Lecture #8 - Alfred's Clay Store

Firing
Lecture #8

Firing
• The final frontier
– Work either makes it or breaks it
• Even when forming and drying conditions
are ideal, we can still loose work at this
stage
• Understanding the firing process can help
us avoid costly mistakes
• Before looking at firing, we will look at
silica, as it is integral to the firing process

(alpha)
(beta)
(alpha)
(beta)
(no inversion)
• A changes from one
phase to another is
called a silica
conversion
• Conversions are not
reversible - they are
one way streets
• Within the same
phase, a change from
one form to another is
called a silica
inversion
• Inversions are
temperature dependent
and are reversible

Quartz
• Also known as free silica
– “free” distinguishes this silica as not being chemically
bound to other molecules (i.e. the silica in our Kaolinite
crystal is not “free”; it’s bound with alumina)
– If you had very small tweezers and steady hands, you
could remove free silica from the clay
• The major impurity found in clay
– Up to 85% of a naturally occurring clay can be quartz
• Quartz is also added to a body through flint or
sand

Determining free silica in clays
• Pure kaolin contains
46% SiO 2 ; 39%
Al 2 O 3
– Silica above 46% is
free silica
• Secondary clays
contain more free
silica than primary
clays
– They have picked up
silica in their travels
% SiO 2 Al 2 O 3
Grolleg 47.7 37.7
OM4 53 30.8
Red Art 54.3 16.4
Hawthorn 55.1 39.11
KT Stone 67.1 20.7

Alpha to Beta Quartz
1064°F
• 573 °C (1063 °F ) is known as the quartz inversion point
• Quartz below 573 °C is always alpha quartz
• Quartz above 573 °C is always beta quartz
• Every time we pass the quartz inversion point, quartz
inversion takes place

Effect of quartz inversion
Alpha
• As we approach the
inversion temperature,
the angle of bonding
between silica
tetrahedra changes
– There is a straightening
the the silica molecules
Beta

Effect of quartz inversion
573°C
1%
volume
change
• The straightening of the silica molecules
accounts for approximately 1% volume
expansion from alpha to beta quartz
• This affects all quartz in the body

Alpha/Beta quartz expansion
Inversion Point
• Volume expands progressively quicker with
temperature until it reaches the inversion point

Cristobalite inversion
• Like quartz, cristobalite has two forms
– Alpha and Beta
• Inversion from Alpha to Beta occurs at 226 °C (439 °F)
– Occurs more rapidly than quartz inversion
– Like all inversions, this is reversible
• Up to 3% volume change!
• Happens at lower temps. when the body is rigid
• Porous bodies handle cristobalite inversion better than
dense bodies
– Porous bodies can absorb shock better than tighter bodies

Alpha/Beta cristobalite expansion
• Volume expansion increases suddenly at the
inversion point

Poorly mixed
Well mixed
Silica
Clay
Glass
Cristobalite
Cristobalite can form
Cristobalite won’t form
• Cristobalite formation is promoted by poor mixing
– Clumps of silica in poorly mixed bodies can form cristobalite
(because silica inside the pocket is isolated)
– Isolated silica grains in a well mixed body form glass (each silica
particle is surrounded/coated by clay)
• We can avoid cristobalite by ensuring no two quartz
particles touch
– there is enough clay in any recipe to do this (assuming proper
mixing takes place, i.e. Shar it!)

Cristobalite
• Forms at higher temperatures
– Free Quartz can convert to cristobalite above 1100
°C (2012 °F ) cone 02
• Temperature varies based on fluxes and time
• Soaking above 1100 °C increases cristobalite formation
• Conversion to cristobalite is irreversible as
silica bonds of quartz are destroyed and
reformed as cristobalite
– Once cristobalite forms, it won’t go back to quartz

Glass
• Glass forms when silica is melted and cooled
quickly enough so that the silica tetrahedra cannot
establish a pattern of structure
– Crystalline matt glazes require slow cooling for this
same reason
– Glass is also referred to as non-crystalline silica
• Glass sources in bodies include flint, feldspar, free
silica and silica from clay minerals (i.e. kaolinite
crystal)
• Unlike quartz and cristobalite, silica glass does not
go through inversions
– No alpha or beta form
– Stable throughout the firing process

• Glass Silica
– AKA Amorphous
silica; fused silica
• Glass has a
random structure
• Cristobalite and
Quartz are
crystalline silica
– Ordered,
repeating
structure
Crystalline form
Quartz
Glassy form
Glass

Glass expansion
• Very low expansion

The Three
Tenors
(Like our three
forms of silica,
they’re related)

Minimizing the danger
• Glass phase
– No inversion…no problem
• Quartz phase
– Avoid adding excess free silica (I.e. silica-rich clays, flint
and especially sand)
– Fire slowly through inversion
– Not as problematic as once thought
• Cristobalite phase
– Is not formed at low temperatures
– Can be minimized at higher temps by proper mixing
– Cool slowly through inversion
– Avoid an excess of free silica / avoid soaking at high
temperatures if possible

• Having examined silica’s phases, we will now take a
chronological look at what happens during firing
• Keep in mind that these overlap and that temperature can
vary from one area of a piece to another during firing
– Pyrometer readings can be off
– Temperature at the pyrometer is different from temperature of
the work
– Temperature gradients throughout the work will vary during
firing
• Also, the extent to which these stages occur vary
depending on mineralogy (Kaolinite is used as an ideal
model, though it is not always the main clay mineral
present in a given body)

Pore water removal
• Up to 120 °C (248 °F)
• The initial stages of firing can be seen as a
continuation of the drying process
(dehydration)
• Drying is completed with the removal of the
last traces of water on particle surfaces
– Pore water is removed up to 120 °C
• AKA “water smoking period”

• Explosions occur
when pore water is
removed too
quickly
– At 100 °C (212 °F)
water turns to steam
• Moisture condenses
on a glassy surface
placed above top
spy hole until all
water is removed
– Hold below 100 °C
until no more water
condenses on glass

Organic decomposition
• At approximately 200 °C (392 °F)
• Organic matter, of mainly plant origins,
breaks down
– May cause a distinct odor (especially in
secondary clays)
• Slight expansion up to 1%
– Not a problem during firing because body is still
porous and somewhat resilient

Burning out
• Carbon, Iron sulphide and Fluorine burn out
– Combine with oxygen to form monoxide, dioxide and trioxide
gases (CO, CO 2 , SO, SO 2 , SO 3 etc.)
• Can lead to blistering and discoloration if fired too
quickly
• Starting point varies substantially with clay type
– Carbon between 650-850 °C (1202-1652 °F)
– Sulphur up to 1150 °C (2102 °F)
• Remedies to bloating
– Thoroughly mix the body
– Slow down the firing – especially when once firing
– Use a less dense body / add filler which allows gas to escape

Metakaolin formation
• Occurs at about 550 °C (1022 °F)
• Chemically bound water (crystal edge water +
water of constitution) leaves the kaolinite crystal
6(Al 2 O 3 • 2SiO 2 • 2H 2 O) 3(2Al 2 O 3 •4SiO 2 )+12H 2 O
kaolinite metakaolinite water
• Kaolin changes into metakaolin (very important)
– Resulting in slight shrinkage
– Very high porosity
– 13.95% weight loss due to water
– Plasticity is lost (a.k.a. “ceramic change”)

• Temperature range varies
– Occurs after the formation
of metakaolin and before
glass formation begins
• Particle are barely held
together
– Not fused but barely
welded at very fine points
of contact
• Overall structure remains
very open
– Very little overall strength
• This is what happens
when we do a low bisque
for glazing
Sintering

Vitrification
• Begins at around 800 °C (1472 °F)
– Composition dependent
– Particle size dependent
• Soda and potash start to flux the quartz
– Glass formation begins
– Quartz grains convert to glass on outer
edges and progress inwards as heat-work
increases
• Shrinkage increases
– Pore volume decreases (voids collapse)
– Density/strength increases

Spinel formation
• Forms around 980 °C (1800 °F)
• Metakaolin layers condense and re-crystallize,
forming spinel crystals
3(2Al 2 O 3 •4SiO 2 ) 3(2Al 2 O 3 •3SiO 2 )+ 3SiO 2
metakaolinite spinel phase free silica
• Silica is released from metakaolinite and
contributes to glass formation

Mullite formation
• Begins above 980 °C (1798 °F)
– Really starts to grow past C. 6
(1200 °C or 2194 °F)
• Alumina from clay and feldspar
dissolves into surrounding glass;
When glass is saturated with
alumina, extra alumina crystallizes
into mullite
• More free silica is also released; more glass is formed
3(2Al 2 O 3 •3SiO 2 ) 2(3Al 2 O 3 •2SiO 2 )+ 5SiO 2
spinel phase mullite free silica

Firing changes

Commercial porcelain after
polishing and etching
Pores
Quartz
Quartz
Primary
Mullite
Secondary
Mullite
10 m

Mineralogical
changes in a
40% kaolinite
30% quartz
30% feldspar
body

Cooling down
• Top temperature down to 573 °C (1063 °F )
– Contraction rate is gradual down to quartz inversion
• At 573 °C
– 1% contraction as beta quartz reverts back to alpha
quartz
• 573 °C - 500 °C (1063-932°F )
– Glazes solidify
• 226 °C (439 °F)
– 3% contraction as cristobalite reverts from beta to alpha
form
– Paper burns at 451 °F

My bisque cycle
Temp °F Rate/hr Hold
0-190 100 Varies (overnight)
190-950 150 -
950-1200 100 -
1200-1940 150 -
1940-2079 100 -
-Suitable for my body (heavily grogged;1/2” walls)
This cycle could easily be sped up for thinner work

Bisque cycle
2500
Final
Soaking
2000
2140 ºF
Temp. (F)
1500
1000
Ceramic
change
500
Steam
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Time (hrs.)

Cooling cycle
2500
2000
1500
1000
Quartz
500
0
Cristobalite
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Complete cycle
2500
2000
1500
1000
500
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Re-firing schedule
(I.e. Glaze schedule)
• Differs from bisque firing because body is
now less elastic (it is more dense)
• Unlike bisque firing, cristobalite and quartz
inversions need to be considered during
heat-up
– Especially true for large/thick work
• Also generally no need for soaking at the
start unless piece is damp from glazing

Glaze cycle
2500
2000
1500
1000
Quartz
500
0
Cristobalite
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Heat transfer during firing (in chronological order)
1- Convection heat
• Air is heated as it passes by the element; hot air rises, cool air sinks; air
currents are created; air currents distribute heat
– Not a very even source of heat (especially at lower temperatures)
2- Conduction heat
• Heat is transferred through direct contact
– The main way to get even heating in a kiln
– Outside wall of a piece heats the inside of the piece (or kiln shelf heats bottom
of piece, which, in turn, heats middle and top of piece)
– Requires time, especially with thicker work
3- Radiant heat
• Heat is transferred from hot to cold areas through radiation
• Requires line of sight (like the rays of the sun)
– something in the shadow of the heat source will not heat up as efficiently
• Objects that heat up, in turn, become sources of radiant heat (I.e. elements,
shelves, and the work itself)
• Pieces that are too close to elements can result in scorching due to too much
radiant heat

Thermal gradient of work during firing
• Outside of work always heats up before the inside
– Analogous to a moisture gradient during drying

• Thermal gradient effect leads to differential
expansion/contraction through the walls of a piece

Thermal gradients
• Very problematic from top to bottom of pieces and
possibly from side to side
– Especially in large pieces
– Can happen both in heating and in cooling
• Why
– Hotter part of the piece goes through quartz/cristobalite
inversion before cooler part
• Stress develops, possibly leading to dunting (firing
cracks)
• Solution
E v e n h e a t i n g

Even heating
• Thermal gradients can be minimized
– Slow down firing rate
– Distribute load in kiln evenly (tall and short
pieces on each shelf) and don’t pack too tightly
• Visualize airflow (Convection heating)
• Visualize mass (Conduction heating)
– Elevate your work off kiln shelves
» Kiln shelves store heat (last to heat up, last to cool)
» Elevate the work using stilts, rolled up balls, soft brick or
other low mass materials
• Visualize lines of sight (Radiant heating)
– Use kilns with multiple pyrometers
• Large electric kilns fire more evenly top to bottom
than medium electrics because of this feature

Stilts
• NOT suitable
for high firing

Stilts
• Metal tipped stilts can bend under too much
weight/temperature – for heavier work and hotter
temperatures, use alumina stilts (what we sell) or
make your own

• Ball-shaped stilts help
minimize heat transfer
and allow work to
expand and contract

Using bars instead of shelves…

• Firing to a hot bisque, and then glazing at lower
temps allows the work to be strategically
supported throughout its shrinkage/slumping
• Work will not shrink further during glaze

Cones / Heat-work
• Temperature is not heat
• Heat, or ‘heat-work’, is a measure of time
and temperature
– We know this through experience
I.e. Cheese on a pizza will not instantly melt
when placed in a warm oven-it will take some
time
• Pyrometers measure temperature
• Cones measure heat-work
– They are sensitive to the firing rate (time) and
final temperature

• Cones are rated for
specific temperatures
at specific firing rates

By setting the base of
the cone on a flat
surface it will lean at an
8° angle. Release it and
it will fall on the
"bending face.“ Use this
to predict where cones
will fall in the firing

Weird reactions
• Can be caused by airflow, or by close proximity to
heat source
– Keep cones away from a flame’s path, elements, and
spy-holes

• Sometimes higher cones bend before lower
ones
– Due to effect of drafts (stalling)
– Avoid drafts on the cones during firing
• Don’t place too close to spy-hole
• Don’t blow into kiln during firing

Check out the eyes on this one…

Crashing kilns
• Note: While the proceeding outlines how to
safely crash-cool work, most kilns
(including those at Alfred) are not designed
for this extreme kind of cooling cycle
– Crashing slowly destroys the integrity of the
kiln, and when done carelessly, can melt
controllers on the side of kilns
– Don’t crash unless you absolutely have to;
think before you act; if you’re not sure, ask!

Re: crashing a kiln
A.K.A.
“dumping heat”
• Correlation between
temperature and color
of the kiln
• Quartz inversion
occurs just below
visible heat
• Our work can be
safely cooled down to
red heat

Safe crashing
• When determining
temperature of ware,
look at the ware, not the
environment
– Color of elements,
bricks, shelves etc. do
not reflect temp. of the
work
• When crashing, extremities cool first
• As edges of the work darken, they are approaching
inversion
• Don’t go past this point!
• Close up the kiln and allow heat to redistribute
• Repeat until work is dark red, then cool normally through quartz
inversion

Safe crashing
• When work has dropped below quartz
inversion kiln can be slightly opened
• Cold air drafts must be kept to a minimum,
as they can upset equal cooling throughout
the piece (especially at lower temps.)
• Close the door well before reaching
cristobalite inversion (439 °F)
• How can you tell if you are safely past
cristobalite

• Paper burns at 451 °F
• Stick paper in the spy-hole; if it even starts to curl or change
color, chances are good that you are too close to 439 °F (i.e.
still too hot to open)

A baffle helps shield the work from drafts during crash cooling

Happy Crashing!

Next week...

Field Trip Next Week!
• Meet next Wednesday outside the
back loading dock of Binns
– We leave at 7 am sharp!
– We will be back by 1 pm
• Bring
– Closed shoes
– A lunch
– A camera

References
Ceramics for the Artist Potter
F. H. Norton
Addison-Wesley Publishing Company, Inc., Reading, MA
1956
Ceramic Science for the Potter
W. G. Lawrence
Chilton Book Company, Radnor, Philadelphia
Second Edition, 1982
Clays and Ceramic Raw Materials
by W.E. Worrall
Elsevier Applied Science Publishers, London and New York
Second Edition, 1986
Cushing’s Handbook
by Val Cushing
Third Edition, 1994

References
Elements of Ceramics
by F. H. Norton
Addison-Wesley Press, Inc., Cambridge, MA
1952
Fine Ceramics: Technology and Applications
by F.H. Norton
Robert E. Krieger Publishing Company, Malabar, Florida
1978
Glass Phase Composition in Porcelains and Correlation with Pyroplastic Deformation
by William Carty
Presentation for the 104 th Annual Meeting of the American Ceramic Society
2002
Pottery Science: materials, processes, and products
by Allen Dinsdale
Ellis Horwood Limited, Chichester, England
1986

References
Structural Clay Products
by W.E. Brownell
Springer-Verlag, Wien and New York
1976
The Potter’s Dictionary of Materials and Techniques
by Frank and Janet Hamer
A&C Black Publishers Ltd., London, England
Third Edition, 1993
WWW.ORTON.COM
Knowledge library
2003