Processing of ceramic

GLAZE
Introduction
A glaze is a specially formulated glass applied to ceramics. It involved
the coating of the fired objects with an aqueous suspension consisting of
finely ground quartz sand mixed with sodium salts (carbonate,
bicarbonate, sulfate, chloride). The ware would then be refired, usually at
a lower temperature, during which the particles would fuse into a glassy
layer.
Application of glaze
In the case of electrical insulators the glaze must maintain specific
electrical properties even in inclement weather. For chemical porcelain it
must have high chemical durability. For fine china, porcelain, and semivitreous
china, it must have high gloss and be resistant, to dish washing
chemicals. In all cases it must be resistant to thermal shock.
Raw materialsof Glaze
1- Lead Glaze :the addition of lead reduces the melting or fusion point
of the glaze mixture, which allows the second firing to be at an even
lower temperature. Lead is a component of fine china glazes because it
“fixes” many of the application and flow problems of glazes and adds
high gloss. Lead is not used as it once was, in the white lead form,
which was desired in dipping glazes. All most all lead is now
contained in frits. A frit is a special glass used in compounding glazes.
It ties up toxic and soluble materials and sometimes coloring oxides.
Clays and insoluble oxides and or carbonates may be added to the frit
to form the glaze composition.
2- Potassium feldspar: is a single-component high-temperature glaze
for chemical porcelain.
Classifications of Glaze
1- porcelain process the body is porous after a low-temperature fire.
Therefore it is easy to dip. This process is automated for dinner plates
and such in modern factories.
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2- china process the ware is vitrified and it must be heated during the
glazing process. This is usually done by burners in the first section of the
glazing tunnel.
3-electrostatic spraying this is a good way to put enamel on
household appliances. Glazes can be applied in powder form by
electrostatic praying
Defects of Glazing
1-Rough Foot-unglazed
If the glazed ware is fired on refractory setters glaze may cause
sticking during firing. The solution is to clean the foot carefully after
glazing, often using a sponge belt. The foot can be waxed prior to
glazing. This can be done by melting paraffin in a flat electric frying
pan with a sponge bottom. Waxing belts are also available. After
firing, the foot should be checked for roughness and smoothed using
a burnishing stone or a foot burnishing machine.
2-Pinholes Pinholes can be caused by over firing, impurities in
the glaze, or contaminants in the body. Closed setters can be used to
protect the ware from flame impingement during firing.
3- Crazing Cause by a glaze / body mismatch where the coefficient
of expansion of the glaze is higher than that of the body. Addition
of silica or a low-expansion frit usually corrects the problem.
4- Crawling crawling of the glaze from the substrate can occur before
or during firing. During firing viscous glaze can pool and overcome
surface tension forces binding the glaze to the ware. Over-grinding
can increase the tendency to crawl due to the formation of colloidal
factions that tend to shrink more cracking the glaze on drying. The
glaze formula may have to be changed to reduce or increase the fired
viscosity of the glaze. Too-fluid glaze may extract silica and alumina
from the body during firing changing the glaze composition. Fine
clays can cause excessive drying shrinkage. Opacifiers and some
underglaze colors can cause crawling. Ware should be clean before
glazing. Binder additions can help crawling. PVA can also be used.
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5- Peeling caused by the opposite of the above. The glaze is in so much
compression that it shivers off. Curved or sharp surfaces enhance the
effect. This is not a common default and is the easiest fault to resolve
simply by increasing the thermal expansion coefficient of the glaze.
Simply lowering the flint content (or grinding the flint coarser) is the
common cure. A more fluid glaze also can reduce peeling.
6- Blisters fast firing or incomplete firing. Non oxidation of carbon
compounds from organic binders due to too little heat in the preheat
zone and or non-oxidizing conditions. Impurities such as limestone
or SiC from grinding or clay storage operations. Hardening of the
glaze may help.
7- shivering caused by too low a thermal expansion coefficient for a
glaze.
Firing Conditions of Glazing
The firing curve naturally has heat up and cool down periods. In between
these slopes is a flat or modified soaking time/temperature. During heat
up, the binders are removed from the glaze. They must be completely
removed and they must not be reduced to carbon during heat up (preheat).
This does not mean that you can’t approach reducing conditions. Some
compounds like MnO and FeO can greatly improve melting although they
are often in the glaze only in tiny amounts as impurities. These
compounds do not form under oxidizing conditions.
Cooling Conditions of Glazing
Cooling is important to both the glaze and ceramic ware. You can cool
quickly to just above the silica phase transition and then cool slowly
through the transitions. This may not be good for the glaze which is
trying to get rid of the bubbles created after the glaze melted. This can put
you between a rock and a hard place. You have to be able to control the
whole cooling zone of a tunnel kiln from the minute the ware comes out
of the hot zone.
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Notes on Glaze Formulation
1- It is interesting that silica can raise the thermal expansion of a body
and lower the thermal expansion of the glaze.
2- For all practical purposes the thermal expansion coefficient of the
glaze must be lower than that of the body. This keeps the glaze in
compression after firing. Glazes always fail in tension. Keeping the
compressive forces higher than the tensile forces.
3- Kiln contamination can be a problem in some operations. Pits, pin
holes, and pocks caused by impurities dropping on the glaze during
firing can be removed by grinding. The ware is resprayed with a thin
coat of glaze and then refired.
4- If body impurities are still releasing gases on refiring.
5- Every element you add to a glaze impacts the final-glaze properties.
For example, too much alkali content will increase the solubility of a
glaze, raise its thermal expansion, and in general raise havoc. A
minimum amount of alkali content will give the glaze fusibility. Also,
individual alkalis act differently in extent. Therefore one would use
more than one alkali and the minimum amount of each that yields the
best balance in properties.
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Slip Casting
Slip casting is the most conventional method of producing varied pieces
that can have complex forms (culinary, sanitary, refractory materials,
technical ceramics). This method consists of casting a suspension (slip) in
a porous mold, generally made of plaster. The capillary migration of the
liquid into the pores of the mold results in the formation of a consolidated
layer of particles on the mold surface. The main advantages of slip
casting are:
i) the complexity of the forms that can be produced
ii) its low cost
iii) the use of perfectly dispersed suspensions in technical ceramics
leading to dense and homogenous green microstructures.
Its major disadvantage is its low production capacity.
Aqueous suspensions are the most common materials (MgO, CaO,
La2O3, etc.) and non-oxides (SiC, Si3N4, AlN, etc.) are cast in an organic
environment (alcohol, ketone, thrichlo-r ethylene). The surfaces of these
non-oxides can nevertheless be “hydrophobated” .
Slip Casting Mechanics
Some of the early analyses treated the mechanics of slip casting in terms
of a diffusion process. The slip casting process involves the flow of liquid
through a porous medium.
J=
??(
????
????
)
???
where J is the flux of liquid, K is the permeability of the porous medium,
dp/dx is the pressure gradient in the liquid, and is the viscosity of the
liquid.
In slip casting, the pressure gradient that causes flow arises from the
capillary suction pressure of the mold. As the consolidation of the
particles proceeds, the filtrate(i.e., the liquid) passes through two types of
porous media: (1) the consolidated layer and (2) the mold
The zeta potential (?)
A charged particle constituting an electrokinetic entity and subjected to
an electric field (E), will move at a speed( v) (electrophoretic velocity).
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The electrophoretic mobility(? ) (? in m2s-1V-1) is defined, for spherical
particles, by:
? = v/E
The zeta potential is calculated using the following expression:
?=
??? ??
?r ??
where ? is the viscosity of the solution and ? r the dielectric constant of
the liquid, ??the dielectric constant of thediffuse layer .
The Henry constant fh is a function of the ratio of the particle radius to
the thickness of the diffuse layer. The fh constant is equal to 1.5for a
value of this ratio lower than 1 (broad diffuse layer). The particle is then
regarded as an isolated charge. If the contrary happens (ratio higher than
100), which is the case of environments with low dielectric constant, the
diffuse layer is regarded as a plane state and the fh dielectric constant is
equal to 1.
The value of the zeta potential is often correlated with the stability of the
suspensions.
Molds
The mechanism of absorption of the liquid in the suspension is linked to
the capillary suction effect of the porous mold. The porous network of the
mold is therefore of vital importance for the setting rate and the
characteristics of the green part. The most widely used molds are made of
gypsum (CaSO4, 2H2O) formed by reaction of plaster (CaSO4, 0.5H2O)
and water. Plaster presents an inter-connected network of gypsum needles
and plates which confer on it its mechanical strength. Plaster allows the
easy and low-cost manufacture of complex.
Molds with good surface quality, high porous volume (40 to 50%) and
pore size lower than 5 ?m. On the other hand, plaster has a low abrasion
resistance and high solubility in water, as well as a drop of its mechanical
properties due to dehydration, above 40°C, which limits its necessary
drying between two castings so as not to increase the setting time. The
surface of the mold can be covered to facilitate the demolding and to
reduce attacks by acid suspensions (silica) or by organic suspensions
(alcohol), using alginates, talc or by graphite.
Other materials are used for the production of casting molds with high
mechanical strength and high hardness, for instance, epoxy resins filled or
with ceramic powders.
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Casting suspensions
Obtaining ceramic parts, with satisfactory properties in a reproducible
way by slip casting requires a judicious choice of the grain size and the
control of the particle surface chemistry. The behavior and the viscosity
of the suspensions in fact depend directly on the grain size of the
powders, the inter-particle interactions (state of dispersion) and the
particle concentration. The powder grain size distribution, which will
influence the arrangement of the particles during the consolidation, varies
considerably according to the type of ceramic and the final properties
desired.
In the case of technical ceramics, the particle size is generally low
with a narrow distribution to ensure satisfactory sintering reactivity and a
homogenous microstructure. On the other hand, in the case of traditional
ceramics, the mixture is typically made up of fine clay platelets, as well
as silica and feldspar particles of about a few tens of micrometers.
Traditional clay ceramic suspensions and technical ceramic suspensions
differ by the degree of particle dispersion . A high viscosity at low shear
rate and/or a sufficiently high yield stress is favorable for avoiding the
sedimentation of the particles at rest, before the casting, but especially in
the mold during the setting. The minimal value of the threshold stress ?s
which balances the sedimentation forces of particles with diameter d and
density ?p in a suspension of density ?s is expressed by:
?s = ??
??d (?p – ?s)
Emptying–demolding
The consolidated layer must have sufficient mechanical strength so that it
will not flow out with the suspension during the emptying of the mold
and allow its demolding. In partially coagulated systems containing clay
(traditional ceramics), the coagulation forces give a sufficient cohesion on
the piece. The cohesion of pieces realized with dispersed suspensions
(technical ceramic) can be increased by the addition of binders
(carboxymethyl cellulose, ammonium or sodium alginate) which can also
contribute to the dispersion. The addition of binders nevertheless
presents the disadvantage of increasing the setting time through an
increase in the viscosity of the suspension and a reduction in the
permeability of the consolidated layer.
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Flaws
The main flaws encountered in a green part obtained by slip casting are:
i) the presence of large pores (pinholes) due to a poor degassing of the
suspension.
ii) a preferential orientation of anisotropic particles (clay platelets, mica)
to a low thickness on the piece’s surface which results in a differential
shrinkage and stresses during drying and sintering.
iii) a non-homogenous microstructure due to a sedimentation of coarse
particles.
Pressure casting
The supply of an additional pressure, compared to the low capillary
pressure of the mold (< 0.2 MPa for plaster), decreases the setting
time. In this respect, the pressure casting consists of applying a pressure,
generally lower than 5 MPa, to the suspension in the porous mold. The
pressure gradient thus created (?P) will force the fluid through the
porous network and the formed layer, considerably reducing the setting
time compared to slip casting. Pressure casting reduces the water content
in the green part and increases its density and its cohesion. The mono- or
multi-cavity molds are made of plaster or polymer with a mechanical
strength and porosity greater than plaster, as well as an elasticity allowing
a water tightness of the mold under low clamping force.
Pressure casting able process provides high productivity. It is now quite
widely used in the industry for production of ceramic sanitary ware and
tableware. On the other hand, the application of this process to technical
ceramics is still relatively new. Elastic stress-strain relation of the form:
p = ??? 2/3
where p is the stress, ? is the strain, and , ? is constant for a given
particulate system.