HOW TO Rework Clay & Fire Without A Kiln   BELOW

HOW TO

Dig Your Own Clay

Scroll these photos all the way down for all you need to know.

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Updated July 7, 2016, Marvin Bartel author bio
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by Marvin Bartel, Emeritus Professor of Art, Goshen College, Indiana, USA
July 7, 2016 update © Marvin Bartel. - CONTACT the author

These high school ceramics students are clay prospecting. They not only made pottery from the clay, they built a kiln and wood fired the pots in the kiln. Eric Good Kaufmann, their teacher, is an accomplished potter and teaches art at Bethany Christian High School, Goshen, Indiana, USA. Mr. Kaufmann is an alumnus, class of '97, Goshen College.

PROCESS to REWORK good clay that becomes too dry to use. Processing self-dug clay follows below and self-dug clay is shown in the photos to the left on this page.

  1. To reprocess hard clay it must first be totally dry.  There is no need to break up dry clay.  
    PRECAUTIONS:
    Be sure it has no plaster chips in it - plaster causes pop-outs when bisque fired.  Leather hard clay or moist clay does not slake well because it is not porous like dry clay is.  Instruct students to handle the dry clay without making dust.  Airborne dust is not healthy to breath. Other contaminants such as paper, sponges, and so on, may cause mold to form in the clay, in addition to being a nuisance in the clay.
     
    see health hazards page
  2. Place the totally dry lumps in clear water in something like garbage cans. Use enough water so clay is totally under water. Just let it set in clear water.
  3. Never stir it.  Stirring clogs up the porosity and prevents good slaking (soaking to mush).
  4. In a few days or less, even huge chunks of dry clay will slake to mush. Go to step 6 below and dry it enough to use as in steps 7, 8, and 9.

SELF-DUG CLAY VARIATION
If you dig clay yourself, it often has impurities that need to be removed.
Most kids love to help with this and there are few better learning experiences. If you are a teacher, invite students to bring in samples for testing. If it works well, ask them to bring more.

  1. Let the clay become totally dry.
  2. Slake it as described in 3 above.
  3. When it is all soft and mushy, stir it until it is a slip. I use a mixer on an electric drill or a blunger.  Add water if needed to liquefy it. 
  4. Pour the slip through ordinary window screen available at any building supply store.
  5. The screening removes stones, roots, and other trash that causes trouble. The chief culprit is limestone. Limestone, like plaster, pieces cause pots to break after firing.
  6. When the clay has settled and turned to mush, remove extra water from top.  Dip water off or siphon it off.
  7. Spread the mush a few inches thick on clean dry porous surfaces. I use, dry plaster, clean concrete, canvas, denim, etc.  Smooth the top to avoid getting small dry pieces on the surface. 
  8. If you want it to dry faster, use a fan and/or set it all on a wire rack to allow air under it.
  9. When it is nearly dry enough, I make coils as thick as my arm and set them around like big arches (a foot tall) and they are ready to wedge and use in 24 hours or less.  This clay can be stored forever in an airtight plastic. 
    In ancient China, potters stored moist clay in caves for the next generation to improve the plasticity of the clay. 
    If it is to be stored long-term, double wrap it.  Double wrapping in plastic bags from the supermarket works.  Students can bring in hundreds of these. 

    notes on digging clay

WHERE IS CLAY? - Check stream banks, construction sites, roadway cuts, and any place that gets slippery after a rain and sticky as it starts to dry. When dry, it is nearly rock-hard. Many of us can find clay under the topsoil in our back yards.

PLASTICITY - Some clay is too sandy and some is too sticky. When I prospect I look for clay that can be rolled between my hands into a pencil thick coil of soft clay and wrapped around my finger without cracking. If the coil cracks, it may be too sandy or its clay particles may be too large. Sticky clay tends to be cling to my hands too much. It will often have severe drying shrinkage and tend to crack during drying. Potters often blend several clays to get the right properties. See photos on left.

Commercial clays can be added to balance the mix. Commercial ball clay adds plasticity (so it is less apt to crack when bending it). On the other hand, crude ground fireclay, china clay (kaolin) fine sand, and/or grog reduce plasticity (make it less sticky and shrink less). Do a web search of "ceramic chemicals and clay" for sources of commercially available clay types near you. See photos on left.

IMPURITIES - Most common clay contains impurities, often in the form of iron oxide, sand, roots, and other debris. Troublesome impurities can be removed by making a thin slip. The sand settles to the bottom first. Allow the sand to settle a short time. Then decant the clay water (the good slip from the top down to the sand) and discard the sand in the bottom. Allow the clay (slip) to settle and process it as described in the 9 steps above.

Iron impurities are very common and not easily removed. Iron gives it the reddish brown color when fired and causes the clay to melt more easily. It may not work for stoneware, but most common clays are fine for earthenware. Most of it will fire to cone 05 without problems.

GOOD USES OF IMPURE CLAY - Potters who make high fire stoneware sometimes add small amounts of impure local clay to their clay body to add character and blemishes. I regularly add some common brick clay to add character to my pottery. Color and iron spots look more natural and give a warmer feeling. Stoneware potters also use local clay as a source of glaze material. These "slip glazes" have been used for hundreds of years for lining jugs and traditional crockery.

AESTHETICS OF SELF-DUG CLAY: Most native self-dug clay fires to look like common clay flowerpots. Some potters burnish it (rub the nearly dry pieces with a polished stone or back of a spoon). Some Native American potters make beautiful polished black pottery from self-dug clay. Black is achieved by smothering the fire at the end with ashes so that no air reaches the hot pottery and the carbon from remaining fuel blackens the pottery. Typically, tribal pottery is not glazed and is fired without kilns. Sometimes the potters use colored and white clay (slip) to decorate. Search (Google) Terra Sigillata for more information on how to get a highly polished surface without glaze.

HINT: Clay that is thick or not dry enough often explodes as moisture turns to steam when it heated rapidly. If this happens, make it thinner, dry it better, heat it slower at first, and/or add something like sand to the clay to open the clay body more and let the steam out.

OUTDOOR FIRING WITHOUT A KILN

Responsible Adult Supervision is Required
Never leave an outdoor fire unattended
Never fire if there is any chance of a wildfire
Have emergency fire quenching equipment on hand
Leave the site cleaner than you found it
Obey all laws and codes

WHY IS CLAY FIRED?
Clay becomes pottery at temperatures at about 1,000 degrees F (the beginning of glowing red heat - about 540 C). Traditionally, tribal earthenware is fired to about 1,400 degrees F (760 C). Heat removes the molecular water in the clay. The heat converts clay molecules to molecules that do not dissolve or slake in water. In modern societies pottery and brick is fired in kilns to temperatures ranging from 1,800 F to 2,400 F. Most of the common clays like clay shown here on the left found in our back yards start to deform and melt if they are fired higher than about 1,900 F. Modern toilets are fired from clay that has fewer contaminants. It is fired to 2,300 to 2,400 F., making it very strong and impervious.

FIRING WITHOUT A KILN
Kilns were invented to contain heat to reach higher temperature with less fuel. In tribal settings it is traditional to use an outdoor bonfire type of firing that is fueled with enough wood kindling under the pottery to exceed red glowing heat during the burn. The tempreatures of the pottery reach 1,000 F and hotter.

WHERE and how to do it SAFELY. Consider fire safety and local fire codes. Many cities and communities are very strict about open fires. In any case, do not do this where there is any chance that the fire will spread from your firing. Have an ample supply of water close at hand. Have a shovel and dirt that can quickly be used to put out an accidental fire. Do not leave it unattended. Teach careful and strict safety habits to children and students. Temperatures are much hotter than a cooking fire.

THE 'UNKILN' CONSRUCTION STEPS
1. The 'unkiln' firing begins with a pile of dry kindling wood. Some potters put this in a shallow pit or within a ring of steel, brick, or stone. In any case clean the area to prevent fire spread.
2. A stack of pottery is carefully piled on top of the kindling wood. Stack it so you think it will survive as the wood burns and your pots tumble into the ashes. Optionally, you can try supporting the pottery pile with some carefully placed supporting stones, bricks, or some old pieces of fired pottery; but leave plenty space for kindling to fuel the fire.
3. OPTIONAL:  Some potters place broken pot pieces over the pottery pile. You can also cheat with some scraps of tin roofing, flattened tin cans, etc. Leave a generous exhaust opening at the top and several combustion air opening at the base around the perimeter.
4. Cover it with a thick layer of natural material such as tall green swamp grass or animal dung to hold the heat in. Some moisture in the dung and grass keeps it from burning off too soon. This insulating layer holds the heat in long enough to fire the clay, but it does also burn toward the end of the firing.
5. OPTIONAL:  In some cases, this insulation layer is smeared with a coating that forms a thin shell. This shell can be made of a clay/sand/straw or grass mixture.
6. A generous exhaust hole is provided at the top of the mound and several vent openings are provided around the bottom so the wood gets air and burns with enough gusto that the clay gets red-hot. The size depends on how large your firing is. The openings around the bottom provide a place to ignite the wood and allow adequate combustion air to enter. The top opening needs to be large enough to allow rapid air flow to enter at the bottom and small enough so the heat is contained.

FIRING
1. Light the kindling with some wads of paper at the vents. OPTIONAL (if worried about breakage):  As soon as you are sure the wood is burning, you can cover the top vent partially with some tin or pottery shards to restrict the burn and heat the contents slower at the beginning. Open this up soon enough to allow most of fuel to burn rapidly and very hot. Most of the fuel is needed to reach a high enough temperature to fire the clay.
2. OPTIONAL:  When the fuel is all burned, cover it immediately with a layer of dry dirt (if you have wood ashes these also work). This chokes off the air so the pots come out smokier and darker. Some potters can get totally black pottery this way.
3. When it has cooled to about 500 F or cooler, feel free to use sticks to carefully probe and role out your hot treasures.

TROUBLE SHOOTING
Mistakes happen, but enjoy the process. Think about it and try again. Many mistakes turn into new ideas and possibilities.

Breakage problems?
Experiment and learn. Steam pressure is what breaks most pots. If pots are not made to a uniform thickness, they sometimes crack because the drying shrinkage varies. If pots break it may mean they are too thick or the clay needs some opener. Sand or grog in clay is an opener. It allows the moisture to steam out (to escape easier) at the early stages of heating. Sand must not include any limestone. After firing, clay pieces will pop off as the pieces of lime contaminants expand by absorbing atmospheric moisture.

Modern computer controlled electric kilns use a prolonged heating stage at 200 degrees F. This is just below the point at which moisture turns to steam. Clay that is fired fast must be TOTALLY dry before it hits the steam forming temperature. This prevents the clay explosions that often happen when clay is heated to too rapidly.

When firing without a kiln, it may help to pre-dry you clay pieces in a kitchen oven set to 190 degrees F. With a kitchen oven, the pots are dried by "baking" below the boiling temperature of water for several hours. I set the oven to 190 F. This is NOT firing the pots, but it dries them so they can be fired in an outdoor bonfire or pit firing with less breakage caused by steam explosions.

CAUTION: A kitchen oven cannot be set hot enough to fire pots. Firing pots in any indoor stove is never recommend. It may cause a house fire. The temperatures needed to fire clay are too hot (1,000 F degrees and hotter). This temperature would make any stove red hot and it would exceed the safety designed into any stove. This is much hotter than a self-cleaning oven reaches when it burns the residues in a dirty oven. Clay does not change to pottery unless it is fired to 1,000 F (red hot) or hotter.

What if the fired pots dissolve in water?
This means that the fire was not hot enough. Tribally fired pottery is often fired to about 1,400 F. Clay converts to pottery at about 1,000 F. The water that evaporates as clay dries is simply physical water. However, at about 1,000 F, the chemical water is removed. This produces a molecular change--making the clay into a stone-like substance that no nolonger softens in water.

What if you don't like the color and texture?
Pit fired and bonfire pots have natural variations. These are not defects. Experiment. Pay attention to everything. Try burnishing. Try coatings. Never use toxic stuff on the inside of pottery that might be used for food or drink by anybody now or in the distant future. If you like boring and reliable uniformity, use an electric kiln.

What if water seeps through the pot?
This is not a defect. Pit fired pots without glaze on them will all be porous and some water will soak through, but the structure of the clay will be okay if it was fired hot enough. If I want to use a porous pot for a vase, I warm it in an oven and then seal the inside with melted wax by pouring melted paraffin wax in and out of the pot. Porous pottery is used for self-cooling water jars that keep the contents cold by evaporation on the exterior. Porous pottery is also used to filter water. Colloidal silver is added to water filters to help eliminate bacteria. If you do this, buy it from a reputable business (some websites have sold unsafe fake materials as colloidal silver). Water filters are typically fired in kilns in order to reach the correct temperature to function properly. This link has more on porous pottery filtration for drinking water.
http://www.potterswithoutborders.com

 

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Also see Marvin Bartel videos
https://www.youtube.com/results?search_query=Marvin+Bartel

Also see this web page about safely working with clay.
http://www.goshen.edu/art/DeptPgs/Hazards.html

Also see this web page about safely cleaning up in a clay studio or classroom.
http://www.goshen.edu/art/DeptPgs/clean.html

 

Visit my ideas for teachers and parents
Art & Learning
to Think & Feel

- Table of Contents - links to essays & lessons -

Teaching Thinking & Feeling
Conversation Game to get ideas for artwork and learn to make friends   

Sources of Authentic Inspiration where artists get ideas
Ideas for Art Content and Topics
Teaching with Questions for thinking strategies and how to set up experiments

Lessons to Teach Thinking
How to Plan Art Lessons that teach Thinking, Feeling, Creativity, including Practice, Art History, Aesthetics, and Art Criticism
Sources of Ideas for Art Lessons
Idea generation as art curriculum
Lesson Idea Development
Art and Word
First Day of Art Class
Kids and Clay reprinted from Studio Potter

Thinking With Clay
Learning from the Clay

Personal Box
using Clay
Surreal Animals using Clay
Sculpture: Gargoyles using Clay
Abstract Expression using Clay
Dominic's Egg using clay

   
contributed by Lisa Blackburn
Glass Pendants
grade 4 fusing glass
   
contributed by
Peter Dobbins 2011
Learning to Throw a free potter's wheel tutorial

Drawing Lessons
Online Book


Free Links on Learning Drawing
Teaching Blind Contour
Using a large bear named Ralph
Teaching Drawing to Children
--ideas for Parents and Teachers
Observation Shading
Rabbit Drawing  using blinders
Nature Drawing  using viewfinders
Ink wash Drawings: Developing
    contributed by Rebekah Short
Dramatic Mood Using Value 
    contributed by Rebekah Short
Cubism as Experience vs Examples
How to Draw an Orchid at age 4

Sculpture and Collage Thinking
Cut Paper Self Portraits
Wire Sculpture
Montage Self Portrait Lesson
Teaching with Artwork from the Internet
Art History Web Quest 

Drawing as Visual Thinking
NOT "how to draw"

Why NOT Draw on Kid's Work
Learning to Draw Made Easier

Drawing Portraits and Figures

Learning Skills to Learn to Draw

The Blinder Drawing Game

Drawing Lesson
with viewfinders
All the Skills Needed to Draw
Teaching Observation Drawing  
Teaching Shading in Drawing
Drawing is Basic
by Unsworth
Drawing for the "untalented"
Learning to Learn to Draw
Practice Shading
in drawing
Cubism Lesson
process centered

Sixth Grade Sketches
Drawing
at age 5
How to Draw an Orchid at age 4

Design and Composition Thinking


Elements and Principles
Creatively Teaching Elements and Principles
Percy Principles of Composition

Teaching Creativity (how to)
Creative Thinking vs Imitation

Idea Generation
as Art Curriculum
Generating Ideas for art Lessons
- More Sources of Ideas for Art Lessons
Creativity Killers in the classroom
Creatively Teaching Multicultural Art
Nurturing Divergent Thinking

Creativity Links
Learning to Learn
Teaching with Questions
Conversation Game
Teaching for Transfer of Learning
Critique in the Art Class 
Critique Notes
printable 
Critique Form
  printable

Assessing Thinking & Feeling
How to write art tests
requiring creativity
Rubric - Assessing Artwork
printable
Rubric - Assessing Art Talk
printable
Teamwork Rubric

Team Rubric
Sketchbook Evaluation
SmartArt Exhibition Awards

 

 

Questions from Readers
Learning to Learn Creatively in short time sessions 2011 update
Learning from the Clay (has a video on experimentation)
Copying: Creativity Killer #10


Growing the Preschool Mind
- Healthy Feeling and Thinking -

Scribbling on the Wall
How to Draw an Orchid at age 4

Preschool Art - letter to preschool teacher
Preschool, Kindergarten, and Art
Scribbling is a good thing
Drawing at age 5
Typical Drawings

Art Connections
Everyday Life Art Choices

Aesthetics and Ethics in Everyday Life  
Art and National Tragedy 

Art Teachers

Observing an Art Teacher
Good and Bad Art Teaching - student's paper

Safety Hazards in Art
Working Safely with Art Materials
Hazards Working with Ceramics
Working and Cleaning with less dust

Art Education Advocacy
Advocate Link Page
A letter to an administrator
To Whom it May Concern Letter

Successful Third Grade

Build Goodwill with Exhibitions
of Student Work
How to Tape Work to the Wall

Practical Stuff
How to mount temporary art displays with tape
How to
copy slides with a digital camera
How to take images from the Internet to use in teaching art history
Designing Art Classrooms

What architects need to know


Specific Art Courses

Teaching Photography  list of links
Teaching Ceramics  list of links  
Art for Children course syllabus

Teaching House Design
Creative Computer Drafting


Sponsorship
Goshen College Goshen IN - USA
Art Department at Goshen College
Art Gallery Exhibit Schedules 

 biographical information
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