More Resources
Note: The Pottery and Ceramics section contains four pages.Skip to Page 2, Page 3 or Page 4
Books
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
Websites
Where does clay come from? by Jenny Gulch
Clay's important features by F.H. Norton
Principal Clay Types Used in Ceramics
Hammill & Gillsepie
Types of Raw Clay and Methods of Extraction (PDF)
EPA
Clay and Ceramics Info
Clay Times
Raw materials dictionary
Sheffield Pottery
Description of raw clays
Claymaker.com
Removing sand/gravel from clay
discussion thread
What is Feldspar?
IMA-NA
Understanding Feldspar Minerals
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)
Stoneware Clay
DigitalFire.com
How to test clay recipes
Pottery-on-the-wheel.com
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
Things to remember about Coil Pots
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
Cat litter, antacid and other
peculiarglaze materials
Ceramics Today
Make Pottery Glaze with Simple Household Chemicals
ehow.com
Kiln Firing Chart (PDF)
Cone Temperature Chart
BigCeramicsStore.com
Country Skills and Native Arts
larger image
"Water Under the Bridge". This mini-exhibit created for a college art class highlights the cycle of clay dating back to antiquity. The cycle begins with wet chunks of clay scavenged along a riverbank. It's then formed into shapes like pinch pots and adobe bricks (clay mixed with sand) to build homes. Over the course of time, these creations eventually fall into ruin, becoming shards and eventually dust. Thanks to Ianna Frisby for the idea behind the display.
Pottery and Ceramics
Since prehistoric times, clay shaped and baked in fire has given civilization its cookware, storage containers, plumbing and conduit, roof and floor tiles, works of art and much more. A 5000-year-old terra cotta museum piece and that porcelain crown in your mouth both got their start from a lump of clay. And the first cuneiform writing discovered in Mesopotamia around 3000 B.C. was inscribed on clay tablets.
Much easier to acquire and shape 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, if and when apocalyptic events transpire, ceramics skills will not only avail you to the creature comforts of life, they'll pave the way for learning other critical trades, like masonry, glass-forming, chemistry, and smelting.
Keep in mind that today's ready-made clay formulations and glazes won't be accessible after a mega-disaster strikes. You'll need to learn the older and simpler methods of working and firing clay. This tutorial, divided into six steps, starts from scratch.
In the Northern U.K., a coastal area knowns as the Dunans yields a deposit of green clay in mudstone. Photo: UKGE Limited
Step 1: Find the Clay
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 clay 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. Platelets have a way of sliding along and past each other with just a tad of of moisture, clay has the ideal, a property known as plasticity. Besides that, when the water between the platelets dries out, the platelets compact together to create an almost impermeable barrier. Together, plasticity and impermeability make clay an invaluable material to build with.
When you first start prospecting for caly, you might confuse it with silt, the second smallest particle in the geologist's rock case. A grain of sand is bigger than silt. Gravel, which is pulverized rock, is bigger still. Most of the time you'll find all this stuff mixed up together, whether in the terrain alongside a river, or the ground beneath your feet. Fortunately, pristine deposits of clay are easy to spot once you know what to look for. For instance, when wet, clay has a certain gleam, though it's not quite the degree of luster you get with metal. One tool that facilitates the hunt is a jeweler's loupe. It's a compact, eyeball-sized magnifying glass designed specifically for mineral and particle identification. (You can buy one on Amazon for about $4.)
A sample of rock particles 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 start groaning. This clay is the bane of all plants, shovels and irrigation systems. Depending on where you live, you may be able to dig it up and have a ready supply to use for pottery-making. However, in developed areas, you're not likely to find an un earth-moving equipment has intermixed the different stratas of the soil horizon together. When that happens, the clay becomes contaminated with sand, silt and humus -- a combination better known as dirt.
Another way to tell whether or not you've found clay is to pound a dry specimen down in a tray and examine its texture. It should look and feel like flour. If you wet some clay and roll a ball of it, you can then test its plasticity by coiling a small wad around your finger. Don't be deterred if the material breaks up, though. Some of the most durable porcelain in the world started out from non-plastic clay. For traditional potters, mixing and matching clays to get the right blend of plasticity and strength is an everyday routine.
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, followed by chlorite, illite and montmorillonite (the source of the popular bentonite clay). When any one of these 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 clay platelets, clay minerals can only be seen under an electron microscope. With time and practice, potters learn the subtleties of the different clays, eventually dividing them up into different uses. One type of clay (or clay mix) might work best for tiles, another for constructing water cisterns, another for dishware, and so on.
When clay is mined or dug up near the place of its birth, it's known as kaolin, or primary clay. Kaolin is named after a mountain in China, where porcelain first earned its widespread reputation. In fact, kaolin's toughness and strength are what make a porcelain crown the next best thing to still having the original tooth in your mouth. Kaolin can also be taken internally as an antacid or used as an external treatment for skin infections and inflammation. (In fact, clay as a healing agent is probably one of the best-kept secrets of the modern age, but that's a how-to article for another day.)
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 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, which lies just beyond the foothills of the Western Sierras.
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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
Nevertheless, most people don't have access to primary clay. Rather, it's the secondary clays used to make earthenware, terra cotta and stoneware that are easiest to get at the lower altitudes. Like gypsy travelers, clay minerals in kaolin break off from their childhood haunts in the mountains and tumble downstream along with gold, quartz and other gems. Eventually, the trip ends and they get embedded 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.
With its telltale rubbery look, a secondary clay is easy to spot in its saturated state along the edge of a modern creek or stream. Needless to say, wet deposits are also a lot less work to excavate than the dry stuff. Yet dry clay is lighter, so you can collect larger amounts of it on an expedition. The U.S. Bureau Of Mines classifies six categories of raw clay: kaolin, ball clay, fire clay, bentonite, fuller’s earth, and common clay and shale. (Details about specific clays will be discussed throughout this article.) When it's mined commercially, most clay is first crushed, then stockpiled for several months in the open air before being cleaned and processed. You can learn a lot about the different varieties available by logging onto a the online sites that sell it.
Here are what potters define as the standard classification used 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
Like generations of potters that came before you, you won't have the luxury of shopping around online for ready-mix clay bodies in the wake of a potential doomsday playing out. By far, the simplest form of 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 is trickier. 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. Another place to find raw, undisturbed clay is at sites of hydrothermal activity, like hot springs.
To expedite any searches, here are a few suggestions you have available now:
Do a Google keyword search for mining operations and clay deposits in your area.
Contact the geology department at an area college and ask the instructors if they know of any kaolin deposits nearby.
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 using backroads or trails.
Check a geologic map of your area. (It may be available online.)
With regard to this last option, geologic maps are not particularly user-friendly when it comes to their legends. "Alluvial", for instance, refers to sedimentary depositions, which are an excellent source of secondary clays. Rock formations consisting of primary clay 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. Once you identify potential areas to search on a map, finding a pristine clay deposit can still be a needle-in-the-haystack proposition, so any extra time spent on research will greatly reduce the sweat factor in the field. Needless to say, if there's a clay mine in the region, start your hunt as close as possible to that property.
One major drawback of any prospecting venture is that dry clay can be a challenge to extract without dynamite and backhoes. Hence the conventional wisdom about road cuts and construction sites. Once a deposit of raw clay has been exposed and loosened by somebody else, all that's left to do is fill your buckets.
Dry clay isn't usually this easy to spot but sometimes you get lucky.
Other Cautions and Suggestions
As noted earlier, beware of limestone contamination and sedimentary deposits with high sand or gravel content. Too much 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 good secondary clays if you're patient.
In rural communities, open fields and pastures may have been farmed or used for drainage in the past, which can make them problematic as well. Silt or sand are often mixed with the clay to aid planting and irrigation. That's why the pristine landscape of a roadcut always trumps a construction site.
A few other cautionary notes for secondary clay prospecting:
- Examine the clay for sand and other granular material. If you see a lot of this in your jeweler's loupe, or if the clay feels gritty to the touch, it may not be useable. This is especially true when limestone gets mixed in with clay. (When you fire the clay, the lime swells and will eventually break the object.)
- Wet some clay, then let it dry. If a scum or whitish color appears, it may be contaminated with what are called "soluble alkalies".
- If the clay shows an excessive amount of organic matter mixed in, you may not be able to get it all out when you clean the clay.
Don't forget, while you're on the hunt, keep a vigilant eye out 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) and other rock minerals that provide 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 (discussed on 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 the 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 well invite negative repercussions should you decide to dig there.
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Continued on Page 2... (Producing a Workable Clay Body)
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