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TECHNICAL PAPER # 13
UNDERSTANDING CLAY RECOGNITION
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 . Fax: 703/243-1865
[C]1984, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in
Assistance to provide an introduction to specific
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their
They are not intended to provide construction or
details. People are
urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and
almost entirely by VITA Volunteer technical experts on a
Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.
VITA staff included Leslie Gottschalk
and Maria Giannuzzi as editors, Julie Berman handling
and layout, and Margaret Crouch as project manager.
Miska Petersham, the author of this VITA Technical Paper and
second one, "Understanding The Small-Scale Clay
has worked in the field of ceramics for many years.
is also a designer in glass and wood and a wood carver, and
considerable experience in these fields in developing
Reviewers Daniel Rhodes and Gerald Rowan are also experts in
and ceramics. Daniel
Rhodes is a professor emeritus at Alfred
University, New York, in ceramics.
He is the author of four books
on ceramics, and has experience with pottery design, glazes,
kilns, molds, clay refining, etc.
Gerald Rowan is the chairman
of the art department at Northampton Community College,
He has a wide knowledge of ceramics, clay, brick making,
kiln building, glazes, owner made equipment, etc.
VITA is a private, nonprofit organization that supports
working on technical problems in developing countries.
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster
volunteer technical consultants; manages long-term field
and publishes a variety of technical manuals and papers.
CLAY RECOGNITION AND PROCESSING
By VITA Volunteer Miska Petersham
I. ABOUT CLAY IN
Clay occurs naturally almost everywhere in the world and is
formed by the action of weathering on several kinds of
This process takes many thousands of years, but it happens
the rocks are exposed to the natural forces of wind, water,
frost, etc. The
rocks change very slowly in both physical and
Physically, they break down into smaller and
smaller bits; chemically, elements are added and taken away.
After a long, long time, some of the rock changes to
longer the geological period of time, the more clay is formed.
There are several different kinds of clay minerals and most
deposits contain more than one kind.
"Clay" is the general term
that is used for all the clay minerals.
Some of these clay minerals
or clays are of greater use to the potter than others.
takes difficult laboratory tests to determine just which
minerals are present in a particular clay.
As practical potters,
however, we are more concerned with how the clay works in
rather than exactly what is in it.
All of these clay minerals are a variation of the one called
Kaolin is the most pure and is a hydrous silicate of
This means that it contains aluminium oxide, silicon oxide,
and water linked chemically.
The other clay minerals often contain
more water and also have some impurities, such as potassium,
Clays are made up of many small, flat particles.
The size of
these particles affects the way the clay behaves.
If the clay has
been carried long distances by water, the particles are
and smoother so that the resulting clay is usually more
In a very simplified way, Figures 1 through 7 show what
when wet clay is dried.
The clay particles and water molecules
are actually too small to be seen, except in a special
Note: Terms in bold face are defined in the glossary in the
of this paper.
When clay is fired at 800 degrees Centigrade or more, it
longer slake (absorb water) but remains hard and permanent
of the glass that is formed.
The most common clay minerals are Kaolin, Illite,
and Halloysite (or disordered Kaolin).
Clays from the temperate zones are weathered slowly from
rock, which is common in these areas.
Because these clays
have been slowly weathered, they tend to consist of the more
stable clay minerals (Kaolin) and to be uniform in
temperate clays are most often Kaolin alone or with small
of Illite and/or Montmorillonite.
Owing to the longer geological
time period, temperate clays are often transported long
distances by water, thereby collecting impurities and being
Temperate clays, therefore, represent a rather
orderly progression from pure Kaolin, weathered on site
to common surface clays carried long distances by water
Table 1 presents some well-known kinds of temperate clay.
Table 1. Some
Established Categories of Temperate Clays
None to Short
High 1300 - 1400
Short to Long Ball
Medium 1250 - 1300
Medium to Fire
Medium 1250 - 1300
High 1000 - 1100
Tropical clays are of volcanic origin and are quite
They are weathered relatively quickly because of the high
humidity, and acidic-conditions.
Because of the shorter geological
period and less movement physically, they are often a
of several clay minerals.
These usually are the less stable ones.
The clays are younger and contain more Illite,
and Holloysite in relation to Kaolin.
Tropical clay deposits vary
greatly in constituents and physical characteristics over
They do not show the orderly progression of temperate
clays because the mixtures are more varied, and
has seldom taken place.
Almost all contain iron as a basic
constituent, since the parent rock is largely basaltic, with
high iron content.
They also often contain a high proportion of
Tropical clays have maturing temperatures of from under 1000
Centigrade to over 1400 degrees Centigrade; that for most
falls between 1100 degrees Centigrade and 1200 degrees
Plasticity is often medium to high, owing to the presence
Shrinkage is high and the color is usually
dark buff to red. It
is impractical to relate them to temperate
clay categories or to seek a pattern by which to set up a
II. USING TROPICAL
Due to the presence of clay minerals other than Kaolin,
excess chemical water in the clay.
This water is given off at
different temperatures depending upon the minerals present;
can be given up as late as 1000 degrees Centigrade.
The water is
often released rather suddenly causing potential
* Dry pots slowly
* Fire bisque
slowly up to 1000 degrees Centigrade.
Do not stack pots inside or on top of each
* Shrinkage is
high and, therefore, any temperature gradient
warpage. High iron content causes
occurs or where flames touch. High iron
can also cause
To prevent excess shrinking or warping:
* Shape must be
* Maintain a
clean, even firing cycle (oxidation only).
* Protect pots
With a slow, clean firing, most tropical clays fire to a
hardness and can be glazed successfully.
It is very difficult
to reduce absorption below 5 to 10 percent without causing
Thermal shock resistance of tropical clays is good to
With sand or grog added, most tropical clays can be brush-
pit-fired (approximately 800 degrees Centigrade) without
loss. The resulting
pottery is rather soft and, therefore,
works well as cook pots or on an open fire.
Brush-fired clay does
not travel well, due to its fragility, but works for stove
water filters, cook pots, small decorative items, bricks,
etc. If the firing
temperature is too low (under 700 degrees Centigrade),
the fired pot will eventually crumble if exposed to water.
When fired at over 1000 degrees Centigrade, most of the
much more durable. A
1000 degrees Centigrade fire is a
bright red orange color.
To reach 1000 degrees Centigrade or
over, it is necessary to construct a kiln to contain the
Excessive shrinkage can be reduced by adding as much silica
or grog as can be added and still use the clay.
plasticity, and absorption can be changed by blending with
other clays. For
example, to lower absorption, add a clay with a
lower maturing point, talc, feldspar, or ground glass.
plasticity, age as long as possible (minimum one week),
blend with more plastic clay, or add a small amount of
Bentonite is mostly Montmoillonite and is highly
plastic. Do not
use over 5 percent.
To decrease plasticity, blend with a short
clay or add sand or grog; this also helps to prevent
Never use beach sand.
It is calcium carbonate and turns to lime
in the heat. This
will destroy the pot. Instead, use
river sand or any inland deposit not associated with the
shells, or coral.
Grog is ground up, fired clay, usually broken pots from the
Never use glazed pieces.
To make grog, crush broken pieces of pot
in a yacona pounder or with a hammer or a stone.
Sift the crushed
pieces through a fine screen.
Remove any piece left in the
screen, crush again, and rescreen.
Grog that passes through the
screen can have fine dust removed, if necessary.
Too much dust
sometimes causes cracks in the finished pot.
If it causes no
trouble, leave it in and do not worry.
To remove the dust, winnow
as you would remove chaff from rice.
In a windy open area, pour
grog from one container to another, as shown in Figure 8,
wind to blow dust away.
Any larger particles stay.
this two or three times.
III. LOCATING AND
In the tropics and on islands with a volcanic history, clay
deposits are younger, smaller, and often part of the
rock. They also
occur in river deltas and low areas.
many clay deposits on hillsides that are the result of the
weathering of a rock mass; thus you will often find clay
bottom to middle of low hills, as shown in Figure 9.
Low lying areas, especially if water does not drain easily,
have clay under 1-4 feet of peat or muck.
Sometimes a field
will have several feet of clay 1-2 feet below the top
and stream banks often show clay deposits under 1-2 feet of
Sometimes a deposit of sand occurs close to the water, so
digging about 20 feet from the water.
Roads and irrigation
ditches often cut through clay deposits, giving easy access
the material. Clay
in easily recognizable when wet because it is
slick and shiny and has water puddles on it.
When it dries, clay
cracks and has a hard smooth surface, as shown in Figure 10.
Banks erode in rivulets, not smoothly, and fine clay is
down to the bottom where it cracks and curls when dry, as
in Figure 11.
Two simple field tests will help to establish whether a
is actually clay.
The only true test is in the fire, but a lot of
non-clay material can be discarded by performing these
First, moisten a lump of test material and knead it until it
free of large lumps and the consistency of putty or bread
Squeeze an egg-sized piece in one hand, as shown in Figure
the lump holds together, does not crumble, and retains the
impression of your hand, as shown in Figure 12, it may be
Second, take another small piece of the kneaded material and
out a pencil-sized coil.
Send this around one finger. If
without cracking or only cracks slightly, as shown in Figure
it may be clay.
IV. PROCESSING CLAY
The materials you will need to process clay include a
piece of window screen mounted on a frame, two or three
other large containers, several pieces of cloth (cotton
or muslin) and plastic bags.
Build a 1 foot x 1 foot frame out of 2 inch x 2 inch lumber
inch x 2 inch lumber, as shown in Figure 14.
Firmly tack the window screen on one side of the frame so
are no gaps, as shown in Figure 15.
You are now ready to make clay in quantity.
Remember that the
longer it can sit in the plastic state, the better it will
when you use it.
Follow these instructions:
1. Break up lumps to golf ball size or smaller and spread
If collecting large quantities, store in
spread to dry as
2. When clay is completely dry, put it into water to slake.
Use a 44-gallon
drum or large pail half filled with water.
Clay should not be
3. Let stand without stirring until the clay softens.
vary from a few
hours to a few days, depending on the clay.
4. Stir vigorously with a paddle or by hand, adding water as
the clay is the consistency of thin cream.
The clay is
5. Dip out slip and screen through a window screen or a
This depends on clay and tooth desired.
6. Some clays will settle readily at this consistency, if
allowed to stand
for several hours. This leaves clear
on top which may
then be siphoned or poured off to make
Clays can be mixed before processing or after they are made
slip. Use the
following procedure to mix before processing: If
the clay contains a high proportion of rock fragments or
large non-clay particles, and you wish to blend it with
clays, it helps to know the amount of material that will be
by the screening.
Suppose you want to blend two clays, A
and B. Assume Clay A has 20 percent residue and Clay B has 5
residue. You can
blend the two clays before processing
(which is much easier) by adding 20 percent extra of Clay A
percent extra of Clay B to whatever blend you are
making slip and screening, the proper proportions of the
To determine the amount of non-clay residue, follow these
1. Weigh out 100 grams of dried clay or measure out by
10 small measures
(such as spoonfuls).
2. Add weighed or measured amount of dry clay to water in a
Water should cover clay fully.
Allow to slake
from 1 to 24
hours, depending on how quickly the clay breaks
down in the
water. When slaked, stir until no lumps
Add water, if
needed, until consistency of cream is reached.
3. Pour through a screen into a second container.
(what is left in
screen). Extra water may be poured over
residue in screen
to wash away any clay remaining.
4. Weigh or measure residue, as shown in Figure 16.
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To mix clays after they are made into slip, you must know
weight of the materials in a known amount of slip.
The dry weight
of ingredients can be calculated using the following
= P - 20(g)
the dry weight needed
the weight in ounces of one pint of slip
the specific gravity of the solid
the weight in ounces of one pint of water.
The specific gravity of pure water is about 1.
The specific gravity of clay is about 2.6.
The specific gravity of potash spar is about 2.56.
The specific gravity of flint (silica) is about 2.65.
If two or more local clays are to be mixed as slips,
the correct proportion by the above formula.
It is not necessary
to return the slips to the mixer, just stir them to assure a
When adding dry ingedients such as feldspart determine dry
of clay in the slip.
Start with a small amount of water.
known amount of the slip.
Then, slowly add other dry ingredients
as needed to complete the correct proportion.
Add water, as needed,
to retain slip consintency.
When mixed, dip out and allow to
settle. It should
not be necessary to screen at this point.
Which ever method is used, you should now have a slip that
all of the desired ingredients, including grog, if called
DRYING CLAY SLIP
There are several popular methods of drying clay slip: (1)
plaster bats; (2) clay bats; (3) drying clay with bricks;
drying clay in a cloth bag; (5) drying clay with cloth and
and (6) drying clay in a frame.
Make large plaster bats over a lump of plastic clay approximately
12 to 18 inches by 24 to 30 inches by 4 to 6 inches.
of burlap dipped in plaster to strengthen and make wall
1 inch thick. See
Figure 17. Fill with clay slip.
Several bats will be needed as they must be dried after each
It takes two days or more to dry the clay.
Bats take considerably
longer to dry unless a kiln is running.
Clay bats can be made and bisque-fired at a low temperature,
good porous body is available.
They should be smaller than the
plaster ones and fired no higher than 900 degrees
They work quite well and have the advantage of not
the clay with non-clay materials.
See Figure 18. Large
clay bowls can also be used, provided they are at least an
Drying Clay with Bricks
Using low-fired commercial brick or homemade ones, build a
raised on bricks, set crosswise to give air circulation
Set other bricks at the edges to contain the clay and pour
slip inside. Cover
with other bricks so drying is even.
See Figure 19.
Drying Clay in a Cloth Bag
As shown in Figure 20, make a bag big enough to hold a
out of thin canvas or sheeting; fill with slip and tie up
open end with a rope.
Hang where water can drip out.
is quite effective but often uneven, leaving dry edges and
Combined with bats, it works well, since much of
the water can be removed before putting in the bats.
Drying Clay with Cloth and Sand
As shown in Figure 21, scoop a shallow hole in dry sand and
the cloth in it.
Fill the hollow with slip. In a
dry area and on
a dry day this works quite well.
It takes from one to three days
to dry to a plastic consistency.
Drying Clay in a Frame
As shown in Figure 22, make a 2 foot square out of 2 inch by
inch wood. Cover one
side with cloth and then wire mesh, such as
chicken wire, to keep the cloth from sagging.
Make a rack or
arrange bricks to hold up the edges of the frame so cloth
not touch. Water drips
out and if watched for uneven drying, this
works quite well.
Store plastic clay in airtight plastic bags or
plastic garbage cans.
The longer, the better, since clay improves
If a mechanical mixer is available, set it up in a separate
and add water to cover blade.
Start mixer in water and add slip
clay slowly with additional water as needed to make a thin,
Mix until lumps are gone. Once
started, do not stop and restart, as the lumpy slip can cause
motor to burn out.
Strength of the motor will determine the
amount of clay you are able to mix at one time.
will mix about one-third (1/3) of a drum, if used with
caution so as not to overload.
A typical clay mixer is shown in
Warning: Do not stop and restart mixer in slip; start only
mixing, once started, should be completed before
shutting off mixer.
If a mixer is to be built, use a shaft length and clamp to
container to be used.
The motor should be 1/2 to 1 horsepower.
Use a 3-inch propellor made out of 1/4 by 1-1/2-inch
steel welded to the shaft.
Balance is most important to reduce
vibration. Set the
top blade so the clay is lifted and the
bottom blade so the clay is forced down.
The propellor may have
two, three, or four blades.
Sucking in of fluid (water retention) due to
Attraction of liquid molecules to the surface
of a solid; electrical bonding.
Oxide of Aluminum [A1.sub.2][O.sub.3].
Dark igneous rock of volcanic origin and contains
Large, shallow, porous containers.
Clay that has been fired once or the first
Combination of elements into new substances
or the reverse; molecules from
atoms or atoms
Alteration product of igneous rock, hydrous
silicate of alumina [A1.sub.2][O.sub.3]Si[O.sub.2]2[H.sub.2]O.
Mineral composed of alumina, silica and either
potassium, sodium or calcium, for example:
[K.sub.2]O/[A1.sub.2][O.sub.3]/6Si[O.sub.2] is called Potash Feldspar.
Rock composed primarily of feldspar.
Any substance that lowers the melting point
of the mix.
A controlled glass bonded to the surface of a
Crushed, fired clay.
Powdered glass, such as bottles crushed to a
Disordered Kaolin particles often tubular in
Clay mineral [A1.sub.2][O.sub.3]/2Si[O.sub.2]/2[H.sub.2]O.
Refractory containers for heating ceramic
The temperature at which the clay obtains
optimum hardness and durability without melting.
The smallest grouping of atoms to which a
substance can be reduced
without losing its
Clay mineral capable of both absorption and
adsorption of water.
Conditions of burning (kiln firing) with an
excess of oxygen.
Original rock from which a clay is weathered.
Calcined and ground gypsum (calcium sulphate).
Ability to bend without cracking.
Able to absorb liquid.
Clay weathered in place and not transported
Conditions of burning (kiln firing) with a
lack of oxygen atmosphere of free carbon or
CO or [CO.sub.2].
Clay transported in water.
The opposite of plastic; cracks on bending.
Oxide of silicon Si[O.sub.2]; also known as quartz
Particles of quartz or Si[O.sub.2].
The absorption of water by clay to make a
Watery clay thin enough to pour.
Sagging or deformed from its own weight.
Mineral containing silica; used
as a body
Middle latitudes of the northern and southern
Reaction to suddenly applied heat such as
open flame or sudden chill.
The amount and character of grit in the clay.
Action on a substance by natural forces, such
as rain, wind, freezing, and sun.
SUGGESTED READING LIST
Buchanan, W. Hand Moulded Burnt Clay-Bricks: Labour
Malawi Ministry of Trade, Industry, and Tourism
Industrial Development Organisation, Project
Cardew, M. Pioneer Pottery.
New York: St. Martin's Press, 1976.
Green, D. Pottery Glazes.
New York: Watson Guptill Publishing,
Leach, B. A Potter's Book.
Hollywood, Florida: Transatlantic
Parry, J.P. Brickmaking in Developing Countries.
Division, Building Research Establishment, UK.
Watford, United Kingdom: Building Research Establishment,
University of California.
Division of Agricultural Sciences.
Agricultural Experiment Station Extension Service.
Construction Method. Manual 19
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