Specifications

structure or binding material for the flux and glass-formers to
bind together. The primary refractory material is alumina.
Rather than add pure alumina high alumina clays, such as fire
clay or kaolin, are blended into the clay body to increase the
alumina content.
Fillers
These gritty, granular materials that improve a clay body
by enhancing the forming strength, decreasing shrinkage,
provide for more even drying and greater thermal shock
resistance. A common filler material is grog – a material
formed by grinding previously fired clay into a sandy grit.
Plasticizers
Occasionally a clay body will need accessory plasticizers
to improve the moldability and flexibility of the clay. Certain
porcelain bodies and pure kaolins benefit from additives such
as bentonite, Veegum T, or macaloid.
The combination and proportion of these ingredients will
affect the general properties of the clay – how it will behave
prior to firing and perform after firing.
Clay Materials Properties
It is a common practice of materials scientists to classify
and evaluate materials by the properties the material contains
or exhibits when manipulated and used. Every material can
be evaluated by objective criteria, such as chemical, electrical,
physical characteristics and behaviors.
There are two fundamental properties that really defines a
clay: its ability to be molded and shaped and how it fires.
Some clays are more flexible, or plastic, than others, some can
be fired to high temperatures while others cannot. Other
attributes one may consider include color (of the clay body
itself), porosity, vitrification (firing characteristics), glaze fit,
shrinkage, maturing temperature, and so on. Some of the
terms and concepts used to describe clay follow:
Plasticity
Refers to the ability of a clay to be molded and shaped.
There are several factors that affect clay’s plasticity: mineral
particle size, acidity levels, amount of water, amount of non
plastic additives, etc. The state of clay particles “sticking
together” or “separating” has a technical, chemical expla-
nation. Rather than digress into a scientific discussion on
positive or negatively charged particles let’s just come to
understand these opposite processes as follows:
Flocculation
The process of adding an acidic substance which causes
clay particles and minerals to attract one another (flock
together). This process increases the stickiness or plasticity of
a clay body.
Deflocculation
The process of adding an alkaline substance to a clay
mixture that causes the particles to repel one another. While
not commonly added to clay compounds deflocculants are
used to help liquefied clay (slip) stay in suspension and flow
better.
Vitrification
The process under which the clay body experiences
chemical and physical changes during firing. The changes
that take place during firing take place in stages. During the
first firing stage the clay is fired to a red heat and the particles
are now stuck together permanently - but the glass-forming
processes have yet to begin. The clay body is said to be
sintered. A sintered body is considered fired – it has now
become bisque. Bisque bodies have strength – not has much
as a vitrified body – and are very porous. At this stage the
clay can no longer be slaked down (reconditioning dry clay to
a more moist state by adding water). For some clay bodies
this stage IS the final stage of firing. Earthenware, for
example, matures at low temperatures and does not vitrify.
Earthenware clays remain porous when bisque fired, requiring
glazing to make a waterproof surface.
As temperatures climb past the sintering stage the fluxes
and glass-formers begin to interact - the particles actually fuse
together, forming glass between and around the mineral
particles. The glassy materials strengthen the sintered connec-
tions between the refractory particles and gradually consume
the air spaces in the clay body. When almost all the air spaces
are filled and the glass-forming stage is complete the body is
said to be vitrified. A vitrified body is impervious to water,
very strong and dense (a vitrified body shrinks and condenses
as the air spaces are filled with glassy material).
Each clay body has a maximum firing range whose top
temperature, if exceeded, will result in deformations.
Bloating, warping, slumping, or complete loss of structure are
the physical signs of an overfired body.
Porosity
Porosity refers to a material’s ability to absorb moisture
(absorption is the common term used by clay users). Porosity
can easily be measured by weighing a mature fired but
unglazed piece of clay or clay body, then place the sample in a
pan of water, bring to a boil, let cool overnight. Blot off the
excess water and weigh again. The percentage increase in
weight represents the porosity of the clay. Earthenware
typically has an absorption rate of 5% to 14%, stoneware 2%
to 6%, and porcelain 1% to 3%.
Shrinkage
Shrinkage occurs in all clay bodies as they are dried and
fired. Considerable shrinkage occurs as water evaporates
from the wet greenware stage to the bone-dry greenware
stage. The more plastic the clay body, the greater the drying
shrinkage. A typical shrinkage rate for wet greenware is
between 4% and 10%. Shrinkage from the firing of a clay
body in a large part depends on the flux content and size and
quantity of refractory materials. A pure kaolin clay or clay
body can shrink as much as 8% during firing, whereas
refractory bodies may have extremely low shrinkage. The
addition of grog material can greater aid in reducing the
shrinkage due to firing of a clay body. Grog is finely ground
up bisque added to the clay to reduce the shrinkage or for
thermal shock.
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Clay