Many properties of raw materials, whether fired or unfired, correlate with their chemical or mineralogical composition.
Yet other parameters, such as size and shape of particles, their arrangement and the way they come into contact with each other the very structure of the ceramic body are equally important. Such factors strongly influence the chemical and physi-cal changes that take place in the presence of the energy input associated with firing.
For this reason, most ceramic materials need to be broken down to a size that will optimise subsequent production processes: this goal can be achieved via a series of methods, collectively known as COMMINUTION.
A comminution process consists of applying energy to particles until they break or separate to form smaller particles: the reactivity of individual particles, of course, will vary as a function of compression and abrasion resistance, hard-ness, elasticity and the nature of the particle as dictated by the type of deposit, how it is fractured and the whole geological history behind it (pressure, tempera-ture) etc.
Without entering into an in-depth description of the principles behind the grind-ing process (a subject that will be dealt with in Volume 2), let us examine the main means of comminution.
Crushing or compression of particles, between the very hard surfaces in the cru-shing or milling machine. In theory, the effectiveness of this comminution pro-cess varies as a function of the compression resistance of the ceramic material, yet the irregular shape of the particles also considerably influences the forces that shatter them. Crushing generally yields relatively uniform sized particles and little powder.
Grinding of particles, in which comminution occurs by rubbing and shattering the particles against each other and use of fixed or mobile grinding media, resul-ting in gradual reduction of average particle size and simultaneous production of finer powder and a correspondingly wider particle size distribution.
Impact fracturing, instead, involves new comminution principles: force is applied on particle edges/corners and thus works efficiently along the fissure lines in the particles themselves. Breakage usually occurs along the natural lines of we-akness of the structure (associated with the mineralogical nature of the mate-rial) or where different minerals meet.
Generally speaking, raw material comminution machines (to be described in greater depth in Volume 2) employ a combination of the above-described principles.
The main problem is the sharp drop in grinding efficiency that occurs as the par-ticles get smaller.
It is thus common practice to employ return-feed screening systems that filter out already-ground material as soon as it forms.
Particle size distribution
A raw material made up of particles of different size and shape may be defined as a particulate system: size distribution of the grains in such a system is enor-mously important, yet very difficult to measure.
Were they perfectly spherical, it would be easy to sort them via dimensional or gravimetric selection (assuming constant sphere density): yet with a clayey system the matter is somewhat more complex, making it difficult to define exactly what is meant by the word particle.
While it has already been described how a clay is made up of extremely small particles (micelles), it is also a known fact that these micelles, in the ceramic bodies and raw materials themselves, are aggregated and agglomerated, thus making them difficult to isolate in their free form.
Particles produced by grinding clays, then, will essentially be made up of many small agglomerates of irregular form and variable surface/peripheral charge, thus influencing behaviour in aqueous suspensions and during firing.
The first problem, then, in attempting to measure the dimensions of ceramic particles and their distribution, is to decide whether to concentrate on the dimen-sions of the individual particles or, vice versa, the agglomerates.
Similarly, the choice of pre-analysis dispersion systems will be decisive, as these could break down the agglomerates/particles or even cause further agglomeration:
particle size analysis on a material dispersed in water with rheological additives gives results that differ substantially from measurements made on the same powder in its dry state.
Further complications are found in the enormous variety of granulometric sizes within a ceramic body, especially where it has been dry-ground: however, even wet-ground materials contain particles ranging in size from just a few tenths of a µm, to tens of mm, and there is no instrumentation capable of providing adequate accu-racy over such a wide range.
Finally, the particles of a ground ceramic material, especially where clayey, are far from spherical, often plate-like.
These particles are mixed in with other materials that break down to form
par-The main dimensional classification methods are listed in the following table, which orders the methods according to minimum and maximum detectable particle size.
Method Detection interval (µm)
Sieving > 50
Micro-sieving/filtration 0.2 - 50
Optical microscope 25 - 2500
SEM - Electronic microscope 0.5 - 1000 AFM - Atomic force microscope 0.001 - 5
Sedimentation 1 - 50
Elutriation 2 - 50
Centrifugation 0.05 - 5
X-ray dispersion 0.05 - 100
Laser diffraction 0.05 - 200
Gas permeability 0.1 - 300
Powder classification is always expressed as per national standards on the basis of equivalent sieve classification, according to ISO (international), BS (British), UNI (Italian), DIN (German) or NF (French) values. However, the MESH ASTM classi-fication (USA) is used extensively all over the world and the equivalent sizes are given on the following page:
Mesh A.S.T.M. Micrometer (µm) Mesh / cm2
The above table highlights the limitations of sieving methods, their use being confined to rather approximate discrimination: on the other hand, hydrodynamic methods, long used for finer resolutions, are complex and lengthy (especially so for very fine particles). The best compromise is to use short wavelength incident radia-tion (X or laser) interacradia-tion instruments through the applicaradia-tion of modificaradia-tions to Stokes law.
This is expressed by the following equation:
2 (ρ1 - ρ2) g r 2 V = ____________________
9η
where V, the speed of a falling particle in cm/sec, is obtained from the values:
At constant temperature, where all constant parameters are known and the sedi-mentation speed of a set of particles is measured, it will be possible to calculate the average dimensions of the particles themselves.
By applying Stokes equation values to a particle with a density of approximately 2.5 g/cm3 and a diameter of 1 mm, it can be deduced that it will drop through water at 20 °C, in the absence of any interaction with the water itself, at just under 1.2 cm/hour. The appropriate practical modifications to the equation, as a function of a real particle, reduces this speed to about 0.3 cm/hr, thus giving a clear idea of just how long hydrometric particle size analysis can take where a sedimentation of at least 20 cm (and thus a wait of some 64 hours) is required.
Whichever system is used, particle size distribution can be represented by two graphs (see Fig. 95): an actual % greater or less than a particular size or a cumula-tive % against a logarithmic scaled size on the x axis.
Industrial production involves a series of particle size distribution tests designed to ensure the desired outcome: it is, in fact, necessary to:
a) check the dimensions of the raw materials destined for grinding so as to optimi-se control of the process itoptimi-self.
b) remove coarse (sieve-retained) materials resistant to the grinding process, elimi-nating them or recycling them back into the grinding process.
Fig. 95. Example of differential and cumulative particle size distribution curve.
c) run a series of checks, with appropriate sieving, so as to ensure smooth opera-tion of the spray drier pumps, the spray drier, the presses, and the glaze/engo-be/silk-screen application stations along the production line.
All this is essentially based on a no larger than principle through use of appropriate sieves.
At this point it is important to highlight, with particular reference to grinding (and homogenisation), the influence of variations in particle size both throughout the process and on final product characteristics and classification.
The understanding and control of particle size is essential to ensure optimum body texture and homogeneity and the required porosity, specific weight, modulus of rupture.
The texture of the material is strongly influenced by the type and degree of grinding. This is because grinding alters the size and shape of the individual grains and causes variations in the way in which they associate, thus influencing particle
packability and final density.
Various studies on maximum attainable packing density have been carried out with regard to both spherical particle systems with no particle deformation and for isomorph sphere systems: these studies have yielded an ideal distribution that gives rise to two possible solutions: open packing with spheres arranged by cubic symmetry, where empty gaps account for 48% of the entire occupied vol-ume and closed tetrahedral-symmetry packing with gaps accounting for just 26% of total volume.
Of course, systems featuring mixtures of different, always-spherical grains will give rise to ever-more complicated theoretical models which optimise use of space enormously: for example, a tri-modal system based on appropriately calibrated spheres (having an assumed diameter ratio of 50:8:1) of large (62% by volume), medium (24%) and small (9%) size will, in theory, fill the space very efficiently, leaving an air-gap residue of just 5% (62 + 24 + 9 + 5 = 100%).
While the individual particles in a ceramic body are generally angular rather than spherical the above-explained general principle may be taken except where particles are particularly elongated in one direction as being applicable, albeit inexactly as there are many other inconveniences linked to the fact that the irregu-lar shape of the particles stops them sliding against each other.
On the other hand, that irregularity will, statistically, give rise to the real possi-bility of forming more extensive areas of tighter agglomeration and, all in all, lowering average porosity.
called sieve residue) serves only to establish whether production activities com-pleted thus far have remained within operational parameters. Fig. 96 shows two bodies (the particle size distribution curves of which are illustrated) with an identi-cal residue of about 2% at 63 µm: yet they clearly differ substantially in terms of particle size composition, with body A having a Gaussian distribution around 15 µm, and body B having bimodal distribution centred around 0.6 µm, with an aver-age dimension of about 9 µm.
Fig. 96. Particle size distribution analysis of two bodies having same residue but different particle size distribution.
Slip sample A (AVG. DIAMETER = 15.3 MICRONS)
Slip sample B (AVG. DIAMETER = 9.1 MICRONS)
Particle Size (Microns)
Particle Size (Microns)
The technological behaviour of the two bodies, which seem identical in the light of residue-only analysis will actually be radically different, as B is much more compactable than A.
The unpackability of ground raw materials and the forces applied during the forming process (pressing, extrusion ) make the production of a ceramic body without any empty spaces at all virtually impossible, no matter how accurate selec-tion and particle size distribuselec-tion may be.
All ceramic materials, except for some glasses, have pores or empty spaces: the porosity of a material is, then, defined by the quantity of air it contains. In scientific literature there are six different kinds of porosity (see Fig. 97):
a) closed pores
b) capillary canals connecting closed pores c) blind pores
d) interconnected pores e) open or ink bottle pores
f) micropores (so small they prevent entry of water or any other liquids).
There are two types of closed pore: those formed by pressing the semi-finished item and those originating from open pores that are sealed by material that melts or Fig. 97. Different types of porosity. From: Grimshaw, Chemistry and Physics of Clays-Benn.
beyond the vitrification point, the pressure exerted by the enclosed gas may cause expansion of the material itself and create bubbles which, in turn, form new macroscopic pores.
Dimples or pin-hole defects seen on fired glazes often stem from closed pores too: these are formed by carbonates, sulphates and other pyrolysis-affected com-pounds that emit gas at high temperatures: if this occurs when the glasses in the glaze are about to become plastic the glaze will block the escape and elimination of such gases.
Open pores (i.e. those connected to the exterior by capillary channels of varying length and width) may be formed by the original particle packing configuration, elimination of water vapour during drying or the initial stages of firing, elimina-tion of gas during firing, micro-structural alteraelimina-tions in the pieces during drying etc. The presence of such pores is closely tied to the size of the body particles and how they are packed.
In general, since fluids (rainwater with regard to freezing resistance and inks, oils etc. for staining and cleaning parameters) can enter and exit open pores, it is preferable for a fired product to have open pores of either a very large average diameter (>300 µm, to allow easy evacuation of water or introduction of deter-gent solutions) or a very small one (<40 µm, the smaller the better) so as to make fluid entry difficult and prevent any water contained therein from freezing as a result of extremely strong capillary forces: it thus follows that materials with a substantial amount of medium-size open pores (from 40 to 300 µm) remain at risk.
Such a wide range of scenarios makes it possible to measure two different body parameters: total porosity and apparent porosity.
The true (or real) porosity of a body is the ratio of all (open and closed) empty space volumes to total volume, while the much more frequently-used apparent po-rosity parameter is defined as the ratio between the volume of water the body can absorb under certain conditions (during prolonged boiling or vacuum treatment) and the total volume of the piece.
Such porosities will clearly be influenced by the dimensions, shape and packing of the particles, the chemical nature of the materials making up the body, and the technological treatment applied to materials during production: an often-underesti-mated parameter is the intrinsic porosity of the individual particles.
As stated in the introduction, depending on their mineralogical formation, clays possess widely differing structures with interlayer spaces ranging from 5 to 20 or more Å; in this respect they differ greatly, of course, from quartz or other compact material particles.
For this reason a clay-based body containing large quantities of sands and feld-spars, especially where the latter are of suitable particle size, will be less porous than that of a predominantly clayey body.
In addition to these morphological features, body texture and particle size distri-bution will play an important role in physical and chemical reactivity during firing, as will be illustrated further on.
Fig. 98. Progressive fusion-reaction hypothesis.
Any particle size variation, in fact, will have an influence on the specific surface area of the powders and an immediate influence on the inter-particle surfaces, sharply increasing that reactivity surface area, which, via solid-solid or solid-viscous liquid reactions (high-temperature ceramic bodies), is of primary importance for proper sintering.
The example in Fig. 98 shows a ceramic body matrix made up of relatively coarse feldspar (F) and quartz (Q) particles contained in a brick-like framework of clayey particles:
The sintering process begins at the particle contact points, where the fluxing action of the feldspars first acts (the black dots in the far left drawing).
Following initial reactivity, which tends to involve the smaller particles in closer contact with the fluxing agents, a liquid phase forms: this goes on to involve, par-tially, the surrounding clayey structure.
During the process, and especially during cooling, crystalline neo-phases form and reinforce the sintering phenomena in the mass.
The importance of particle size distribution in ceramic manufacturing is not just limited to the need for proper grinding: there must also be proper control of the powder agglomerates sent on to the pressing department.
As seen in the introduction, ceramic technology allows us to form ceramic tiles by pressing powders of low moisture content (3-6%).
Such powders may be produced by dry grinding (i.e. with dense, irregularly-shaped and often sharp-edged solid particles), followed by appropriate addition of water, or wet processing followed generally by spray drying that produces (some-times hollow) smooth spherical particles which can agglomerate.
In both the former case (particle size distribution either natural or induced by raw material grinding) and the latter (particle size distribution of semi-finished items), it is essential to evaluate the relative amount of the different granule classes so as to obtain the best possible mix and optimum compaction.