Hands on Clay: An Introduction to Ceramics by Charlotte Speight and John Toki Buy now...
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 Buy now...
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 Buy now...
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
Where does clay come from? by Jenny Gulch
Clay's important features by F.H. Norton
Principal Clay Types Used in Ceramics
Hammill & Gillsepie
Clay and Ceramics Info
Raw materials dictionary
Description of raw clays
Removing sand/gravel from clay
What is Feldspar?
How to Dig and Process Your Own Clay video Ceramic Arts Daily (scroll down)
How to test clay recipes
Ancient Chinese Method of Processing Porcelain Clay and Creating Ceramics
Nanhai Marine Archaeology
"Ancient Chinese pottery confirmed as the oldest yet found." Guardian, U.K. 6/28/12
Steps for Making Traditional Borger Cordmarked Pottery
Texas Beyond History
How to Make a Pinch Pot video
How to use an extruder
Create Your Own Homemade Foot-Powered Extruder
Ceramic Arts Daily
A Photographic Tour of Firing Pottery (using the open fire method)
The art of ceramics
Learning to Throw Pottery
Kiln Firing Chart (PDF)
Cone Temperature Chart
This mini-exhibit created for a college art class highlights the cycle of clay. It all starts with wet chunks of clay scavenged along a riverbank (lower left corner). The clay is formed into shapes like pinch pots and adobe bricks (clay mixed with sand) to build homes. Over the course of time, these creations fall into ruin, becoming shards and eventually dust. Thanks to Ianna Frisby for the idea behind the display.
Since prehistoric times, clay shaped and baked has given civilization its cookware, storage containers, plumbing, roof and floor tiles, works of art and so much more. Whether it's a 5,000-year-old terra cotta amphora in the Louvre,or that porcelain crown in your mouth, all ceramics begin from a simple lump of clay. Not only that, the earliest cuneiform, dating to around 4000 B.C. was inscribed in clay and fired as tablets.
Much easier to acquire and form than either metal or wood, clay also involves fewer tools and no brute strength whatsoever. With little more than your two hands and a campfire, you can create products that will last for millenia. More importantly, should apocalyptic events transpire again on Earth, those who survive to start over will have an easier time if they know something about this age-old profession. Besides the ability to fabicate much needed commodities, ceramic skills provide a bridge to other trades, like masonry, glass-forming, alchemy (chemistry), smelting and casting.
Keep in mind that today's ready-made clay bodies and glazes probably won't be accessible once a mega-disaster strikes and you have to evacuate. Consequently, you'll need to learn older, traditional methods for mining, mixing and firing clay. This tutorial, divided into six steps, provides a portal for getting up to speed quickly.
In the Northern U.K., a coastal area knowns as the Dunans yields a deposit of green clay in mudstone. Photo: UKGE Limited
Clay has many definitions, so knowing when you've found the right stuff is not the slam-dunk task you might think. Field geologists describe it vaguely as the smallest of rock particles. Under a microscope, these particles are a hundred times longer than they are wide. That's why chemists prefer the term platelet when discussing clay. Moist platelets have a way of sliding along and past each other, a property known as plasticity. Then, when the water between the platelets dries out, the platelets compact together to create an impermeable barrier. Together, plasticity and impermeability make clay an invaluable material for construction.
When your hunt for clay begins, you might confuse it with silt. Silt is the second smallest particle in the ground beneath our feet. A grain of sand is bigger than a particle of silt. Gravel is bigger still. As elementary as all this seems, it's unfortunately the case that you will often these different substances all mixed up together. That's why the goal of your search is to find an undisturbed deposit of clay. One tool that facilitates the hunt for it is a jeweler's loupe. This compact, eyeball-sized magnifying glass is designed specifically for mineral and particle identification. (You can buy one on Amazon for about $4.) Also, when wet, clay has a certain greasy gleam, though not quite the same luster of metal.
A sample of rock matter from oil drill cuttings, magnified ten times. An ideal clay deposit will contain as little of this stuff as possible - especially the limestone, which disrupts the firing process of clay.
Now, if you ask gardeners for their definition of clay, they'll point to the hardpan under the topsoil in their backyards and groan with misery. This clay is the bane of all plants, shovels and irrigation systems. But it's still clay and you might be able to put it to good use. In developed areas, you're not likely to find a pristine deposit though, since earth-moving equipment intermixes the different stratas of the soil horizon together. When that happens, the clay becomes contaminated with sand, silt and humus, a combination otherwise known as dirt.
One way to identify a workable clay is to roll a wet ball of it, and try coiling a small wad around your finger. Don't be deterred if the material breaks up. Some of the most expensive china in the world started out from a non-plastic clay. It's quite common to mix a couple different clays and add a few other things to make it just right for whatever project you're working on. But we'll get to that subject in a minute. Suffice to say that potters who have access to a naturally plastic clay that can be easily formed, accepts a glaze and fires easily in a kiln without any tweaking have it pretty easy.
A potter in New Zealand extracts gray terra cotta clay from a ledge with a spade.
Photo: Danny Holland
Types of Clay
Mineralogists equate clay with four minerals. Kaolinite is the most common, and derives from the feldspar family, which includes minerals like the ones you find in granitic rocks. Over the course of a long time the feldspathic minerals break down into a crumpled heap. This happens because the rocks become exposed, or rainwater seeps down through the formation, or hot gases rise up from beneath. Other clay minerals include chlorite, illite and montmorillonite, like the famous bentonite clays found in Wyoming. When any one of these minerals is found in high concentrations, the clay is said to be "fat". If not, the deposit is considered "lean" and classified as common clay. As far as distinguishing which is which, your jeweler's loupe won't be much help. Like platelets, clay minerals can only be seen under a microscope. With time and practice, however, potters learn the subtleties of the different clays, eventually dividing them up for different uses. One type of clay (or clay mix) will work best for making flat tiles, another for constructing water pipes, another for dishware, another for thin-walled pots thrown on a wheel, and so on.
When clay is mined or dug up near the place of its birth, it's known as primary clay. The kaolinite described above fits in this category. If the color is white, then the clay is often classified as kaolin. This word is taken from a mountain in China, where the field of ceramics developed a lot sooner than in Europe and elsewhere. Porcelain is one of the highest grades of kaolin, or white clay. When it's used to make dishes, we refer to it as china. Dental crowns are also made of porcelain. The hard part about making china, even if you can find porcelain clay, is that you need to get your kiln really hot in order for it to mature. This was the big secret of the Chinese, and only in 1710 did a ceramics shop in Meissen, Germany, finally figure it out.
Ancient amphorae recovered from a ship that sunk around the time Jesus of Nazareth lived. Amphorae were already being mass produced from terra cotta clay. The shape was designed for easy packing in the hull of ships.
Sprinkled around the United States, kaolin and other white clay deposits may be lodged in plutonic rock formations, sleeping next to coal deposits, or buried in all kinds of other interesting places. This includes California's 200-mile-long Ione Formation, located at the base of the foothills that rise into the Western Sierras.
At left, a commercial kaolin mine. Notice the red earth that covered the clay before earth-moving equipment stripped the cover bare. Excavating your own small supply of clay usually means sleuthing out an already exposed deposit. At right, a handful of kaolin. As a test, see if you can get a small piece to stick to your tongue. Kaolin is extremely water absorbent. Photo/right: Pyromasse Montreal
Another category of white clay is fire clay. This material contains refractory minerals like mica and quartz, as well as sulphur. Geologists refer to it as "underclay" when it's in a layer beneath a coal deposit. Fireclay is not very plastic but can be added to ones that are to produce stoneware clay. Most of the everyday dishware we use is produced from inexpensive but high-fire stoneware Fire clay by itself is widely used to make bricks that line the inside of kilns, large ovens and chimneys. Its refractory properties make it ideal for use in places that get really hot.
As mentioned earlier, bentonite is a clay made from montmorillonite. It's incredibly plastic. Of course, the down side of plastic clays is that they shrink a lot more than other clays when fired. Shrinkage causes warpage and cracks, so only a little bit of bentonite can be added to a less plastic clay to make shaping it easier.
Ball clay is more often used in conjunction with nonplasic clays like kaolin to make a workable clay body. It's a fine-grained clay with a lot of organic matter, which is what makes it so plastic. It's also known as a secondary or sedimentary clay, since it's not found near its parent rock. Secondary clays are carried down a river or on the back of a glacier, eventually deposited in strata, or layers, along riverbanks, or at the bottom of a marsh or lake. The bodies of water in question may be ancient ones that have long disappeared. Or they may be waterways flowing today. Ball clay got its name in England, when miners use to roll it up in balls for transport.
Earthenware and terra cotta are two other examples of sedimentary clays. Becuase of the long trip downstream, these clays contain a lot of iron and other impurities. That causes them to mature in a kiln at much lower temperatures. They're not as dense and heavy as stoneware, and they have a reddish color after being fired.
For its part, the U.S. Bureau Of Mines classifies six categories of raw clay: kaolin, ball clay, fire clay, bentonite, fuller’s earth, common clay and shale. When it's mined commercially, most clay is crushed, then stockpiled for several months in the open air before being cleaned and processed.
Here's what potters define as the standard classifications of clay in ceramics work:
Earthenware: Made from the most abundant secondary clays on Earth, earthenware is associated with flower pots, roof tiles and terra cotta pottery. It usually contains a lot of iron, so will mature between 1750 and 2150 degrees farenheit, making it a low-fire clay. The raw material may be red, orange, buff, green, gray, white or brown.
Stoneware: Also derived from secondary clay, stoneware has less iron and more refractory (i.e. heat-resistant) material added, like feldspar, mica and quartz. This makes it stronger than earthenware. Since it also fires at a higher temperature, it creates a non-porous, impermeable ceramic. Most dinnerware is manufactured from stoneware. It matures between 2200 and 2400 degrees farenheit.
Porcelain: Made primarily from kaolin, this primary clay is usually excavated near where the original rock/mineral deposit formed. In its pure state, its color is white. Porcelain dishware (aka china) is made from kaolin, which is why some people refer to it as china clay. The most heat-resistant and durable of all clays, porcelain starts maturing at 2400 degrees farenheit, making it a high-fire clay.
Locating clay deposits
By far, the simplest method of clay prospecting is to search the banks of lowland marshes and streams, at the bottom of lakes and ponds, and along creek and riverbank ledges where erosion has cut into the rock strata.
Identifying dry clay can be tricky. For one thing, primary clay deposits are far more limited in distribution than secondary clays used for stoneware and earthenware. When traveling in higher altitudes, check roadcuts for exposed rock and be ready to identify kaolin, which is colorless and sticks to your tongue. Another place to find raw, undisturbed clay is at sites of hydrothermal activity, like hot springs. You can look for deposits of fine-grained clay minerals in sedimentary rocks (like shale and mudstone), or metamorphic rocks like slate.
To expedite your search for clay deposits, here are a few suggestions :
Do a Google keyword search for mining operations and clay deposits in your area.
Check a geologic map for your area.
Use Google Earth to search for roadcuts, construction areas, rivers or other geography where a clay layer may be exposed. You can also use the map to figure out how you can access remote regions via backroads or trails.
Contact the geology department at an area college and ask the instructors if they know of any kaolin deposits nearby.
Geologic maps are not particularly user-friendly when it comes to their legends, but they do get the job done. "Alluvial", for instance, refers to sedimentary depositions, which are an excellent source of secondary clays. Rock formations consisting of primary clay, on the other hand, are referenced by geographical names rather than rock type. For instance, in California the "Ione Formation" is mentioned, but not the fact that it's the motherlode of kaolin in the state. Needless to say, if there's a clay mine in the region, start your hunt as close as possible to that property without trespassing or violating any mineral extraction laws that may apply.
One major drawback of any prospecting venture is that buried clay deposits can be a challenge to extract without dynamite and backhoes. For small amounts, you're best bet is to scavenge road cuts and construction sites. Once a deposit of raw clay has been exposed and loosened by somebody else, all that's left for you to do is fill your buckets.
Dry clay isn't usually this easy to unearth but sometimes you get lucky.
Other Cautions and Suggestions
As noted earlier, beware of limestone contamination and sedimentary deposits with high sand, gravel or sulphur content. Too much of any of these materials can make the clay's extraction more trouble than it's worth. Creeks and streams that run downhill of gold or coal mining operations (even long-ago mining) are prone to this mingling of ingredients. A geologic map labels such terrain as "tailings", although you can sometimes find uncontaminated secondary clay deposits in the area.
In rural communities, open fields and pastures may have been farmed or used for drainage in the past, which can make them problematic for clay prospecting as well. Silt and sand are often mixed with the clay to aid planting and irrigation.
Here are the general rules of thumb regarding clay prospecting:
Don't forget, while you're on the hunt, keep a vigilant eye open for other raw materials you can use for ceramics. These include flint, quartz, feldspar, talc, volcanic ash, sand, dolomite, calcium carbonate, iron oxide (i.e. rust) . Rock minerals (in moderation) add strength to a clay body, offer color potential (for painting or mixing with the clay body to change its color), or refractory properties (i.e. heat resistance) to help it survive firing. Potters routinely grind these materials down and incorporate them into their clay bodies to serve various purposes. Anything added to clay is known as temper. More about this on Page 2. Some of these ingredients are also used to produce waterproof glazes (see Page 3).
Tools to bring along on a prospecting trip include a pick, shovel, buckets or trash bags, a jeweler's loupe, dust mask or scarf, safety glasses (to wear while chipping at rocks), towels, and water (to test for plasticity). Don't forget a first aid kit, especially disinfectant, so you can immediately treat cuts and scratches that occur frequently in field geology. Make sure you keep your tetanus booster up-to-date.
One other caviat: Entering someone's private property to prospect, or mining clay in public parks, is illegal unless you get prior written permission. You're likely to have more leeway for small-quantity extractions on non-park lands controlled by the Bureau of Land Managment, the U.S. Forest Service, state fish and game departments, and local municipalities or counties. Most waterways, even where they run through private land, have an area on either side that's designated public property. Prior to an expedition, check with whoever has jurisdiction for any rules or prohibitions that may be in force. If you see a sign identifying terrain as a wildlife habitat, it may invite negative repercussions should you decide to dig there.
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