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
What is fire clay and where to get it
Clay and Ceramics Info
Raw materials dictionary
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
Pottery Making DVD by Cindy Clarke
Kiln Firing Chart (PDF)
How to make Egyptian Paste
The Great Linford Brick Kilns, U.K. Photo: theparkstrust.com
Step 5: Fire the Clay in a Kiln or Open Fire
Although we all wish it were so, the job of firing clay objects is not as straightforward as baking a Betty Crocker cake mix in the oven at 350 degrees for half an hour. Granted, primitive people on remote islands produced fired clay items everyday with their eyes closed. But for every generation of potters, the learning curve can be steep. That's why it's a good idea to understand what happens to clay after the kiln door is shut and the heat inside starts to climb.
As a rule, the temperature is slowly raised, which allows any residual moisture to get "smoked" out of the pots. Water escapes as vapor into the air once the temperature reaches its boiling point at about 212 degrees farenheit.
At 600 degrees F, any organic materials in the clay start burning off afte. If a dead grasshopper happened to get mixed into your clay body, there will now be a cavity in your pot shaped in his exact dimensions. (This is why it's important to wash and screen raw clay. A little organic debris is no big deal, but a lot of it can leave your pot porous and susceptible to water seepage after it's fired.)
As the temperature inches past 1050 degrees F, quartz inversion takes place. This is the nail-biting portion of a kiln firing. Any silicates in the clay actually expand a little in advance of the inevitable shrinkage to come. To mitigate against cracks and distortion in their meticulously created wares, modern potters may take eight hours or more to reach the maximum heat inside their kilns. (It's also why they make every effort to use the right clay formulation in the first place.)
At 1700 degrees F, the clay and refractory particles partially melt and fuse together. This is called sintering. As the particles compact, the clay object shrinks, hopefully throughout the piece at the same rate, so there won't be any cracks. Making sure the heat hits all sides of the clay (including the bottom and inside) evenly is paramount to the success of a firing. The end result is a strong, permanent single mass of ceramic that can never again be dissolved with water. Many potters cut the heat at this point so they can add a glaze and fire the piece again. When they do this, the first pass is called a bisque firing.
But the process of firing is not yet complete. The next to last step in the firing schedule is referred to as partial vitrification. Here, the maximum temperature is reached (see the numbers below), causing the clay particles to bind together as tightly as physically possible. From the potter's perspective, the ceramic has now reached its peak hardness and strength. If any glaze was added, that concoction vitrifies as well, permanently cementing itself to the ceramic walls. At this point, the clay walls of a pot should become non-porous, which is essential to limiting or preventing seepage once the new pottery gets used.
The last step of any firing is cooling down the temperature slowly. As in steelmaking, maintaining heat in a newly-formed ceramic - rather than suddenly quenching it with water or air cooling - tempers it, and that means long life and durability. The ancient Romans first perfected the art of tempering on the swords used by rank-and-file soldiers. (Before that, the swords broke frequently and had to be replaced.) In the case of a fired ceramic, potters withdraw the heat gradually or simply leave the kiln door closed for several hours after firing. (If the "kiln" happens to be an open fire, the potter covers the ceramic with soil or ash once the fire is put out.)
Depending on whether you're using earthenware, stoneware or a porcelain clay body, the maximum temperature for hardening, or maturity, varies:
Earthenware: Since it contains a lot of iron, which is a flux, earthenware and terra cotta mature between 1750 and 2150 degrees F, making them low-fire ceramics.
Stoneware: With less iron, this secondary clay (or combination of clay types) matures between 2200 and 2400 degrees F. That makes it far less permeable to water absorption than earthenware and less brittle as well.
Porcelain: The most heat-resistant and durable of all clays, kaolin starts maturing at 2400 degrees F, making it a high-fire ceramic.
Dry Your Clay Objects Thoroughly Before Firing
Before submitting pots or other clay objects to the heat, however, it's critical to dry out the work thoroughly. The pitfall that all potters shudder to think about is the explosive power of water vapor when heat rises above 400 degrees farenheit. A pot or other clay object will burst if there's too much moisture in it. Greenware (another term for unfired pottery) takes a long time to dry, longer than most people think. If you're living in an arid climate, it's less of a problem than if you're anchored in San Francisco during the summer, or hunkered down for winter in most other places. Humidity essentially determines the time frame. What you're shooting for is pottery that's bone-dry.
Guard against uneven drying: Typically ceramicsts store their greenware in plastic sacks, a closet or some other enclosed space near the kiln. This promotes even drying throughout the forms. If objects sit out in the open air, the exteriors dry more quickly, potentially causing cracks or warping. For thick-walled objects, keep the exteriors damp for the initial phase of drying. That will equalize the drying rate overall.
Hollow out thick clay centers: Another pre-firing step that's critical: Any solid clay objects, like sculpted artworks, must be hollowed out to avoid an explosion in the kiln. A pear pitter or other loop tool is used to dig into the middle of the object before it dries and extract all the excess clay. Afterward,use your needle tool to poike holes into the piece. This provides any vapor that gathers in that hollowed center a pathway out. Remember, gases expand when the temperature rises.
Hollow out thick clay pieces (left) then poke holes (if necessary) to provide a path for vapors to escape (right). Photos: Ceramic Arts Daily (left) and Taiga Works (right).
Drying with fire: In some cultures, potters expedite the drying process by setting afire leaves or other materials inside pots. alternatively, the greenware is stacked around an outdoor fire and turned constantly so that the pieces dry evenly. (This timesaver can also reduce the potential for thermal shock when the actual firing begins.) Just remember, dry clay objects are extremely vulnerable to breakage. Try to minimize their transport distance to the kiln prior to firing. You can also pack greenware in pillows or other makeshift cushions to mitigate against vibration and movement.
An example of the open-fire technique. (1) After a warming fire has dried the pots, they're stacked in the center of the fire (between or on top of rocks), as the hot coals continue to burn around them. (2) Kindling is then very quickly added to the fire and logs set up in a tee-pee style, enclosing the pots. (3) Once most the logs have burned down (i.e. 30-90 minutes later), moss or other damp plant material is piled over the blaze (an optional step) to suffocate the oxygen and create a reduction atmophere, which turns the undecorated portion of the pottery black. Wait for the pots to cool a little before removing them. Photos: NativeTech.org
Open fire and fire pit: Long before the the first kiln was invented, potters fired their wares in far less ideal conditions to induce maturity. In the event of a technological collapse, you may need to consider a few of these earlier methods. Firing began with open fire clay bakes, where pots were wrapped with leaves, branches or palm fronds to keep the clay safe from the flames. The items were placed on twigs or dried rocks, or some type of grate in order for the heat of the fire to reach the bottom of each piece as well as the top, sides and inside.
Open fires are still widely used in many places. It takes 30 minutes to two hours for low-fire earthenware to get fired. Stoneware and porcelain are another story. To get the high heat necessary for these clays requires some resourcefulness. Native American potters enclose their wares with sheet metal and light the fire around this makeshift kiln. Because metal conducts heat just as it does electricity, this approach evenly radiates heat despite the use of an erratic fire.
If you build an open fire, you'll have to use whatever fuel is at hand. Cow dung (or dung from others hooved animals) burns more steadily than wood and is an excellent choice. Primitive potters often heap the dung right onto the pots before setting their fire. In some locatings, bundles of grass are used as a fuel source. These, too, burn more evenly than wood, but it takes a lot of them to keep the blaze going for two hours. Coal is another good source of fuel, but may darken the clay
The big challenge with open firings is controlling the wind. Even a slight breeze can trigger a spike in temperature that will cause the clay to melt or slump. One way to mimimize wind gusts is to dig an above-ground fire pit. The surrounding wall of earth will block the breeze and allow you to better control the flames. On the other hand, potters in Fiji take advantage of tradewinds to increase the temperature deliberately. This enables them to use a wider range of refractory materials and stoneware clay bodies.
The second big problem with open fires concerns flatware. Clay items like plates, casserole dishes, pot lids, tiles and protrusions (like legs, handles, and spouts) are unlikely to survive the open-fire experience. Uneven heat distribution means uneven expansion (during quartz inversion) and uneven shrinkage through the rest of the firing. Enclosed pots manage to avoid this danger because their mass is shaped in a way that circulates the heat. Not so with flat objects. To get around this limitation, you can experiment with attaching two or more pieces together to simulate the enclosure of a pot. Make the joints loose enough (like a serration), so you can cleave the pieces apart easily after they're fired.
Underground fire: The next innovation to come along in the history of ceramics was the underground fire. Primitive tribes stacked pottery wares at the bottom of a dugout hole, then lit a wood fire on a platform directly above them. Alternatively, the fire might be built underneath the platform, with holes perforated in the floor to allow the heat to move upward and out through a narrow chimney. Either way, the greenware now fired more evenly in a more controlled environment. The platforms also prevented contact with the flames, so pots wouldn't be singed.
A fire clay deposit (left) and what it looks like up close. Photos: TraditionalOven.com
Kiln firing: Eventually, the first above-ground kiln was built. Already fired ceramic material called firebricks proved the ideal construction material for the ovens because of their high heat resistance. The bricks are made of fire clay, which is higher in alumina content than other clays -- around 30 per cent, compared to a silica content of 60-70 percent. This type of raw clay is extremely common in nature and has a whitish-yellow, pink or very light brown color. It's melting point is nearly 3000 degrees farenheit, making it ideal for chimney, furnace and kiln construction. You can also mix the powder clay with sand and water to produce your brick mortar.
At left, a typical ancient Greek kiln was beehive-shaped. It used an updraft technique, with a rectangular fuel box (see left corner of right image). First, the kiln was heated with an ample wood fire, good air circulation (aka oxidation) and all its peep-holes open. When the temperature reached about 850°C, indicated by the cherry-red color of the fire, the chimney and peep-holes were blocked, producing a reduction atmosphere. That influenced the colors on the ceramic exteriors. At right, a Greek pot fired using the reduction method. The unpainted surface turned black when a reduction atmosphere was present. Left photo: University of Illinois Urbana-Champaign
How to measure the temperature: Knowing what the actual temperature is inside a kiln at any given moment presents a long-running conundrum for the potter. In centuries passed, it was deduced from the color of flames. A cherry red color from burning wood, for instance, puts you in the ballpark of 850 degrees farenheit. In more modern history, pyrometric cones (aka Orton cones) came into use. These are small but tall, narrow pyramids that droop and melt at specific temperatures, anywhere from 1000 degrees F to 2500 degrees. Looking through the kiln's peep hole, the potter gets a rough idea of the current temperature by noting the cones that are tipping over or have already melted, versus those that remain in tact.
At left, cones on top shelf before firing. At right, cones visible inside a glass kiln. When each cone slumps over, it signifies that a temperature threshold is reached. Photos: Gary Jackson (left).
Unfortunately, you're not likely to be packing any pyrometrics in your bug-out bag after a doomsday strikes. Instead, experiment with ways to create these cones, using your different clay bodies and recipes. In a primitive situation, a temperature reading is relative anyway, and that's the idea behind using a disposable clump of clay to gauge it. Make a series cones that each melt at a different temperature, then write down the recipes. That way, on days when you're not mining clay or shaping pottery, you can work at building up your cone inventory.
Firing temperatures and times: Otherwise known as the firing schedule, these vary for different forms and clay bodies. Greenware is said to mature when it hardens into a ceramic. To arrive at the right firing schedule, the potter factors in the type of clay body to be fired and a general idea about how much mass (the total number of objects) that will be packed into the kiln. Pottery books and websites offer charts to get you in the ballpark, but test firings are the most reliable way to determine how your forms will react to the heat. In modern practice, the heat is slowly raised to 450 degrees F over the course of two to eight hours. It's customary during this time to leave peep holes and vents open so steaming water vapors can move out of the kiln.
Once the steam threshold is crossed, the heat is raised again to the final firing temperature. It may be kept there for several hours more. Then the heat is withdrawn. Usually when the temperature goes below the 200-degree will the potter enter the kiln and start moving the ceramics out.
A kiln operating in a Burmese refugee camp near the Thai border. Info. Photos: CESO
Generally speaking, earthenware matures between 1750 and 1950 degrees farenheit, depending on the iron, flint and feldspar content. Stoneware and porcelain clays require a temperature between 2200 and 2500 degrees to mature.
An optional step in kiln firings is to cut off oxygen for a time in order to affect the coloring of the pottery. While the fire continues to burn, oxygen molecules are sucked out of the pottery, causing the clay to change color. The ancient Greeks made good use of what chemists call a reduction atmosphere to change the base color of their pottery black. That allowed the painted images to stand out.
Step 6: Add a Glaze
Although pottery hardens in a kiln, earthenware and stoneware fire at too low a temperature to induce waterproofing. Instead, the clay remains porous enough to allow water or other liquids to seep through the walls. To address this shortfall, potters in the past developed a next step in the ceramics process: glazing. A glaze is liquid glass, formed by combining silica, alumina and a flux agent (often a metal oxide). While the modus operandi is similar to the formulation of a clay body, the outcome is much different. When a glaze vitrifies in the kiln (or open fire), the result is a transparent sealer permanently cemented on the ceramic's walls.
A typical glaze recipe might include feldspar, silica (like sand, quartz or flint) and a little kaolin. But into the mix potters routinely toss borax, bone ash, talc or Epson salt, all in an effort to insure a smooth firing process. Of these additives, talc is probably the most popular. It furnishes stabilityand consistence to low-fire glazes for earthenware (i.e. it stops the glaze from dripping), while at the same time reducing the stress of thermal shock. Extracted most efficiently from soapstone, talc is also found in white and blue schists. Most people are familiar with it in the form of talcum powder. Talc's high magnesium oxide content is key to its contribution in ceramics, but dolomite, calcium carbonate (aka whiting) and a few other carbonates serve much the same purpose in a glaze.
Unfortunately, the standard recipes used for mixing glazes today read like the EPA's most-wanted list. Lead glazes are a long-time favorite, but the lead itself is toxic and not something you want your water, wine or food to come in contact with. Another popular glaze ingredient, barium oxide, is likewise hazardous. To make things worse, there's the practice of burning sodium in kilns to stimulate a natural glaze on the ceramic. This produces toxic fumes that can and do escape out into the environment.
There are better ways to waterproof your pottery. In the case of a simple water container, for example, glazing may even be counter-productive. That's because on hot days, unglazed ceramic walls allow much needed evaporation and condensation, which keeps the water from heating up inside the vessel.
Wine and milk, on the other hand, do require waterproof storage. High-fire clays like stoneware and porcelain have much lower absorption rates (i.e. porosity) after firing than earthenware, so you may not need to glaze them at all. In addition, porosity naturally decreases over time in any ceramic that holds liquid. That means you can take older water vessels and reassign them after about a year for perishable liquid storage.
Natural Ash Glaze
Another healthy option for stoneware and porcelain is to use an ash glaze. The ash is generated from burnt plant matter - grass, straw, wood, stalks, seaweed, etc. Depending on your source of material, these ashes may be more alkaline or more acidic. It helps to know the pH, since acid ashes work better for a high firing range, while more alkaline content has a flux effect, reducing the amount of heat that the glaze can handle. (See Basic Chemistry for an explanation of acids and alkalis.) At any rate, you can perform test firings on square tiles to see how the glaze performs, then adjust your ash source or firing schedule if something goes wrong.
To make an ash glaze, light a campfire and burn the plant material. Ashes are extremely caustic, so wear a dusk mask whenever handling them dry. After they cool down, soak them in water. Any unburned fragments should be skimmed off. . A fine-particled ash should be your end result, so if you've got a sieve or mesh screen, put it to use. (The procedure may require several soakings.) A simple formula calls for half ashes, half feldspar (and/or talc or other carbonates). Alternatively, you can use 20 percent kaolin and the other 80 percent divided between ashes and feldspar/carbonates. Mix these ingredients into water and brush them onto your ceramic ware.
Like most glazes, the ash formula is applied to bisque-ware, which is already fired pottery. Once glazed, the ware is fired again at a higher temperature for a shorter period, just long enough for the glaze to vitrify and attach to the ceramic walls.
Glazing a pinch pot.
In some instances, potters dry-dust the glaze ingredients over the fired ceramics, In others, it's applied in liquid form. Care must be taken to include a heat-resistant holder or kiln spurs to catch any glaze dripping down off the ware as it cooks in the kiln.
Check potter books on the internet for more information on glazing. Glass itself, incidentally, is created from a melted mixture of white sand, soda ash and lime.
Egyptian Paste: The Alternative to Glazing
Long before the first glaze was invented, potters in the Nile Valley stumbled upon a far simpler solution to sealing their wares. Nowadays, it's known as Egyptian Paste. Originally, it was not a separate product from the clay body, but a chemical reaction naturally triggered when the clay dried. Potters noticed that soluble salts migrated out from the middle of clay to the surface. When the clay was fired, that salt generated a glossy water-resistant seal on the walls.
In other words, Egyptian Paste is a self-glazing, low-firing clay body. Sand, clay, potash, feldspar and soda ash are typically included in the clay body recipe to achieve this effect. Needless to say, once you leave a newly formed clay object to dry, it's essential not to disturb any surfaces as the salts begin forming on them. You'll also have to place the greenware on stilts during firing, so the paste doesn't drip down and glue itself to other solids. Watch out for toxic fumes the salt may produce during firing.
Although it's not entirely constructed from easy-to-find materials, here's a recipe for a clay body that replicates the Egyptian Paste effect:
36 gm feldspar
12 gm kaolin
2 gm bentonite
6 gm Natriumbicarbonate
2-3 gm copper carbonate
As you can see below, the copper produces a natural torquoise color, but this isn't a great recipe for making pottery, so you'll have to experiment to find the right blend of ingredients.
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