Tip #31: Clay, the Drying and Firing process
Many potters, especially beginners, use prepared clays without much
thought. However, there comes a point when it is useful to understand
the process that clay goes through during drying and firing. This can
help troubleshoot many of the problems you experience with your pieces.
Plus it is darn interesting!
Air Drying
Wet clay contains a large amount of water, a minimum of 25% water. When
clay starts to dry, water evaporates from it. As this happens, the
particles of clay are drawn closer together resulting in shrinkage. Many
problems with clay are formed by uneven rates of drying, which create
stresses in the clay. Sometimes these stress show up right away as
cracks or warpage, other times not until during or even after firing. So
it is important to ensure drying is even. This is done by ensuring
uniform thicknesses throughout the piece, drying slowly, and even
slowing down the drying of certain parts. This has been discussed in
several previous tips.
Clays which have very fine particle sizes will shrink more than clays
with larger particle sizes. Porcelain clay has very fine particle sizes
which makes it very plastic and also shrinks the most. These bodies have
the most strength in the dry state. Groggy clays such as sculpture
bodies shrink the least. (Grog is clay which as already been fired and
then ground to various particle sizes.) These bodies shrink less because
they have lower water content to start with, and also provide channels
through which moisture can escape toward the surface. These are called
“open” bodies.
When the water has evaporated from between the clay particles, and all
the remaining clay particles are in contact, drying shrinkage is
complete. This is called the leather hard stage. The particles
themselves are still damp, but their drying will not cause any
additional shrinkage.
Initial Kiln Drying
Complete drying doesn't take place until the piece is in the kiln. This
happens when the boiling point of water has been reached (100 degrees C,
or 212 degrees F.) This must happen slowly, or the formation of steam
within the body of the clay may cause it to burst. For this reason, the
early stages of firing are done slowly, and with a peephole or lid open
for steam to escape.
Dehydration
The next change which occurs is at about 350 degrees C (662 degrees F),
the point where the chemically combined water of the clay is driven off.
This is water that is part of the molecular structure of the clay, not
the previously described water that is between the particles of the
clay. This drying is completed by about 500 degrees C (932 degrees F).
After this point you could no longer mix the dried clay with water to
make new wet clay. An irreversible chemical change has taken place,
known as dehydration. No shrinkage is observed during this stage.
Burn-off
Another thing which happens up to about 900 degrees C (1652 degrees F)
is the burning off of organic and inorganic materials, such as carbon
and sulphates. These are the fumes that it is important not to breathe,
and the reason a kiln should be well ventilated even during bisque
firing.
Quartz Inversion
After dehydration, the next change that happens is Quartz Inversion,
which happens at 573 degrees C (1064 degrees F). At this point, quartz
crystals rearrange themselves into a slightly different order. A slight
and temporary increase in volume occurs at this point. This is why you
always need some space around pieces during firing, as they will expand
somewhat. Firing should proceed slowly during this Quartz inversion. A
large percentage of ware that is cracked during firing happens from fast
firing through this stage. The factory set program on electronic kilns
usually slow down the firing at this stage for you.
Vitrification
The next stage that happens is vitrification. This is the hardening,
tightening and finally the partial glassification of the clay.
Vitrification results from fusions or melting of the various components
of the clay. The strength of fired clay is increased by the formation of
new crystalline growth within the clay body, particularly the growth of
mullite crystals. Mullite is an aluminum silicate characterized by a
long needlelike crystal. These lace the structure together, giving it
cohesion and strength.
Shrinkage happens at the vitrification stage. This is due to diminished
size of the particles as they approach fusion and to the closer
arrangement of particles in their glassy matrix. The firing shrinkage of
a clay is usually about the same as the drying shrinkage. Total
shrinkage will usually be about 8-12%.
Clays vitrify at various temperatures depending upon their composition.
A red clay high in iron and other impurities might fire to hardness at
about 1000 degrees C (1832 degrees F) and melt to liquid at 1250 degrees
C (2282 degrees F). A kaolin body which is very free from impurities
might not melt until over 1800 degrees C (3272 degrees F)! By mixing the
ratios of different types of clays that melt at different temperatures,
clay bodies are developed for different firing temperatures.
Melting
If you fired high enough, the clay would first swell up (bloat) then
fuse into a liquid which would cool as a glass. Or course in ceramics we
don't fire that hot; we stop at the point where we have just enough
fusion and hardness for durability, but not too much so we cause melting
or deformation of the ware. This point is called the maturing of the
clay.
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