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
"Water Under the Bridge". This mini-exhibit created for a college art class highlights the cycle of clay. The cycle begins with wet chunks of clay scavenged along a riverbank. It's 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 and conduit, roof and floor tiles, works of art and much more. Whether it's a 5,000-year-old terra cotta museum piece, or a porcelain tooth, these items got their start from a simple lump of clay. Even the cuneiform writing that dates back to 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 endure for millenia. More importantly, if and when apocalyptic events should transpire on Earth, ceramics skills will not only avail you to the creature comforts of life, they'll pave the way for the development of the other critical trades. These include masonry, glass-forming, chemistry, and smelting and casting.
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 mining, mixing 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
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. 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 you first start your hunt for clay, you might confuse it with silt. Silt is the second smallest particle in the world of geology. A grain of sand is bigger than a particle of silt. Gravel is bigger still. Much 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. And you can't really separate the ingredients. That's why the goal of your search is almost always a pristine deposit of clay. One tool that facilitates the hunt for it is a jeweler's loupe. This a compact, eyeball-sized magnifying glass designed specifically for mineral and particle identification. (You can buy one on Amazon for about $4.) Also, when wet, clay has a certain gleam, though not quite the same luster of metal.
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 groan with misery. 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 a pristine deposit, 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.
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 a 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. 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 platelets, clay minerals can only be seen under an electron 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. This is also referred to as residual clay. If its color is white, then the clay is generally classified at kaolin. This word is taken from a mountain in China, where ceramics developed in sophistication far ahead of European and other cultures. 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. A lower grade of primary clay is known as stoneware. This material contains other things beside clay, usually minerals like calcium, feldspar and iron. It's used to make everyday dishes and coffee cups.
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 neither plastic nor suitable for ceramics work, but is widely used to make bricks that line the inside of kilns, large ovens and chimneys.
Finally, Bentonite is an impure clay made from montmorillonite. This is a highly plastic clay. However, since it shrinks a lot more than other clays when fired, it's generally added in small quantities to a less plastic clay to make shaping it easier. Bentonite is the main ingredient of most cat litter and also used extensively as a drilling mud. Also, this clay may be taken internally as a laxative, applied externally to combat some forms of dermatitis, or mixed with degraded soils to produce higher crop yields . (According to a 2012 article in Natural News, Pascalite is a creamy white-colored form of bentonite clay that has been found to heal a wide variety of ailments ranging from brown recluse spider bites, to skin necroses and digestive tract ulcers. Cataract patients applying a clay paste to their eyelids and making eye drops from water filtered through the clay claim their cataracts have been dissolved.)
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
Primary clay is a lot rarer than secondary clay, which is found in current or ancient river beds. Also known as Sedimentary (or placer) clays, these deposits are composed of layers of fine clay particles that started out at a higher elevation. Like gypsy travelers, the clay minerals in kaolin broke off from their childhood haunts in the mountains and tumbled downstream along with gold, quartz and other gems. Eventually, the trip ended and they were 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.
Found in abundance worldwide, these clays are not nearly as hard and durable as porcelain, but they're more plastic and therefore easy to work with. Earthenware and terra cotta clays are sedimentary clays with a huge dose of iron oxide mixed in, accounting for their reddish color. These clays are more fragile and porous when fired. Ball Clay is a fine-grained, sedimentary clay that may include a lot of organic matter. lt has more plasticity than terra cotta clay, and for that reason a small portion of it if frequently mixed with primary clays to create a more workable clay body.
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.
To summarize (sort of), 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 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
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 spot 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.
- - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - - - - - - - - - - - -
Return to Mega-Disaster Planner
- - - - - - - - - - - - - - - - - - - - - - - - - -