Whitewares

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 Contents
 Introduction
 Classification
 Properties
 Physical properties of whitewares
 Raw materials
 Whiteware Processing
 Process Description
 Firing: What Happens to Whiteware in a Firing Kiln
 Glazes for whitewares
 Glaze Application

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 Introduction
 Any of a broad class of ceramic products that are white to off-white in appearance and
frequently contain a significant vitreous, or glassy, component. Including products as diverse
as fine china dinnerware, lavatory sinks and toilets, dental implants, and spark-plug insulators.
 Technologists apply the name “whitewares” to most fine ceramic products, and the term
simply reflects the fact that most of the products are white (or near white) in color where
“ware” is the technical name for a ceramic object that is sold in commerce.
 Whitewares are often referred to as triaxial bodies, owing to the three mineral types—clay,
silica, and feldspar—consistently found in their makeup. Traditional raw material mixture for
whiteware (porcelain) production covers kaolin or/and kaolin clay, quartz and feldspar rock
at a composition about 50:25:25 wt.%.
 Clay (tissue) is the plastic component, giving shaping abilities to the unfired product and also
serving as a glass former during firing.
 Flint (Bone) (the common name used in the industry for all forms of silica) serves as a filler,
lending strength to the shaped body before and during firing.
 Feldspar (Blood) serves as a fluxing agent, lowering the melting temperatures of the mixture.
 Clay is the most important of the ingredients, and the most important clay used in fine
whiteware products is kaolin, also known as china clay. Kaolin is the only type of clay from
which a white, translucent, vitreous ceramic can be made. It is a refractory clay, meaning that
it can be fired at high temperatures without deforming, and it is white-burning, meaning that
it imparts whiteness to the finished ware.
 Classification
 Whiteware products are often differentiated into three main classes—porous,
semivitreous, and vitreous—according to their degree of vitrification (and
resulting porosity). Proceeding from porous to vitreous, more particular product
categories include earthenware, stoneware, chinaware, porcelain, and technical
ceramics.
 Earthenware is defined as glazed or unglazed non-vitreous (porous) clay-based
ceramic ware. Applications for earthenware include artware, kitchenware,
ovenware, tableware, and tile. Earthenware is non-vitreous and of medium
porosity. It is often glazed to provide fluid impermeability and an attractive finish.
Specific products include tableware and decorative tile ware.
 Stoneware is vitreous or semivitreous ceramic ware of fine texture, made primarily
from nonrefractory fire clay or some combination of clays, fluxes, and silica that,
when fired, has properties similar to stoneware made from fire clay. Applications
for stoneware include artware, chemicalware, cookware, drainpipe, kitchenware,
tableware, and tile.

Vitrification (from Latin vitreum, "glass" via French

vitrifier) is the transformation of a substance into a
glass, that is to say, a non-crystalline amorphous solid
 Classification
 Porcelain is defined as glazed or unglazed vitreous ceramic ware used primarily for technical
purposes. Applications for porcelain include artware, ball mill balls, ball mill liners,
chemicalware, insulators, and tableware. Electrical porcelains are widely used as insulators in
electrical power transmission systems due to the high stability of their electrical, mechanical
and thermal properties in the presence of harsh environments. They are primarily
composed of clay, feldspar and a filler material, usually quartz or alumina. Typical
properties of porcelain body are low porosity (below 0.3%), high mechanical strength
(bending strength over 40 MPa, Young Modulus over 60 GPa), firing temperature about
1300°C and high whiteness and translucency.
 Technical porcelains, like china, are vitreous and nonporous. They are similarly strong and
impact-resistant, but they are also chemically inert in corrosive environments and are
excellent insulators against electricity.
 Technical ceramics include vitreous ceramic whiteware used for such products as electrical
insulation, or for chemical, mechanical, structural, or thermal applications. Applications
include chemical ware, dental implants, and electric insulators, including spark-plug
insulators in automobile engines.
 Classification
 Chinaware is vitreous ceramic ware of zero or low absorption after firing that are used
for nontechnical applications. Applications for chinaware include artware, ovenware,
sanitaryware, and tableware.
 China is vitreous whiteware for nontechnical applications. Because of its high glass
content, it can be used unglazed, though it also can be glazed for aesthetic appeal. China
is known for high strength and impact resistance and also for low water absorption—all
deriving from the high glass content. Typical products include hotel china, a lower grade
of china tableware with a strength and impact resistance suiting it to commercial use; fine
china (including bone china), a highly vitreous, translucent tableware; and sanitary
plumbing fixtures.
Classification
 Properties
 Whitewares all depend for their utility upon a relatively small set of properties:
 imperviousness to fluids,
 low conductivity of electricity,
 chemical inertness, and
 an ability to be formed into complex shapes.
These properties are determined by the mixture of raw materials chosen for the
products, as well as by the forming and firing processes employed in their
manufacture.
 Normal triaxial whiteware bodies consist essentially of the following microscopic
phases as described by (Hoffman, 1990):
 A glassy groundmass intimately associated with minute mullite crystals and
small included voids.
 Quartz grains which may show rounding and which may be bounded by clear
solution rims.
 Glassy zones bearing the pseudomorphic outlines of feldspar grains (slender
needles of mullite may appear in this glass).
 Fissures from the shrinkage of clay away from the non–plastics (these round out
and decrease in size at the maturing temperatures).
 Spherical blebs originating at higher temperatures in feldspar glass grains. These
blebs grow in size in the over firing range
 Properties
 The general relationship in whitewares is that low porosities are associated with high
mechanical strength, high resistance to chippage (or breakage), and excellent durability.
Adjusting the composition or mineral constitution and increasing the amount or degree of
vitrification in firing achieves low porosity in whitewares. With respect to composition,
mixtures of clay, flint (ground silica), and feldspar minerals have been found to achieve the
highest strengths of clay based-whitewares, and these products are called “triaxial
whitewares: due to the three components in their composition. Further improvements in
strength are noted when aluminum oxide (alumina) and/or other minerals, such as zircon,
are added to the composition.
 Vitrification is achieved in kilns as the ceramic composition melts as temperatures achieve
“red heat”, i.e. rise above about 950oC. As temperature is further increases, more melted
phase is formed, and this glass is “distributed” by capillary forces throughout the ceramic.
As temperatures generally increase above 1000oC, a new mineral named mullite appears.
Simultaneously, the ceramic ware exhibits shrinkage effectively increasing density and
reducing porosity. After cooling in the kiln, the fired product has mechanical properties
reflecting the degree of vitrification and densification achieved in the firing process. The
degree of vitrification is the percentage of melting of the original constituents that has
occurred in manufacturing. In whiteware ceramics, the amount of melting may be as high as
40-60% while in glass products the extent of melted phase approaches 100%. In
manufacturing of whitewares, measurements of firing shrinkage and porosity (water
absorption) reflect the quality of the fired product.
 Properties
 Whiteware products are typically glazed in order to increase their utility and improve their
appearance. A glaze is a glass coating over the ceramic surface formed by melting or fusing a
coating that has been applied to all or part of the ceramic ware’s surface. Glazes provide a
smooth surface that can easily be cleaned. Glazes also prevent water to be absorbed by the
product thereby promoting sanitation and/or improving performance. Glazes also can be
colored using pigmenting oxides providing enhanced appearance.
 One advantage of glazing over a white surface is that is it relatively easy to achieve light
colors without “bleed through” of color from the substrate. This is a distinct advantage of
whiteware products as compared to red clay (red body) ceramics. With red clay ceramics, it is
frequently necessary to apply double layers of glaze (a white under glaze layer topped by a
colored outer layer) to achieve a desired light color.
 Physical properties of whitewares

Stain resistance is defined as the ability to resist

the contamination from atmospheric dust, and it
is an important performance of exterior coatings.
Physical properties of whitewares
 Raw Materials
 Raw materials: clay, flint, and feldspar. These raw materials or minerals can generally be
broken down into three categories, which are clay, flux and “fillers” (or non-plastics).
 The clay is the component that provides plasticity or cohesion for forming, while the flux
promotes vitrification. The filler is typically inert, but it also may serve to modify the glass
viscosity during vitrification, which effectively extends the range of firing temperatures of
the composition.
 Kaolin is formed principally of the mineral kaolinite, a hydrous aluminosilicate with a fine,
platy structure; its ideal chemical formula is Al2(Si2O5)(OH)4.
 China clays are composed mostly of well-ordered kaolinite, with no impurities. Lower-grade
whitewares are usually made of ball clays, which incorporate ordered and disordered
kaolinite plus other clay minerals and impurities. These impurities—particularly iron
oxides—render the fired ware off-white to gray or tan in colour.
 Clay is a hydrous sheet silicate that is a product of the weathering of feldspathic parent
rock. Clays are defined by their plasticity when mixed with water, and they exhibit an
extremely small particle size in an unagglomerated state.
 When mixed with water, clay becomes a plastic substance that is formable and moldable,
When heated to a sufficiently elevated temperature (firing ), clay fuses into a dense, strong
material, Thus, clay can be shaped while wet and soft, and then fired to obtain the final hard
product.
 Raw Materials
 There are three main groups of clay minerals of kaolin, illite and montmorillonite. Kaolin
clays are the most common group and they have the approximate composition of
Al2(Si2O5)(OH)4. White colored clays of this composition are simply called “kaolins”.
Kaolin clay is also typically the major clay mineral in “ball clay” – a highly plastic and
cohesive form of kaolin.
 Fluxes promote the formation of a glassy bond during vitrification. Fluxes provide alkaline
(Na2O and K2O) or alkaline earth (CaO or MgO) to the composition, which promote glass
formation and reduce glass viscosity during firing which serves to enhance vitrification. The
level of fluxing components must be optimized to achieve the desired fired property in the
selected firing range. There are several types of feldspar that are used commercially. These
include soda feldspar (albite, NaAlSi3O8), potash feldspar (microcline or orthoclase,
KAlSi3O8) and lime feldspar (anorthite, CaAl2Si2O8). Feldspar typically does not exist as
pure albite or pure anorthite, but it is founds as mixtures of these major feldspar types.
 Raw Materials
 Feldspar rocks are used in the fine ceramic industry as a fluxing agent to form a glassy
phase for accelerating of sintering process. Feldspar rocks are a mixture of pure feldspars,
quartz and mica especially from the mineralogical point of view. Pure feldspars are divided
into potassium feldspars (orthoclase, microcline), sodium feldspars (albite) and calcium
feldspars (anorthite). Solid solutions between K‐feldspar and albite are called alkali
feldspars, and solid solutions between albite and anorthite are plagioclase feldspars. The
plagioclase series follows according to percentage of anorthite in parentheses. Feldspar
rocks are usually used as a source of alkali oxides (Na2O, K2O) and alumina (Al2O3)for the
preparation of glazes. Suitable choice of feldspar rock can significantly affect the properties
of the ceramic body , firing temperature and soaking time. The densification of green body,
cleanability and the stain resistance of polished sintered ceramic tiles is influenced by
particle size distribution of used feldspar rocks. Feldspar rocks may be successfully replaced
by LCD waste glass. Wollastonite is very suitable material for acceleration of sintering
process in porcelain body. Only 1 wt.% addition of wollastonite is able to decrease firing
temperature (about 25°C) in the mixture with kaolin, quartz and potassium feldspar rock.
 Raw Materials
 The non-plastic or filler portion of the whiteware composition usually consists of flint
(ground quartz), but it may include alumina (Al2O3), pyrophillite Al2Si4O10(OH)2 or sericite
depending on desired function of the filler. Ground quartz used in whiteware compositions
is frequently called “potter’s flint. In the case of quartz, the silica particles are partially
melted at high temperatures (above red heat) and enter the glassy phase formed by the
fluxing components. One result is that the viscosity of the glassy phase is increased
allowing for a relatively “wide” firing range characteristic of porcelain compositions. The
residual or “unmelted” quartz serves as a “skeleton” preventing the product from slumping
under its own weight at elevated temperatures.
 Available naturally in various forms, most important is quartz The main source of quartz
is sandstone •Low in cost; also hard and chemically stable •Principal component in glass,
and an important ingredient in other ceramic products including whiteware, refractories,
and abrasives.
 Whiteware Processing

 Typically, pressing is employed in the forming of tiles, chemical ware, and

technical porcelains; extrusion in the forming of tiles and sanitary ware
(including pipe); and slip casting in the forming of plumbing fixtures and some
tableware. In addition to these standard processes, jiggering is employed in the
manufacture of tableware.
 Jiggering involves the mixing of a plastic mass and turning it on a wheel beneath
a template to a specified size and shape.
 Most whitewares are fired in continuous tunnel kilns. The porous varieties are
fired at lower temperatures (1,100–1,250 °C, or approximately 2,000–2,300 °F),
whereas china and true porcelains are fired at 1,250 to 1,300 °C (2,300 to 2,400
°F).
 Porous and semivitreous whitewares may be glazed in a second firing to produce
an impermeable glass coating for decorative or functional purposes.
 Whiteware Processing
 As an example, in a typical feldspar-clay-silica composition for porcelain, a
whiteware with a particularly high glassy component, small grains of feldspar
would begin to form liquid at temperatures as low as 990 °C (1,810 °F), and
large feldspar grains would be molten by 1,140 °C (2,080 °F). Because of the
high viscosity of the liquid formed, there would be no change in the shape of
the ceramic piece until approximately 1,200 °C (2,200 °F).
 Above this temperature the feldspar grains would react with surrounding clay
particles to form glass, and “needles” of mullite (a crystalline aluminosilicate
mineral formed during the firing of clay-silica mixtures) would grow into the
liquid regions. In addition, the surfaces of silica particles would begin to
dissolve and form solution rims, or envelopes of glass surrounding the
crystalline particle.
 Process Description
 The basic steps include raw material procurement, beneficiation, mixing,
forming, green machining, drying, pre-sinter thermal processing, glazing,
firing, final processing, and packaging.
 Raw Material Procurement - The raw materials used in the manufacture of
ceramics range from relatively impure clay materials mined from natural deposits
to ultrahigh purity powders prepared by chemical synthesis. Naturally occurring
raw materials used to manufacture ceramics include silica, sand, quartz, flint,
silicates, and aluminosilicates (e. g., clays and feldspar).
 Beneficiation - The next step in the process is beneficiation. Although
chemically synthesized ceramic powders also require some beneficiation, the focus
of this discussion is on the processes for beneficiating naturally occurring raw
materials. The basic beneficiation processes include comminution, purification,
sizing, classification, calcining, liquid dispersion, and granulation. Naturally
occurring raw materials often undergo some beneficiation at the mining site or at
an intermediate processing facility prior to being transported to the ceramic
manufacturing facility.
 Process Description
 Comminution entails reducing the particle size of the raw material by crushing, grinding, and
milling or fine grinding. The purpose of comminution is to liberate impurities, break up
aggregates, modify particle morphology and size distribution, facilitate mixing and forming, and
produce a more reactive material for firing. Primary crushing generally reduces material up to 0.3
meter (m) (1 foot [ft]) in diameter down to 1 centimeter (cm) (0.40 inch [in.]) in diameter.
Secondary crushing reduces particle size down to approximately 1 millimeter (mm) (0.04 in.) in
diameter. Fine grinding or milling reduces the particle size down to as low as 1.0 micrometer
(µm) (4 x 10-5 in.) in diameter. Ball mills are the most commonly used piece of equipment for
milling. However, vibratory mills, attrition mills, and fluid energy mills also are used. Crushing
and grinding typically are dry processes; milling may be a wet or dry process. In wet milling,
water or alcohol commonly is used as the milling liquid.
 Several procedures are used to purify the ceramic material. Water soluble impurities can be
removed by washing with deionized or distilled water and filtering, and organic solvents may be
used for removing water-insoluble impurities. Acid leaching sometimes is employed to remove
metal contaminants. Magnetic separation is used to extract magnetic impurities from either dry
powders or wet slurries. Froth flotation also is used to separate undesirable materials.
 Sizing and classification separate the material into size ranges. Sizing is most often accomplished
using fixed or vibrating screens. Dry screening can be used to sizes down to 44 µm (0.0017 in.,
325 mesh). Dry forced-air sieving and sonic sizing can be used to size dry powders down to 37
µm (0.0015 in., 400 mesh), and wet sieving can be used for particles down to 25 µm (0.00098 in.,
500 mesh). Air classifiers generally are effective in the range of 420 µm to 37 µm (0.017 to
0.0015 in., 40 to 400 mesh). However, special air classifiers are available for isolating particles
down to 10 µm (0.00039 in.).
 Process Description
 Calcining consists of heating a ceramic material to a temperature well below its melting point to
liberate undesirable gases or other material and to bring about structural transformation to produce the
desired composition and phase product. Calcining typically is carried out in rotary calciners, heated
fluidized beds, or by heating a static bed of ceramic powder in a refractory crucible.
 Liquid dispersion of ceramic powders sometimes is used to make slurries. Slurry processing facilitates
mixing and minimizes particle agglomeration. The primary disadvantage of slurry processing is that
the liquid must be removed prior to firing the ceramic.
 Dry powders often are granulated to improve flow, handling, packing, and compaction. Granulation is
accomplished by direct mixing, which consists of introducing a binder solution during powder mixing,
or by spray drying. Spray dryers generally are gas-fired and operate at temperatures of 110° to 130°C
(230° to 270°F).
 Mixing - The purpose of mixing or blunging is to combine the constituents of a ceramic powder to
produce a more chemically and physically homogenous material for forming. Pug mills often are used
for mixing ceramic materials. Several processing aids may be added to the ceramic mix during the
mixing stage. Binders and plasticizers are used in dry powder and plastic forming; in slurry processing,
deflocculants, surfactants, and antifoaming agents are added to improve processing. Liquids also are
added in plastic and slurry processing.
 Binders are polymers or colloids that are used to impart strength to green or unfired ceramic bodies.
For dry forming and extrusion, binders amount to 3 percent by weight of the ceramic mixture.
Plasticizers and lubricants are used with some types of binders. Plasticizers increase the flexibility of
the ceramic mix. Lubricants lower frictional forces between particles and reduce wear on equipment.
Water is the most commonly used liquid in plastic and slurry processing. Organic liquids such as
alcohols may also be used in some cases. Deflocculants also are used in slurry processing to improve
dispersion and dispersion stability. Surfactants are used in slurry processing to aid dispersion, and
antifoams are used to remove trapped gas bubbles from the slurry.
 Process Description
 Forming - In the forming step, dry powders, plastic bodies, pastes, or slurries are consolidated and
molded to produce a cohesive body of the desired shape and size.
 Dry forming consists of the simultaneous compacting and shaping of dry ceramic powders in a rigid
die or flexible mold. Dry forming can be accomplished by dry pressing, isostatic compaction, and
vibratory compaction.
 Plastic molding is accomplished by extrusion, jiggering, or powder injection molding. Extrusion is used
in manufacturing structural clay products and some refractory products. Jiggering is widely used in the
manufacture of small, simple, axially symmetrical whiteware ceramic such as cookware, fine china, and
electrical porcelain. Powder injection molding is used for making small complex shapes.
 Paste forming consists of applying a thick film of ceramic paste on a substrate. Ceramic pastes are
used for decorating ceramic tableware, and forming capacitors and dielectric layers on rigid substrates
for microelectronics.
 Slurry forming of ceramics generally is accomplished using slip casting, gelcasting, or tape casting. In
slip casting, a ceramic slurry, which has a moisture content of 20 to 35 percent, is poured into a porous
mold. Capillary suction of the mold draws the liquid from the mold, thereby consolidating the cast
ceramic material. After a fixed time the excess slurry is drained, and the cast is dried. Slip casting is
widely used in the manufacture of sinks and other sanitaryware, figurines, porous thermal insulation,
fine china, and structural ceramics with complex shapes. Gelcasting uses in situ polymerization of
organic monomers to produce a gel that binds ceramic particles together into complex shapes such as
turbine rotors. Tape casting consists of forming a thin film of ceramic slurry of controlled thickness
onto a support surface using a knife edge. Tape casting is used to produce thin ceramic sheets or tape,
which can be cut and stacked to form multilayer ceramics for capacitors and dielectric insulator
substrates.
 Process Description
 Green Machining - After forming, the ceramic shape often is machined to eliminate rough surfaces
and seams or to modify the shape. The methods used to machine green ceramics include surface
grinding to smooth surfaces, blanking and punching to cut the shape and create holes or cavities, and
laminating for multilayer ceramics.
 Drying - After forming, ceramics must be dried. Drying must be carefully controlled to strike a
balance between minimizing drying time and avoiding differential shrinkage, warping, and distortion.
The most commonly used method of drying ceramics is by convection, in which heated air is
circulated around the ceramics. Air drying often is performed in tunnel kilns, which typically use heat
recovered from the cooling zone of the kiln. Periodic kilns or dryers operating in batch mode also are
used. Convection drying also is carried out in divided tunnel dryers, which include separate sections
with independent temperature and humidity controls. An alternative to air drying is radiation drying in
which microwave or infrared radiation is used to enhance drying.
 Presinter Thermal Processing - Prior to firing, ceramics often are heat-treated at temperatures well
below firing temperatures. The purpose of this thermal processing is to provide additional drying, to
vaporize or decompose organic additives and other impurities, and to remove residual, crystalline, and
chemically bound water. Presinter thermal processing can be applied as a separate step, which is
referred to as bisque firing, or by gradually raising and holding the temperature in several stages.
 Glazing - For traditional ceramics, glaze coatings often are applied to dried or bisque-fired ceramic
ware prior to sintering. Glazes consist primarily of oxides and can be classified as raw glazes or frit
glazes. In raw glazes, the oxides are in the form of minerals or compounds that melt readily and act as
solvents for the other ingredients. Some of the more commonly used raw materials for glazes are
quartz, feldspars, carbonates, borates, and zircon. A frit is a prereacted glass. To prepare glazes, the raw
materials are ground in a ball mill or attrition mill. Glazes generally are applied by spraying or dipping.
Depending on their constituents, glazes mature at temperatures of 600° to 1500°C (1110° to 2730°F).
 Process Description
 Firing - Firing is the process by which ceramics are thermally consolidated into a dense, cohesive body
comprised of fine, uniform grains. This process also is referred to as sintering or densification. In
general: (1) ceramics with fine particle size fire quickly and require lower firing
temperatures; (2) dense unfired ceramics fire quickly and remain dense after firing with
lower shrinkage; and (3) irregular shaped ceramics fire quickly. Other material properties
that affect firing include material surface energy, diffusion coefficients, fluid viscosity, and
bond strength.
 Parameters that affect firing include firing temperature, time, pressure, and atmosphere. A
short firing time results in a product that is porous and has a low density; a short to
intermediate firing time results in fine-grained (i. e., having particles not larger than 0.2
millimeters), high-strength products; and long firing times result in a coarse-grained
products that are more creep resistant. Applying pressure decreases firing time and makes it
possible to fire materials that are difficult to fire using conventional methods. Oxidizing or
inert atmospheres are used to fire oxide ceramics to avoid reducing transition metals and
degrading the finish of the product.
 In addition to conventional firing, other methods used include pressure firing, hot forging,
plasma firing, microwave firing, and infrared firing.
 Conventional firing is accomplished by heating the green ceramic to approximately two-
thirds of the melting point of the material at ambient pressure and holding it for a
specified time in a periodic or tunnel kiln. Periodic kilns are heated and cooled according to
prescribed schedules. The heat for periodic kilns generally is provided by electrical element
or by firing with gas or oil.
 Process Description
 Tunnel kilns generally have separate zones for cooling, firing, and preheating or drying. The kilns may
be designed so that (1) the air heated in the cooling zone moves into the firing zone and the
combustion gases in the firing zone are conveyed to the preheat/drying zone then exhausted, or (2) the
air heated in the cooling zone is conveyed to the preheat/drying zone and the firing zone gases are
exhausted separately. The most commonly used tunnel kiln design is the roller hearth (roller) kiln. In
conventional firing, tunnel kilns generally are fired with gas, oil, coal, or wood. Following firing and
cooling, ceramics are sometimes refired after the application of decals, paint, or ink.
 Advanced ceramics often are fired in electric resistance-heated furnaces with controlled atmospheres.
For some products, separate furnaces may be needed to eliminate organic lubricants and binders prior
to firing.
 Ceramic products also are manufactured by pressure firing, which is similar to the forming process of
dry pressing except that the pressing is conducted at the firing temperature. Because of its higher costs,
pressure firing is usually reserved for manufacturing ceramics that are difficult to fire to high density by
conventional firing.
 Final Processing - Following firing, some ceramic products are processed further to enhance their
characteristics or to meet dimensional tolerances. Ceramics can be machined by abrasive grinding,
chemical polishing, electrical discharge machining, or laser machining. Annealing at high
temperature, followed by gradual cooling can relieve internal stresses within the ceramic and
surface stresses due to machining. In addition, surface coatings are applied to many fired
ceramics. Surface coatings are applied to traditional clay ceramics to create a stronger,
impermeable surface and for decoration. Coatings also may be applied to improve strength, and
resistance to abrasion and corrosion. Coatings can be applied dry, as slurries, by spraying, or by vapor
deposition
 Firing: What Happens to Whiteware in a Firing Kiln
 Crystal bound water has to escape during bisque or single fire. At earlier stages,
mechanically bound pore water, that is water between clay and mineral particles, is expelled.
However, H2O is bound right into the clay crystal itself, as well as into other minerals that
may be in the clay body.
 Quartz is a crystalline form of silica in that it has a three dimensional regular pattern of
molecular units. Quartz is made of a network of triangular pyramid (tetrahedron) shaped
molecules of silicon combined with four oxygens.

 A change to another phase is called a “silica conversion”. The most significant phases are
quartz, tridymite, crystobalite, and glass.
 Changes which occur between these are reversible, that is, the change which occurs during
heat-up is inverted during cool down (they are thus called “quartz inversions”). These
inversions, unfortunately, often have associated, rather sudden, volume changes.
 Firing: What Happens to Whiteware in a Firing Kiln
 Two inversions are important because of their sudden occurrence and extent of volume
change involved. The first is simply called ‘quartz inversion’ and it occurs quite quickly in the
570°C range. In this case, the crystal lattice straightens itself out slightly, thus expanding 1%
or so. The second is crystobalite inversion at 226°C. This is a little more nasty because it
generates a sudden change of 2.5% in volume and it occurs at a temperature within the range
of a normal oven.
 Burnout : Almost all bodies contain some organic matter that must decompose and then
burn at some point to produce carbon gases (the dark color of ball clays, for example, is due
to their coal content).
 1) The first and most important of these reactions is the dehydroxylation. During
dehydroxylation, the crystalline water is evolved from the clay mineral above 500°C.
2) After dehydroxylation, the clay mineral is in a disordered form existing a residual
material called “metakaolin”.
3) A second reaction, around 1000°C, and it is associated with mullite formation, as
characterized by a large exothermic peak. In this regime, a portion of the clay mineral
reorganizes to form mullite (3Al2O3.2SiO2).
 Firing: What Happens to Whiteware in a Firing Kiln
 Clay goes through several physical changes when fired. The first step is the evaporation of
water from between the clay particles. This part of the firing, up to 212°F (100°C) is called
water smoking, which causes the water in the pores between the clay crystals to evaporate.
 The next stage is to drive off the water which is chemically combined with the clay (remember
the formula for clay is Al2O3•2SiO2•2H2O). This is known as dehydroxylation and occurs up
to 1022°F (550°C). Once this chemical change has taken place, the process cannot be reversed
and the clay cannot be returned to its plastic state.
 At 1063°F (573°C), the crystalline quartz in the clay body increases in volume by 1%. This
may cause cracking if the temperature in the kiln is increased too rapidly.
 Organic matter in the clay is burned and oxidized to carbon dioxide, and fluorine and sulphur
dioxide from materials in the clay body are driven off at 1292–1652°F (700–900°C). At this
point the biscuit firing is completed. The clay particles are sintered or welded together. The
ware has undergone little shrinkage since the bone-dry stage, but it is durable enough to
withstand handling and glazing.
 Above 1652°F (900°C), the clay body begins to shrink and vitrify. The silica starts to melt,
filling the spaces between the clay particles and fusing them together. The fired clay is known
as metakaolin.
 At 1832°F (1000°C) the clay crystals begin to break down and melt. At 1922°F (1050°C),
needle shaped crystals of mullite 3Al2O3•2SiO2 begin to form, giving the fired clay strength
and hardness. When mullite forms from metakaolin Al2O3•2SiO2, extra free silica is released.
 Firing: What Happens to Whiteware in a Firing Kiln
 Above 2012°F (1100°C), any free silica (not chemically combined) in the clay changes to
cristobalite, which has a different structure from that of quartz.
 When the kiln is cooled down, cristobalite contracts suddenly by 3% at 439°F (226°C). This
can cause cracking if the kiln is cooled too rapidly by opening too soon, causing some areas to
drop in temperature and stressing the ware. It is advisable not to open it until it has cooled
down to below 212°F (100°C).
 Metastable sanidine formed from decomposition of the feldspar at about 600°C and dissolved
at about 900°C. Liquid formation at 1000°C was associated with melting of feldspar and silica
discarded from metakaolin formation via the K2O–Al2O3–SiO2 eutectic.
 Fine mullite and γ-alumina crystals precipitated in pure clay relicts and larger mullite crystals in
mixed clay-feldspar relicts at 1000°C.
 Typical final microstructures of fired porcelain bodies consist of 10%–25% mullite, with
composition ranging from 2Al2O3.SiO2 to 3Al2O3.2SiO2, 5 – 25% α-quartz (SiO2), and 0 –8%
pores dispersed in 65 – 80% potassium aluminosilicate glass. Bodies with a high percentage of
quartz also may contain cristobalite
Firing: What Happens to Whiteware in a Firing Kiln
Firing: What Happens to Whiteware in a Firing Kiln

Figure presents volume content of

the phases and porosity present in a
in a typical clayware at different firing
temperatures. Firing is one of the
most important processes in the
manufacture of ceramics because it
determines largely the physical
properties of the product. The
properties such as strength, porosity,
frost resistance, thermal expansion,
thermal conductivity and hardness
depend on the relative abundance
and texture of the various phases
resulting from the firing process.

Volume content of the phases and porosity

present in a in a typical clayware at different
firing temperatures.
 Glazes for whitewares
 Glazes are thin glass coatings applied to the whiteware surface.
 Glazes may simply serve as a decorative layer or they may serve a mechanical purpose such
as making the surface impervious so that it resists water penetration and staining. Glazes
may also serve to increase the electrical resistance or the mechanical strength of a
particular product.
 Glazes are generally applied as a suspension (slip) of ceramic particles in an aqueous
matrix, but dry applications are also available for special applications. The glaze should also
have a similar thermal expansion as the body to avoid defects on cooling.
 Variety of materials are used in glaze formulations that may include the basic materials
used in the body formulations such as clay, feldspar and silica, but other components such
as coloring agents, zinc oxide, carbonates of barium and strontium and borates may also
be present.
 Silica (SiO2) is the backbone of the glass formulation since it provides the network
structure of the glass. The silica used to form the silicate structure is derived from quartz
(flint), clay, feldspar or any other silicates in the formulation. Borates are unique in that
they serve as network formers and typically decrease the melting temperature of the melt.
Borax (Na2B4O72*10H2O) is the primary source of borates, but other minerals such as
colemanite (Ca2B6O11*5H2O) are also available. Traditionally lead (as PbO) was an integral
part of any glaze formulation. The addition of lead to the glaze imparted the glaze with a
lower firing temperature and improved coverage. Zinc oxide also acts as a flux and typically
enhances the effect of other fluxes.
 Glazes for whitewares
 The silicate structure is modified by alkaline and alkaline earth components which act as
fluxes (reduce the melting temperature). These components serve to disrupt the silicate
structure which reduces the viscosity of the melt. The more alkaline (Li, Na, K) and
alkaline earth (Mg, Ca, Sr and Ba) in the glaze, the lower the melting point, but care must
be taken because excessive levels can result in durability or devitrification problems.
Alkalines are usually introduced into the glaze from silicates such as feldspar, but may also
come from soda ash or other carbonates and hydroxides. In general, higher levels of alkali
and alkaline earth components increase the thermal expansion of the glaze. As the thermal
expansion of the glaze increases, the likelihood of defects in the fired product increases. In
general, higher levels of alkali and alkaline earth components increase the thermal
expansion of the glaze. As the thermal expansion of the glaze increases, the likelihood of
defects in the fired product increases.
 Glaze Application
 Glazes are usually applied in liquid or paste form and therefore require that the ceramic
components be suspended prior to application.
 Depending on the starting particle size of the raw materials, glazes can either be milled to
achieve the proper particle size, or if the components are fine enough, there are simply
mixed with water in a high intensity mixer to disperse the suspension.
 Glaze formulations that require milling are usually ball milled which reduces the particle
size and disperses the components to give a homogeneous suspension.
 Like slips for spray drying, the residue, solids content and viscosity are monitored during
milling of glaze suspensions.
 Depending on the application technique, the specific properties of the glaze, specific
gravity or viscosity for example can vary widely with fluid glazes used for spraying and
thick pastes used for screen applications. Additives such as binders or suspending agents
are commonly added to glazes to achieve desired effects. Organic binders such as cellulose
(CMC) are commonly used as well as bentonite or hectorite clay minerals which thicken
the glaze and improve green strength. Suspending agents such as kaolin, bentonite or
colloidal silica are used to avoid settling in the glaze before application.
 The oldest methods of glaze application are either dipping the ware into a bath of the
glaze composition. This simply leaves a uniform coating of the glaze on the surface of the
ware and works well with intricate shapes such as toilets. Handling of the glazed ware,
especially for large or heavy pieces, can be somewhat problematic. Brushing is another
simple technique where glaze is applied with a brush to achieve a uniform coating.
 Glaze Application
 During the maturing of the glaze in the kiln, a number of processes take place which
include, decompositions, solid state reactions, vitrification, fusion, and crystallization.
 Before the glaze composition melts, it undergoes vitrification like the ceramic ware. During
vitirication, a glassy phase forms. The formation of this glassy phase is a kinetic process
which means that it is a function of both temperature and time.
 As the quantity of glassy phase increases due to heat work, the viscosity of the glaze
decreases and eventually the glaze becomes molten. The surface tension of the glaze
allows it to wet the surface of the ware, which promotes adherence of the glaze to the
ceramic surface.
 Conclusions

 What is whiteware ceramics

 The classification and properties of whiteware ceramics
 The raw materials, products and process description of whiteware ceramics
 The firing of tri-axial body
 Glaze and their application