Clay as a constituent of soil is defined as being less than 0.002 mm. in diameter. These particles adhere together to form a sticky mass when wet and hard clods when dry.

Clay particles less than 0.001 mm. in diameter exhibit colloidal properties due to the very small size, these particles also possess a tremendous amount of surface area per unit mass, and also exhibit electromagnetic charges which attract water, and positive and negative ions.

These characteristics along with the specific minerals present in a given clay give rise to most of the chemical and physical activities of soils.

    These are the things I learned: Share everything. Play fair. Don’t hit people.
    Put things back where you found them. Clean up your own mess.
    Don’t take things that aren’t yours. Say you’re sorry when you hurt somebody.
    Wash your hands before you eat. Flush. Warm cookies and cold milk are good for you.
    Live a balanced life. Learn some and think some and draw and paint
    and sing and dance and play and work every day some.
    -Robert Fulghum, All I Ever Really Needed to Know I Learned in Kindergarten

Play Clay

This is a pure white homemade modeling clay that dries very hard. It's a non-toxic recipe made with ingredients that most people have on hand and is inexpensive. Make it for rainy days, holidays or for the child in all of us.

Blend the baking soda, cornstarch and water in a saucepan. Now is the time to add the food coloring, or you could add it after the dough has cooked and cooled. Stir over low heat for about fifteen minutes until it looks like the consistency of mashed potatoes. When it’s cool the dough can be kneaded to make the texture smoother, and/or to add food colorings. Make four small balls out of the dough. Add food coloring to each ball and knead it in until the color is even. If the dough is too tacky you can add some flour or if it’s dry, add water to smooth it out. This can also be cooked in the microwave which takes only 3-3½ minutes, but it has to be stirred every thirty seconds to break up the lumps.

The clay will dry in about twelve hours if the creations are ¼ inch thick or less; larger pieces may take a few days to be completely dry. If color is added they may bleed and stain the surface that it’s being worked upon, keep a paper plate or newspapers underneath with the project until absolutely dry. It’s best to wait until then to paint the piece with acrylics. This will make a two pound batch and the recipe can be doubled.

Helpful Hints:

No food coloring? You can toss in a package of Kool-aid for some color. Warning: this smells great, but tastes awful! Play dough tools are fun to use in making free form shapes. With a rolling pin and cookie cutters or knife, you can roll and cut out creative play clay to make holiday ornaments, mobiles, and three-dimensional pieces. Use colors to reflect the seasons. You could make candle holders and decorate them with shells, flowers, beads, shards of pottery and old keys by pressing them into the clay while it’s still soft. Or they can be glued on after the clay dries. How about a hot plate or refrigerator magnet? It can be molded into almost anything; flowers, animals, birds and fruits.
  • To make realistic indentations in strawberries or oranges, roll the molded shapes up the fine-shred side of a cheese grater.
  • For apple or pear stems, use a real stick or twig.
  • To make hair for a goofy face, squeeze the clay through a garlic press.

You can also make this clay using dried tempera paint powder or it can easily be painted with enamel after it’s dry. If you decide to do this make sure for every two tablespoons of tempera paint added to the mix, decrease the baking soda by the same amount or the clay will turn out crumbly. When they are completely dry, brush with shellac or clear nail polish. Shellac is optional for figures colored with food coloring. Stored in an airtight container this clay will keep for several weeks.

Sources: My recipe box.

Clay Types

All clay can be classified as either residual or sedimentary. Residual clays remain more or less at the site of the original rock formation. They tend to be less plastic than sedimentary clays and contain larger-sized particles. Sedimentary clays have been transported far away from the parent rock. This type of clay has finer particles and therefore is the more plastic of the two types.

It is very uncommon for a single type of clay to be used as a clay body. This body might be made up any combination of the following clays. The ratio of these types of clay determine the plasticity, firing temperature, color and texture.

Kaolin is a very pure form of clay, it is near to the ideal formula of Al2 O3. 2SiO2.2H2O . It is never used alone as a body, but can be used to compare other clays. It can be fired to very high temperature, and can also be used in glazes. It is usually white. The major area where you can find this clay in the USA is the Southeast.

Ball clay is similar to kaolin after firing, however, in its unfired state, the color is a dark grey, due to the presence of organic materials. This clay can be found in swampy areas. Gasses and organic acids break down the clay into smaller particles than the sedimentary kaolins. Ball clay gives increased plasticity and strength when used in the clay body. Thus its throwing qualities are improved. Similar to kaolin, ball clay must be fired at a very high temperature. Tennessee and Kentucky have the largest deposites of ball clay in the US.

Stoneware clays are used quite often by the potter. These clays are generally plastic and are fired from 1222° C, to 1305° C. The color differs from buff to grey. When compared to kaolin, stoneware clays have many impurities such as calcium, feldspar, and iron. These impurities lower the maturing temperatures and give color to the clay. Stoneware clays can be found from New York to Illinois, and also on the Pacific Coast. When fired, stoneware clays become very strong.

Fire clay is a high-firing clay. It is usually used for insulating bricks, firebricks, and kiln furniture. Their physical characteristics vary from a fine plastic quality, to coarse and granular quality. They generally have an iron impurity. Fire clays can be found near coal veins. They are common to most states. However, they are rare in mountainous regions and the east coast.

Earthenware clays are a group of low-firing clays that mature at temperatures ranging from 955° C to 1120° C. Because of the low temperature, these clays are generally porous and fragile after firing. Some can not even hold water. They contain a high percent of iron oxide, which is a flux. Because of this flux, Earthenware clays can not be fired at a higher temperature than 1150° C or else it deforms and blisters. It can be found in almost all of the US. They can be shale.

Slip Clay are clays that have sufficient fluxes to be used as a glaze without any additions. While blue and white slip clays do exist, the most common are red tan, or brown-black. The best-know commercial slip clay is mined in small pits near Albany, NY. Slip glazes are easy to apply, and they have a long firing range.

Bentonite is a clay that is used in small amounts as a plasticizer. Deposits are found in most of the western mountain states. It has the finest particle size of any clay known. It is composed mainly of silica. It should be mixed dry, as it becomes quite gummy when mixed alone with water.

Mixing clay

Clays that are completely satisfactory to fire seldom happen in nature. You can change the plasticity, color, and texture by mixing different types of clay. To make a clay body, you start with a fairly inexpensive clay that has no major faults. It should also be moderately plastic. Although, with the addition of Bentonite or ball clay, the plasticity can be improved. In some cases, the clay may be too fine to dry without cracking. You can remedy this problem by adding grog or a silica sand. (Grog is coarse, crushed, fired clay) When this is added, it opens up the clay body and makes it more porous so it will dry uniformly.

Fluxes such as feldspar, talc, dolomite, nepheline, syenite and bone ash can be added to a clay body if the clay lacks sufficient fluxes to fire hard enough at the desired temperature.

It is much cheaper to make your own clay. Homemade clay bodies cost about 10 cents per pound to make. While factory made clays tend to cost around 60 cents per pound. With homemade clay bodies you can also choose what color, plasticity, and texture you want much easier.

Carbonic acid exists because carbon dioxide dissolves in water. CO2 + H2O = H2CO3.

Reactions between carbonic acid and primary minerals, like silicates and other shales, produce secondary minerals, like clays.  In natural systems, much carbonic acid accumulates from rainwater falling through atmospheric CO2.

CO2 is one product of the oxidation of carbon. This process can happen quickly, as in fire, or slowly, as in rust, or in metabolic processes.

Mitochondria in cells oxidize sugars teased from an inward-spiraling line of food breakdown.  The product is movement.  The by-product is breath--carbon dioxide, heat, and water.

In natural systems much atmospheric CO2 and transpired water are the products of metabolic processes.

Clay is breath on stone.




Two very broad bits of context re: the spatials of clay and water:

1. Skeletal soil--soil without organic matter--is an aggregate of three things: sand, silt, and clay.

2. Erosion of rock happens two ways: mechanical, chemical.

Sand and silt come from mechanical erosion, which comes from force: wind, water, moving debris, expanding ice crystals, soakage and drainage, etc

But, clay.

Clay, as described, is a product of chemical erosion--the re-assembly of molecules.  So, while sand and silt can be understood to be little rocks, clay is a thing apart.

Clay particles are a) crystalline and b) the smallest of the aforementioned particles by a factor of ten.  This is important because a) crystals possess straight edges and regular angles and b) small particles expose much surface area relative to volume. Water molecules bond to the crystals' sides, pushing them apart. The clay dissolves; it's colloidal.

This effective spike is the reason good cob (sand, clay, straw) includes just enough clay to hold together particles of sand. The clay shrinks as it dries; every granule is pulled against every other granule.

The effect in soil is similar. The consequence is cemented roots. 

This is mitigated with organic matter.  In natural systems this is introduced through volunteer or pioneer plant species that have evolved (or adapted) to exploit specific poor soil conditions and secure missing nutrients.




This relationship between clay and water exists mostly because water is dipolar. This is a rare thing for a molecule to be. 

Dipoles are easy enough to imagine for this purpose as magnets (though, you know, illustration only).  The two hydrogen atoms make up the positive end, the one oxygen the negative.  Clay particles are almost always negatively charged.  Conveniently, so is organic matter.

The capacity to hold and release ions--molecules with any charge--is described with the term cation exchange capacity or anion exchange capacity, depending.

Two things:

Positively-charged molecules are also calle d cations.  Mineral nutrients, like calcium and magnesium, are cationic and also alkaline.

Negatively-charged molecules are also called anions.  Humus--organic matter--is anionic and acidic.

By-products of life are attracted to mineral nutrients. 

There is indeed a clasp in the carbon cycle.






Robert Leo Smith, "Elements of Ecology, Fifth Edition." HarperCollins, 1992.

Allan Kahn, "Studies of the size and shape of clay particles in aqueous solution."

Michael Astera, “Cation Exchange Capacity in Soils, Simplified.”

Monash University Department of Civil Engineering.  “The Characteristics of Clay.”\





Clay (?), n. [AS. cl�xd6;g; akin to LG. klei, D. klei, and perh. to AS. clam clay, L. glus, gluten glue, Gr. glutinous substance, E. glue. Cf. Clog.]


A soft earth, which is plastuc, or may be molded with the hands, consisting of hydrous silicate of alumunium. It is the result of the wearing down and decomposition, in part, of rocks containing aluminous minerals, as granite. Lime, magnesia, oxide of iron, and other ingredients, are often present as impurities.

2. Poetry & Script.

Earth in general, as representing the elementary particles of the human body; hence, the human body as formed from such particles.

I also am formed out of the clay. Job xxxiii. 6.

The earth is covered thick with other clay, Which her own clay shall cover. Byron.

Bowlder clay. See under Bowlder. -- Brick clay, the common clay, containing some iron, and therefore turning red when burned. -- Clay cold, cold as clay or earth; lifeless; inanimate. -- Clay ironstone, an ore of iron consisting of the oxide or carbonate of iron mixed with clay or sand. -- Clay marl, a whitish, smooth, chalky clay. -- Clay mill, a mill for mixing and tempering clay; a pug mill. -- Clay pit, a pit where clay is dug. -- Clay slate Min., argillaceous schist; argillite. -- Fatty clays, clays having a greasy feel; they are chemical compounds of water, silica, and aluminia, as halloysite, bole, etc. -- Fire clay , a variety of clay, entirely free from lime, iron, or an alkali, and therefore infusible, and used for fire brick. -- Porcelain clay, a very pure variety, formed directly from the decomposition of feldspar, and often called kaolin. -- Potter's clay, a tolerably pure kind, free from iron.


© Webster 1913.

Clay, v. t. [imp. & p. p. Clayed (?); p. pr. & vb. n. Claying.]


To cover or manure with clay.


To clarify by filtering through clay, as sugar.


© Webster 1913.

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