Basic Clays and Clay Bodies: Tutorial II (CeramicsWeb)

Text by Robert Fromme, (c) 1994; contact Robert Fromme at <rfromme@tenet.edu>
Original from CeramicsWeb: Basic Clays and Clay Bodies
For educational purposes only. 

Introduction

Why is a basic understanding of clay important when we are learning about ceramics, surfaces and glazes?

* Sometimes the art object requires no glaze and the organic unit depends upon a natural or manipulated clay surface as part of the composition.

* Many problems in forming and drying the objects can be traced back to the composition of the clay.

* The color and character of many glazes depend upon its relationship to the clay body under it. (bleeding iron, manganese, etc.)

* The fit of the glaze on the clay body (expansion and contraction of both) is an important consideration for anyone working with glazes or clays.

* The composition of the clay related to problems with the inversion of quartz and other forms of silica in the heating and cooling kiln which may cause major problems for anyone trying to fire their work.

* An understanding of the nature of clay is critical for most forming and firing techniques and if the crafts person doesn't understand what is going on throughout the total process, the greatest glaze in the world will not insure the efforts.

* Clays such as the kaolins, ball clays and slip clays are used in glazes as one of the raw materials. The Alumina and Silica which can be found in clay are two critical ingredients in most ceramic glazes.

* I am sure some of you can come up with some other reasons for taking a little of our time for notes on clays.

Where Does It Come From?

Scientists tell us that our world was once a body of molten lava. The outside cooled and a variety of chemical elements blended and stratified into a crust made of mostly igneous rock. The rock surfaces of our earth began to erode as our atmosphere began to form moisture along with the other forces we associate with weathering. In time, the rock began to disintegrate into clay. The gasses and pressure from below worked up through the crust while the expansion, shrinkage and squeezing of the rigid exterior continued to work erode the surface stone. Of course, the procedure continues even today.

The Clay Particle

When we look at clay through an electron microscope, the clay particles seen as a thin hexagonal plate, approximately 100 times longer than it is thick. (Note: Richard Burkett has placed an image file of clay particles as seen under an electron microscope in another directory of the Ceramics Gopher at SDSU.) When we add the water of plasticity to the dry clay, moisture between the flat plates creates a surface tension attraction so that the particles do not easily pull apart, but they slide easily over one another. The flat shape of the clay particle and the surface tension when water is added gives the strength and plasticity which we associate with clay in its workable state.

When the clay object is formed and the material is dry, the tiny, flat, packed particles seem to lock closely together giving the greenware its structural soundness. No doubt, the same forms created from inorganic earthen materials which have little clay in the mixture would crumble even before the drying process was finished.

Clay vs. Clay Body

At this point, let us make a distinction between the terms 'clay' and 'clay body'. We will use the term clay to refer to those materials of a plastic quality which are formed by natural forces and which are to be found in nature. The term 'clay body' will be used to indicate a mixture of clay like materials with other inclusions for a specific ceramic technique. In other words, a 'clay body' may have several different kinds of clay, fluxes, silica, grog, and other ingredients for color,plasticity, warping, cracking, shrinkage, porosity, firing temperature,texture and etc. A single clay from the natural world will seldom have all of the characteristics which the potter will need for a particular ceramic technique. The principles of forming a body are the same regardless of whether it is earthenware, stoneware, or porcelain.

Plasticity (workability or elasticity)

The basic clay may be overly plastic or not plastic enough for a particular use. Clays that are too plastic (tiny clay particle size) may not hold their shape when throwing. The mix will be very sticky and in the wet state the mixture may not have the strength to stand while being thrown. While throwing a cylinder with this kind of clay, the form will appear to squat near the bottom after each pull up the form.

Clay that is not plastic enough will not throw or move with ease when it is under pressure. It may tear or rip when thrown or crack when bent, even when very wet. Many Handbuilding processes do not need a clay body that is as plastic as the typical throwing body. Handbuilding clays are often made to be less plastic to reduce problems with fabrication and drying. Clay that is very plastic will almost always shrink more than a less plastic clay, often leading to drying problems such as warpage and cracks.

The old standard test for a clay with enough plasticity for throwing is to wrap a 1/2 inch thick clay coil around your finger and if it cracks exorbitantly, it is not plastic enough for the requirements of the forming technique.

Ancient Chinese potters discovered that they could increase a clay's plasticity by aging it, thus developing a more intimate relationship between particles, water, and fillers. There is the old story of organic inclusions such as milk being added to the clay as it is prepared and stored for the coming generation while using clay which had been prepared years earlier by their ancestors. Such organic additions can cause problems in studio clays as they spoil and smell bad in the short run.

The potter can reduce plasticity by adding fillers such as grog, sand, or flint if it is not possible to use a less plastic clay in the body mixture. The inclusion of at least a small amount of fine-particled, plastic clay will usually improve the dry (green) strength of a clay body.

Clay Body Color

When your locate a clay in nature, the fired color may be lighter or darker than desired. Metallic inclusions such as iron or manganese often 'bleed' out from the body and into the glazes on the surface.

For a light or near-white clay body, a relatively pure clay is needed to start the mixture and only small amounts of colorant are necessary to alter it to a tan or buff color. One can not work backwards and lighten or bleach a darker clay to make it lighter.

If the potter wants to darken a body, the most common addition is iron. Some additional colorants are: cobalt (blue), copper (green), chrome (green), and manganese (brown). Be warned that many metal oxides and compounds are considered hazardous. A better solution may be to add natural dark clays such as Barnard clay to the clay body instead of metal oxides. Iron oxide is usually a safe addition. In addition, oxides like copper, manganese and cobalt may add unwanted flux to the clay body. Most fired clays range in color from buff to black, depending on how much iron is present-the more iron,the darker the fired clay body.

Texture

Often, a clay as it is found in nature may fire too open or too tight and contain more or fewer visual impurities than desired. When the clay is too rough or visually coarse, you can add clay of a finer texture.

Keep in mind that the non-plastic material you add will affect a clay's plasticity. If the clay needs to be adjusted the other way, grog, Kyanite,sand, and other aggregates (such as vermiculite) can be added. A number of organic inclusions have been tried throughout history, rice, sawdust, hair, cattail fuzz, paper pulp, nylon fiber, fiberglass and may other materials can be added to the clay body. The organic inclusions will burn out in the firing and while the more traditional things like grog and sand will survive the heat. Other ideas for visual texture, have included red brick grog, or white porcelain grog, shale, iron shavings and granular ilmenite.

Shrinkage (Warping or Cracking)

Clay body shrinkage refers to the loss of size that occurs throughout the total drying and firing process. There are three distinct periods of shrinkage.

The first is when the piece air-dries to become bone dry, the second is during bisque firing, and the third is during glaze firing.

As a general rule, we can assume that the more plastic a clay body the more shrinkage. Clay with finer particles absorbs more water, expands more, and when the water is forced out by drying or firing, one can expect more shrinkage.

Less plastic clays and fillers such as grog or sand can be added to "open" the body and help with a shrinkage problem. Such additions in large amounts can also reduce plasticity.
Spodumene or wollastonite can be added instead of flint or fluxes in a clay body in order to lower the shrinkage even further.

When the clay artist reduces shrinkage, other problems such as warping, and cracking during drying will also improve. Warping results from the shrinkage of clay which is highly absorptive or it may also result from poor technique which creates uneven walls, uneven drying, unsupported weight, uneven moisture consistency of the clay,uneven firing support and the plastic memory of the clay.

Firing a clay body at too high a temperature can also lead to warping as the body is heated beyond its maturation temperature and begins to loose structure in the melt.

Cracking may also result from poor technique. It can be caused by having clay which is too wet, excessive shrinkage, or uneven drying. Cracks may appear when moisture is not consistent between different parts of the object, when the wall thickness is not even, when force-drying causes uneven drying, when you use thick glaze applications, or thick slip applications.
 
One must also remember that the expansion and contraction of materials as they are heated and cooled and chemical changes such as the quartz inversion during firing also has an effect upon the warping and cracking during and after firing. (Quartz inversion is a transformation of crystalline silica that occurs at about 1060 deg. F /571 deg. C and that causes the body to shrink considerably as it cools.)

Clays can shrink as little as 4% or as much as 25%. A clay body that meets all a potter's needs and shrinks no more than about 12% is acceptable for most techniques.

Maturation Temperature (Porosity)


After firing, some natural clays remain too porous or become too dense at the chosen firing temperature. The higher a particular clay body is fired, the more vitreous it becomes. However, other factors in formulating a body will affect the maturation. Color, plasticity, texture, or shrinkage desired in a clay body may call for the addition of materials or clays which affect the final maturing temperature. Vitreous clay bodies can be made at any temperature, but are most often found at the higher firing ranges such as stoneware or porcelain.

Adding lower melting ingredients lowers the maturation temperature of the clay body and decreases the potential for water absorption in the fired ware, while at the same time increasing firing shrinkage. Typically, fired porcelain absorbs from 0-3%, stoneware 1-5% and earthenware 4-10% liquid. In general, we add ingredients to lower the maturation temperature if we want to make a clay body less porous,more vitreous and with a better ring when tapped. However other factors like thermal shock or freeze resistance may also affect how vitreous one would want the clay to be when fired.

Residual and Sedimentary (or Primary and Secondary Clays)

Clay types are identified by the way they geologically formed. We have two main classifications, primary and secondary. These are sometimes called residual and sedimentary. In other words, the primary or residual clays remained at their original location and the secondary or sedimentary clays were moved from the primary site to a new location by wind, rain,or ice. Natural weathering has relocated most of the surface of our world and primary clay deposits are relatively rare. Of course, the amount of mineral impurities and organic matter would be greater in those clays which had been transported from their original location and subjected to a mix of miscellaneous other inclusions as thy were being moved and relocated. The grinding action of clay particles in water,wind, and ice created, as a consequence, very fine particle sizes, making secondary clay extremely plastic. As the materials settled in river and lake beds, the weighty, coarser particles settled first, leaving the minute,more plastic particles on the surface.

Kaolin(China Clay)

[Kaolin]



[Ball Clay]



[Red Earthenware]



[White Earthenware]



[Stoneware]



[Fire Clay]



[Saggers inside a kiln at the Pottery Museum]



[Clay Slip]



[Casting Slip]



[Volcanic Clay]



[Adobe]



[Gumbo]




Deposits of kaolin occur in Europe, England, North America, and Asia. Scholars suggest the name 'kaolin' comes from the Chinese words 'Kao' and 'Ling' which means 'high hill', giving reference to the location of original deposits of the material. Seldom in nature will we find any materials which is pure, however, the primary kaolins are often 95% pure (free of impurities such as iron, manganese, alkalies and other inclusions which may be found in the darker sedimentary clays). The kaolins have been exposed to less weathering so the particles size is usually larger. This means that the kaolins can be thought as generally less plastic than some of the other types of clays.

The lower melting alkalies are among the original inclusions in the igneous rock. These and other lower melting inclusions are water soluble and end up being leached out as the rock is reduced to clay. The loss of the lower melting inclusions results in a clean, near-white firing clay that matures at elevated temperatures (Approx. 3275 deg. F / 1805 deg. C).

Some locations have been commercially developed for the extraction of deposits of secondary kaolins. These clays are those that have been transported at some time in history by a mode that did not cause the inclusion of a large amount of impurities. However, because of the additional physical activities involved with the development of these clays, the particle sizes of secondary kaolins are usually finer,making the secondary kaolin easier to work with (more plastic) than kaolin from a primary location. Secondary kaolins usuallyloose some of the purity as they are relocated to the second deposit. The typical formula for kaolin is the same formula that we associate with kaolinite, the crystalline form in most clay like materials, Al203.2SiO2.2H2O.

Ball Clay

After the discussion of secondary or sedimentary clays, the origins of Ball clay will be easy to understand. Thousands of years past, in the swamps and low flat lands, these clays seem to have been laid down along with organic layers which would later be mined as coal. The name 'ball clays' comes from the practice in the mines involving the removal of layers of clay from the coal deposits. It seems that the clay was rolled into balls, loaded onto the back of the pack animals or carts, and transported out of the area where the coal was being mined. Like most sedimentary clays, the ball clays have fine particles. They are very plastic and are important additions for clay bodies used in throwing and other forming techniques.

Although, the ball clays are often good additions to other clay bodies to add plasticity, there are some problems which we should keep in mind. With the fine particle size, the shrinkage and tightness of the body create forms which often shrink a great deal, dry slowly and do not allow for the passage of steam out of the ware in the early stages of the fire. Ball clays in excess may add a sticky quality to a clay body. Also, the amounts of organic material (coal dust, etc.) and extra sulfur present in ball clay account for its brown color and early firing odors. In spite of the dark raw color, the clays are usually fairly low in iron and other impurities and fires to a light buff color. Alkalies and other lower melting impurities, as well as the small particle size of the ball clays render the materials lower in maturation temperature than the cleaner kaolins (china clay). Ball clays
mature at around(2345 deg. F / 1285 deg. C).

Ball clays can be found in many locations and the commercial locations of extensive mining are in Tennessee and Kentucky. Kentucky ball clay,Old Mine no. 4 is quite pure and may be fond in many raw batch weight laze recipes as well as in some porcelain formulas to help increase plasticity.

Earthenware (Red Clay)

Natural earthenware deposits are usually found in outcroppings or in sedimentary patches along streams or rivers. You may also discover these clays along highways that cut through layers of secondary earth. This sedimentary clay is probably the most common clay in nature where numerous lower melting inclusions (impurities such as alkalies and iron) lower its maturation temperature. We use the term 'flux' to refer to the lowering of melting temperatures. Earthenware clays melt at such low temperatures that they seldom become vitreous and the ware continues to be porous after firing (1850 deg. F/1010 deg. C). For this reason, the work is usually glazed if it is to contain liquids. The body usually fires a dark red because of the high iron content in the clay. Seldom will a naturally deposited earthenware fill all of the requirements of a particular forming or firing technique and characteristics such as particle size and plasticity often vary a great deal. Like the other clays in nature, earthenware also require other materials to meet the full set of potter's requirements maturing point, texture, color, and plasticity).

Earthenware Clay (Low-fire White or Buff Clay)

We should note that recent interest in low firing techniques have given rise to the use of the term earthenware when talking about any clay body which has been formulated to mature at the earthenware temperature range. The origin of these white clays can be traced to Europe and the early efforts to duplicate the imported porcelain which was being traded from the Orient. These European clay bodies (Soft-Paste Porcelain, bone china, etc.) were formulated using large quantities of fluxing inclusions, lowering the melting temperature for the relatively clean mixtures of kaolin and ball clays. The best descriptive term for this kind of clay body is 'low-fire clay' or 'low-fire whiteware' in order to distinguish it from red earthenware in nature. Another term, 'white earthenware', is also used to distinguish the man-made clay bodies from the red earthenware of nature.

Stoneware Clay

Stoneware clays are even less pure than the ball clays. Some of the impurities, calcium, alkalies, iron, and feldspar drop the maturation temperature a bit lower (2300 deg. F / 1262 deg. C).

The fired color for these clays becomes much darker because of the additions of metals like iron in the sedimentary mix. One additional difference between the ball clays and stoneware clays involves the size of their particles. Here, ball clay usually beats the stoneware in plasticity and finer particle size.

In the raw form, stoneware clays usually require the addition of other clays and chemicals to adjust their properties to match the demands of specific forming techniques. The adjustments usually involve lowering shrinkage, improving plasticity fired color, texture, and adjusting for a specific maturation temperature. There are always the old potter's stories of looking for the clay nests of 'mud dauber wasps' and running them though a firing in order to see if a usable stoneware clay deposit might be in the vicinity. As one would expect, local deposits of stoneware clay will seldom match all  of the technical requirements of maturation temperature, color, plasticity etc.

Fireclay

Fireclays are characterized by large particle size (low plasticity),elevated maturation temperature (2650 deg. F / 1454 deg. C) and large inclusions of impurities such as iron. Aside from their inclusion with other clays to raise the maturing point for the clay body, lesson drying shrinkage,open the body so it  will dry quicker, add color, add texture and to lesson plasticity.

Sagger Clay

The clays which have been given the name 'Sagger Clays' are usually fireclays or coarse stoneware clays which are of fairly large particle size and which mature a high temperatures. The clays get their name from the practice of using clay containers, saggers, in the kiln to protect the ware and allow for stacking the work. With new developments in fuel, kiln building and the manufacture of refractory kiln shelves and posts, saggers are seldom used today. Sagger clay had to withstand many firings and support the weight of additional saggers and ware in the loaded kiln.

Wad Clay

In addition to sagger clay for throwing the containers to stack the kiln, a cheap source of low grade clay was needed in the kiln stack to level the stack and seal potential cracks in the kiln walls and around bricks of kiln  doors. Like sagger clay, this 'wad clay' was usually a coarse stoneware or fireclay with good strength and low shrinkage. It does not have to be plastic and it does not require the traditional quality standards of preparing clay for specific forming techniques.

Slip Clay

Throughout history, slip clays have provided the basis for very good glazes for stoneware and porcelain. Fine particles and large quantities of impurities (flux) cause slip clays to melt around 2250 deg. F/1235 deg. C. The common slip clay formerly used by potters is Albany clay with its high iron content and beautiful rich brown to black shiny surface. With little or no alteration, an acceptable surface can be obtained anywhere between 2235 deg. F/1225 deg. C and 2380 deg. F/1305 deg. C. As Albany slip is no longer mined, other substitutes include Alberta slip from Canada and various recalculations of recipes using locally available earthenware clays.

Casting Slip

We must make a distinction between 'slip clay' which is found in nature and 'casting slip' which is a man-made mixture of clay, water and various chemical deflocculants (electrolytes) such as sodium silicate or soda ash which makes the mixture into a liquid, but with less water and subsequent shrinkage. This makes a slip which is useful for pouring and casting in plaster molds.

Clay particles have a natural attraction to one another(positive and negative charges) that brings them together or to 'flock'. The attraction requires large amounts of water to break up the particles and render the clay into a slip under normal conditions. To alter the charges so the particle will act like the backs of magnets and repel each other, an electrolyte is used. The use of these chemicals reduces the amount of water needed to disperse the particles and make a fluid,cartable mixture.

Volcanic Clay

We have various kinds of clays of volcanic origin and all of them ensue from the weathering of volcanic glass or deposits of volcanic ash. Their particles size ranges from that of most clays to some of the finest of all clay particles.

Bentonite, the most common volcanic clay (Al2O3.4SiO2.H2O), is often used as a plasticizer for other clay bodies. At four to five times more plasticity than other clays, only a small percentage (2-3%) is needed to add workability to another clay body which is in need of plasticity. Bentonite should always be added to the dry ingredients of a glaze or clay (body) before adding water, or it should be thoroughly slaked (or blended in a blender) with a portion of the water to be added to the clay. If you do not do this, you will be forced to struggle with a gummy mass of bentonite which will not mix into the other ingredients easily.  Bentonite in a glaze (1-3%) will help hold most glazes in suspension, assist glaze adhesion, and harden the dry surface without noticeably affecting the fired glaze.

Adobe Clay

The clay we call 'adobe clay' is usually clay from near the earth's surface and which has proven acceptable for making the sun-dried bricks. It is usually quite sandy and not very plastic. Some clay artists have experimented with the addition of petroleum products in the adobe mixtures for their sun dried sculptures.

Gumbo

This is a common term for surface deposits of clay which show up in farmer's fields and other locations where the plastic material can cause problems for vehicles and farm implements.


Final Note

You will want to consult the many fine ceramics texts which are available. This file is intended to help the student establish a very basic understanding of clays and clay bodies as they relate to forming techniques, glazing and other forms of surface decoration. The temperature ranges and percentages in this file are approximate and based on experience in the craft. I can not control the conditions of your clay and glaze testing so I am not responsible for any problems or damage which may result from your use of this basic information. I hope you will continue your research into the specific areas of your interest using texts and additional on-line educational materials. I hope these notes will help you in your search for mastery of ceramics.
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