Lecture #2 - Alfred's Clay Store

Mineralogy
and Clay
Lecture #2

Lecture #2
• Basic chemistry
• Silicate structures
• Clay Minerals
• Clay profiling

Ex. Oxygen
Atoms

• Un-combined atoms are
neutral in charge
• Same number of electrons (-)
and protons (+)
Diagram of
complete outer
rings
•However, atoms need
their outermost shell to
be full of electrons in
order to achieve stability
•In most cases, their outer
shells are not completely
filled when they are
neutral so…

Ions
An atom completes its outermost shell by losing, gaining, or sharing
electrons from other atoms, in the process upsetting its original
neutral charge and becoming a charged atom or “ion”
_
Atoms with many
electrons in outer shell
gain electrons to attain a
complete outer shell
+
Atoms with few
electrons in outer shell
loose electrons to attain
a complete outer shell
Neutral
Atom
Unstable
Charged
Ion
Stable

Valency
• An atom’s valence refers to how many charges (electrons) an
atom must loose or gain in order to fill its outermost ring
• Elements in the same columns share a common valency

Valence
• In order for atoms to balance each other during interaction,
their valences must cancel out (must equal zero)
E.g. Silicon = +4 Silicon = +4
Oxygen = -2 so 2 x Oxygen = -4
+2 0
SiO SiO 2
unstable
stable

Ionic size is related to the number of
electrons and the ionic charge
Ionic Size
+
_

Co-ordination Configurations
•The physical size of an atom dictates its stable arrangement with
other atoms
•The number of cations which can surround the smaller anion is
called the co-ordination number; it is expressed as a ratio
between the size of the cations and the anion

Si +4 radius = .41A
O -2 radius = 1.32A
Radius ratio = .41/1.32
Or .31
So…
Crystals involving
silicon and oxygen will
always join in four-fold
coordination
(Tetrahedron)

The Silica Tetrahedra:
the silicates’ most basic building block
Tetrahedrons
always have a
–4 charge
4 oxygen = -8
1 silicon = +4
Total charge –4
Two separate
Tetrahedra have
A combined
charge
Of -8
When they
combine, they
share an oxygen,
and their
combined charge
drops
7 oxygen = -14
2 silicon = +8
Total charge -6

• The more silica tetrahedrons combine and
share oxygen with other tetrahedrons, the
lower the combined negative charge
becomes and the closer the structure comes
to achieving electric stability (net zero
charge)
• When not enough tetrahedra are available to
achieve a neutral charge through sharing of
oxygen alone, neutrality is achieved through
the presence of cations (positively charged
ions)

Isolated Structures
Olivine
No oxygen is shared. Silica tetrahedra (-4) are bonded
and neutralized by abundant Mg (+2) or Fe (+2) cations.

Single Chain Structures
Spodumene
Two oxygens of each tetrahedron are shared
with adjacent tetrahedrons in a chain
formation, reducing their net negative
charge and the need for cations.

Sheet Structures
Kaolinite
Each tetrahedra shares three of its oxygens, forming a
sheet. The left over negative charge is satisfied by a layer
of aluminum, oxygen and hydrogen (A.K.A. Gibbsite)

Framework
Quartz
Feldspar
All four oxygen atoms are
shared by other tetrahedrons

Clay Minerals
•Affect many properties of clay
Plasticity
Drying properties
Composition
Firing characteristics
Color
•We will look at kaolinite, illite, and montmorillonite as
these predominate most of the clays we use

Kaolinite
(Fine)
Illite
(Finer)
Montmorillonite (Finest)

Kaolinite Crystal
Tetrahedra
•The main clay
mineral in most clays
(esp. Kaolins)
Silica
Alumina
Silica
Alumina
Silica
Alumina
Octahedra

Kaolinite Crystal
-Exploded View-
Al
Si
•Each Aluminum shares
Oxygen with two Silicon
atoms, creating a strong
internal bond
•Hydroxide (OH) mediates
the charge difference
between Si and Al layers
•1:1 sheet silicate
1 Si :1 Al
Chemical formula
Al 2 O 3 • 2SiO 2 • 2H 2 O

Kaolinite Particle
•Kaolinite particles are ten
times wider than they are
thick
•They exhibit a positive and
a negative charge on
opposite planes which
encourages stacking of
individual layers

Stacking
Individual crystals in a stacked formation

Kaolinite
However, the attraction of
crystal faces is relatively weak
(easily separated with high
energy blunging in water)

Feldspar
Mica
•The three
minerals that
make up a
granite rock
Quartz

Kaolinite Minerals Often Result
From Feldspar Weathering

Kaolinization of feldspar
• Example: Potash feldspar (K 2 O • Al 2 O 3 • 6SiO 2 )
• Feldspar is soluble in water (H 2 O) and even more so in
carbonic acid (H 2 CO 3 )
• Water, carbon dioxide, and carbonic acid dissolve feldspar
• Carbon, along with most potassium and some silica is
carried away
• Feldspar becomes kaolinite:
Al 2 O 3 • 2SiO 2 • 2H 2 O

Environmental Factors Aiding in
• Climate
Kaolinite Formation
– Rainfall/groundwater (carbonic acid/water)
– Temperature (encourages organic activity)
• Soil/vegetation
– Thickness of soil layer (moisture retention)
– Organic activity (physical breakdown/increase
acidity)
• Length of exposure
– Age of deposit

Montmorillonite
• Found in Bentonite (also in
smaller amounts in some
kaolins, ball clays,
earthenwares, etc)
• Produced by the weathering of
volcanic rock, esp. fine
volcanic ash
• Finer/more plastic than Illite or
Kaolinite
• Expands in water
• High Mg and Fe impurities
within the crystal structure act
as fluxes (lower melting temp.
than kaolinite)

Montmorillonite
Silica
Alumina
Silica
Silica
Alumina
Silica
Alumina
Silica
Silica
Alumina
Silica
Alumina
Silica
Silica
Alumina
Montmorillonite
2:1
Sheet structure
Kaolinite
1:1
Sheet structure

Silica
Alumina
Kaolinite
1:1
Silica
Alumina
Silica
Montmorillonite
2:1

Kaolinite vs. Montmorillonite
-
+
-
+
-
+
Kaolinite
Silica
Alumina
Silica
Alumina
Silica
Alumina
Stronger
Oxygen/Hydrogen Linkage
Non-expanding
Montmorillonite
-
Silica
Alumina
Silica
Silica
Alumina
Silica
Silica
Alumina
Silica
+
-
-
+
-
-
+
-
Weaker
Interlayer space (water)
Expanding
•In Montmorillonite,
faces are of similar
charge
•Negative edges are
weakly bonded by
positive cations (Na,
Ca, or Mg)
•Weakness of bond
and large size of
cations permits easy
separation of stacked
crystal faces
•Water permeates
layers (plasticity
increases)
•Crystal lattice
expands (bentonites
swell in water)

Illite
• Usually comes from weathering of
Mica
– But contain more water and are much
finer
– Can also form from K-spars and
Smectites
• Finer/more plastic than Kaolinites
• Often found in high % in lower-fire
dark clays (I.e. Redart)
• Illite’s presence is indicted by high K
in clay analysis
• High K acts as a flux (lower melting
temp. than kaolinite)
• 2:1 silicate (like Montmorillonite)
Various different forms of Illite

Raw Clay
• Most clays, as dug from the ground, contain
few amounts of pure clay minerals
• Before processing, a raw clay contains:
• Quartz
• Micas
• Feldspars
• Clay minerals
– Kaolinite
– Montmorillonite
– Illite
– And many others …
• Mined clays are processed
80-85%
10-15%
Impurities
Clay
– impurities are removed to increase the proportion
of clay mineral content
• Differences among raw clays reflect the different proportions of these
clay minerals and impurities

Primary & Secondary Clays
• Primary clays
– Some kaolins
• Clays that are found where they were formed
• Involve little or no transportation
• Also known as residual clays – they are the residue of
their parent rocks
• Secondary clays
– All other clays (most kaolins, ball clays, stone
wares, fireclays and earthenwares)
• Involve transportation and deposit of primary clays
• Also known as sedimentary clays
• Much more common than primary clays

Clay Distribution
Secondary Kaolins

Kaolin
• Kaolinite: The predominant mineral in many clays
Kaolin: A type of clay; A.K.A. China Clay
• Originally comes from Chinese “kao-ling” meaning
high-ridge (where it was mined)
• The raw clay that most closely resembles “ideal”
Kaolinite mineral
Al 2 O 3 • 2SiO 2 • 2H 2 O
– Lowest impurities of all clays (fired whiteness)
• Very refractory (melts @ cone 34-35; 1770°C)
• Mature @ ca. cone 12-18

• Mainly coarse particles
• 1 micron = 1/1000 mm (60 mesh screen = 250
microns)
• Ball clay range: 0.02 - 1 microns
• Kaolin range: 0.5 - 10 microns
Ball Clay
0.02 1
Kaolin
0.5 10
Kaolin
• Relatively non-plastic
• Widely used in china production BUT principally used
as a white coating on paper (80% of kaolin market)

Primary/Residual Kaolins
Decompose where their parent rock once was
-Little or no transport / minimal impurities
– Fire white/off-white
• Low plasticity
– Generally coarse particles (10-5 microns)/well
crystallized (large crystals)
– Uniform particle size and shape
• Major deposits: Japan, China, Korea, India,
England, Germany
Ex. Grolleg, SSP, Polaris etc…(no major US production)

Secondary/Sedimentary Kaolins
More transport/more impurities than primary kaolins
• Finer particles/greater range of particle sizes (aka.
Particle size distribution) due to geological transport
– Particle range: 5-.5 microns
– Higher plasticity: A.K.A “Plastic Kaolin” (small
Montmorillonite content)
• More contaminants, especially Ti & Fe
• Fire white/off-white
• South Carolina and Georgia are the main US sources
(others include NC, KY, TN & FL)
Ex. Tile 6, Helmer, EPK etc…

Primary Kaolin Deposits

Primary Kaolin Deposits
Secondary Kaolin Deposits

Kaolins
• Primary
– Grolleg
– SSP (Super Standard Porcelain)
– Polaris
• Secondary
– EPK (Edgar Plastic Kaolin)
– Tile-6
– Helmar
– Velvacast
– Peerless
– Kaopaque

Kaolin
• Pros
– White (low impurity levels)
– Coarse (improves drying characteristics)
• Cons
– Typically expensive
– Coarse (low plasticity/low green strength)

Ball Clay
• “Ball” originates from the early English
mining practice in which plastic clay was cut
from its deposit in the form of large balls.
• Mainly Kaolinite minerals but can also contain
large amounts of Illite and Montmorillonite
• Transported and deposited from earlier kaolin
deposits
• Fine particle size (60-70% under 1 micron)
– Highly plastic (higher than kaolins)
• Rarely used alone in a clay body

Ball Clay
• Contain higher amounts of impurities (iron,
calcium, quartz) than kaolins
– Mostly off-white to cream color after firing
– Vitrify in the cone 03-4 range (but can go as
high as cone 10)
• Most contain 1-3% organic materials (because
they are sedimentary)
• Organics add to ball clay's plasticity
• Readily accept ions from deflocculants, making
them valuable in casting slips
• Most US ball clays are mined in Kentucky and
Tennessee

Ball Clays
• OM-4 (Old Mine #4)
• XX Saggar
• Spinks Blend
• Jackson
• Tennessee #10
• KT Stone
• C&C

Ball Clay
• Pros
– Increase plasticity
– Help suspend slips
– Increase wet and dry strength (particle packing)
– Easy to deflocculate (slip casting)
• Cons
– Increase dry shrinkage
– Often too dark for a true “White” body
– Too sticky by themselves
– Can impede evaporation when leatherhard (slower
casting and drying rates)
– High amounts of free silica (firing related issues)

Fireclay
• So named because some (though not all) resist fire
-Firing range 1300-1800 C (cone 6-22)
• Usually more Illite than Kaolinite minerals
• Generally coarse particle size
– Provide firm, toothy quality to bodies
– Open structure / quick drying
Originally acted as soil for vegetation and are
therefore usually found underneath coal seams
– Plants remove potassium and other soluble salts
– Plants also remove some trace silica but no alumina.
Thus fireclays are high in alumina and often refractory

Fireclay
• Fireclays contain volatile impurities, including
sulphur, carbon and carbon dioxide
• Carbon derives from decomposed organic activity
– Up to 3% carbon
– Some fireclays actually contain carbonaceous matter
like old plant roots, remnants of trees and other organic
matter
• Other impurities include
– Free silica (quartz), sulphur (iron pyrite), mica, iron,
and calcite (calcium carbonate)

Fireclay
• Large pieces of Calcite in unrefined fireclays can lead to
problems after firing
– I.e. CaCO 3 pop-outs
• Volatile impurities (carbon/sulphur) can be trapped if not
fired properly
– I.e. black coring or bloating
• Typical post-fired color ranges from whites to pinks to
reds
– Iron leads to reds
– Presence of calcium tints (leaches) iron, tinting red colors towards
pinks
• US sources
– Kentucky, Ohio, Missouri, Pennsylvania

Fireclays
• Narco (no longer carried here)
• Hawthorn Bond 20
• Hawthorn Bond 35
• Hawthorn Bond 50
• Gold Art
• Christy Gold
• A.P. Green (R.I.P.)

Fireclay
• Pros
– Refractory
– Often available in a wide range of particle sizes
(HB 20, 35, 45, 50 mesh)
– Cheap! (inexpensive)
– Cheaper alternative to grog
• Cons
– Lime pop-outs/carbon impurities (can be minimized by
using 50 mesh and finer grades)
– Not very clean
– Limited plasticity/fineness

Stoneware
• Not to be confused with a stoneware body (we
will look at bodies latter)
• Kaolinite and Montmorillonite minerals
• Midway between ball clay and fire clay in
plasticity, firing range, texture and color
• Mixed particle size distribution
– Toothy, yet plastic
• Can be used w/o other clays in a clay body
– Acceptable working characteristics “as is”
• Extensively used throughout history
• Mature between cone 4-10
• Colors range from buff to tan to brown

Stonewares
• Foundry Hill Cream (FHC)
• Yellow Banks #101

Stoneware
• Pros
– Does the job of ball clay and fire clay
– Inexpensive
• Cons
– High impurity levels

Earthenware
• “Common Blue Collar Clay”
– World wide use throughout history; often used straight
from the ground
• Also called “Brick Clay”
• Varying amounts of Kaolinite and Illite
• Extensive weathering and transport + varying
sources and depositional environments
– Wide range of chemical/physical properties
• Typically contain high amounts of Iron (5-15%)
along with Calcium and Titanium
• Prefired colors range from black to ochre to red
• Post fired colors range from bright orange to buff to black
• White or cream colored when little Iron is present
• Start to vitrify below 1200 (Cone 3-4)

Earthenware
• Often contain soluble salts and organics
• Wide distribution of particle sizes due to transport
– Range from .5-5 microns
• Wide distribution leads to good workability
(combination of strength and plasticity)
– Like stoneware clays, these can be used in large
amounts “as is”
• Widely distributed and mined throughout the US

Earthenwares
– Red Art
– Lizella
– Barnard Black Bird (Substitute)
– Alfred Shale
– Newman Red (Substitute)
• Slip Clays:
• (Higher flux levels of Ca, Mg & Fe)
– @ Cone 04 • can be used for bricks
– @ Cone 4-10 • used for glaze
– Albany Slip (or Albany Slip Substitute)
– Alberta Slip Clay
– Sheffield Slip Clay

Earthenware
• Pros
– Good plasticity and drying characteristics for
modeling/throwing (due to fine/coarse particles)
– Rich color variations
• Cons
– Scumming (soluble salts)
– High Mn (Manganese) in some varieties may pose
health risks
– High temperatures and/or reduction atmospheres may
cause excessive bloating/melting
• Not suitable in large amounts for high temperature clay bodies
– Can be difficult to deflocculate (slip-casting)

Bentonite
• Named after Fort Benton, a major source of bentonite in
the US
• Typically originates from the decomposition of volcanic
ash or lava flows
• Contains montmorillonite rather that kaolinite
• Extremely fine particle size (ten times finer than ball clay)
– So fine that they are not measured in diameter but in surface area
– Kaolin 16 m 2 /g (50lbs = 363,200 m 2 = 81 football fields)
– Bentonite >100 m 2 /g (50lbs = 2,270,000 m 2 = 509 football fields)
• Typical bentonite softens at 1040 C (cone 06-05) and fuses
around 1336 C (cone 11)
• Mainly mined in Wyoming and S. Dakota (ancient
volcanic activity)

Bentonite
• When added to water, bentonites swell (increase in
volume) dramatically
• Additions:
– Typically 1-3% to improve plasticity
– Up to 6% can be added to non-plastic porcelains
• Macaloid / Vee-Gum
– Based on the clay mineral Hectorite
– Stronger effect on plasticity than bentonite (twice as
strong)
– Much whiter than bentonite
– $$$$
– Vee-Gum and Macaloid are trade names; both are a
blend of Hectorite and organic gums

Bentonite
• Pros
– Add plasticity
• Cons
– Most are too dirty for true “white” bodies
– High drying shrinkage
– Must be thoroughly mixed and/or blunged (can
cause drying problems if not properly
dispersed)

Scanning Electron Micrograph (SEM)
Note the well developed hexagonal crystals in these Georgia kaolins

AP Green at the same scale as OM-4
OM-4
AP Green
Ball clay (OM-4) contains much finer crystals than fireclay (AP Green)

Wide particle distribution range of an earthenware (Red-Art)

Montmorillonite crystals in this bentonite are too fine to be seen

Chemical analysis of clay (cheat sheet)
• Low SiO 2 (30%)
– Refractory (resists melting)
• High K 2 O (>2.5%)
– High Illite, and possibly quite plastic
• High MgO
– Possibly high in montmorillonite (and therefore, plastic)
• High Al 2 O 3 (>30%) and less than 3% of K 2 O CaO, MgO, and Na 2 O combined
– Probably a high refractory clay
• Low Al 2 O 3 (

Clay type Product name SiO 2
Al 2
O 3
Fe 2
O 3
TiO 2
Na 2
O K 2
O MgO CaO
% % % % % % % %
Kaolin Velvacast 45.21 38.75 0.33 1.42 0.11 --- 0.18 0.24
Kaolin SSP 46.51 37.55 0.38 0.02 0.11 1.09 0.22 0.05
Fireclay Hawthorn Bond 50.2 31.2 1.5 1.95 0.2 0.8 0.3 0.4
Fireclay Gold Art 56.71 27.3 1.42 1.75 0.16 1.87 0.42 0.22
Ball Clay OM #4 53.00 30.80 0.80 1.60 0.30 2.84 0.30 0.40
Stoneware Foundry Hill Crème 66.58 20.59 1.61 0.53 0.55 0.67 0.51 0.5
Earthenware Red Art 64.28 16.41 7.04 1.06 4.07 0.4 1.55 0.23
Bentonite Bentonite 64.3 20.7 0.11 0.11 2.9 0.09 2.3 0.45

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
Clays: their occurrence, properties and uses
by Heinrich Ries
John Wiley and Sons, Inc.,
Third Edition, 1927
Cushing’s Handbook
by Val Cushing
Third Edition, 1994
Physical Geology
by Charles C. Plummer and David McGeary
Wm. C. Brown Publishers, Dubuque, IA
Seventh Edition, 1996
Rocks and Minerals: a guide to field identification
by Charles A. Sorrell
Western Publishing Company, Inc., Racine, WI
1973

References
Science of Whitewares II
edited by William M. Carty and Christopher W. Sinton
The American Ceramic Society, Westerville, OH
2000
Soil Clay Mineralogy: a symposium
edited by C. I. Rich and G. W. Kunze
The University of North Carolina Press
1964
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

References
Understanding Earth
by Frank Press and Raymond Siever
W.H. Freeman and Company, New York, NY
Third Edition, 2001