It is common to apply a surface coating to tiles which, when fired, produces a vitreous layer that is hygienic, easily cleanable and provides aesthetic qualities. Bear-ing this in mind, let us now take a look at the most common techniques and materi-als used in such technology.
Firstly, it is important to define just what is meant by the term frit (a name for a major component in most glazes.
A frit is obtained by melting a mixture of materials in either continuous (basin) furnaces or intermittent (rotary) furnaces.
Glazes contain combinations of one or more frits with other additives to obtain various effects to be described later.
The pre-melting or fritting of materials serves to produce an amorphous, homo-geneous compound, free from any non-dissolved raw material residues or bubbles, which, on subsequent grinding will become even more homogeneous.
Fritting aims to:
1) make certain components (boron derivatives, alkaline salts, lead derivatives) in-soluble: these would otherwise dissolve during the grinding stage if used in their raw state.
2) completely eliminate all organic impurities, thus removing all volatile compo-nents via reactions that would otherwise occur during firing of the finished prod-3) disperse certain impurities (of ferrous and/or metallic nature) evenly within theuct.
mass: even where such impurities are present in very low percentages they can still cause local defects. Melting ensures that these contaminant particles are dispersed throughout the molten mass and incorporated in the composition, thus eliminating defects such as iron, copper specking etc.
4) aid reactions at high temperatures (i.e. 1400 °C or more), forming vitreous com-pounds that would otherwise develop at standard body firing temperatures.
Raw materials for melting of frits
In selecting raw materials for the production of ceramic frits the following crite-ria must be taken into consideration:
consistency of chemical composition over time
consistency of particle size distribution over time
low Fe and Cr content
absence of difficult-to-melt minerals (kyanite and sillimanite) that, remaining in the frit as unmolten elements, compromise its quality
last but not least, materials should be economically viable and readily avail-able.
The most important and thus the most commonly used materials are:
quartzes and sands
ulexite (sodium and calcium borate)
calcium carbonate
In lesser quantities, the following products are also used:
potassium nitrate
sodium carbonate
potassium carbonate
magnesium carbonate
titanium dioxide.
Such a variety of raw materials means there is a need to make basic choices in terms of storage and handling solutions. It should be noted, in fact, that some of the above-cited raw materials cannot be stored in silos (or, rather, not in a way that is straightforward).
Moreover, materials such as barium carbonate, titanium dioxide and potassium nitrate are not available in bulk.
Zinc oxide differs from other materials in that, in its light form, it cannot be stored in, or, rather, extracted from, silos: consequently, it is now common practice to employ so-called heavy zinc oxide, a material without any particular handling problems.
Given the above difficulties and the simple fact that they are often utilised in small quantities these raw materials are, then, generally stored in sacks rather than
Technological aspects of frits and glazes
Final ceramic surface coatings are, then, made up of thin vitreous layers. They are applied in aqueous suspensions obtained by water grinding the frit and any other raw components in the composition onto fired or dried unfired ceramic bodies.
A certain glass type, known as cristallina, is, on some wall tiles, applied on already glazed and decorated tiles to obtain brighter and deeper colour effects.
Another characteristic glaze is maiolica, highly opaque, usually applied thickly to give a rich white and glossy tile surface. The glaze get its opacity from the frit opacifier, usually, zirconium silicate.
There are also frits for specific, finely-targeted use, such as those employed in monoporosa. In addition to opacity and brilliance, these must also have very high softening points (1020-1050 °C) so as to favour emission of the gas (CO2) emitted from the body during firing.
With glazes, instead, the diffusion of vitreous single firing products has meant that, in addition to the concept of opacity other factors such as surface appearance (matt, semi-matt) should be considered.
This chapter will deal first with the theoretical aspects of the nature of the glass, and then provide a review of the different types of frit and, finally, glaze compounds.
Glass formation and formulation mechanisms
All vitreous masses come from the fusion of various constituents (see Tab. 8):
vitrifying agents
fluxes
stabilizers
opacifiers
de-vitrifying agents.
Tab. 8.
The key physical characteristic of glasses is that they are isotropic while crys-talline-structured solid bodies are known to be anisotropic.
In the past, this led to the belief that glasses were amorphous substances; how-ever, recent studies by Zachariasen and Warren have established that the character-istic tetrahedral coordination of silicon is maintained in glass too.
While these tetrahedrons are, in crystals, arranged as a strictly regular geomet-ric construction, in glasses they are arranged chaotically, without periodicity or symmetry.
So while we can speak of a glass lattice, it must be born in mind that this is a disordered, contorted lattice essentially made up of silicon and oxygen.
Two-dimensional illustrations of the tetrahedral arrangement in crystal-type silica and molten silica are shown in figures 88 and 89 respectively.
Like silica glass, common glass also features this irregular arrangement of tet-rahedrons; moreover, in the latter the ions of the other constituent elements fill the gaps left by the silicon and oxygen.
Fig. 88. Crystalline structure. Fig. 89. Vitreous structure.
For every temperature, then, there is a corresponding glass structure that char-acterises it. During solidification, as the temperature gradually falls, the glass re-establishes the bonds and tends to take on the structural state associated with lower energy levels. However, as the increase in viscosity around the transition point is somewhat rapid, the internal state of the solidified glass corresponds to that of higher temperatures. Consequently, structural instability arises: while this instabil-ity necessarily tends to evolve it does so over a very long time interval (Tab. 9).
Tab. 9. Some crystalline components in de-vitrified frits.
Anorhtite Gehlenite Sphene Gahnite Willemite Cristobalite Tridymite Spodumene
Magnesium titanate Wollastonite Rutile Cordierite Forsterite Enstatite Diopside
Zirconium silicate Zirconium oxide Celsian
Leucite
The cations, which, in the oxide state, can be obtained in the vitreous state sim-ply by heating and are thus known as lattice-forming cations are Si4+ and B3+.
While silicon forms apex-linked tetrahedrons, the coordination 3 boron forms equilateral triangles at the centre of which lies the B3+ ion.
Since the silicon ion has four bonds, while the boron ion has just three, it can be seen how boric glass is less viscous and thus more fusible.
The fluxing cations, also known as lattice modifiers, split the links between the tetrahedrons once they are added in the oxide state:
O O O O O Si O Si O by introduction of Na2O O Si O O Si O O O O Na Na O
These ions generally take up positions in the interstices between the siliceous polyhedrons.
The greater the number of sodium ions introduced, the greater the number of splits, and so on, thus diminishing the viscosity of the glass. Furthermore, the high number of splits between the tetrahedrons ends up compromising the existence of the vitreous state itself because the more freedom the tetrahedrons gain, the more marked is their tendency to take on the regular structure of crystals and, conse-quently, of devitrified glass (Tab. 9).
O O O O
O Si O Si O by introduction of CaO O Si O O Si O O O O Ca O
Stabilising cations are lattice modifiers too: unlike alkaline cations, which, be-cause of their weak ionic potential are only loosely linked to the lattice and thus easily removed, resulting in alteration of the glass, alkaline-earth cations have twice as much ionic potential and therefore reinforce the lattice structure of the glass and act as stabilisers.
The replacement of a modifier ion (Na) with another of higher electrical charge (Ca), having more or less the same dimensions, causes:
an increase in density, because the greater attractive force exerted on the adja-cent oxygen ions gives rise to greater compactness
an increased refractive index as a result of the increased density
a reduction in electrical conductivity stemming from reduced mobility of the cations, in turn caused by increased bond energy
Therefore in exclusively boric glasses the lattice is made up of equilateral trian-gles, apex-connected by oxygen atoms which act as bridges; in silica-boric glasses, instead, increasing quantities of B2O3 first lead to the formation of BO4 tetrahe-drons (boron coordination switches from 3 to 4), giving rise to a structure analo-gous to that of a strongly silica glass. When the B2O3 exceeds a certain threshold the triangular structures characteristic of pure boric glass, rather than BO4 tetra-hedrons, are formed.
Aluminum ions cannot, in themselves, be called lattice-formers; nevertheless, studies of feldspars have shown that they can, in the presence of electropositive ions, substitute the silicon ions, forming a corresponding number of tetrahedrons.
Aluminum may behave similarly in the glass and this reinforcement of the tetra-hedrons makes the glass more viscous, more chemically resistant and the vitreous state more stable.
This behaviour of aluminum means that a net distinction between lattice-form-ing and lattice-modifylattice-form-ing ions cannot be established. Under certain conditions even the latter may exist as lattice-formers. Numerous examples of such dual behaviour are provided by the coloured ions, depending on whether they exist as formers or modifiers.
In this respect it can generally be stated (Dietzel) that cations having high coor-dination strengths with respect to the oxygen anion (Tab. 10) behave as lattice-formers (Si4+, B3+) while those with the lowest values act as lattice modifiers (Pb2+, Ca2+, Ba2+, Li+, Na+, K+) and, finally, those that have intermediate values may per-form both functions (Fe3+, Be2+, Mg2+, Ni2+, Zn2+, Co2+).
Standard raw materials and their influence on the characteristics of glass
1 - Silica (SiO2)
Introduced in the form of quartz, quartz sands, feldspathic sands and feld-spars.
Silica is the prime component of vitreous compositions as it has the property of vitrifying under the effect of the fluxes within a very broad temperature range. The fluxes or modifiers are: PbO, B2O3 , K2O, Na2O and Li2O. Glazes rich in silica are highly resistant to chemical agents and are extremely hard. The higher the silica content in a glaze the higher its firing temperature.
2 - Diboron Trioxide (B2O3)
Introduced as boric acid, sodium borax, colemanite.
Because of its vitrification properties, boron is, after silica, the most important element. However, it cannot be used on its own as it would give rise to strongly soluble glasses . It acts as a flux in silica glasses; indispensable in glasses that are lead-free with low melting points; dissolves many colorants, confers glossiness, reduces viscosity and lowers the expansion coefficient in those glasses to which it is added.
3 - Lead monoxide (PbO) Lead monoxide gives glass:
high fusibility
Tab. 10. Bond strengths calculated for certain oxides.
MODIFIERS INTERMEDIATES LATTICE FORMERS Degree
of oxidation
Dissociation energy (Kcal/g.atom)
Coordination Strength of single bond (Kcal/g.atom)
high toxicity, proportional to lead content and as a function of the form in which the glass is bound
high sensitivity to acid attack where oxide content exceeds a certain propor-tion.
4 - Alkalis (K2O, Na2O, Li2O)
Introduced as nitrates, chlorides, carbonates or feldspars. Alkalis are lattice mod-ifiers: their introduction weakens the lattice structure of the glass by lowering the melting point.
The Na+ and K+ ions occupy positions in the interstices separating the tetrahe-drons. The K+ ions, which are larger than the Na+ ions, form stronger bonds, thus giving rise to the easy alterability of sodium glasses.
Highly sodium glasses are easily soluble. Alkalis generally increase glass expan-sion coefficients, except for lithium, which, as it is highly fusible, can produce the same results while being used in extremely low percentages (much lower than sodi-um or potassisodi-um).
Alkali, especially lithium, give the glasses gloss; however, on their own they are unable to constitute the entire basic part of a glass composition owing to a tenden-cy towards devitrification and the solubility of the formed silicates.
5 - Calcium oxide (CaO)
Introduced in the form of calcium carbonate, dolomite, wollastonite, anorthite.
Calcium oxide is a stabiliser: added to an alkaline silicate it eliminates the alter-ability of the glass.
On its own it would form silicates with a high melting point (above 1400 °C);
mixed with other silicates it gives rise to the formation of vitrified masses.
It is thus obvious that high percentages of this oxide would lead to devitrifica-tion (matt CaO). Introduced in the right propordevitrifica-tions (5-10% by oxide analysis), the calcium not only gives stability but also improves bending strength and body-glaze adhesion.
Also lowers viscosity in glasses fired at high temperature.
6 - Alumina (Al2O3)
Introduced as calcined alumina or alumina hydrate, feldspars, kaolin, corun-dum.In glazes in appropriate proportions (4-8%) it confers, in the case of low-temper-ature glazes, the following:
increased viscosity
reduced tendency to de-vitrify (crystallization)
increased bending strength
reduced expansion coefficient
increased resistance to acids
improved opacity (introduced in high percentages in concordance with the glaze firing temperature).
The quantity of Al2O3 introduced into a glaze increases or decreases in propor-tion to the firing temperature. Quantities will thus be higher in matt or satin-finish glazes and lower in high-gloss glazes.
The quantity of Al2O3 introduced into a glaze also depends on its particle size distribution. As the particles become finer the percentage that can be introduced drops, while larger particles allow the introduction of higher quantities.
Since it is an amphoteric substance this oxide has the capacity to combine as much with silica as with basic oxides. It is thus the most efficient of the stabilis-ers.
7 - Barium oxide (BaO)
Normally utilised by introducing barium carbonate (BaCO3) into the composi-tion. This oxide increases density and refraction, thus giving the glaze shine. It is also an excellent flux in the fusion of silicate glasses, a property that, in part, allows it to substitute lead oxide effectively; it is, however, highly toxic.
Where present in significant percentages (over 0.3% in molecular equivalents), this oxide hardens glazes and induces de-vitrification. A barium glaze melts much more rapidly than a calcium one and is less viscous too.
8 - Magnesium oxide (MgO)
Introduced by means of dolomite, magnesium carbonate and talc.
Magnesium oxide behaves in glasses in much the same way as calcium oxide.
The only difference is that it gives rise to more viscous glasses. It cannot be used in overly high percentages as it would otherwise raise the firing temperature of the glass. The magnesium reduces the expansion coefficient yet heightens the surface tension of the glass.
9 - Zinc oxide (ZnO)
In acid glazes with a high alumina content, zinc oxide plays a fluxing role.
Depending on the percentage used, this oxide has a range of effects:
a) in low percentages: increases the brightness of glasses and colours except for greens and blues; together with alumina it improves the opacity and whiteness of the glazes as long as CaO content is low: in the absence of B2O3 it reduces the expansion coefficient.
b) in high percentages: devitrifies from the vitreous mass, giving the glaze surface a characteristic matt finish, brought out where the glaze is basic.
c) in very high percentages: crystallizes and individual crystals made up of ZnO silicates separate. Glasses rich in this oxide are extremely vulnerable to acid
Adding TiO2 colours the glass: even at just 2% yellowing is observed. Simulta-neously the surface of the finishing glaze takes on a matt appearance, becoming hard and coarse as oxide percentages increase.
It has opacifying properties that improve in the absence of B2O3 and especially where the glass composition is rich in Al2O3 and added in the mill; under these conditions the colour fades.
These characteristics are particularly evident if TiO2 is introduced into the glass as anatase, while, where introduced as rutile, it loses its de-vitrifying characteristics until, in high percentages at high firing temperatures, it gives rise to needle-shaped crystals.
Crystallisation occurs mainly in high-fusibility glasses.
11 - Stannic dioxide (SnO2)
Although a superb opacifier even at low percentages (6-10%) it is rarely used owing to its high cost.
The opacity stems from suspension of the oxide in the vitreous mass as finely dispersed particles. Its opacifying potential thus depends on the purity of the oxide, the fineness of its particles and the nature of the vitreous mass to which it is added.
Alkalines and boron may prove detrimental to good opacification of this oxide.
The tin is best introduced into the mill during grinding stage and not in frit melting.
12 - Zirconium dioxide (ZrO2)
Used in the form of zirconium silicates of varying particle size distribution. An excellent opacifier (not quite as efficient as stannic oxide but much more economic) and undoubtedly the one preferred by industry.
High percentages of this oxide raise the firing temperature of the glass into which it is introduced. Zirconium silicates lend themselves as opacifiers in all glaze types firing between 940 and 1300 °C.
Only a part of the introduced zirconium silicates combines with other compo-nents, the majority remaining as it is.
The part that combines heightens the crazing resistance of the glaze. Zirconium also has the property of being a colour stabiliser. Calcium and/or barium oxide (no more than 0.2 molecular equivalents) help the zirconium improve opacification as do zinc and alumina.
Three different types of zirconium silicate are commercially available: these large-ly differ in terms of particle size distribution:
micronized zirconium silicates (very fine)
zirconium silicate powder (coarser)
zirconium silicate sand (very coarse).
The most commonly used glaze opacifiers are micronized products, while pow-ders are used in frit melting and sands are used as hardeners or fillers. Low firing temperature glazes, opacifiers with zirconates, often have non-smooth surfaces, un-doubtedly in relation to their highly viscous nature.
Table 11 provides an overview of the mineral types allowing the introduction of different oxides.
Table 12, instead, shows the most significant ceramic oxide parameters. Such parameters are:
– molecular weight – surface tension – expansion coefficient.
Types of frit
As seen, the term “frit” is commonly used to indicate, in industrial production processes, a vitreous mix cooled suddenly in water.
TABLE SUMMARIZING THE COMMONEST MATERIALS FOR THE GLASSES AND GLAZES COMPOSITIONS Oxides Used raw materials SiO2 Quartz, Feldspars, China-clays B2O3 Boric acid*, Borax*, Colemanite PbO Minium, Litharge
Na2O Feldspars, Borax*, Na2CO3, NaCl*
K2O Feldspars, KNO
3* LiO2 Feldspars, Li2CO3
CaO Wollastonite, CaCO3, Feldspars, Dolomite
BaO BaCO3
MgO MgCO3, Talc, Dolomite
Al2O3 Al2O3, Al(OH)3, China-clays, Feldspars ZnO Zinc oxide
SnO2 Tin oxide
Quartz, Feldspars, China-clays, Zirconium Silicates
Wollastonite, CaCO3, Feldspars, Dolomite, Colemanite Li2O
Tab. 12.
Frits are used as bases in finishing glazes and in low-temperature glazes to ren-der the components insoluble.
The market offers a wide range of frits having different fusibility, glossing, opaci-fication and matting performance speciopaci-fications.
They can be grouped as follows:
1 - Glossy transparent frits
a - for traditional double firing (a slow glost cycle)
Characterised by low temperature fusibility. They are made up of a high
per-TABLE SUMMARIZING THE CHARACTERISTICS OF THE MAIN CERAMIC
per-TABLE SUMMARIZING THE CHARACTERISTICS OF THE MAIN CERAMIC