The Quest for Clay

by Estabon Fire

Neolithic – The latter part of a Stone Age culture, and is often the period when ceramic technology is achieved.

Slip – A paint-like mixture of fine clay particles suspended in water. Slips can be applied, like paint or glaze, to pottery for color variation, to give the pot a smoother texture and to decrease water porosity. It may also be used like glue to help attach wet clay forms together.

Organic (vegetable) and Mineral pigments – Paint made from plants and/or earth minerals and applied to pottery for decoration.

The development of pottery represented a significant advancement for primitive cultures. The Americas have rich and unique traditions in primitive pottery and provide inspiration and opportunities to learn from observing the results and techniques of ceramic-using cultures, which developed over thousands of years. Pottery cultures that evolved over such time periods were very familiar with which clays and pottery shapes most consistently survived the journey from clay to fired pot. Generations of potters would intimately know the strengths, weaknesses and limitations of their clays and firing methods.

The Anasazi, Seris and Pueblo people represent several examples of primitive North American cultures that developed vigorous pottery legacies. The Anasazi, a prehistoric agrarian people from the northern Southwest US, made gray utilitarian pottery and perfected the black on white organic pigment style pottery. Much of the oldest known examples of North American pottery are from the Anasazi, who are believed to be the ancestral culture of the Pueblo people. The Seris, the least numerous indigenous people of the Sonoran Desert in Mexico, are one of the very few nomadic people to embrace pottery making. Most Seris people today have given up their tradition of hunting, fishing and making trade-crafts and instead make trinkets, primarily ironwood carvings, for tourist. The Pueblo people from Southwest US were one of the few Neolithic cultures to develop glazed pottery. The glossy glaze was lead based and used only as decoration. This technique died out around 1700 AD, with preference toward vegetable or mineral pigments and slip decoration. Pueblo pottery culture still thrives today, although emphasis has changed from primarily utilitarian and trade goods to tourist, collectors and art gallery markets.

Earth mineral – Various inorganic substances found mostly underground that are naturally occurring or today, commercially obtained and refined through mining.

Clay mineral - A particular group of earth minerals composed of minute particles that have a crystalline structure.

Clay – A combination of minute earth mineral particles in varying amounts that when combined with water becomes pliable and can be shaped, dried and then fired, after which it no longer dissolves when in contact with water. Kaolinite is the world’s most abundant clay mineral and consists primarily of silicate of aluminum.

Ceramics (also Keramics) - Clay forms permanently altered by heat (the ceramic change) and the production and study of this subject.

Clay body - A term referring to a particular clay, its earth mineral makeup and the characteristics of that particular clay.

Primitive pottery refers primarily to hand built ceramics made from naturally occurring clays with simple tools and fired to a temperature, which induces the “ceramic change,” where the fired clay no longer dissolves when in contact with water. The temperature that produces this ceramic change generally occurs between 950 and 1150 degrees F depending on the clay body. This temperature can generally be reached after a few hours of firing in an open pit fire. The lowest temperature where red glow can be seen in the dark is approximately 470 degrees C (878 degrees F). The temperature range at which the ceramic change takes place is dependent on the earth mineral variations found in the clay body. Primitively built and fired ceramics have their own qualities, challenges and limitations. Expectations that primitive pottery can or should achieve the look or durability of higher fired ceramics are not realistic. This lower fired pottery will likely be fragile, brittle, and porous compared to higher fired pottery. This fragile nature can require continual replacement of pots broken during daily use. However with careful handling, primitively fired pots may last for years, decades and even millenniums.

CLAYS AND THEIR TEMPERAMENT

Commercial clay - Clays made from a combination of refined earth minerals.

Natural clay – Clays that are useable in their natural occurring state.

Recipe clay - Clays homemade from a blend of natural and/or refined earth minerals.

Flux – A substance that aids fusion and can lower the firing range of a clay body.

Rarely in the Pacific Northwest have I found naturally occurring clays already moist and pure enough to use without the need to filter, hydrate, and work them before forming into desired shapes. The time required to remove impurities such as rocks, gravel and organic matter can certainly be worth the effort to ensure success. However, some clays become so time consuming to process that it may be better to continue looking for more readily usable clays. Natural clays vary in characteristics depending on how they were laid down geologically and their mineral variations. Once wet clay is shaped it must withstand drying without cracking or falling apart and then being heated to red-hot temperatures and finally cooled.

Most natural clays, which I have tested in the Pacific Northwest, are not user friendly for primitive pottery making. Many if not most natural clays contain a percentage of Kaolinite or other clay minerals which fire best at temperatures beyond the primitive firing range. Fluxing agents, like talc or feldspars, or even some low fire clays, in or added to the clay body can bring down its firing range. Ball clays, which are light gray in color or bentonites (decomposed volcanic ash) can give a clay body more plasticity, if needed, making it easier to work and form. Natural clays are underfoot almost everywhere. The goal is to find a clay that is readily accessible and with the right ingredients, which allow it to be worked easily and fired at bonfire temperatures.

Testing and experimentation can determine if clay is usable for making primitive pottery. I favor only a few of the numerous natural clays that I have tested. When a good natural clay can not be readily found some commercial clays can be used while you search for your own natural clay mother lode and become familiar with the pottery process. Working with commercial clay along with natural clay can help develop sensitivity toward qualities to look for in natural clays.

TEMPER, TEMPER

Temper is an addition to a clay body, which alters some of its characteristics and can make a clay more workable; this is often referred to as “opening” the clay body. Natural or commercial clays may or may not need the addition of a temper, which adjusts the clay body in a number of ways. The unfired pots will dry and shrink more evenly and during firing will have increased resistance to thermal shock and fluctuations in temperatures. Some clays already contain temper, such as sand, in their natural state. Pots made without enough temper will crack or blow up during firing. Sand and grog are commonly used tempers. Grog is fired clay such as pottery shards that have been ground into fine particles, which is then added to the clay as temper. The amount of temper added to a temperless clay body is usually between 10 to 30% and can reach up to 50% by wet volume. River sand often works well and other possible tempers include volcanic ash, crushed shell dust (calcified), ground mica, some fibrous organic materials and even other clays that contain high percentages of temper.

Good places to look for natural clays can include any place that the vegetation and topsoil have been removed, like road cuts and river edges.

TESTING, EXPERIMENTATION AND OBSERVATIONS

Aging – Storing of moist clay which can improve workability.

Plastic state – The unique characteristic of clays combining the strength of a solid and the fluidity of a liquid.

Investing in a notebook to keep track of your progress is a good idea. Start naming and testing clays. Use the pottery journal to record firing information and results. Make drawings of your pots detailing clays, decorations and firing notes. This enables adjusting and fine-tuning in the future.

Once you have found a potential clay work it to a plastic state where it can be easily shaped and formed. The clay should be moist enough to shape easily yet not crack excessively, and preferably not so sticky wet that it rapidly builds up on your hands when working the clay. Turning and massaging the clay to the correct consistency through out is called wedging which is similar to kneading bread dough. Correctly wedging the clay forces out air bubbles, helps align clay particles and evenly mixes the particles and moisture. Air bubbles not removed from the clay will cause your pot to explode during firing. Incorrect wedging can put more air bubbles into the clay than are being eliminated.

A quick and easy plasticity test that can be done on site; start by working a small amount of moist clay into a ball. Roll the ball into a coil with your hands to about the diameter of a pencil. Then gently lay it around a finger. Few cracks or no cracking along the coil is a sign of plasticity. Many cracks and breaking apart along the coil tends to designate low plasticity. Be cautious about giving up on a clay too soon. One of the clays that I favor has almost no plasticity but a pot made from this clay can be formed, dried and fired on the same day, which is very rare. The fine particle size and mineral content of this same clay makes it a good temper for other clays which are going to have a polished surface.

Clay test tiles or bowls are quick to make and work well for testing and experimentation. These tiles/bowls can be made in a variety of ways. For a shrinkage test you can make standard size tiles which can help determine the amount of shrinkage from the wet state through the firing, and enabling color, shrinkage and other comparisons between fired clay bodies. Drying a few tiles at different rates will help in understanding the nuances of various clays and determine which might need more temper. A white scum deposit left on the tile during drying probably indicates alkalizes in the clay body and can be a hindrance in low fired pottery. Various tempers can be tested by making and firing the tiles, as can slips, polishing and other surface texture and decoration. If your tile surfaces have craters after firing it is likely due to a high lime content or other particles in the clay that expanded after firing blowing out chunks of ceramic. Particles of pyrite often cause this problem and usually a shinny particle of pyrite can be seen embedded in the pottery at the bottom of the crater. Testing a couple of natural clays with a commercial or recipe clays can rapidly help develop the ability to judge qualities to look for when testing new clays. Numbering test tiles or bowls and making notations on the under side as well as in your journal helps track your progress.

A test during firing which is fun and entertaining, especially when you fire after dark is a test for the thermal shock resistance of a clay and it’s temper. Place the tiles/bowls or pots to be tested in positions where they can be easily found toward the end of the firing process when they are glowing red hot. Pull them out of the fire and immediately dunk them in a bucket of water. If they survive they have a high amount of thermal shock resistance, which is desirable in the primitive firing process where temperatures fluctuate greatly and can indicate a clay will be good for making cooking pots.

Once the tiles are fired it will be more clear whether a clay is promising. Make enough tiles so that after firing you can have a few to try breaking, giving you an idea of the clays ceramic strength.

When a clay looks promising and larger quantities are needed, filtering should be considered to remove unwanted debris and help insure success through the firing. Some potters prefer to gather dry raw clay and then pound it to dust and process it dry. Water is then added to bring the dry powdered clay to a workable condition. Some clays are so pure they may not need processing. I prefer two methods of filtering. The water filtering system is simply filling a five gallon bucket half full of clay then filling it with water. The water and clay is mixed well before allowing it to sit for a few minutes, which allows the heavy particles to settle to the bottom, after which the top water is carefully removed to another bucket where the fine particles settle to the bottom and the clear water is poured off and the remaining water is left to evaporate until the clay reaches the correct consistency. This filtering technique is good to help remove to much or unwanted temper. The second filtering method consist of making a slurry of the raw clay and water, then filtering it through a screen door size screen into a pillowcase, which I then tie off. The clay is left to sit, draining and evaporating water until the clay reaches the correct state for working. This takes about 5 days during hot dry weather. I then transfer it to a watertight container. During the slurry step I like to add a little milk. The milk encourages the aging process. However, too much milk can give an overpowering sour odor that can last for months. This filtering method allows for natural tempers to stay in the clay and is the method I most often use.

For those wanting to make their own recipe clay from refined ceramic materials, the following recipe has worked successfully with primitive firing methods.

Mix these ingredients thoroughly with enough water to make a slurry. Filter it into pillowcases and monitor until it achieves the preferable moister content. Turning and wedging the bags can help keep the consistency even.

Always show consideration and appreciation for the earth when gathering clays.

Dig deep.

First published in “The Bulletin of Primitive Technology”
Updated: September 2009