Lecture #3 - Alfred's Clay Store

Fluxes
Lecture #3

Plastic
Clays
Non-plastic
Fluxes
Fillers

In order to achieve a strong, dense clay
body we rely on glass formation during
firing…
Silica - Is our main source of glass
Forms glass by itself at 1710° C (32)
Present in virtually all materials (clays, fluxes and
fillers)
Alkali – First 2 columns of the periodic table
Reduce silica’s m.p.
E.g. Na 2 O, Li 2 O 3 , K 2 O, CaO, MgO etc.
Alumina - Controls viscosity of the glass
Very refractory / raises m.p. / thickens the melt

Fluxes
• The alkalis present in clay help account for
the wide range of melting temperatures
– Clays with more alkalis tend to mature at lower
temperatures
• Alkalis are also found in many non-clay
materials
– Often in much higher concentrations than in
clays
– We refer to these classes of materials as
“fluxes”

• Fluxes are oxides that promote fusion by
interacting with other oxides in the presence
of heat
• Fluxes often contain varying amounts of
SiO 2 and Al 2 O 3
• Together, the amounts of silica and
alumina, and the amounts and types of
alkali determine the fired characteristics of
fluxes. These include:
– M.P. (melting point)
– Firing range
– CTE (coefficient of thermal expansion)

Roles of Flux in a Clay body
• In clay bodies, fluxes promote
– Glass Formation (Density)
• Fluxes interact with and melt silica; without fluxes,
clay bodies would need to be fired at much higher
temperatures
– Strength
• As glass content increases, so does clay body
strength
– Fired shrinkage
• As the amount of fluxing or melting increases,
density increases and volume decreases

Flux additions in a body
shrinkage
absorption
As flux amount increases, absorption decreases
22.15
22.97
23.59
23.14
24.18
23.44
20.4
17.15
%
14.48
9.57
As flux amount increases, shrinkage increases
2 2
1
0.5
1
0.5
1 1
3
4
7
1.49
1 2 3 4 5 6 7 8 9 10 11
% additions of frit in a body

• Glazes and clay bodies share similar oxides in
their makeup
– SiO 2 , Al 2 O 3 , Na 2 O, K 2 O, Li 2 O, CaO, MgO, Fe 2 O 3 + many others
• One of the main compositional difference between
glazes and bodies is in the amount of these oxides
R 2 O
Alkali
RO
weight % SiO 2 B 2 O 3 Al 2 O 3 Na 2 O K 2 O Li 2 O CaO MgO ZnO BaO SrO
Glaze 51.00 0.00 13.44 1.09 5.43 0.00 10.26 0.06 6.51 0.00 0.00
Clay body 54.70 0.00 29.62 1.67 1.43 0.00 0.64 1.96 0.00 0.00 0.00
V.C. Satin White glaze ; V.C. Stoneware clay body (both cone 6)
•Lower alkali levels in bodies than glazes

• Clay bodies contain more alumina than
glazes
– Al 2 O 3 stiffens / stabilizes melts (melts are more
viscous/less runny)
– Alumina makes clay bodies more refractory
Note higher levels of alumina in clay body (3.13 SiO 2
:1 Al 2
O 3
) compared with glaze
Cone 6
Glaze
Cone 6
Clay body
SiO 2 B 2 O 3 Al 2 O 3 Na 2 O K 2 O Li 2 O CaO MgO ZnO BaO SrO Fe 2 O 3 TiO 2
2.50 0.00 0.39 0.05 0.17 0.00 0.54 0.00 0.24 0.00 0.00 0.00 0.00
SiO 2 +2B 2 O 3 / Al Ratio Alkali Metals Alkali Earth
6.44 0.22 0.78
SiO 2 B 2 O 3 Al 2 O 3 Na 2 O K 2 O Li 2 O CaO MgO ZnO BaO SrO Fe 2 O 3 TiO 2
8.93 0.00 2.85 0.26 0.15 0.00 0.11 0.48 0.00 0.00 0.00 0.09 0.15
SiO 2 +2B 2 O 3 / Al Ratio Alkali Metals Alkali Earth
3.13 0.41 0.59
Alkalis normalized to 1
• Together, Alkali and silica/alumina levels help determine the
quantity, viscosity, and melting point of glass

SiO 2 B 2 O 3 Al 2 O 3 Li 2 O Na 2 O K 2 O CaO MgO SrO BaO ZnO
SiO 2 +2B 2 O 3 / Al Ratio Alkali Metals Alkali Earth
+1

SiO 2 B 2 O 3 Al 2 O 3 Li 2 O Na 2 O K 2 O CaO MgO SrO BaO ZnO
SiO 2 +2B 2 O 3 / Al Ratio Alkali Metals Alkali Earth
+2

Of the Alkali, which tend to be stronger fluxes
SiO 2 B 2 O 3 Al 2 O 3 Li 2 O Na 2 O K 2 O CaO MgO SrO BaO ZnO
SiO 2 +2B 2 O 3 / Al Ratio Alkali Metals Alkali Earth
+1
+1
-2 -2
+2
oxygen
–Metals R 2 O (Two elements per oxygen)
–Earths RO (One element per oxygen)
• Alkali Metals tend to be stronger fluxes than Alkali Earths
– Alkali Metals are more easily separated from their bond with
oxygen (each element only has a +1 bond with oxygen)
– The +2 bond of the Alkali Earths requires much more energy to
break down

• The alkalis’ strength as fluxes does not
always correlate with their m.p. (melting
point)
– I.e. Zinc melts earlier than Calcium, yet it is a
weaker flux

Eutectic
A proportioned mixture of two or more materials,
which melts at the lowest possible temperature
Liquid
Solid
Solid
E.g. An RO (e.g. MgO, CaO, ZnO) and SiO 2 form a
eutectic when mixed 60/40.

A eutectic
More melting occurs in the middle
of the blend than at either end
100% Glaze
100% Spodumene

Some combinations of materials can have multiple Eutectics; if the
gaps in your line blend are too large, you may miss out on these…
E.g. PbO & CuO

The SiO 2 -Al 2 O 3 eutectic

Phase Diagrams
Typical Industrial
Porcelain Compositions
Phase Diagrams depict the
various different liquid,
solid and mixed phases of
mineral mixtures at various
different temperatures.
SiO 2
K 2 O
Al 2 O 3
Introduced through
different raw
materials

CTE…
@ High T
On Cooling
CTEGCTEB
Glaze in Compression Glaze in Tension
Coefficient
of
Thermal
Expansion
body
glaze
body glaze body glaze
A change in volume
based on temperature
(Low CTE glaze/
High CTE body)
Shiver
(High CTE glaze/
Low CTE body)
Craze
• Everything we put into a body will affect its overall CTE
• Fluxes/materials that exhibit very low or very high CTE
may limit the use of some glazes (or at the very least,
glazes may not “fit” properly)

All oxides have
a specific
Coefficient
of Thermal
Expansion
(CTE)
We will revisit
this later…

High Metals High Earths
Classification Of Fluxes
• Feldspar
• Frit
• Talc
• Whiting
• Bone Ash
• Dolomite
• Wollastonite
Primary
Secondary
Strong / stable enough
to be used alone
Used in combination with
primary fluxes
–Primary fluxes are higher in R 2 O (metals) (talc is an exception)
–Secondary fluxes are higher in RO (earths)

Feldspars
Clay Al 2 O 3 •2SiO 2 •2H 2 O
Potassium spar K 2 O•Al 2 O 3 •6SiO 2
Soda spar Na 2 O•Al 2 O 3 •6SiO 2
Lithium spar Li 2 O•Al 2 O 3 •4,8SiO 2
• Feldspars involve alkali (Na 2 O, K 2 O, Li 2 O, CaO, MgO )
with alumina and silica
• They are the most common flux above 6
• Will form a glass alone at high temperatures
• In general, provide fluxing action over a long firing range
• Forgiving if over/under fired
• Inexpensive ($0.12/lb. vs. $0.80/lb. for frits)

Feldspars
Metals
Earths
Al 2 O 3 • 2SiO 2 •2H 2 O
K 2 O•3Na 2 O•4Al 2 O 3 •9SiO 2
Na 2 O•Al 2 O 3 •6SiO 2
K 2 O•Al 2 O 3 •6SiO 2
Clays
Soda
Spars
Potash
Spars
SiO 2 Al 2 O 3 Na 2 O K 2 O Li 2 O CaO MgO Fe 2 O 3
EPK 43.44 35.46 0.40 0.05 0.16 0.51
Redart 54.30 16.40 0.40 4.07 0.23 1.55 7.04
Neph Sy 61.95 22.10 10.29 4.40 0.36 0.03 0.04
Kona F-4 68.50 18.56 6.22 4.61 1.45 0.01 0.07
NC-4
Custer 69.60 16.24 2.10 10.33 0.32 0.08 0.26
G-200 75.03 12.79 3.65 7.31
Cornwall 74.73 14.28 2.95 3.72 1.47 0.11 0.20
• Much higher alkalis than clays
• Higher silica than clays (6SiO 2 vs. 2SiO 2 )
• Usually contain multiple alkalis (classification is
based on predominance of Na or K)
– I.e. Soda Spars always contain some Potash and vice versa

Soda Spars
Na 2 O•Al 2 O 3 •6SiO 2
•More powerful and melt earlier than potash spars because
soda is more reactive (Neph Sy. is lowest melting of all spars)
•Begin to melt at 4 (potash begins at 6)
•Shorter firing range than potash spars
•Soda is more soluble than potash and acts as a deflocculant
•Can deflocculate clay bodies (decrease plasticity)
•Bodies that contain high concentrations of Neph Sy. can
shorten over time

Potash Spars
K 2 O•Al 2 O 3 •6SiO 2
• Potash volatilizes latter in the firing than soda
– more stable at higher temperatures
• Wider firing range than soda
• Dissolves silica more easily than soda spar
– Can improve translucency in porcelains
• Cornwall Stone
– Transitional material between feldspar and kaolin
– Relatively plastic compared to other feldspars but dirtier

10 R.
10 Ox.
N/A
6
04
Custer G-200 PV clay Cornwall
Stone
Kona F-4 NC-4 Minspar
Neph Sy
Potash
Soda

Custer Kona F-4
@ C. 6
Note: Single
material samples
like these do not
account for
eutectics!
Neph Sy

• High Temperatures
– Porcelain
Feldspars In Bodies
• High percentages are used to create dense, glassy bodies
• Normally 10-30% additions
– Stoneware
• Normally up to 10% additions (often less dense than
porcelains)
• Stoneware bodies get higher amounts of alkalis from dirty
clays than porcelains do (and therefore require less flux)
• Mid Temperatures
• Often used in combination with lower temperature fluxes
(I.e. frits, talc etc.)

• A purification process in which
a mineral is coated with a
specific organic compound
• The compound is hydrophobic
(doesn't like water)
• The organic compound (and the
targeted mineral) attaches itself
to air bubbles that rise; they are
removed at the surface
• Uncoated minerals sink to the
bottom and are reprocessed with
different compounds
Froth Floatation:
Differentiation revisited
•Chronology of purification for a feldspar: Mica, Iron, Feldspar and Silica

Lithium Feldspars
Petalite Li 2 O•Al 2 O 3 •8SiO 2
Spodumene Li 2 O•Al 2 O 3 •4SiO 2
• Contain high levels of lithia along with trace
amounts of the other alkalis
• Exhibit very low CTE because of lithia
• Increase resistance to thermal shock
• Ideal for oven ware and flame ware
• When used alone, require large amounts
– Displace clay in the recipe / this reduces plasticity
• More expensive than other fluxes
– Usually only used for flame ware / ovenware bodies

Petalite
Li 2 O•Al 2 O 3 •8SiO 2
• Petalite
– Lowest CTE (coefficient of thermal expansion)
of all lithium spars
• Increases resistance to thermal shock
– High temperature flux ( 9-14)
– Short firing range
– A powerful auxiliary flux in combination with
soda / potash spars, talc or whiting
– Used in flame ware bodies (in combination
with spodumene – up to 60%)

Spodumene
Li 2 O•Al 2 O 3 •4SiO 2
• Spodumene
– Dirtier source of lithia (Li) than petalite
• Can contain up to 2.5% Fe
– More refractory than Petalite (more Al 2 O 3 )
• M.p. of 14-15 (Petalite m.p. 11-12)
• Contains only 5-8% alkali (most other spars >12%)
– Very low CTE, but still five times higher than
petalite
– Gives warm orange-brown color to low iron
stoneware bodies
– Is expensive (usually only used for flame ware)

C.10 re.
C.10 ox.
C.6
C.04
Petalite
Spodumene

MgO•4SiO 2 •H 2 O
Talc
• A.K.A. Magnesium Silicate
• Common low-temperature material
– Begins interacting/decomposing at 09
– Melts at 12
– Also used at higher temperatures but not as common as
feldspars or frits
• Readily forms eutectics with other materials
100% Stoneware body Stoneware body + Talc

Talc
• Encourages formation of the mineral Cordierite at high
temperatures Cordierite (has low CTE; like lithium
bodies, talc bodies are used for thermal shock-proof
situations)
• Ovenware / flame ware
• yellowish kiln shelves are made of Cordierite
• Promotes fired whiteness (Talc is low in Fe)
– Actually, not in Alfred anymore
• We now use a dirtier Talc… the really clean stuff potentially
contained asbestos-like minerals
• Inexpensive to use compared to frits
• Longer firing range than frits
• Some talc contains carbonates
– May cause bloating if fired too quickly

Talc
• Large amounts are used at low temperatures (unless
other fluxes are present)
– Large amount of talc limits clay content in clay body
Decrease in clay = decrease in plasticity
– Hi Talc bodies are therefore relatively non-plastic and are
best suited for casting or other situations where little
plasticity is needed
• At low temperatures produces Enstatite
– Enstatite has a high CTE (can help prevent crazing)
– Controls moisture expansion in non-vitreous bodies (which
can also prevent delayed crazing)
– But may lead to cooling cracks (not great for Raku)

Talc In Bodies
• High Temperatures
– Used for shock-proof bodies at very high
temperatures (w/high amounts of talc)
– Powerful auxiliary flux for high-fire bodies (3-5%
addition)
• Mid Temperatures
– 2-5% addition with Feldspar produces lower firing
range and tighter, stronger body (eutectic)
– Can lower maturing temp by 2-3 cones, or from
high-fire to mid-fire
– 40-50% required to flux to vitreous state (e.g. 50% talc 50%
ball clay is vitreous at cone 6)
• Low Temperatures
• Extensively use, probably functions as a filler (definitely
not vitreous when used alone!)
• Varying amounts used (up to 50%), along with other fluxes

Frit
• The main low temperature flux
• Pre-fired glass made from several different materials
• Melt at lower temperatures 017-02
– energy has gone into melting them
– Silica and alkali have combined to form glass
– less energy/heat required to melt again
• Frits are very expensive because of their preparation
• Much narrower melting range and quicker melting
than feldspars (lower alumina levels)
– A 2 cone difference in a firing can have drastic effects
• Used at mid and sometimes high temperatures but in
smaller amounts

SiO2 Al2O3 Na2O K2O Li 2 O CaO MgO Fe 2 O 3 B 2 O 3
EPK 43.44 35.46 0.4 0.05 0.16 0.51
Kona F-4 68.5 18.56 6.22 4.61 1.45 0.01 0.07
Custer 69.6 16.24 2.1 10.33 0.32 0.08 0.26
Frits
3110 69.43 3.93 15.52 2.18 6.26 2.68 2.59
3124 55.31 9.90 6.24 0.68 14.11 13.77
3134 47.42 10.32 21.94 20.32
3195 58.70 10.87 6.52 15.22 8.70
• Frits introduce higher amounts of soda and
calcium than feldspars
• Frits also introduce boron
– Boron plays an important role in lowering the melt
temperature (boric oxide m.p. 017)
– Boron acts as both a flux and glass former
• Lower amounts of alumina gives more fluid
melts compared to feldspars

Frits
• Like soda feldspars, hi Na frits can be soluble
in water (aged clay can loose plasticity)
• The most common body frits are 3124 and 3195
– 3110 has lower boron content (less slumping)
– 3124 has less soda (may not deflocculate over time
as much = better plasticity)

C.10 re.
C.10 ox.
C.6
C.04
3124 3195
3110

Frits In Bodies
• High Temperatures
– Rarely used (feldspars are less expensive/more
forgiving)
• Mid Temperatures
– Often used in small amounts (up to 12%) in
combination with feldspars and/or talc
• Low Temperatures
• Extensively use as a primary flux (up to 30%)
• Also used in smaller amounts when talc or
wollastonite are present

Secondary Fluxes
• Secondary fluxes are low in alkali metals
(Na, K, Li) and hi in earths (Ca, Mg +
others)
• Often more refractory, and less active
• Used in combination with primary fluxes

Bone Ash
3CaO•P 2 O 5
• A.K.A. Calcium Phosphate
• Melts alone at 33
• Contains 55% calcium and 40% phosphorous
pentoxide
– (Like SiO 2 )P 2 O 5 is a glass former
– Unlike SiO 2 ,P 2 O 5 vitrifies with little softening or slumping
– Very high viscosity melt
• Usually made from calcined cattle bones
• Small carbon content (1%) after calcination may add
some plasticity
– However, is still non-plastic compared to clay
– Often used in casting bodies / non-plastic forming

Bone Ash
3CaO•P 2 O 5
• At High Temperatures
– Extensively used (up to 50%) in bone china for
its anti slump properties, whiteness, and added
translucency
– Increases fluxing of feldspars in smaller
amounts (due to Ca)
• At Mid Temperatures and below
– Ineffective as a flux

Whiting
CaCO 3
• Generally used as a secondary flux
• Soluble in water (migrates to surfaces during drying)
• High Temperature
– Very powerful flux
• Difficult to control
• Small additions (1-2%) can improve translucency (Ca may enhance
translucency)
• Small additions may also reduce the need for feldspar
Ex: Cone 10 50% Clay
50% Clay
30% Spar
20% Flint
=
20% Spar
20% Flint
+ 2% whiting
8% more
clay can be
added

Whiting
CaCO 3
• Mid Temperatures
– Useful in this range as a secondary flux
• Low Temperatures
– Prevents warping and increases whiteness of
fired bodies
– But does not flux
– If not well mixed, larger clumps can absorb
moisture after firing, swell and cause pop-outs

Wollastonite
CaSiO 3
• Considered a flux and a filler
• As a flux
– Ca as a silicate is very stable – no chance of CaO
hydration, or lime popping (unlike CaCO 3 )
– Insoluble compared to CaCO 3
• Improves fired strength
• Improves thermal shock resistance
• High aspect ratio (fibrous) needlelike crystals
provide high fired strength

Wollastonite’s needle-like crystals within a silica melt

Wollastonite
CaSiO 3
• However
– Limited to fluxing at high temperatures (Ca
present)
– Slightly higher m.p. than CaCO 3
• As A Filler
– Reduces drying shrinkage
– Needle shaped crystals increase green strength

Deciding on Flux amounts…
White Translucent
50% Clay
50% Flux/Flint
Off white to dark
80% Clay
20% Flux/Flint
• White clays are refractory
• Require large amounts of
flux and silica to create
glass
• Not very plastic (50% clay)
• Darker clays naturally contain
some flux
• Require smaller amounts of flux
and silica to create glass
• Much more plastic (80% clay)

Deciding on Flux types…
• High Temps
– Feldspars
• Mid Temps
– Feldspars
– Talc
– Frits
• Low Temp
– Frits
– Talc

References
Ceramic Industry
(Periodical)
Business News Publishing Co, Troy, MI
January 2002
Ceramic Science for the Potter
W. G. Lawrence
Chilton Book Company, Philadelphia, New York, London
First Edition, 1972
Clay Mineralogy
by Ralph E. Grim
McGraw-Hill Book Company, New York, St. Louis, San Francisco
Second Edition, 1968
Clays and Ceramic Raw Materials
by W. E. Worrall
Elsevier Applied Science Publishers, London and New York
Second Edition, 1986

References
Cushing’s Handbook
by Val Cushing
Third Edition, 1994
Science of Whitewares II
edited by William M. Carty and Christopher W. Sinton
The American Ceramic Society, Westerville, OH
2000
The Potter’s Alternative
By Harry Davis
Methuen Australia Pty Ltd, North Ryde, NSW
First Edition, 1987
The Potter’s Dictionary of Materials and Techniques
by Frank and Janet Hamer
A&C Black Publishers Ltd., London, England
Third Edition, 1993