More Resources

The Pottery and Ceramics section contains five pages. Skip to Page 3 or Page 4 or Page 5.

Books

Hands on Clay: An Introduction to Ceramics by Charlotte Speight and John Toki

The Potter's Studio Clay and Glaze Handbook: An Essential Guide to Choosing, Working, and Designing with Clay and Glaze in the Ceramic Studio by Jeff Zamek

Clay and Glazes for The Potter by Daniel Rhodes

Primitive Pottery by Hal Riegger

Alternative Kilns & Firing Techniques: Raku, Saggar, Pit, and Barrel by James C. Watkins

Hand Building Techniques by Joaquim Chavarria

The Complete Potter's Companion by Tony Birks

Handbuilt Pottery Techniques Revealed: The secrets of handbuilding shown in unique cutaway photography by Jacqui Atkin

Websites

Where does clay come from? by Jenny Gulch

Clay's important features by F.H. Norton

Photo of clay rock deposit

Principal Clay Types Used in Ceramics
Hammill & Gillsepie

Clay and Ceramics Info
Clay Times

Clay minerals

Clays and raw materials used to formulate clay bodies
DigitalFire.com

Raw materials dictionary
Sheffield Pottery

Description of raw clays
Claymaker.com

Removing sand/gravel from clay
discussion thread

Watch how feldspar weathers into clay particles video

Digging up and creating pottery with your own clay
eHow
video

How to Dig and Process Your Own Clay video Ceramic Arts Daily (scroll down)

How to Make a Pinch Pot video

Things to remember about Coil Pots

Clay coiling tutorial

How to use an extruder
GlynnisLessing.com

Create Your Own Homemade Foot-Powered Extruder
Ceramic Arts Daily

Introduction to pottery making

A Photographic Tour of Firing Pottery (using the open fire method)
NativeTech.org

The art of ceramics
NALIS

Learning to Throw Pottery
Marvin Bartel

How to make pottery

Cat Litter, Antacid and other non-traditional glaze materials
Ceramics Today

Make Pottery Glaze with Simple Household Chemicals
ehow.com

Kiln Firing Chart (PDF)

Cone Temperature Chart
BigCeramicsStore.com

Pyrometric Cone Charts

Pottery and Ceramics

Continued from Page 1

Sedona red clay ready for forming.

Step 2: Produce a Workable Clay Body from Raw Clay

For definitions of terms highlighted below, use the ceramics glossary.

Processing newly mined clay begins with drying it out in the sun. Many traditional potters store clay outdoors for a year or more. Eventually, it's crushed or sifted into a fine powder. Wear a mask or scarf for this task, as clay dust is bad for your lungs.

Recycled clay from ceramics, as well as any new clay that doesn't need to be cleaned or tempered, can be slaked rather than turned into powder. Slaking is an easy process. The clay chunks are added to a bucket of water and allowed to sit for a few days. During this time the clay particles dissolve into the water. Then the excess water rises to the surface. After pouring off the excess, the potter spoons out clumps of clay onto a board covered with canvass or another absorbant cover ( or an uncovered sheet of plywood). These wet chunks will sit outdoors a few more days until they harden into the consistency of a workable clay body. It helps to turn them once a day to allow for even drying.

Cleaning and Filtering

Once newly mined, raw clay is in a powder state, it's easy to pick out visible debris, rocks, roots and other organic material. If you can find one, use an old window screen or any other type of sieve to filter gravel and hard clumps out of your powder.

After the clay is pounded down and thoroughly dried, it can be stored for future use, mixed with other ingredients, or slaked (as described above).

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Digging clay beneath a topsoil, then pounding it down once it dries. Photos: Pheasant Place

Drying freshly dug raw clay. Photo: Michael Mahan

What Makes a Good Ceramic Clay?

Most clay used in pottery comes from rocks and sediments rich in chemically-weathered feldspar. Water penetrating into the feldspar breaks it down to form the clay minerals mentioned above. Unfortunately, when it comes to identification, feldspar is about as vague a term as clay. Rather than a single entity, it represents a long list of silicate-based minerals that are divided into two main rock families - plagioclase and orthoclase - plus a few inlaws and other relatives, like lithium feldspars.

In terms of pottery-making, the most useful feldspar mineral to remember is microcline. It's always a good idea to gather some of it while you're out prospecting. Modern ceramicists like to add it to their clays because it gives the material more "tooth", or strength. A member of the orthoclase family, microcline is found in high percentages (about 65 percent) in a granite-like rock called pegmatite. The other 35 percent of pegmatite is composed of quartz and mica, minerals which are also commonly added to ceramic clay bodies. Another place to locate microcline is in a vein running through other rocks, known as an aplite dyke.

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Feldspar, pictured at left as red microcline crystals in the rock pegmatite. The other crystals in the rock are mica (gray) and quartz (white), which may also be added to a clay body for strength. In fact, you can probably just ground up the whole rock and keep a supply of this material for your ceramics work. At right is a photo of a commercially sold potash feldspar.

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At left, an aplite dyke. In the center photo, a specimen of potassium feldspar. Check pottery supplies stores online (like Clay World), and you'll see that feldspar is sold as a bleached powder, as shown in the photo on far right. Photo/far left: Michael Jefferies

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Here's a little more explanation about why feldspar figures prominently in pottery:

When clay is fired at a high temperature, a chemical change locks in its permanence as a ceramic. Since feldspar minerals melt gradually over a range of temperatures, they ease the clay through this rather traumatic process. Clay objects are highly susceptible to thermal shock, which causes them to slump, melt or break apart. That's why getting the clay body recipe right before you begin shaping and forming an object is a pretty big deal.

Primary clays tend to be more heat-resistant than secondary clays. This is because kaolin naturally contains a higher concentration of feldspar and other refractory (i.e. heat-resistant) materials. Flint, quartz, mica and even common river sand are all considered to be refractory.

The third fundamental quality of a good clay is the naturally-occurring (or physically added) presence of what's called a flux agent. The function of a flux is to lower the temperature at which clay hardens, or matures when it's fired. Without a flux, you would have to heat your clay forms to 3000 degrees farenheit. That's hotter than most primitive outdoor fires get and well past the melting point of most clays. Red clay, on the other hand, starts melting after 2100 degrees. As it turns out, the clay is red because there's a lot of iron oxide in it, and the iron is a flux. Feldspar and iron are the two most common fluxing agents in ceramics.

In case you're wondering, pottery that doesn't get fired at the required temperature (between 1800 to 2500 degrees farehnheit) will eventually crumble back into dust, or turn muddy when it gets wet. The bricks in an adobe house aren't fired, for example. For that reason, they must be constantly covered with fresh coats of lime or plaster to guard against deterioration.

Developing A Pottery Recipe

Here are the basic requirements for an ideal clay body:

Let's look more closely at a few of these qualities:

Plasticity: Try bending a coil of moistened clay around your finger to see if it cracks badly. If it does, further processing is needed. Nowadays, artists invariably rely on a chemically-modified mix of secondary clay called ball clay, which they combine with their own clay to increase its plasticity. Pottery thrown on a wheel requires a lot more plasticity than coiled pot, and slabs require a lot less. (Shaping is discussed on the next page.)

Of course, primitive potters and survivalists don't have that option of purchasing specialty clays. Ball clays in their natural form are found in swamps and low flat lands, often beside layers of coal. In the United States, most of this raw material is commercially mined in Kentucky, Tennessee and Mississippi, but Texas, California and Maryland also have their own deposits. If you don't have access to that, check clays found on the bottom of lakes, marshes and slow-moving currents. These mayll be more plastic than other clays due to bacterial action. (Ball clay, incidentally, got its name because of the way it was initially extracted in the United Kingdom. Hand spades were used to roll the clay into balls to expedite transport.)

Another clay that's highly plastic, bentonite, is found around volcanic ash. Also used for medicinal healing, bentonite is five times more plastic than ball clay. However, you don't find it in too many places. The Black Hills of Wyoming supplies the country with most of this product. (Petroleum drillers also use bentonite to help them deal with mud and water.) Since it produces excessive shrinkage and poor color in a clay body, experts warn not to let it account for more than three percent of your total mass.

If you don't have access to either ball clay or bentonite, and your clay doesn't bend well, don't fret. Another tried-and-true method of achieving plasticity is to store it wet for several months (or years) after it's cleaned and soaked. This stimulates fermentation. Trillions of bacteria spawn in the clay and then release enzymes that break it down into those slip and slide platelets that give clay its plasticity.

Primitive potters knew all about fermentation, often storing their clay underground, wrapped in leaves or cloth to keep it damp and separated from the dirt. (They didn't, however, seal it so tight that the microbes couldn't get in.) Some potters even speed the process up by spreading a little vinegar or milk on their long-term storage clay before packing it underground.

Instead of burying their future clay underground, potters in many cultures try the opposite approach. They leave the clay out in the Sun for several months. This finishes the job of chemical weathering and breaks the clay down through that .

Density: When preparing a clay body, another important, though less apparent concern is the clay's density. You don't want your pots to burst at the time they're fired because water vapors couldn't escape outward from the tightly packed clay. To alleviate that threat, potters temper their clay bodies by mixing in coarser material. Sand, beach shells (crushed into a powder), volcanic ash, gravel, mica and grog (already fired clay that's ground up) are typical additives. Sand and grog as the most common.

Heat Resistance and Flux: As discussed earlier, the addition of refractory materials like flint provide strength and allow clay to survive high temperatures. There's also a type of clay, called fireclay, that's know for it's ability to withstand high heat. (More about fireclay on Page 3.) Flux is an agent like iron or feldspar that delivers vitirification at a temperature much lower than 3000 degrees F. The more flux in your clay, the lower your firing temperature.

Shrinkage: All clays shrink when they dry and get fired, some losing up to 20 percent of their mass. That's primarily the water going away, but organic matter in the clay also contribute to the loss. Too much shrinkage or loss of mass can cause a clay object to crack and warp. Potters who throw their wares on a wheel are more likely to encounter problems than hand-builders. It may sound counter-intutitive, but one way to mitigate against the shrinkage is to add even more organic matter -- like straw, paper and sawdust. These materials absorb water in the clay to limit shrinkage. However, when these combustible fillers go away during firing, there's always the potential for leaving lots of holes, so be careful if you include them.

It's a good idea to learn a few standard recipes for clay bodies and their firing requirements. Depending on the raw materials available, there are many ways to go in this regard. Here are a couple of examples:

1. Porcelain clay body: Mix about 25% each of ball clay, kaolin, feldspar, and silica (e.g. quartz). Be careful with the silica, as it decreases plasticity. You might also try using a higher percentage of kaolin for strength. Keep in mind, however, that the maturing temperature of kaolin is 3275 F if no flux is used, which is really high. Including feldspar insures that the clay body will fire well.

2. Stoneware: Mix 70% fire clay, 20% ground pegmatite (a feldspar) and 10% sand. Natural stoneware may also contain kaolin or a little ball clay (or bentonite) to add plasticity. Ball clay and bentonite will increase shrinkage, so add them sparingly (especially the bentonite).

More tips on mixing a good clay body:

Since any variety of clay is hyper-sensitive to environmental factors like heat and lack of humidity, devising the correct blend of ingredients is paramount to your success. Likewise, you're sure to discover that working with clay on a hot summer day is a much different venture than working with clay on a cloudy and damp April morning.

The type of water you use will also affect the clay body. Tap or river water introduces new minerals into clay which can mess up its chemistry. Tap water can be especially problematic, since chlorine and other disinfectants limit healthy bacterial growth on storage clay (which makes it more plastic), or may effect the clay during firing. If possible, set up some barrels to catch rainwater during storms, or use distilled water. Distilling involves evaporating a liquid into vapor by boiling it, then condensing it back to liquid. Sediments and minerals, which are incapable of turning to vapor, will remain at the bottom of the boiling pot. (To build a passive solar still for this task, see Wilderness Survival - Water.) If you can't use mineral-free water to mix your clay, just be sure to screen any river or other water you use for sediments and debris.

Traditional potters think they'll improve their chances of a good outcome by sprinkling maize or by making some other ritual offering at the site where they excavate the clay. The gesture couldn't hurt, but you should also set aside plenty of time to experiment by mixing up different clay recipes to discover what works best for a particular project. Potters typically prepare and fire test tiles (small wafers of clay) to see how they handle the heat. While the tests may seem like a time-consuming chore, once this R&D phase is finished, you can write down the recipe for posterity and never worry about your handiwork falling apart in the kiln.

Any prospected clay specimens you bring home should be labeled and stored with your other raw materials. Information you should document includes specific directions back to the location where the clay was collected, the date the batch was retrieved or prepared, and whatever properties or ingredients the clay (or clay body) may contain. Remember, the not very plastic red clay you collect one day may complement another, overly plastic clay you discover the next. Non-plastic clays can also be used for bricks and tiles, or to make clay slip for casting and painting.

To learn more about the different categories of clay, download this article from CavemanChemistry.com.

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