Specifications

Clay is a compound of minerals and organic material
resulting from the natural decomposition of certain igneous
rocks (for example, feldspar and granite are two common
sources of clay minerals). As these mineral deposits age and
get moved around by natural forces (wind, water, glaciers),
chemical changes occur which cause the materials to become
clay.
The two major classes of naturally occurring clay
deposits are primary clays and secondary clays. Primary
clays are those that remain at the physical location where the
parent rock decomposed. These clays tend to be the most
pure, but tend to be less plastic than secondary clays.
Secondary clays are deposits that have been transported by
wind, water, or glacial activity. These clays, while still very
pure, generally have had other materials introduced into their
basic composition that change their performance character-
istics (color, plasticity, etc.)
Types of Clay
Clays can be grouped or classified several ways:
according to the way they are found in nature, by their
physical and chemical properties, by the way they are used to
make finished properties, and so on. One of the first ways to
classify clay is by the methods used by Mother Nature to
create clay deposits. The major types of naturally occurring
clay are as follows:
Kaolin or China Clay
Chemically known as Al2O3-2SiO2-2H2O this clay is
almost pure white as a primary clay and slightly less white
but more plastic as a secondary clay. These clays are a major
component of most high-fire porcelain clay bodies and are
frequently used in stoneware to lighten the fired color.
Ball Clay
These are secondary clays that have been transported to
swampy areas where organic acids have broken down the
mineral particles to ultra fine size. These clays are extremely
plastic – if used alone they will shrink quite a bit, causing
severe cracking.
Earthenware Clay
Earthenware is the most common surface clay found
throughout the world. These clays usually contain high
amounts of iron, which gives the fired wares the character-
istic terra cotta color. True earthenware clay cannot vitrify,
which means the clay body remains porous after firing.
Stoneware Clay
Stoneware tends to be kaolins that contain more
impurities - usually calcium, feldspar, and iron - resulting in
clays that have finer particle sizes and higher flux content.
The flux materials cause the clay to vitrify at lower firing
temperatures.
Fire Clay
Similar to stoneware clays, fire clays generally contain
less flux (especially calcium and feldspar). Fired alone these
clays won’t fully vitrify – even at high fire temperatures.
Bentonite Clay
Bentonite is formed from the decomposition of volcanic
ash. Bentonite has the finest particle size of any natural clay.
It is very useful as a plasticizer but it must be used in moder-
ation – too much of it in a clay body will result in cracking
during the drying process.
Slip Clay
These are naturally occurring clays that have a high iron
content. At high temperatures these clays melt to form a
glaze; no additives needed.
Pure natural clays almost always have shortcomings – in
a potter’s eyes. The production processes and ultimate use of
the piece dictates the properties the clay needs to have –
either during the forming stage or as a finished piece. A clay
body is a mixture of clay and other materials designed to
meet the needs of the user. Design objectives for a clay body
may involve making it more plastic during the throwing
stage, improving the body’s stability in large-scale work,
helping the body resist thermal shock from firing, and
improving general properties such as vitrification and density.
The “other materials” in a clay body perform a specific
function to make the final product work better for the clay
user. A general description of each component and its
purpose are discussed below.
Components of a Clay Body
All clay bodies involve combining clay (many recipes
call for several types of clay) with non-clay additives. The
basic types of additives and the purpose for each are as
follows:
Flux
These materials act as melting agents, helping to lower
the maturing temperature and assist in the formation of glass
- the essential binder in all ceramics. Some clay contains
higher concentrations of fluxes naturally–feldspar and iron
are the most common.
Glass-Formers
These materials react with fluxes to form glass. The
most common glass-former is silica. Pure silica melts at very
high temperatures – the proper mix of flux materials and
silica allows for glass to be formed at more manageable
temperatures. This balance must be carefully achieved – too
much flux produces a weak glass, too much silica can lead to
reduced thermal shock resistance.
Refractories
These materials stabilize the body, providing the physical
What is Clay?
Clay & Clay Bodies
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