Clay
Why It Acts The Way It Does

by F.H. Norton

This article first appeared in Studio Potter, Volume 4, Number 2 (Winter 1975/76).
Copyright © 1976 by Studio Potter. All rights reserved.

Introduction

In this article an attempt is made to describe, in terms understandable by the average potter, the structure of clay and the reasons why it acts in such a unique way when used in a pottery body.

Types of Pottery Clay

The purest type of clay is kaolin, found in great quantities in our southern states. There are several kinds of kaolin, the most plentiful are the sedimentary kaolins which may be used as mined or put through a washing process to remove unwanted minerals such as quartz, feldspar or iron oxide. Then there are residual kaolins which must always be washed as they contain large amounts of non-clay minerals. When properly treated, however, they become our whitest kind of clay. One other kaolin of interest, found in northern Florida, has high plastic properties.

Kaolins are almost never used as the complete pottery body because of low plasticity, high drying shrinkage and very high firing temperatures to vitrify. They are white burning, however, and serve as the basis for white earthenware and porcelain after an additive of quartz and feldspar.

Another class of clays is the ball clays found in Kentucky and Tennessee. These are fine grained clays containing organic matter. They are not used as the complete body, but rather in additions of 10% to 20% to give added plasticity, better green strength and improved casting properties. These clays are not so white burning as the kaolins, but when used in small amounts have little adverse effect on the fired color.

Another class of clays used by the potter is stoneware clays. These clays burn buff or gray and usually contain enough nonplastic material to give good working and firing properties. In some cases, however, these clays are washed and some additions made to give enhanced properties.

The early pottery used in this country was made from red-burning glacial clays found in nearly all villages in the eastern United States - the same clay as was used for making bricks. These clays are quite plastic and are still used for some types of pottery, but it is difficult to fire them into a water-tight body.

Another clay similar to the last one, known as slip clay, is used as a glaze for stoneware and electric insulators. The traditional source is Albany, New York.

Lastly there is an extremely fine grained clay called bentonite that is used as very small additions to bodies for enhancing their plasticity.

Nature of Clay

If one takes any finely grained non-clay mineral and mixes it with water, a crumbly mass will be produced with almost zero formability. If the same is done with clay, however, there is produced a mass that is readily formed into any desired shape and, most interesting of all, it will retain that shape under the force of gravity. In other words, the clay mass has three unique properties; first, it may be deformed without cracking; second, when the deforming force ceases, the shape will remain fixed; and further, when the clay mass is dried, it has considerable strength.

It should be of interest to the potter to learn the reasons why the clay mass behaves in this way. This can be done by examining the individual clay particles as to shape and size and their reaction with the associated water.

The Clay Particle

The optical and electron microscope have made it possible to examine the clay particles of all sizes. It is found that each particle is a crystalline plate with an hexagonal outline as shown in Fig. 1. The average diameter of this plate is one micron [one millionth of a meter], a size so small it can only be observed by a high power microscope. Other particles in the clay are as large as fifty microns and as small as one tenth micron in diameter. The thickness of the clay particle is about one tenth the diameter with the faces flat and smooth.

In the clay mass most of the particles are stacked together somewhat as shown in Fig. 2. In most clays the clay material is kaolinite with the formula (OH)4 Al2 Si2 O5, but other clay minerals occur in small amounts.

The Clay Particle and Water

In a batch of moist clay, the water forms thin films between the faces of the clay plates. These films are very thin, averaging six thousandths of a micron in thickness. Of course, the more water added to the clay mass, the thicker will be these films. A sort of equilibrium exists in the clay mass with the particles pulled together by attractive forces and at the same time held apart by the water films. Therefore the wetter the clay mass, the thicker will be the water films and the more easily the clay particles can be forced to move in relation to each other. This explains why a wet clay may be molded with less force than a dryer one.

It is interesting to note that when dry clay is mixed with a non-polar liquid, such as kerosene, a workable mass cannot be produced. This is due to lack of attractive forces which occur with water.

Plasticity

This term may be defined as the ease by which the clay may be formed. If the amount of extension of a clay-water specimen is plotted against the force to move it, curves such as those in Fig. 3 are obtained. It will be seen that the specimen elongates elastically as the force is increased until point (a) (the yield point), then flows plastically, and point (b) (the breaking point) is reached. The most highly plastic masses have both a high yield point and a long extension range before fracture.

Much work has been carried out by ceramists to develop a quantitative measure of plasticity, but nothing has really taken the place of the hands of the experienced potter.

Drying, Shrinkage and Warpage

When a plastic clay dries, the following steps occur: (1) the water in the layers between clay particles gradually diffuse to the surface where it evaporates until finally the particles touch each other and the shrinkage stops; (2) the remaining water in the pores then dries out with no further shrinkage; and (3) absorbed water on the particle surface disappears. This explains why a formed clay article must be dried slowly during stage ito prevent cracking, whereas stages 2 and 3 may safely take place quite rapidly. This is shown diagrammatically in Fig. 4 where the black areas represent the water.

Warpage in drying thin ware is due to uneven shrinkage. This can be prevented by slow overall drying or retarding the drying of certain parts by covering. The question often comes up as to why warpage occurs when a newly formed piece has a uniform water content and the dry piece has a low, but also even content. This can be explained by a drying tile resting on a smooth surface. The upper face will dry first and the tile will curl up because the upper face is smaller than the lower face. Later when the tile is completely dry the curl remains as the body is too rigid to completely straighten out. This warpage is shown in Fig. 5.

Casting Slips

While casting slips may be made of nearly an inorganic powder, slips containing a major portion of clay are used by the potter. To produce a good casting slip, it is necessary to add more than water. The clay particles must be completely dispersed so each one is free to move about. This requires that the normal attractive forces between the particles be changed to repulsive forces which can be done by adding a very small amount of chemicals with an alkaline reaction, such as a combination of silicate of soda and soda ash. Thus normal clay paste with 20% to 30% of water would be in the plastic condition, whereas with the proper amount of alkalinity (deflocculation) the mix is fluid enough to be easily poured. In Fig. 6 is shown the viscosity of a clay slip with varying amounts of deflocculant. This shows the great decrease in viscosity by the proper use of deflocculating agents.

Green Strength

A non-plastic powder, for example, potter's flint, has practically no green strength. On the other hand, clays have relatively high strengths running from 20 psi to 1000 psi. The reason for this is not entirely clear but is probably due to the ionic attractive forces between the flat clay plates.

The potter must have a green strength sufficient to handle the green ware safely, but the great strength required for heavy pieces, like glass pots, is usually unnecessary.

The Influence of Non-Plastics on Clay

Of course, most pottery clays contain natural non-plastics such as finely divided quartz or feldspar, but most pottery bodies have non-plastics added to the clay for various purposes, such as to reduce drying shrinkage or enhance firing properties.

As shown diagrammatically in Fig. 7, the non-plastic particles simply displace an equal volume of the clay-water mixture. As the non-plastic particles are stable in size, the overall drying shrinkage is reduced. For example if the pure clay has a drying shrinkage of 5%, then 10% of added non-plastic should reduce the shrinkage to 4.5%. Actually the picture is more complicated than this due to the size and shape variations of the non-plastic.

The non-plastic has another effect in that it often makes a sticky clay more workable in the plastic state.

CONCLUSION

Moist clay is a material of many moods. The better we understand the underlying structure the more closely it can be adapted to our needs. The right clay used in the right way can be a faithful servant, but the wrong clay and the wrong use turns it into an exacting tyrant.

Crystals from a washed kaolin showing hexagonal plates (electron microphotograph, 32,000X).

Prof. Frederick H. Norton is the author of "Elements of Ceramics, Ceramics for the Artist Potter," and other books. He is retired from teaching at MIT and presently lives in Gloucester, Massachusetts. (1976)