4.1. Introduction
Silicate ceramics are generally alumino-silicate based materials obtained from
natural raw materials. They exhibit a set of fundamental properties, such as chemical
inertia, thermal stability and mechanical strength, which explain why they are
widely used in construction products (sanitary articles, floor and wall tiles, bricks,
tiles) and domestic articles (crockery, decorative objects, pottery). They are often
complex materials, whose usage properties depend at least as much on
microstructure and aesthetics as on composition. Silicate products with an
exclusively technical application (refractory materials, insulators or certain dental
implants) will not be explicitly discussed in this chapter.
To distinguish silicate from technical ceramics, it is useful to qualify these
products as traditional ceramics. This term refers to the centuries-old tradition that
still strongly influences the classification of this type of materials and the vocabulary
attached to them. However, it does not reflect the considerable evolution of a sector
of activity where progress relates more to the manufacturing technologies (raw
material mixtures, drying, sintering, etc.) than to the products themselves.
These products of terra cotta, earthenware, sandstone, porcelain or vitreous china
are generally widely marketed materials. They represent a predominant share in the
total sales turnover of the ceramic industry. In 1994, the fields of roof tiles and
bricks, wall and floor tiles, crockery and ornamentation, and sanitary products
Chapter written by Jean-Pierre BONNET and Jean-Marie GAILLARD.
96 Ceramic Materials
accounted for, respectively, 28, 14, 13 and 13% of the turnover of the French
ceramic industry (technical + refractory + traditional) [LEC 96].
4.2. General information
Silicate ceramics can be formed in various ways: by casting in a mould aqueous
suspension called slip, by extrusion or jiggering of a plastic paste, or by
unidirectional or isostatic pressing of slightly wet aggregates. The quantity of water
contained in the sample therefore depends on the method of forming. Generally,
water is eliminated during a specific drying treatment. The raw part is transformed
into ceramic by sintering, also called firing, carried out under suitable conditions of
temperature, heating rate and atmosphere. Depending on the application considered,
this ceramic, also called shard, can be dense or porous, white or colored.
Clay is the basic raw material for these products. Mixed with water, it can form a
plastic paste similar to the one used by the potter on his wheel. Although easy to
form, this paste often exhibits insufficient mechanical strength to enable handling
without damaging the preform. Owing to the clay colloidal nature, a relatively pure
paste is low in solid matter. It thus shrinks significantly during drying and sintering,
which makes it difficult to control the shape and dimensions of the final piece. To
limit all these effects, non-plastic products known as tempers can be added to the
paste. They then form an inert and rigid skeleton that enhances the mechanical
strength of the preform, favors the elimination of water during the drying stage and
limits sintering shrinkage. Among the commonly used tempers, we can mention
sand, feldspars, certain lime-rich compounds or grog (a paste sintered and ground
beforehand).
Given the complexity of the composition of argillaceous raw materials, the
appearance of a viscous liquid during firing can be observed. The addition of fluxes
to the starting mixture amplifies this phenomenon. These compounds, which also
behave as tempers, generally contain alkaline ions (Na, K, sometimes Li). The
examination of the phase diagram of Al2O3-SiO2-K2O represented in Figure 4.1
highlights the role of flux of a potassium feldspar (orthoclase with the composition
K2O,A2O3,6SiO2) with respect to the deshydroxylation product of kaolinite, whose
composition in equivalent oxides Al2O3,2SiO2 is symbolized by point MK. At
equilibrium, the addition of a small quantity of feldspar leads to decrease the solidus
temperature from 1,590 to 985°C. Under certain temperature and composition
conditions, iron oxides and a few calcium-rich compounds, such as chalk, can also
contribute to the formation of a liquid phase. In the presence of a sufficient quantity
of molten matter, the heat treatment can be pursued until the almost complete
disappearance of the porosity. The shards thus obtained are rich in vitreous phase
and exhibit good mechanical strength. The quantity of liquid formed during partial
Silicate Ceramics 97
vitrification must remain sufficiently low or its viscosity must be high enough so
that the piece does not become deformed under its own weight.
Figure 4.1. Phase diagram Al2O3-SiO2-K2O [LEV 69]
Some products are covered with a vitreous enamel film intended to modify the
appearance of the ceramic and/or to waterproof it. This layer can be deposited on an
engobe whose role is to mask the color of the shard and/or to facilitate the adhesion
of the enamel. Depending on the case, the enameling operation is carried out on a
green support, on a partially fired part during a so-called bisque firing (maximum
temperature lower than that of enamel firing) or on a biscuit (completely fired shard
at a temperature higher than that of enamel firing). The low temperature enamel
intended for the protection of porous ceramics, such as earthenware and potteries, is
also called glaze. Transparent glaze is the name used to denote enamel obtained by
melting at the sintering temperature of the porcelain shard or the underlying
stoneware. The enamel coloring is obtained using metallic oxides.
98 Ceramic Materials
4.3. The main raw materials
4.3.1. Introduction
Each mineral raw material has a specific influence on the rheology of the paste, the
development of the microstructure, the phases formation during the heat treatment and
the properties of the finished product. The manufacture of all silicate ceramics requires
such a large number of raw materials, which cannot be discussed here. Only those
most commonly used, i.e. clays, feldspars and silica, will therefore be described.
4.3.2. Clays
4.3.2.1. Common characteristics
Clays are hydrated silico-aluminous minerals whose structure is made up of a
stacking of two types of layers containing, respectively, aluminum in an octahedral
environment and silicon in tetrahedral coordination. Their large specific surface (10
to 100 m2g-1), their plate-like structure and the physicochemical nature of their
surface enable clays to form, with water, colloidal suspensions and plastic pastes.
This characteristic is largely used during the manufacture of silicate ceramics insofar
as it makes it possible to prepare homogenous and stable suspensions, suitable for
casting, pastes easy to manipulate and green parts with good mechanical strength.
By extension, the term clay is often used to denote all raw materials with proven
plastic properties containing at least one argillaceous mineral. The impurities present
in these natural products contribute to a large extent to the coloring of the shard.
4.3.2.2. Classification
All clays do not exhibit the same aptitude towards manipulation and behavior
during firing. Ceramists distinguish vitrifying plastic clays, refractory plastic clays,
refractory clays and red clays.
Vitrifying plastic clays, generally colored, are used for the remarkable plasticity of
their paste. They are made up of very fine clay particles, organic matter, iron and
titanium oxides, illite (formula Si4xAlx)(Al,Fe)2O10(OH)2Kx(H2O)n) and micaceous
and/or feldspathic impurities. These clays are also characterized by a high free silica
content; sand can represent up to 35% of the dry matter weight. The product called
“ball clay” is widely used for its plasticity and its particularly low mica content.
Although it contains the same argillaceous mineral as kaolin, this clay has much higher
plasticity because of the much smaller size of the kaolinite particles [CAR 98].
Refractory plastic clays are rich in montmorillonite (formula (Si4-xAlx)(Alx-
vRx)O10(OH)2M2v(H2O)n with R = Mg, Fe2+ and M = K, Na), kaolinite or
halloysite (Si2Al2O5(OH)4(H2O)2).
Silicate Ceramics 99
Refractory clays are used in high temperature processes. Their composition is
rich in alumina. Kaolins are the most refractory among these clays. Always purified,
they contain little quartz, generally less than 2% alkaline oxides in combined form
and a small quantity of mica. Their plasticity is ensured by kaolinite and, if
necessary, a little smectite or halloysite [CAR 98]. Very low in coloring element,
they are particularly suited for the preparation of products in white shard.
Red clays used for the manufacture of terra cotta products are actually natural
mixtures with a complex composition. They generally contain kaolinite, illite and/or
other clays rich in alkaline, sand, mica (formula Si3Al3O10(OH)2), goethite
(FeO(OH)) and/or hematite (Fe2O3), organic matter and, very often, calcium
compounds. The latter, just like the micas and the other alkaline-rich compounds,
help lower the firing temperature of the shard.
4.3.3. Kaolinite
4.3.3.1. Structure of kaolinite
Kaolinite, Si2Al2O5 (OH)4 or Al2O3,2SiO2,2H2O, is the most common among
the argillaceous minerals used in ceramics. A projection of its crystalline structure is
represented in Figure 4.2. It consists of an alternate stacking of [Si2O5]2- and
[Al2(OH)4]2+ layers, which confer to it a lamellate character favorable to the
development of plates. The degree of crystallinity of the kaolinite present in clays is
highly variable. It depends largely on the genesis conditions and the content of
impurities introduced into the crystalline lattice.
Figure 4.2. Projected representation of the structure of kaolinite
100 Ceramic Materials
4.3.3.2. Evolution of the nature of phases during heat treatment
Figure 4.3. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA)
of two kaolinites with different degrees of crystallinity
Silicate Ceramics 101
During the heat treatment, kaolinite undergoes a whole series of transformations.
The variations of exchanged heat and the corresponding mass changes are indicated
in Figure 4.3. The departure of water, which occurs from 450°C onwards, is a very
endothermic phenomenon. The amorphous metakaolin, Al2O3, 2SiO2 then formed,
exhibits a structural organization directly derived from that of kaolinite. The
exothermic transformation observed between 960 and 990°C is a structural
reorganization of the amorphous metakaolin, sometimes associated with the
formation of phases of spinel structure like Al8(Al13,332,67)O32 (γ variety of Al2O3)
or Si8 (Al10,675,33)O32. In these formulae, represents a cation vacancy. Between
1,000 and 1,100°C (often around 1,075°C), these phases are transformed into mullite
stoichiometry ranging between 3Al2O3,2SiO2 and 2Al2O3,SiO2. During this
reaction, amorphous silica is released. The surplus amorphous silica starts to
crystallize in the form of cristobalite from 1,200°C onwards. It should be noted that
the impurities present, the degree of crystallinity (see Figure 4.3) and the speed of
heating influence each of these transformations.
4.3.4. Feldspars
Four feldspathic minerals are likely to enter the composition of silicate ceramic
pastes. They are:
− orthoclase, a mineral rich in potassium with the composition K2O,Al2O3,6SiO2;
− albite, a mineral rich in sodium with the composition Na2O,Al2O3,6SiO2;
− anorthite, a mineral rich in calcium with the composition CaO,Al2O3,2SiO2;
− petalite, a mineral rich in lithium with the composition Li2O,Al2O3,8SiO2.
Orthoclase and albite, which form eutectics with silica, respectively, at 990 (see
Figure 4.1) and 1,050°C, are widely used as flux. Anorthite is rather regarded as a
substitute to chalk. The use of petalite, especially owing to its negative expansion
coefficient, is marginal [MAN 94].
Potassic feldspar is particularly appreciated by ceramists because its reaction
with silica leads to the formation of a liquid whose relatively high viscosity
decreases slightly when the temperature increases. This behavior is considered as a
guarantee against the excessive deformation of the pieces during the heat treatment.
Natural feldspars used for the preparation of ceramics are mineral mixtures.
Thus, the commercial potassium products can contain between 2.5 and 3.5% of
albite mass, whereas anorthite and a small quantity of orthoclase, between 0.5 and
3.2%, are often present in the available sodium feldspars [MAN 94]. They can also
be incorporated into the paste in the form of feldspathic sand. When these natural
products are heated, mixed and homogenous feldspar is formed. This compound,
102 Ceramic Materials
called sanidine, occurs at a temperature which varies according to the
sodium/potassium ratio and ranges between 700 and 1,000°C. Then, in the presence
of silica, the formation of the liquid takes place. Above 1,200°C, mullite is formed
in the still solid part of the feldspar grains.
A rather recent trend among stonewares and porcelain manufacturers consists of
replacing feldspars by nepheline syenite with average composition
(Na,K)2O,Al2O3,2SiO2. This rock, made up of nephelite (composition:
K2O,3Na2O,4Al2O3,9SiO2) and a mixture of potassium and sodium feldspars, is a
powerful flux which makes it possible to decrease the sintering temperature of
ceramics and increase the alkaline content of the vitreous phases [CAR 98].
4.3.5. Silica
Silica, SiO2, is a polymorphic raw material found in nature in an amorphous
(opal, pebbles) or crystallized form (quartz, cristobalite and tridymite). Sand
contains between 95 and 100% of quartz mass. It is the most frequently used temper
in the ceramic industry. To contribute significantly to the mechanical strength of the
raw parts, it must consist of much coarser particles than those of clay. In the modern
manufacturing processes of stonewares and porcelains, it is customary to use
relatively fine sand grains (20 to 60 μm).
Volume expansion (%)
Figure 4.4. Influence of temperature on the expansion of the various forms of silica
Silicate Ceramics 103
When a ceramic is fired, the sand can react, particularly with the fluxes. This
reaction is seldom complete. The transformation of residual quartz into cristobalite
can then start from 1,200°C onwards. It is favored by the rise in temperature, the use
of fine grained sand, the presence of certain impurities and a reducing atmosphere
[JOU 90].
The form in which silica is found determines the thermal properties of silicate
ceramics. Thus, quartz and cristobalite do not have the same influence on the
expansion of the shard (see Figure 4.4). Quartz can also cause a deterioration of the
mechanical properties of the finished product owing to the abrupt variation in
dimensions (ΔL/L ≅ –0.35%) associated, at 573°C, with the reversible transformation
quartz β → quartz α. As the crystal of cristobalite formed from the flux are usually
small, the transition cristobalite β → cristobalite α, which occurs at about 220°C (see
Figure 4.4), often causes less damage to the finished product. It can even contribute to
the shard/enamel fit by compressing it after cooling at room temperature.
4.4. Enamel and decorations
4.4.1. Nature of enamel
The enamel layer deposited on the shard generally has a thickness ranging
between 0.15 and 0.5 mm. Its purpose is to mask the porosity and/or the color of the
shard, to make the surface of the piece smooth and brilliant and to improve the
chemical resistance of the ceramic. This layer, transparent or opaque, white or
colored, is obtained from a silica-rich ceramic composition capable of developing
glass during the heat treatment. The composition of the enamel also contains many
other constituents, in particular alkaline and alkaline-earth oxides. They help to
adjust the melting point, the thermal expansion coefficient, the surface tension and
the viscosity to the enameling conditions, and ensure the wetting and adhesion of the
enamel on the shard. The enamels used for sheet enameling have many common
points with those described here [STE 81].
The properties of the enamel are often analyzed by considering that it is made up
of a combination of acid oxides responsible for the vitreous structure (mainly SiO2
and B2O3), amphoteric oxides (Al2O3) and basic oxides (K2O, Na2O, CaO, MgO,
PbO). It should be noted that the role of flux, traditionally reserved for basic oxides,
is now increasingly played by acid or amphoteric oxides, such as B2O3 or Bi2O3.
Enamel is obtained from a mixture of raw materials mineral and/or frits. The raw
materials used are mainly feldspars, kaolin, quartz and chalk or dolomite. Frits are
close mixtures of components prepared by melting several compounds at high
temperature (T > 1,400°C). After quenching in air or water, the product, markedly
104 Ceramic Materials
vitreous in character, is ground. Combined in this manner, water soluble salts or
volatile oxides can be used without harm in the composition of enamel. We can
distinguish raw enamels formed only from natural raw materials and sintered
enamels. The latter are particularly suitable for low temperature applications that
require flux bases, richer in basic elements, which are non-existent in nature. The
role of frits in the composition of the enamel is all the more important as the firing
temperature reduces and the heat treatment is shortened. Frits are widely used for the
enameling of the tiles in the fast sintering process [ENR 95].
It is customary to classify the various types of enamel, based on the nature of the
flux used. Thus, we can distinguish lead enamels (PbO rich enamels), boron oxide
enamels, alkaline enamels, alkaline-earth enamels, zinc enamels and bismuth oxide
enamels [STE 85]. Lead enamels, historically the oldest, were the most commonly
used for a long time. Because of the toxicity of lead, their future hinges on the
evolution of legislation relating to the leaching of this element. They tend to be
replaced by enamels containing a very small quantity of bismuth oxide (< 5% mass)
and, especially by alkaline borosilicate products.
4.4.2. Enamel/shard combination
For the enamel to remain strongly attached to the shard, the interdiffusion must
be effective and the thermal expansion coefficients of these two parts of the ceramic
must be compatible. When the expansion coefficient of the shard is lower than that
of the enamel, the latter is subjected to tension stresses when the piece cools. The
stresses generated at the interface can cause the formation of cracks in the enamel.
This flaw, also called crazing, is all the more important the more significant the
difference between the expansion coefficients and the higher the modulus of
elasticity of the enamel. To avoid this, it is customary to try stabilizing in the
support, high expansion coefficient phases. On the other hand, when the shrinkage
of the shard on cooling is the highest, the enamel is placed under compression and
its mechanical strength is thereby reinforced. This positive effect occurs only if the
difference between the expansion coefficients is sufficiently low to prevent the
enamel from falling apart due to compression and from flaking off.
4.4.3. Optical properties of enamel
When the vitrification is complete, i.e. after the various components have melted
completely, the enamel is generally brilliant, smooth and transparent. In most cases,
opaque enamel is desired. This is achieved by favoring the formation of crystallized,
vitreous or gas inclusions with an index of refraction different from that of the
vitreous matrix. The differences between the indexes of refraction, the size and the
Silicate Ceramics 105
form of the inclusions are then decisive parameters. The formation of crystallites can
be favored by the presence in the enamel of mineralizers such as ZrO2 and SnO2
Vitreous inclusions occur when a decomposition takes place during the total fusion,
as in the case of compositions like SiO2-B2O3-MO (M = Pb, Ca, Zn, Mg).
A mat appearance and opacity owing to the diffusion of light on asperities can be
observed when the surface of the enamel is slightly rough. This phenomenon occurs
when the melting is incomplete or when the viscosity of the formed liquid is high. It
can be favored by increasing the contents of SiO2, Al2O3, CaO and ZnO.
4.4.4. Decorations
The pigments used for the production of decorations generally consist of colored
frits or stain mixtures crystallized in a vitreous silico-aluminous phase. The main
products used as coloring are oxides of antimony, chromium, copper, cobalt, iron,
manganese, nickel, praseodymium, selenium, titanium, uranium, and vanadium
[HAB 85].
In order to be applied on the parts, the ground pigments are mixed with liquid
organic substances (for example, turpentine oil) which facilitate their adhesion. The
nature of the process of decorating the enamel depends on the desired quality and
the complexity of the decoration. The decalcomania technique is the most efficient,
insofar as a very complex decoration, involving up to 20 colors, can be carried out
by serigraphy. Processes making it possible to print directly on the enamel (direct
transfer through a membrane) or decorate it without firing in the kiln (lazer
sintering) can also be used.
An additional firing is generally necessary to fix the decorations. Depending on
the application envisaged for the piece, it can be carried out below 800°C (low fire
firing) or at round 1,200°C (high fire firing). Low fire firing makes it possible to
obtain a very broad pallet of colors; high fire firing is especially used to fix
decorations likely to change in a highly aggressive environment, a dishwasher for
instance. In view of the interactions between phases existing at high temperature, the
pallet of colors is therefore considerably reduced.
4.5. The products
4.5.1. Classification
Based on the criteria taking mainly into account open porosity and/or the coloring
of the shard, it is customary to distinguish, among silicate ceramics, terra cotta
106 Ceramic Materials
products, earthenware, stoneware, vitreous china and porcelains. The materials treated
at higher temperatures or in the presence of a large quantity of flux are generally the
least porous. Whiteness is primarily the result of the use of raw materials free from
iron and titanium or containing only small contents of transition metals.
The representation given in Figure 4.5 helps locate each of these families. Terra
cotta products and earthenwares are characterized by a porous shard. The strong
coloring in the mass of the terra cotta products has given them the name “red
products”. These porous ceramics can be used just as they are (bricks and tiles) or be
covered with enamel (earthenwares). Among the dense products, stonewares shard is
more colored than porcelain shard. Vitreous china forms an intermediate group
between these two families. Many products are on the border between two of these
groups; their name, which very often differs from one country to another, depends
on the custom and the envisaged application.
Figure 4.5. Representation of the various traditional ceramic families
The nature of the raw materials used for the manufacture and the chemical and
mineralogical compositions of the shards can also be used as additional criteria for
classification.
4.5.2. Terra cotta products
We are referring here to potteries or construction products such as roof tiles,
bricks, flues, drainage pipes or some floor tiles. Terra cotta products were obtained a
long time ago by modeling, drying and firing common clays. Nowadays, the
compositions are more complex; they combine clays and additives, such as coloring,
Silicate Ceramics 107
tempers or agents which make it possible to improve the manufacturing behavior or
the final characteristics. The raw materials added to water form a plastic paste whose
rheology must be adapted to the shaping process (extrusion possibly completed by
pressing). The raw parts are dried in a ventilated cell or a tunnel dryer. The
temperature at the end of firing usually ranges between 900 and 1,160°C.
Terra cotta products are porous and mechanically resistant. They are marketed
raw, enameled or covered with a glaze realized at low temperature, between 600 and
900°C, called varnish. They are appreciated for their esthetic quality, their stability
through time and their hygrothermic and acoustic properties. They represent a highly
automated industrial sector which is the scene of continual technological
developments.
The coloring of terra cotta shards can vary from yellowish white to dark brown.
The variety in the tonality of the tiles present on roofs illustrates the extent of the
pallet available. For roof tile manufacturers, the mastery over colors represents a
commercial stake, insofar as they often constitute a regional specificity or a
decorative element. The coloring of the shard depends on the bonding of iron ions
with inhibitors, such as the calcium ions or with additional coloring (titanium and
manganese oxides). The crystallized phases that are formed during the firing of a
terra cotta product can be described using a ternary system defined by the major
oxides Al2O3, SiO2 and CaO. This includes primarily wollastonite (CaO, SiO2),
gehlenite (2CaO, Al2O3, SiO2) and anorthite (CaO, Al2O3, 2SiO2). A high
temperature heat treatment favors the formation of anorthite to the detriment of the
other two phases. The Fe3+ ions dissolved in the anorthite confer a yellow coloring
to it. Hematite, Fe2O3, is brown-red. The presence of compounds containing Fe2+
ions favors bluish or greenish tonalities. The final coloring of the shard is a
combination of these three effects. In an oxidizing medium, a strong concentration
of iron leads to the formation of a significant quantity of hematite and a brown red
shard. The abundant presence of CaO in the starting mixture is favorable to the
formation of anorthite and thus to the evolution of coloring towards yellow. A
treatment at an excessively high temperature or the use of an atmosphere that is too
low in oxygen can involve the formation of Fe2+ ions and a green or black coloring
of the shard. Based on these considerations and experimental observations, the
following rules may be laid down:
− when the Al2O3/Fe2O3 mass ratio is lower than 3, the shard is red;
− when the Al2O3/Fe2O3 mass ratio ranges between 3 and 5, the shard is pink;
− when the Fe2O3/CaO mass ratio is less than 0.5, a suitable heat treatment (high
temperature and a sufficiently oxygen-rich atmosphere) yields a yellow shard;
− when the CaO/Al2O3 mass ratio is close to 1, the color of the shard is particularly
dependent on all the parameters likely to affect the formation of anorthite. It is also
108 Ceramic Materials
significantly influenced by the other impurities present; MnO produces, for instance,
black reflections.
As an atmosphere that is excessively oxidizing is detrimental to the formation of
anorthite, the stacking density of the parts and the temperature of the final stage of
firing can assume considerable importance, particularly in the last two cases. Thus,
for a given composition, iron can be in the form of hematite at 1,000°C (pink
coloring), dissolved in anorthite at 1,050°C (yellow coloring) and partially reduced
at 1,100°C (coloring turning to green). The tile color therefore depends on the
composition of the raw materials and the firing conditions (temperature, atmosphere
and setting load of the kiln). Today it is often modified by using mineral coloring
deposited, sometimes directly on the surface of the raw parts (colored engobe).
4.5.3. Earthenwares
4.5.3.1. General characteristics of earthenwares
We call earthenware the ceramic products made up of a porous shard covered
with a glaze. This enamel makes it possible to mask the appearance of the shard and
to remedy the high permeability due to the existence of an open porosity ranging
between 5 and 20%. Although present in the form of objects of imagination and
crockery, earthenwares are especially used as wall tiles.
These products are prepared from one or more clays to which quartz, chalk,
feldspar or ground glass are added. Earthenwares are primarily shaped by slip
casting, jiggering of plastic paste and atomized powder pressing. After drying, the
raw product is subjected to a heat treatment called biscuiting, carried out at a
temperature ranging between approximately 900 and 1,230°C. The deformation and
the shrinkage of the shard during this stage are limited because of the refractory
nature of the raw materials used. The porous biscuit obtained is then enameled
during an enamel firing carried out at a temperature lower than or sometimes equal
to that of biscuiting. The third firing, at a lower temperature, is necessary to fix some
decorations deposited on the glaze, in particular those containing gold or platinum
and those known as “low fire” decorations.
4.5.3.2. Common earthenwares
Common earthenwares are found especially in old products. Their production is
very limited nowadays. They are primarily glazed potteries and stanniferous
earthenwares.
Silicate Ceramics 109
4.5.3.2.1. Potteries glazed with low melting temperature argillaceous paste
Potteries glazed with fusible argillaceous paste are very close to terra cotta
products. Just like them, they are obtained from common argillaceous soils,
relatively fusible, and naturally containing a certain quantity of sand. Although their
sintering, carried out between 900 and 1,060°C, takes place in the presence of a
significant quantity of liquid, their shard is still porous.
These products, used in construction (enameled bricks and tiles), for domestic uses
and as crockery (jugs, pots, etc.), are generally covered with an engobe whose pores
are finer than those of the shard. This engobe constitutes a smooth and regular
surfacing intended to mask the coloring of the shard and to be used as decoration base.
4.5.3.2.2. Stanniferous earthenwares with low melting temperature argilo-calcareous
paste
To produce certain decorative objects, it is customary to use an argilo-calcareous
paste obtained by mixing argillaceous marls, limestone and often sand. Magnesium
carbonate (MgCO3) and dolomite ((Ca,Mg)CO3) can also be used. The biscuits are
sintered at a temperature between 900 and 1,060°C. They are generally covered with
a glaze opacified by tin dioxide, which explains the name stanniferous earthenwares.
4.5.3.3. Fine earthenwares
Fine earthenwares are characterized by a white or very lightly colored shard, a
thin and regular texture, high mechanical strength and the brightness and durability
of their glaze. They are widely used as decorative objects and crockery, fields where
the quality of their enamel is highly appreciated.
The argillaceous component of the paste consists of a mixture of kaolin and clays.
The kaolin increases the refractory character of the paste, whereas the plastic clays
contribute to the mechanical strength of the raw parts. Although the selected clays are
very poor in colorings, the presence of very small quantities of impurities in them,
such as Fe2O3 and TiO2, can be sufficient to slightly color the shard. The kaolin/clay
ratio must therefore be adjusted in order to obtain an acceptable compromise between
the whiteness of the biscuit and the resistance of the raw parts. Kaolin generally
represents between 25 and 50% of the mass of all the argillaceous raw materials.
Grog, silica in its various forms and chalk (case of calcareous earthenware) can
be used as tempering raw materials. The overall content of quartz, primarily
introduced with the clays in the form of sand, is generally very high in fine
earthenware pastes (30 to 40% of the mass). The biscuit is fired in an oxidizing
atmosphere, at a temperature ranging between 950 and 1,150°C. The presence of a
110 Ceramic Materials
large quantity of residual quartz increases the shrinkage of the shard on cooling, thus
reinforcing the mechanical strength of the enamel.
4.5.3.4. Feldspathic earthenwares
Feldspathic earthenwares are obtained by firing, at a temperature between 1,140
and 1,230°C, a mixture containing, for example, kaolin (40 to 70% of the mass),
quartz (25 to 58%) and feldspar (3 to 14%). This latter component favors the
formation of a viscous liquid during the high temperature treatment. After cooling,
the biscuit has an increased solidity by virtue due to a significant quantity of vitreous
phase formed during the solidification of the liquid. The porosity, 10 to 15%, is
generally lower than the one observed in other earthenwares. The enamel is fired at
a temperature between 1,000 and 1,140°C. At these high temperatures, it is possible
to obtain glazes that cannot be easily scratched by steel. The increase in the silica
content in the paste, the use of finer silica favorable to the formation of cristobalite
or the reduction in the porosity of the shard contribute to the improvement of the
shard/enamel combination.
Owing to their very high solidity, their particularly scratch-resistant enamel and
their low open porosity, feldspathic earthenwares are particularly suitable for
applications in the tiling field.
4.5.4. Stonewares
4.5.4.1. General characteristics of stonewares
Stonewares have a vitrified, opaque, colored and practically impermeable shard
(0 to 3% open porosity). They are obtained from a mixture of vitrifying plastic clays
and flux, sometimes supplemented by sand or grog. They are formed by extrusion
(pipes, bricks, etc.) or by granulated powder pressing (tiles, slabs, etc.). The firing
temperature generally ranges between 1,120 and 1,300°C and it forms a critical
parameter. In fact, sintering at an insufficient temperature (non-firing) results in the
persistence of a significant open porosity and a treatment at too high a temperature
leads to the deformation of the pieces because of the excessively large quantity and
the low viscosity of liquid formed. If usage requires it, stonewares can be enameled.
A salt glaze during firing can also be carried out (traditional salt-glazed stonewares).
Stonewares are known for their unchangeability, excellent mechanical
performances and resistance to erosion and chemical agents.
Silicate Ceramics 111
4.5.4.2. Natural stonewares
Natural stonewares are obtained from natural vitrifying clays, i.e. capable of
forming a significant quantity of liquid at high temperature. They are used just as
they are or are modified only by adding a kaolinitic refractory clay. Fe2O3 can
represent up to 3% of the mass of the composition of these raw materials.
Irrespective of the firing atmosphere, mullite occurs in an acicular form between
1,000 and 1,100°C and continues to be formed up to 1,200°C. During this treatment,
the viscous liquid dissolves the finest quartz grains. The solidification of this liquid
on cooling leads to the formation of a significant quantity of vitreous phase. In an
oxidizing atmosphere, the color of the shard can vary from ivory to dark brown. This
coloring depends, in this case again, on the iron content and the nature of the other
impurities present in the clays. Thus, titanium dioxide tends to color the shard of
natural stonewares light yellow, whereas manganese oxide favors the development
of darker colors. To avoid the appearance of blisters due to the presence of sulphates
in the clays, the firing of natural stonewares must often be carried out in a reducing
atmosphere. The Fe3+ ions are then reduced above 570°C. The ferrous oxide formed
confers on the shard a grayish color and acts as a very active flux. As this action
compounds that of the alkaline derivatives, a high iron content can lead to a marked
softening and the deformation of the pieces during firing.
The production of natural stonewares is primarily traditional, insofar as the clays
necessary for the manufacture of this type of product are seldom available in large
quantities and their firing is often difficult.
4.5.4.3. Compound or fine-grained stonewares
Fine-grained stonewares are different from natural stonewares because the grains
of flux are no longer contained in the clay, but added in the form of feldspars. They
are obtained from clay very poor in coloring, kaolin, ball clay and a mixture of
orthoclase and albite. Colorings are sometimes added to the paste to develop a
particular color in the mass of the product. Fine-grained stonewares are used as
crockery, walls or floor tiles, antacid tiles and sanitary pipes.
During firing, the deshydroxylation of clays occurs from 450°C onwards. Shortly
before 1,000°C, orthoclase starts to react with silica and the liquid occurs. The
mullite formation begins between 1,000 and 1,100°C. In this temperature range,
certain micaceous phases contained in the clays can start to react with the products
of the decomposition of metakaolin. The interaction of albite with silica begins from
1,140°C. The maximum firing temperature ranges between 1,250 and 1,280°C, so
the amorphous silica derived from the kaolinite and undissolved in the liquid can be
transformed into cristobalite. The degree of crystallization of SiO2 in the end
112 Ceramic Materials
product depends on the thermal past of the stonewares, the nature of the flux and the
mineralizing impurities.
4.5.4.4. Porcelain stonewares
Porcelain stonewares are characterized by an open porosity of less than 0.5%.
This characteristic gives them remarkable mechanical properties and an excellent
resistance to frost and corrosive agents. These products have experienced a rapid
development as floor tiles. The world production rocketed from a few million m2 in
the 1980s to more than 150 million in 1997. This growth is linked to the use of new
processes of grinding (wet process), forming (more powerful presses), sintering (fast
mono-layer sintering roller-hearth kilns) and decoration (polishing, simultaneous
pressing of several layers of enamel powder). These new technologies have reduced
production costs and have considerably improved products’ esthetics.
Figure 4.6. Ranges of chemical composition of various types of stonewares tiles (mass %)
The paste used to manufacture stonewares tiles generally consists of a mixture of
plastic clays, kaolin, feldspathic sand, sodium or potassium feldspar and small
quantities of talc, dolomite and/or chlorite. The overall chemical composition of the
Silicate Ceramics 113
shard is generally less pure than that of fine-grained stonewares tiles, and it is also
richer in Al2O3 (see Figure 4.6).
Expressed in mass % of oxide, it generally corresponds to 66 to 69% of SiO2, 20 to
23% of Al2O3, 0.5 to 5% of MgO, 1.2 to 1.8% of CaO, 2.5 to 3.6% of Na2O, 1.7 to
2.8% of K2O, 0.7 to 1.3% of Fe2O3, 0.4 to 0.9% of TiO2 and 0 to 2% ZrO2 [DON 99].
The maximum temperature of the heat treatment generally ranges between 1,120
and 1,200°C. The evolution of the phases during this firing is very close to the one
described in the case of fine-grained stonewares. The quantity of mullite formed
represents only 50% of what is expected for this type of composition. The porcelain
stonewares shards are mainly made up of mullite, amorphous phase and quartz.
Their composition, after cooling, belongs to the field represented on Figure 4.7.
These shards are free from open porosity and exhibit between 7 and 13% closed
porosity [DON 99]. This microstructure confers on these materials a high Young’s
and rupture modulus (about 75 GPa and 85 MPa respectively) compatible with their
use as floor tiles. These moduli increase with the Al2O3 content, the quantity of
mullite formed and the compactness of the shard (reduction in closed porosity).
Figure 4.7. Representation of the composition range of porcelain stonewares shards
in the ternary mullite-quartz-vitreous phase diagram (mass %)
114 Ceramic Materials
4.5.4.5. Grogged stonewares
Grogged stonewares are obtained from a paste made up of vitrifying clay
sometimes rich in kaolinite or quartz, a small proportion of flux and a large quantity
of grog and/or ground shard (40 to 60% of the mass). It is generally shaped by
casting a slip in porous plaster moulds. The use of large-sized grog grains, up to
0.8 mm, increases the permeability of the rigid skeleton and the speed of drying. It
reduces the capillary forces responsible for the formation of cracks and then slits.
The dry products are engobed and then enameled before firing. The engobe and
the enamel are deposited successively by dipping or pulverization. The operation
must be carried out in several layers in order to obtain a sufficient thickness and
avoid excessively rewetting the dry part. After a final drying, it is fired in single
thermal cycle (process known as single firing). The rigid lattice made up of grog
grains does not allow sufficient shrinkage to eliminate all porosity during sintering.
After a treatment between 1,250 and 1,280°C, open porosity remains considerable
(8 to 15%). The presence of an opaque engobe masks the coloring of the shard,
levels out the surface imperfections and facilitates the fixing of the enamel.
Grogged stonewares, easier to dry than most other products, are particularly
suited for the manufacture of bulky and robust products. They are widely used in
sanitary plumbing (sinks, shower basins, etc.).
4.5.5. Porcelains
4.5.5.1. General characteristics
Thanks to the purity of the raw materials used, porcelain shards are white and
translucent beneath the low thickness. They do not have open porosity (< 0.5%), but
are likely to exhibit some large closed pores (air holes). Their fracture are brilliant
and have a vitreous appearance. After enameling, the surface of the pieces is
remarkably smooth and brilliant.
When porcelain is fired, a liquid phase surrounds the solid grains and dissolves
the finest of them (< 15 μm). During this stage, known as “pasty fusion”, the
viscosity is sufficiently high for the deformation of the pieces to remain within
acceptable limits. The solidification of the liquid on cooling leads to the formation
of a large quantity of vitreous phase.
The manufacturing processes are changing constantly (see section 5.6.2). Thus,
when the geometry of the parts allows it, pressure casting and shaping by isostatic
pressing gradually replace jiggering and casting in plaster molds. Fast firing
techniques are increasingly used for enamel and decorations. They improve the
Silicate Ceramics 115
quality of the parts by limiting the risks of deformations [SLA 96]. A tendency has
also been observed to decrease the sintering temperature [LEP 98].
4.5.5.2. Hard-paste porcelains
Hard-paste porcelains are obtained from mixtures made up almost exclusively of
kaolin, quartz and feldspars. A little chalk (≈ 2% of the mass) can be added to favor
the formation of the viscous liquid. This mixture is very similar to the one used to
prepare fine earthenware. It differs from it only because of the almost exclusive use
of kaolin as clay and the proportions of the various components.
Sintering is carried out at a temperature ranging between 1,350 and 1,430°C. The
use of a reducing atmosphere, by favoring the reduction of the Fe3+ ions to Fe2+,
guarantees being able to obtain a white shard (possibility of bluish reflections) [SLA
96]. In a twice-firing process, this treatment is preceded by a bisque firing, carried
out in oxidizing atmosphere at a temperature ranging between 900 and 1,050°C. The
product then has a sufficient rigidity and the high open porosity necessary for its
enameling. The development of passage kilns with several heating zones, each with
its own atmosphere, has made it possible to prepare certain types of hard-paste
porcelains in a single firing. To prevent carbon monoxide, present in the reducing
atmosphere, from depositing carbon (Boudouard equilibrium) in the still porous
paste, the heating zone ensuring the rise in temperature up to about 1,050°C is
traversed by an oxidizing atmosphere. The reducing atmosphere therefore circulates
only in the sintering zone.
Because of the high compactness of the shard after firing, enameling is done at
the same time as sintering. The enamel is then a glaze with a feldspathic and calcium
composition. The high firing temperature and the reducing atmosphere diminish the
pallet of possible colors during this treatment to green, blue and brown only. Most
decorations are therefore painted or deposited on enamel by decalcomania and then
fired between 800 and 900–950°C.
Hard-paste porcelains are particularly used in the field of crockery (Limoges
porcelain) and as technical ceramics (insulators).
4.5.5.3. Soft-paste porcelains
Soft-paste porcelains differ from the above porcelains by their greater
translucidity and lower sintering temperature. Chinese porcelains and porcelains for
dental implants belong to this category. English porcelains, known as bone china,
constitute a particular class.
116 Ceramic Materials
Fine bone china
These are the most expensive porcelains on the market, particularly appreciated for
their esthetics. They owe their name to the bone ash added to the mixture of raw
materials. The composition of a paste is typically: 37 to 50 % of the mass of bone ash,
22 to 32% potassium feldspar, 22 to 41% kaolin and 0 to 4% quartz. Hydroxyapatite,
Ca5(PO4)3OH, present in the bone ash contributes to the formation of a low viscosity
liquid, whose quantity increases very rapidly when the solidus temperature is reached.
Above 1,200°C, this liquid dissolves the free quartz gradually. At the maximum firing
temperature of the biscuit, between 1,250 and 1,280°C, the material is made up of a
paste that contains only calcium phosphate, Ca3(PO4)2 and liquid. Sintering shrinkage,
highly dependent on the quantity and viscosity of liquid formed, is much more
sensitive to the heat treatment conditions than in the case of hard-paste porcelains and
vitreous china (see Figure 4.8). Mastery over the dimensions and deformation of the
pieces requires the temperature conditions and sintering time to be strictly controlled.
Deformation can be controlled by placing the raw products in a bed of alumina powder
[SLA 93]. As anorthite is formed on cooling, the shard of a bone china porcelain is
primarily made up of calcium phosphate (35 to 45% of the mass), vitreous phase (27
to 30%) and anorthite (25 to 30%).
The fixing of enamel presents difficulties inherent to the absence of porosity of
the shard. Its firing is carried out at high temperature, ranging between 1,120 and
1,160°C. In order to avoid the appearance of efflorescence due to the decomposition
of the enamel, this second firing must be done in a strictly oxidizing atmosphere.
The brilliance of the enamel obtained is highly dependent on the lead oxide content.
Figure 4.8. Temperature influence on the shrinkage speed of a
hard-paste porcelain, a bone china and a vitreous china
Silicate Ceramics 117
4.5.5.4. Aluminous porcelains
The composition of the paste of aluminous porcelains can contain up to 50% of
Al2O3 mass, about half of which can be introduced in the form of calcined alumina
or corundum. The fluxes used are based on alkaline-earth oxides (CaO, MgO and/or
BaO); lithium ions are sometimes added in the form of spodumene
(Li2O,Al2O3,4SiO2). Sintering and enameling are carried out in single firing at a
temperature ranging between 1,280 and 1,320°C.
There are also products called extra-aluminous porcelains whose Al2O3 content
can range between 50 and 95% of the mass. In order to facilitate their forming, 3 to
5% of very pure plastic clays and/or organic binders are added to the paste. When
enameling is necessary, the raw parts are generally covered by dipping in the enamel
suspension. After sintering at a temperature between 1,430 and 1,600°C the shard
exhibits very little vitreous phase.
These porcelains are used for their electrical characteristics (insulators,
disconnecting switches, spark plugs, dielectrics for high voltage) and their hardness
(cutting tools, wire guides, grinding ball). These are technical ceramics whose
dimensions can be subject to considerable constraints because of low tolerances
(machining after firing) or the large size of the piece. Consequently, the conditions
imposed for producing the 4 m height and 1 m diameter aluminous porcelain
insulators used for very high voltage electricity transmission have had to be
mastered.
4.5.6. Vitreous china
The term “vitreous china” denotes dense products obtained from pastes close to
those used to manufacture feldspathic earthenwares. The feldspar content of these
pastes is increased in order to produce, during the firing, a sufficient quantity of
liquid to eliminate open porosity (< 0.5%). Used more particularly to manufacture
sanitary articles and very robust crockery (wash basin, crockery for communities),
vitreous materials are in the middle between white paste stonewares and porcelains.
These products are formed by jiggering, casting or isostatic pressing. A good
mastery of the raw materials and shaping process makes it possible to obtain raw
pieces with a mechanical strength sufficient to withstand the application of an
enamel paste. Sanitary products are generally vitrified and enameled in a single
treatment, carried out in oxidizing atmosphere at a temperature between 1,200 and
1,280°C. A twice-firing treatment is usually used for crockery. The first firing is
thus carried out between 900 and 950°C. The elimination of open porosity and the
formation of the enamel occur du
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