers to how easy it is to shape without problems. If problems do occur, a workable clay would allow you do fix those problems and continue. For example, if you were throwing a large plater on the wheel, the workability of the clay would affect how easy it is to center the clay, open it into a cylinder, and hold its shape when it is formed.

Color

The color of a clay is affected by the materials that are in it. For example, a clay body that has a lot of iron it will be red. A clay with a lot of manganese will be blackish in color. The shade of these colors can change some when you fire it but for the most part, unlike glazes, they remain the same base color. 

Porcelain

A high-firing fine-grained white clay body that fires to a durable, strong, vitreous ceramic. It is usually pure white because of its high kaolin content and lack of other ingredients like iron that can change the color and properties. Since kaolin is the purest form of clay, porcelain lacks some of the additives that make clay more plastic (stretchy) making it not the most plastic of clay. This gives it the reputation of being difficult to work with—though modern formulations have addressed this.

Stoneware

A mid- or high-firing coarse-grained clay body that can be anywhere between buff color to gray to dark brown, because of varying levels of iron and other “impurities”. It is incredibly durable after firing, with little to no absorption (vitreous).

Earthenware

A low-firing fine-grained clay body that is typically found in creek beds and other deposits that have been transported by wind and/or water from their location of origin. This transportation aids in the weathering and breaking down of the source rock into clay, and also adds  “impurities” like iron, which is what makes earthenware brown, orange, or red, both in the raw state, and after firing.

Because it is fired low, it is more porous and relatively less durable than stoneware or porcelain, but that can be an advantage in some cases. Terra cotta garden planters are a type of earthenware clay, for example, and the fact that they are porous and can “breathe” is a design advantage.

Clay is found, in one form or another, in almost every region of the world. It is naturally formed over time as rock (feldspar) is broken down and washed down mountains where it is deposited in lakes, rivers and streams in the valleys below. As the rock washes down the mountain it is combined with different organics and minerals (i.e. iron, sodium, calcium…) along the way which give each regions clay, its own unique characteristics.

Technically speaking in it’s purest state clay is an aluminum silicate mineral kaolinite. Its property of being plastic arises from its composition of microscopic disk-shaped platelettes that give clay its ability to hold its shape when molded.

In early civilizations, potters would go down to the riverbeds where clay was deposited, dig it up, pick out the rocks and sticks and start making pots. Today the process is a little more refined. Clay manufacturers purchase the refined raw materials neccesary to make the clay bodies they offer. These raw materials are first mixed as dry materials, water is then added, and the mixed clay is de-aired in a pug mill where it then is extruded into bags and boxed. 

When dried clay is fired to temperatures between 1800°F and 2300°F, depending on the type of clay, it changes chemically into ceramic, and becomes rigid, durable, and permanent (no longer plastic and moldable).

More to come soon on:

1. Cracks
2. Crawling
3. Pinholes
4. Overfire
5. Underfire
6. Peeling

Sample projects will be available soon.