Felix Singer, Sonja S. Singer (Auth.) - Industrial Ceramics-Springer Netherlands (1963).Pdf

978 94 017 5259 6 Book 1 PrintPDF pdf INDUSTRIAL CERAMICS INDUSTRIAL CERAMICS FELIX SINGER Dr Ing , Dr Phil , M I Chem E , F I Ceram and SONJA S SINGER M A , B Sc SPRINGER SCIENCE+ BUSINESS MEDIA, B V[.]

CERAMICS

INDUSTRIAL CERAMICS

FELIX SINGER

Dr Ing., Dr Phil., M.I.Chem.E., F.I Ceram

© SONJA S SINGER 1963

Softcover reprint of the hardcover 1st edition 1963

First published 1963 ISBN 978-94-017-5259-6 ISBN 978-94-017-5257-2 (eBook) DOI 10.1007/978-94-017-5257-2

PREFACE

with individual sections of ceramics, it was felt that there was a need to try to draw together the threads, and discuss as many aspects of ceramics as possible,

in one volume The word ' ceramics' from the Greek 'keramos ', 'the ter' has, however, come to mean a l'ather ill-defined number of subjects The field considered in this book has therefore to be defined at the outset, and our definition is as follows :

Pot-Ceramics are products made from inorganic materials which are first shaped and subsequently hardened by heat

This definition embraces the European use of the word, meaning ware made with clay, while allowing inclusion of non-clay new developments It does, however, exclude the chemically related subjects of glass, enamels and cements which may be included in the term ceramics in the United States The word 'industrial' in the title is generally taken to mean any ceramic product made in a factory, and usually made in considerable numbers The hand making of individual pieces by artists is not included, although some of the illustrations are perhaps borderline cases

The text has been compiled largely from my father's large and well fied library, collected over fifty years, and also embodies his long practical experience as a ceramic consultant It is very sad that after so many years of collecting data followed by several years of selecting from it he did not live to see the final completion of the manuscript, but I assure readers who remem-ber him well that the choice of matter to be included was very largely his, and that he personally approved the bulk of the text On the other hand I take responsibility for any errors that have not been eradicated, due to my desire

classi-to have the book published as quickly as possible in memory of him

It would be impossible to list here the many people and firms who have helped us by allowing us to quote from their work, sent us information, or made illustrations available Their names are given in the bibliography, which is arranged alphabetically, thus combining it with the author index Where machines, apparatus or products are described or illustrated these are not necessarily the best for the purpose, but it is hoped that they are repre-sentative of the types under consideration Availability of information has often been a factor governing what has been included

The units used throughout are the British ones with the metric units in brackets (except where Continental items are described when the metric unit may be given first) This applies also to pyrometric cone numbers, and where a foreign cone does not correspond exactly to a British cone the two neighbouring ones are given A few tables that do not belong exclusively to any particular chapter are given at the end in the appendix

Finally I want to thank most particularly my late mother and my husband for their encouragement, assistance and also their prolonged tolerance of the large amount of books and papers that have been amassed in their respective homes during the writing of this book

South Croydon

March 1960

SONJA S SINGER

Non-plastic raw materials- Refractories-

Page

Instruments for observing, recording and controlling

CERAMIC PRODUCTS

Appendixes

II Weight solids, % water, % solids for given slip

American Society for Testing Materials, Standards 1317

Chemically, the classical raw materials, clays, flint and feldspar, are compounds of silica The properties which make them suitable for ceramics are precisely those by which they differ from other sub-stances It is therefore well worth considering the basic structures and properties of silicates before proceeding to the raw materials themselves

PART I The General Principles of Ceramic Manufacture

The Raw Materials

INTRODUCTION TO SILICATE CHEMISTRY

ALTHOUGH silica is the commonest constituent of the earth's crust, the study

of it and its compounds baffled investigators for many decades There are several reasons for this Firstly, silicates are almost all insoluble in anything except hydrofluoric acid, so that they cannot be separated or investigated by solution methods Secondly, their thermal reactions of transition, inversion, melting or freezing are sluggish and ill-defined so that no information about compounds and their purity can be inferred from thermal curves Thirdly, the distinctions between compounds and solid solutions and mixtures are

This resistance to the classical chemical methods of investigation in itself shows the silicates to be different types of compounds from the normal oxides, acids, ~ases and salts of inorganic chemistry

Twentieth-century chemistry, physics and, in particular, X-ray lography, have now enabled us to elucidate the problem sufficiently to understand how it arose

crystal-The Chemical Properties of the Silicon Atom

Silicon's place among the elements, determined by its atomic number and, therefore, chemical properties, is expressed in the Periodic Table (see Appendix 1300) It is a tetravalent element forming predominantly covalent l>onds These are directional and in the case of silicon are normally directed

to the corners of~ tetrahedron of which the silicon atom is the centre Its atom is small so that only four other atoms can be packed round it; thus silicon has a coordination number of four

Silicon atoms have no affinity for each other, and they do not form chains like carbon atoms But silicon has a great affinity for oxygen Oxygen

is divalent so that the simplest theoretical silicon oxide molecule is

O=Si=O

For the silicon molecule to take the above configuration, two bonds, normally some 109° apart, would have to be parallel This is virtually impossible with silicon (although it occurs with carbon) so that silicon tends to combine with four half oxygen atoms instead of two whole ones Four unsatisfied oxygen valencies are left, which can further combine with silicons

The ability to form Si-0-Si-0-Si chains of undefined length is the basis of almost all the silicates, and its preponderance over all other combina-tions is unique amongst the elements

The Building Up of Silicates

The fundamental unit of silicate chemistry then is the silicon-oxygen tetrahedron This is found as such in the simplest natural silicates, the orthosilicates These are crystalline ionic compounds of the tetravalent [SiO 4] 4- anion with cations, and the commonest of these is olivine, containing about 85% magnesium and about 10% iron orthosilicates This highly refractory mineral, softening point about 1700° C (3090° F), occurs but rarely near the earth's surface, but when it does it is found in a vast deposit

V M Goldschmidt, • however, believed that below the rocks comprising the accessible crust there is a great quantity of olivine, which must have crystal-lised out first, from which it can be inferred that the orthosilicate structure is very stable

The SiO 4 tetrahedron is also the basic unit for building up the more complex silicates Its representation presents some difficulty The best method is to assume that atoms in combination are approximately spheres in close-packing The average radii for these spheres have been found (P20);

the easiest way to make use of them is, of course, to make solid models Failing this, however, a comparatively simple arrangement can be represented

by a perspective drawing with a portion cut away (Fig 1.1(a)) Where

expanded view (after Hauth, H49, and

Hauser, H43)

confusion could arise from this an expanded view is sometimes given (Fig l.l(b)) In this and other diagrammatic presentations it must be remembered that the 'bond' separating the spherical atoms is there for visual convenience only and does not represent a physical fact

Simpler non-perspective diagrammatic forms are shown in Figs l.l(c) and (d) The last (d) is the commonest schematic form

The fundamental orthosilicate [Si0J4- unit is rarely found independently (olivines, chondrodite series, phenacite, garnet and the sillimanites) The unit readily joins up to form rings, chains, bands, sheets and three-dimen-sional networks The oxygen atoms that are bonded to only one silicon bear

a single negative charge AB the total charge is zero these negative charges

must be balanced by cations, thus forming silicates The double unit

the various possible cyclic complexes only the [Si309] 6- occurring in benitoite BaTiSi309 (Zl), wollastonite CaSi03 (Btl), and catapleite

Na2ZrSi309.2H20 (B128), and the [Si6018]12- in beryl Be3Al2Si6018

The next degree of complexity is found in the chain ions The pyroxenes have twisted single chains of Si04 units giving the composition (Si03),., they include enstatite MgSi03 and diopside CaMg(Si03}z (W9), jadeite NaAl(Si03}z and spodumene, LiAl(Si03}z (Fig 1.2)

The amphiboles have double chains and hence the composition (Si4011), (Fig 1.3)

By sharing three of the four oxygen atoms of the SiO 4 unit with other units, silicon-oxygen sheets can be built up Two kinds of these are known, one containing rings of six silicons, and the other with alternate four-rings and eight-rings The former is of major importance in the structures of micas and clay min~rals (the latter is found in the mineral apophyllite) (Fig 1.4)

The three-dimensional network (Si02), is found in pure silica, feldspars and zeolites

An essential feature of silicon-oxygen structures is that the oxygen atoms are bound by the silicons and take up positions leaving spaces of various sizes This is in contrast with ionic oxides as will be seen The different metal silicates are formed by balancing the negative charge of the silica skeleton with positively charged metal ions These fit into the holes in the network The three-dimensional network can be so arranged as to have no surplus charge (except at the edge of a crystal); it is then pure silica Here the actual arrangement of the SiO 4 tetrahedra has three possibilities, giving quartz, tridymite and cristobalite, and the Si-0-Si angle can vary, giving the low- and high-temperature forms of each isomorph The changes between quartz, tridymite and cristobalite involve breaking and remaking

of Si-0 bonds; these changes are known to be very sluggish The changes between the a- and /3-forms of each variety involve only a small rotation of the atoms and are quick (Fig 1.5)

(a) With the unit [(Si 2 0 5) 2 -]n, cutaway and expanded views (Hauth, H49)

(b) With 4-and B-rings (Wells, W40) FIG 1.4 Silicate layers

There is, strictly speaking, therefore, no independent molecule of any silicate (other than the orthosilicates) The ordered structure of a crystal-line silicate continues to the edge of the crystal The chemical formulae ascribed to such compounds are merely expressions of the ratios of different atoms present

These structures amply illuminate the observed properties of the silicates, their resistance to chemical attack and particularly their sluggish melting and freezing The melting point of a silicate may be so undefined that the term has been replaced by the pyrometric cone equivalent P.C.E., the temperature

at which it attains a certain viscosity In fact the solid merely softens and gradually becomes less and less viscous Because of the unsaturated nature

FIG 1.5 Temperature inversion between a- and fJ-forms of a

silica isomorph (Hauth, H49)

FIG 1.6 The semi-random arrangement of a vitreous

structure

of the SiO 4 basic units, bonds are re-formed and re-broken constantly in the melt When it cools it becomes more and more viscous so that the units have increasing difficulty in arranging themselves in the ordered crystalline lattice network and easily join up with neighbouring units in a random network This action produces a glass (Fig 1.6)

Strangely enough, although impurities often prevent proper crystallisation, the presence of a trace of certain substances, termed 'mineralisers ', in a silicate melt will cause it to crystallise instead of forming a glass It is such impurities that caused the crystallisation of the mineral silicates of the primary rocks on the earth's crust

Other Major Structural Constituents of Silicates

ALUMINIUM

The aluminium atom has a dual role It can only become a positively charged trivalent ion with difficulty, and so quite easily forms three covalent bonds like the four bonds of silicon Its size is such that although the aluminium atom should be surrounded by six oxygen atoms, it can fit into the hole left in the middle of a tetrahedral oxygen arrangement

The mineral gibbsite Al(OH)3 has a layer structure that recurs in several

of the clay minerals (Fig 1.7)

MAGNESIUM

Magnesium is divalent It is larger than aluminium and is always in octahedral coordination (surrounded by six oxygen atoms) The mineral brucite, Mg(OH)2, has a layer structure (Fig 1.8) Such layer structures occur combined with silica layers in a number of minerals of major impor-tance in ceramics, as will be seen

Isomorphous Substitution

The large structures of suitably combined [Si04] 4- tetrahedra, [Al06] 9 and [Mg06]1°-octahedra may also be considered as oxygen atoms so stacked that the holes between them are either tetrahedrally or octahedrally sur-rounded These are then filled with atoms, the small ones in the small holes and the large ones in the large holes until the total charge is zero Thus in an oxygen network of charge 12-, one could insert 4Al3+ or 3Si4+

-A crystalline structure is obtained when the cations are in regularly repeated positions

Bearing this theoretical concept in mind it is easy to see that one atom can

be substituted by another of the same size This is isomorphous substitution The substituting ion may or may not be of the same valency If it is not, some other adjustment such as loss of a hydrogen or addition of a sodium occurs

Thus it is often found that an aluminium atom has replaced a silicon atom

in a tetrahedral hole As aluminium is trivalent, whereas silicon is valent, the charge is neutralised by an additional univalent ion, e.g sodium, which fits in one ofthe larger holes (Fig 1 9)

tetra-PLASTIC RAW MATERIALS

Introduction

In primitive and early times the only raw materials for pottery were the natural plastic clays In modern ceramics many other raw materials play an important role, but that of clay is still a major one The term 'clay' is

a/uminiums (Hauth, H49)

perspective view Large spheres are hydroxyls ( OH) and small spheres are

magnesiums (Hauth, H49)

FIG 1.9 C11taway viero of hexago11al

arrange-ml!lll of ~ilicon-oxygen tetrahedra The two circles i11 the ctmtre space represent potas~ium

and sodium io1zs The dimensions marked are

i11 A,gstrom 1111its (Hauser, H43)

applied to those natural earthy deposits which possess the singular property

of plasticity This property, so easily detected and yet so hard to define, will

be discussed later

Clays occur in deposits of greatly varying nature in many parts of the world No two deposits have exactly the same ' clay' and frequently different samples of clay from the same deposit differ It is therefore worth while to give brief consideration to the origin and mineralogy of clay Firstly, clay

is a secondary ' rock', that is, it has been formed by weathering of certain

other rocks Secondly, clay is a mixture (P14)

Geology

Primary igneous rocks that gave clays on weathering were the granites, gneisses, ·feldspars, pegmatites, etc The weathering of these primary rocks was achieved by the mechanical action of water, wind, glaciers and earth movements working together with the chemical action of water, carbon dioxide, humic acids and, more rarely, sulphurous and fluorous gases, assisted

by elevated temperatures

The weathered rocks have in some cases remained in their original position These are the residual clays More frequently the weathering agents and other influences have transported the small particles and deposited them elsewhere During the transportation sorting by size takes place, also mixing in of weathering products from other sources occurs The deposits from water are always layered Deposits from wind transportation are known as 'loess'; they are not stratified and have a much more porous and crumbly structure

Mineralogy

The basic rocks from which clays are formed are complex aluminosilicates During the weathering these become hydrolysed, the alkali and alkaline earth ions form soluble salts and are leached out, the remainder consists of hydrated aluminosilicates of varying composition and structure, and free silica This remainder is therefore more refractory than the original

igneous rock Unchanged rock particles, e.g feldspar, mica and quartz

remain in the clay, too

This process can be represented by chemical equations, e.g

(1) K20.Alp3.6Si02 + 2Hp~Alp 3 6Si0 2 HP + 2KOH hydrolysis

feldspar

pyrophyllite

(4) Al203.2Si02.H20 + HP~AlP 3 2Si0 2 2H 2 0 hydration

kaolinite

diaspore (6) Al203.H20 + 2Hp~Al 2 0 3 3H 2 0 hydration

gibbsite The liberated silica is probably hydrated (N36)

This is a convenient way of obtaining a picture of the processes It must, however, be remembered that these chemical formulae have little physical significance as most of the substances under consideration exist as giant molecules The actual weathering process must be highly complicated (In general, equations will not be given for any but the most straightforward reactions.)

The hydrated silicates of aluminium are the ' clay substance' which gives the clays their main defined characteristics One of the predominant properties of these substances is the extreme fineness of their particles This factor, so vital to their physicochemical nature, was for a long time a major stumbling block to investigation With the aid of the microscope, the electron microscope, X-ray diffraction and differential thermal analysis it has now been established that clay particles are extremely small flake-like particles of crystalline minerals •

A number of these clay minerals have now been investigated and the ledge obtained makes it possible to divide them into groups The actual structures of the clay minerals have been very clearly put forward by Schofield (S27) and Jasmund (J41)

know-Basically the structures are dominated by the distribution of the

• It will be remembered that at first all 'clay substance' was considered to be amorphous, later only those fine particles small enough to be colloidal were so classi- fied Modem concepts of molecular structure make it difficult to conceive an amorphous nature for giant molecules and the X-ray powder technique has proved the crystalline nature of all clay particles

1.11 (b)

FIG 1.11

(a) Kaolinite from Langley, South Carolina Fresh fracture Electron micrograph of preshadowed carbon replica x 19 300

(Bates and Comer, B20)

(b) Kaolinite from Langley, South Carolina Fresh fracture, showing edge view of kaolinite ~ook Electron micrograph of pre- shadowed carbon replica x 30 900 (Bates and Comer, B20)

(c) Dickite, Schuykill County, Penn Electron micrograph of preshadowed carbon replica x 16 400 (Bates and Comer, B20)

commonest and largest atoms, namely oxygen In between them are hedral, octohedral and polyhedral spaces The way in which these are filled determines how they are attracted and held together The five main clay mineral groups are outlined below

tetra-The simplest clay mineral group is the kaolinite group This includes: kaolinite, dickite, nacrite; anauxite; halloysite, high- and low-temperature forms; livesite Their basic structure consists of oxygen atoms arranged to give alternate layers of tetrahedral holes and octahedral holes Where these layers are filled with silicon in the tetrahedral holes and aluminium in two-thirds of the octahedral ones we get the common mineral kaolinite, and the more perfect and rarer minerals dickite and nacrite (Figs 1.10 and 1.11 ) Where silicons replace aluminiums in octrahedral spaces (hydrogens being ,expelled to keep the charge right) a continuous series is obtained with anauxite, Al203.3Si02, as the end member These crystals are all thin hexagonal plates The basal spacing is 7·2 A They tend to become stacked on top of each other and loosely cemented into aggregates

Halloysite crystals are elongated The adjacent oxygen layers are able to take up a unimolecular water layer, making the basal spacing 10 A A continuous series from kaolinite to halloysite exists, intermediate members are termed livesite (Figs 1.12, 1.13) Inside the crystals there is no excess charge On the surface distortions occur and excess negative charges may be set up Positive ions are then adsorbed to neutralise it

The livesite of English fireclays is described as a kaolin type of mineral randomly oriented along the b-axis, and with ultimate particles much smaller

l c-Axis

I

r-,

· · 1o-2sA

b A x i s Halloysite (OI.fl,2AI 4 Si 4 0 8 2H 2 0

-2Hz0 2(0H)

FIG 1.13 Electron micrograph of surface replica of halloysite from Wendover, Utah, showing tubular morphology of crystals x 84 300 (Bates and Comer, B20)

than those of kaolinite from china clays, a proportion of them being less than O·lp The structure of the crystals is further disordered by the substitution

of Fe3+ and Mg2+ ions for AP+, leaving an inherent negative in the structure The mineral therefore readily attracts cations, mostly Ca2+, to balance this charge (W88)

The montmorillonite group of clay minerals present a very different

picture:

Montmorillonite (Fig 1.14),

Al203.4Si02.H20 + Aq, or (MgCa)O.Al203.5Si02.nH20

Beidellite, (Mg, Ca)O.Al203.4Si02.4H20, or Al203.3Si02,nH20

Saponite, 2Mg0.3Si02.nH20

Stevensite (perhaps a talc-saponite interlayered mineral (B116)

Nontronite, (Al.Fe)03.3Si02.nH20

(Sauconite), 2Zn0.3Si02.nH20

FIG 1.14 (a) Montmorillonite from Montmorillon, France Electron

micro-graph of preshadowed carbon replica x 20 225 (Bates and Comer, B20)

There are two layers of tetrahedral holes to every one of octahedral ones These minerals have the common property of absorbing large quantities of water between adjacent layers, changing the basal spacing from 10 A to 20 A The oxygen lattice is such that its spaces may be filled by different atoms; the octahedral spaces may have aluminium, magnesium, ferric ion or zinc, the tetrahedral may have silicon or aluminium Larger spaces that will accommodate alkali cations are also present The charges are often unbalanced and large numbers of cations are adsorbed and may be easily exchanged (Fig 1.15)

These structures make it easy to split the particles into very fine charged fragments ideally suited to go into colloidal sols

The next group of clay minerals is the illite, or hydromica group (Fig 1.16)

These resemble the micas and they have larger spaces which contain cations

to keep the charge neutral Their finely divided state, however, makes many of these cations accessible for exchange Unlike the montmorillonites water does not enter into the lattice itself and expand it, as adjacent layers are held together by potassium ions (Fig 1.17)

(OH)4K,(Al4.Fe4.Mg4.Mg8)Si8_y.Aly020 withy varying from 1 to 1·5

FIG 1.14 (b) Montmorillonite from Gaura, Siebengebirge, Germany: left, coarse fraction < 2p.; right, fine fraction < 0· Sp x 7000 (Jasmund, J41)

An unusual clay mineral is attapulgite which is fibrous It is found only

in Florida and Georgia (U.S.A.), fuller's earths and clays at Mormoiron, France (G79) (Figs 1.18, 1.19) Suggested formulae are:

(OH2MOH)2Mg5Si8020.4H20, some Mg replaced by Al (B103)

Si3012(Alt.Mg2)H8 (D23)

the former giving rise to the structural diagram shown in Fig 1.20

The term ~allophane' is used to cover non-crystalline mutual solutions of silica, alumina and water Although a constituent of clays this is not a true mineral Future investigation will throw more light on this group

These main groups divide the minerals according to their general ties as well as their chemical composition It may be useful for general reference to include a table of theoretical composition Unfortunately nomenclature of minerals is not universal and so a mineral may occur under two names Some of the named minerals have already been proved to be mixtures, others may prove tQ be so Table 1, however, is a guide to names one may come across

proper-The possible variations of chemical composition of a given mineral have been summarised by Engelhardt (E19), and are presented here in Table 2 Most natural clays are dominated by one clay mineral but also contain smaller quantities of a few others

Clays also have a number of other constituents which are not in themselves plastic The chief one is quartz, which, together with feldspar and mica,

FIG 1.16 Illites A, from Fithian, Illinois B, from N.E

Pennsyl-vania (Jasmund, J41, after Bates, B19)

2(0H)+4 0

Al 4 ·Fe 4 ·Mg 4 ·M96 2(0H)+4 0

4-ySi ·YAI

50

yK

1 !lite (OH) 4 Ky (AI 4 ·Fe 4 ·Mg 4 ·Mg6 )(Sie-Y ·Aiy) 020

FiG 1.17 Schematic representation of the crystal structure

of illite (Grim, Bray and Bradley, G78)

FIG 1.19 Attapulgite, AttaptJlgus, Georgia Electron micrograph of

pre-shadowed carbon replica, parallel fracture surface Some end sections are

are unaltered remainders of the parent rock The hydromicas are partially altered fragments Under certain high-temperature conditions the weather-ing has proceeded beyond the clay stage to give free aluminium hydrates, namely gibbsite and diaspore; these are, however, unusual

Iron compounds, often the oxides, are frequently present, and constitute the main colouring agents in clays Many other minerals occur in clays,

e.g calcite CaC03, aragonite CaC03, dolomite CaC03.MgC03, gypsum CaS04.2H20, rutile Ti02, tourmaline (a complex aluminium borosilicate), glauconite (a variable hydrous silicate of iron and potassium), hornblende (silicate of calcium and magnesium also containing iron, manganese, sodium and potassium), garnet (silicate minerals of special structure), vanadates, wavellite Al6(0H)6(P04) 4.9Hp, manganese oxides, and vivianite Fea(P04h.8H20 Organic inclusions also occur and may play a very important role Many of these are introduced during the transportation

It is therefore hardly surprising that every clay is different Classification

of clays is a big task and leads to a different result according to the viewpoint taken, whether geological, mineralogical, with regard to properties or according

to use A geological classification, as made by Ries, gives some idea about

Chemical Composition of Clay Minerals

(after Engelhardt (E19))

0 - 6·2 2·0- 4·5

0 - 0·6 6·1- 6·9 0·1- 0·5 0·5 6·4- 7·0

e H 2 0 () Hydroxide o Mg or A I

• OH, Q 0 • Si Attopulgite (OH 2 )4 (0H), Mg 5 Si 8 0 20 • 4H 2 0

FJG 1.20 Schematic representation of the crystal structure of attapulgite

(Jnsmtmd, J4J, after Bradley, B103)

the position as well as the nature of clays, and is a useful preliminary guide

to the ceramic industry

A Residual clays Formed in place by rock alteration due to various agents,

of either surface or deep seated origin

I Those formed by surface weathering, the processes involving solution, disintegration, or decomposition of silicates

(a) Kaolins, white in colour and usually white burning

Granite, pegmatite, rhyolite, Blankets; tabular steeply limestone, shale, feldspathic dipping masses; pockets or quartzite, gneiss, schist, etc lenses

(b) Ferruginous clays, derived from different kinds of rocks

II White residual clays formed by the action of ascending waters, possibly of igneous origin :

(a) formed by rising carbonated waters;

(b) formed by sulphate solutions;

III Residual clays formed by the action of downward percolating sulphate solutions

IV White residual clays formed by replacement, due to action of waters, supposedly of meteoric origin (indianite)

B Colluvial clays, representing deposits formed by wash from the foregoing and of either refractory or non-refractory character

C Transported clays

I Deposited in water

(a) Marine clays or shales, deposits often of great extent;

ball-clays, white burning clays;

fireclays or shales, buff burning;

Impure clays or shales non-ca careous 1

(b) Lacustrine clays (deposited in lakes or swamps);

fireclays or shales;

impure clays or shales, red-burning;

calcareous clays, usually of surface character

(c) Floodplain clays, usually impure and sandy

(d) Estuarine clays (deposited in estuaries), mostly impure and finely laminated

(e) Delta clays

II Glacial clays, found in the drift, and often stony May be either

red-or cream-burning

III Wind-formed deposits (some loess)

IV Chemical deposits (some flint-clays)

Ernst, Forkel and Gehlen (Ell) have put forward a method of shorthand description of clays on a mineralogical basis, as they consider this to be a scientific foundation from which properties and uses can be derived For instance, although mineral content cannot be derived from chemical analysis, the reverse can give a fair value Also they have shown that firing behaviour,

i.e P.C.E of mixtures of minerals can be plotted on diagrams and then

compared with phase rule data with which it does not coincide, the former being of greater use to the ceramist

Their proposed system is to use initial capital letters to denote mineral

groups, e.g K, kaolinite, Q, Quartz, F, Feldspar, with index letters when the

mineral is known more accurately The letters are used in the order showing the relative quantities, and/or percentages are given Added to these are

values for M the 'half weight particle size' (i.e such that the particles

making up half the weight of material are smaller than this size and the other half bigger) and the surface factor

Of greater industrial use is a classification according to the properties and therefore uses of the clays themselves as given by Norton (N36):

A White-burning clays (used in whiteware)

(2) Sewer-pipe clays and shales

(3) Brick and hollow tile clays and shales

D Stoneware clays (plastic, containing fluxes)

E Brick clays (plastic, containing iron oxide)

(1) Terra-cotta clays

(2) Face and common brick

F Slip clays (containing more iron oxide)

We will now consider the different important clays

Kaolin

The name is a corruption of the Chinese 'kao-liang ', meaning 'high ridge', a local designation for the area where a white china clay was found The kaolins or china clays, the latter expression often being reserved for the Cornish product, are white-burning clays, generally of low plasticity, and high refractoriness (cone 34 to 35) (1750-1770° C, 3182-3218° F) When mined they are rather siliceous, e.g of the Cornish ~lay rock only about 13% is extracted as china clay But after washing the chemical composition

of the clay approximates to that of kaolinite; this mineral does in fact dominate in kaolins but others are present

pre-Many theories have been advanced about the conversion of feldspar to kaolin The three most satisfactory being: (1) the igneous emanation theory; (2) the surface weathering theory; and (3) the bog or moor water theory According to the first theory igneous gases, originating in the centre of the earth and containing superheated steam, boron and fluorine compounds, carbon dioxide, etc., are the active agents This theory accounts for deep deposits like the world-famous ones in Cornwall and West Devon, the depth

of which is not known

Kaolinisation due to surface weathering, i.e downward percolating water

containing carbon dioxide, is necessarily of limited depth, and there is a graduation from rock base to the fully weathered product The china-clay deposits of Auvergne (France) and Passau (Germany) are supposed to have been formed in this way

The fact that many of the German kaolin deposits are near beds of lignite points to the fact that the drainage water from bogs containing ammoniacal salts and organic acids may have been active kaolinisating agents

The most famous European deposits are the Cornish ones, followed by those at Zettlit:t near Karlsbad in Czechoslovakia, Kemmlitz, Bortewitz and Amberg in Germany

In the United States the main deposits of residual kaolin lie in a band from Vermont to Georgia and up the Mississippi valley with a few scattered deposits in the West, the chief ones being near Spruce Pine in North Carolina Sedimentary kaolins occur in the United States in South Carolina, Georgia and Florida

Less than half of the kaolin produced is used for ceramics, the rest being employed as a filler in the paper, rubber, textiles and numerous other in-dustries Of that used in ceramics some goes to white-burning pottery and some to refractories

montmorillonite attached to the edges of the kaolinite platelets (B83) The name is derived from the English mining method of cutting the clay out

Ball clays are used in whitewares (earthenware, porcelain, etc.) to make the body more plastic and workable

Stoneware Clays

Stoneware clays are refractory or semi-refractory but contain enough

flux to fire to a dense body at comparatively low temperatures (ca 1100° C)

They are comparatively plastic without showing too much air- and shrinkage Stoneware clays include those clays that resemble ball clays in every respect except that they do not burn to a white product

fire-Fireclays

The use of this term has unfortunately become increasingly wide and thereby loose in its application Strictly it should be applied only to refrac-tory clays and shales which occur in hard masses that do not in their natural state take up water and become plastic, but on fine grinding will do so True fireclays occur in Great Britain, Czechoslovakia and in the U.S.A Unfortunately the term ' fireclay ' has been used to cover all types of clay

deposited in swamps or coal basins, i.e associated with coal measures,

without regard to its fusibility or firing behaviour

This has necessitated classification of fireclays The primary classification

is according to physical character into:

(1) Plastic fire clays

(2) Semi-flint fire clays (G73) These resemble the plastic fireclays but develop plasticity only after working and are somewhat more refractory (3) Flint fire clays (the true fire clays) These hard clays break with a conchoidal fracture and are refractory They find great use in the refrac-tories industry

(4) Nodular flint fire clays Deposits of these clays are rare They are flint clays containing nodules of gibbsite or other hydrous aluminium oxides and are therefore the most refractory

Classification according to fusibility is also of importance, the Ries (R28) system being usually used

(1) Highly refractory clays fusing above cone 33

(2) Refractory clays fusing from cone 31-33 inclusive

(3) Semi-refractory clays fusing between cones 27 and 30

(4) Clays of low refractoriness, 'low heat duty clays', fusing between cones 20 and 26

This classification deals only with the upper limit of use, the fusion point

It gives no guidance to the temperatures of incipient or complete vitrification which one needs to know for the manufacture of certain products

Purdy and Moore (P77) and subsequently Purdy (P79) take this need into consideration in classifying clays according to changes in porosity and specific gravity curves for different temperatures

No 1 fireclays show a small regular decrease in porosity from the beginning

of firing to above cone 11

No 2 fireclays show an increase in loss of porosity with consequent early vitrification, the change beginning about cone 02 and becoming marked with increasing temperature

No 3 fireclays show a marked change with increasing porosity at cone 4 and seldom have a fusion point above cone 16 (No.3 fireclays would not be termed fireclays in Great Britain.)

Another practical classification on similar lines is given by Matthews (M35) who considers the temperatures at which the following occur:

(1) Some of the constituents fuse to a glass and cement the more refractory grains together, termed' incipient vitrification'

{2) Sufficient fusion occurs to close all the pores, the maximum shrinkage taking place, termed 'complete vitrification'

(3) So much fusion takes place that the body is just no longer supporting, termed 'viscosity'

self-He groups the clays in the following way:

(a) Clays for which the interval between incipient and complete vitrification

is more than that between complete vitrification and viscosity

(b) Those for which the range is approximately half of the total range from incipient fusion to viscosity

(c) Those for which the range from incipient to complete vitrification is less than that from vitrification to viscosity

Investigation of the constitution of British refractory clays was undertaken

by Roberts and his collaborators (C12, C13, G84, G85, G86, G87, R39) The major clay mineral component was at first thought to be halloysite but later proved to be another mineral of the kaolinite group, namely livesite A secondary clay mineral component is illite The main non-clay component is quartz The third important mineral in British fireclays is hydrous mica, 0·3K20.3Al203.6Si02.4·5H20, which is probably an intermediate product

in the breakdown of mica to livesite, or it may be a mixture of muscovite and livesite Livesite, hydrous mica and quartz make up 90-95% of fireclays Fireclays may also contain some iron compounds, but in general they are very pure and particularly free from soluble salts

The clay mineral constituents of United States clays are principally kaolinite and hydromica, the hydromica probably being what has elsewhere been referred to as illite (G73) Some Ohio deposits contain a large quantity of dickite (R57)

The respective roles of the three main minerals in British fireclays have

been outlined by Grimshaw (G87) Livesite confers refractoriness and a high potential plasticity although this may require weathering to make it active Livesite gives a green strength considerably greater than a corres-ponding amount of kaolinite but less than halloysite and montmorillonite Hydrous mica may assist in developing the plasticity of livesite by opening

up the structure of the mined clay Its chief function is its fluxing action which is greater in high-silica than in high-alumina clays and also depends

on particle size Fine-grained hydrous mica gives a large firing shrinkage

up to 1200° C (2192° F) but a small after contraction on reheating to 1400° C (2552° F) whereas coarse-grained material gives a smaller firing shrinkage but a larger after contraction Quartz acts as grog up to about 1350° C (2462° F) but thereafter acts as an active flux and can lead to rapid contrac-tion of the body

Fireclays are used chiefly for refractories, e.g firebricks, retorts, furnace

linings, and also for sanitary ware and certain tiles

High-alumina Clays

The hydrated alumina minerals diaspore and gibbsite (seep 114 alumina) frequently occur together with kaolinite and may be used for making refrac-tories in the mixture in which they occur The mixtures containing diaspore ' diaspore clays' are preferred to those with gibbsite because they have more favourable shrinkage properties They usually contain over 60% alumina The gibbsite-containing mixtures are termed bauxites, 'bauxitic clay' having less than 50% gibbsite and ' argillaceous bauxite' more than 50%

Bentonite

This clay is derived from volcanic ash It is widely distributed, occurring

in beds from a few inches to ten feet deep

The main clay mineral of bentonite is montmorillonite This makes the clay take up water readily and swell to four or five times its dry volume It is extremely plastic, has a low fusion point, and gives a coloured product The chief use of bentonite is as a plasticiser Addition of 1% bentonite may improve plasticity more than 10% ball clay would, making it particu-larly useful for moulding sands

Brick Clays

Large clay deposits have become such a mixture of various minerals that they will fire to coloured bodies at a relatively low temperature Complete vitrification to a stoneware is not possible, a porous product being obtained Correns (C56) describes the origin and nature of the minerals present Plate-like minerals are predominant, this including not only the clay minerals but also micas and chlorites It is possible that the mica is formed after the sedimentation of the clay Certainly the sulphides are formed in situ with

the aid of organic matter, as are also nitrates, the former being of more concern to the brickmaker than the latter There is also considerable migration of carbonates in the clay leading to local concentrations It is therefore not surprising that brick clays are localised and give rise to ware called by place names

The very important British Fletton brick clays have the great advantage

of containing about 5% carbonaceous matter which reduces the fuel ments for firing the clay by about i

Loess usually matures at low temperatures to products of varying colours

It is largely used for bricks

A Glossary of Trade Names compiled by Robertson (R4l) was issued in

1954 by the clay minerals group of the Mineralogical Society of Great Britain and Ireland and is most useful

Table 3 gives a small selection of analyses of various types of clay, a few details other than the analyses being included So far as possible the data have been obtained from the firms that mine andfor supply the clays Some

of these firms furnish considerably more details as regards grain size bution and drying and firing shrinkage

distri-Nos 1-14, china clays and kaolins

Nos 15-40, ball clays and similar clays

Nos 41-43, off-white fat clays

Nos 44-48, siliceous clays

Nos 49, 50, ferruginous clays

Nos 51-54, red clays

Nos SS-59, stoneware clays

Nos 60-85, fireclays and refractory clays

Nos 86 89, bentonites

Nos 90-91, special clays