|Monthly Tech-Tip |
A history, technical description of the process and body and glaze materials overview of the monoporosa single fire glazed wall tile process from the man who invented it. By Nilo Tozzi
The word “monoporosa” is generally referring to glazed wall tiles which are single fired and it is used to distinguish the process from double firing. From an historic point of view term is used from 1982, namely from the time we obtained first glossy tiles with suitable characteristics for interiors covering, like tiles obtained by traditional process of double firing.
Dr. Nilo Tozzi, during his cooperation with Mr A. Brusa of Sacmi Imola, prepared first frit for this technology which also required new bodies and new firing cycles. Perhaps original idea born in Spain where a couple of companies started single firing of glazed wall tiles but bodies contained chamotte, without carbonates so bodies show a shrinkage and glazes where made by normal frits. Only when Eng. A. Villa and A. Brusa of Sacmi decided its development, idea became an industrial process.
The first tiles obtained by a semi-industrial process (in the Sacmi pilot plant) were shown at 1982 CERSAIE in Bologna. Ceramica Faetano in San Marino Republic was the first factory to produce monoporosa tiles in 1983. To adapt the the process monoporosa process companies needed hydraulic presses (instead of impact presses that had been used for wall tiles) and roller kilns needed more burners in the preheating zone.
However, monoporosa quickly became more popular in Spain than Italy because their clays had a low content of organic matter and higher levels of quartz. Spanish glaze suppliers quickly proposed new frits for monoporosa, early formulations grew from the simple observation that matt glazes never showed defects when used for monoporosa and that a mix of similar percentages of a standard calcium matt glaze with a zinc matt glaze produced an almost glossy glaze. This phenomenon occurred because the rapid cooling in the process prevents re-crystallization and the concentration of crystallizing elements is not sufficient to seed and develop them. The rapid growth of the monoporosa process in Spain created a demand that enabled glaze suppliers to enhance the quality of frits.
Body compositions for monoporosa red or white contain a significant amount of calcium carbonate or dolomite which, during firing, produce porous bodies with almost zero shrinkage. Generally the content of carbonates is in the range 8 – 15% and they can be sourced from clays naturally containing carbonates, from rocks containing carbonates or by natural carbonates.
The role of carbonates in the body is thus twofold: The development of new crystalline phases during firing that boost mechanical strength and the production of a fired product with almost zero shrinkage.
Besides the carbonates the body is composed of:
In the monoporosa system, carbon dioxide release occurs at temperatures lower than normal decomposition temperatures of natural carbonates of calcium and magnesium. For example, calcium carbonate decomposes at about 880C when alone but when mixed into a ceramic body its decomposition starts at about 700C (after which the calcium oxide quickly reacts with the clays). The reaction accelerates as the temperature rises and free calcium oxide disappears in the range 900–1000C. The reaction between the calcium and the products of clay decomposition (like silica and amorphous metakaolin, a product of dehydroxylation of kaolinite) produces crystal phases like wollastonite (CaSiO3), anorthite (CaAl2Si2O8) and gehlenite (Ca2Al2SiO7). From x-ray diffraction patterns at different temperatures we observe that the calcium-based gehlenite is the most important new phase until about 950C, after which it slowly disappears giving way to anorthite.
Dolomite exhibits reactions similar to calcium oxide however the starting temperature of carbon dioxide evolution is around 650C. Magnesium oxide also reacts with metakaolin giving diopside (MgCaSi2O6) and forsterite (Mg2SiO4) but a large amount remains as oxide periclase (MgO) even at high temperature (it is evident in post-fired products using x-ray diffraction). Crystal phases resulting from reactions of calcium and magnesium oxides have needle shapes so that they form a skeleton network that imparts mechanical strength to the ceramic body and prevents shrinkage during firing.
From a practical and technical point of view the behaviours of calcite and dolomite in the monoporosa process are similar (substituting one for the other produces similar mechanical strengths and porosities). Considering that magnesium has lower atomic weight than calcium we could speculate that with dolomite we would obtain a larger amount of new formation phases. However this hypothesis fails in practice because part of the magnesium remains in the oxide form in the body after firing. The key difference is often just the cost because in several countries dolomite is cheaper than calcite.
Post-expansion issues sometimes present a problem, tiles can expand after several months (which of course causes glazes to craze). For a dolomite magnesium oxide process, periclase could be implicated as the cause, however we must consider that this oxide is very stable (we can find it in natural sediments that span geological eras). An alternate hypothesis seems more plausible: Post-expansion results from a poor structural arrangement of new crystal phases, these are prone to re-hydroxylation in humid environments. Post-expansion has also been observed for double fired tiles where both firing cycles are fast, but it is rare where the body is subjected to a mult-hour cycle.
Thermal expansion is an important body characteristic to understand and control. A good planarity after firing is only possible where there is a good fit between the thermal expansions of body and glaze. While some firing adjustments are possible (temperature gradient, bottom rollers) the physical thermal expansion match between body and glaze is the most important variable.
Glossy glazes for monoporosa contain high percentages of frit (having high calcium and zinc oxide contents) so they exhibit a low thermal expansion (during cooling in the kiln a lower expansion body shrinks more than glazes and tiles tend toward convex). Usually monoporosa tiles with glossy glazes show some degree of convexity after firing but the extension of this defect quickly decreases during storage of tiles (because a post expansion occurs caused by atmospheric moisture). The speed of convexity decrease depends on body composition and it can vary from some hours to some days. The process can be enhanced by dipping the tiles in water. Thus we adjust percentages of quartz and sandy clays in order to have a suitable thermal expansion to ensure a good planarity several hours or days after firing. Usually the limit value for thermal expansion is around 65·10-7 cm-1. Of course, considerable technical effort can be required to achieve tiles with the correct degree of convexivity so that the final equilibrium state is flat and uncrazed.
Shaping by press is a relatively simple task and no compensatory measures to adjust for size are needed because the body does not shrink. However, we have to control the apparent specific weight and it must be less than 2.1-2.2 gr/cm3 for the entire area of tiles in order to avoid degassing problems. If the density is too high gases cannot escape at release temperatures and we have blisters on the surface of glaze.
Frit content is in the range 20-40%, depending on maximum firing temperature, while other components are typical of glazes: clays, feldspars, nepheline, kaolin, micronized zircon, alumina and quartz. The content of plastic materials must be suitable to ensure a homogeneous drying of glaze.
Sometimes it is necessary to consider thermal expansion of the engobe in order to avoid excess concavity of tiles after firing. In such cases a strong addition of quartz can help to reduce the problem (because the mineral exhibits a large volume reduction during cooling and reduces the thermal shrinkage of the engobe). It is also possible to employ a frit having a high thermal expansion.
It is often necessary to use an engobe as both a water and light barrier. Large percentages of micronized zircon (usually more than 20%) can be needed to obtain a sufficiently opaque and white engobe. At the same time we have to regulate fusibility of the engobe in order to have a complete sintering so water can’t pass through.
Monoporosa frits for glossy surfaces have unique characteristics because they sinter and melt both within a narrow range and at a higher temperature than typical frits (because they have a high content of calcium and zinc oxides). During firing there is an evolution of carbon dioxide from the body that can continue until 950C. This maximum temperature depends on heating gradient and thickness of ceramic body (because an adequate temperature for carbon dioxide elimination must be reached right to the core). Thus glossy glazes must remain highly porous and permeable even above 1000C to allow gas evolution from body and avoid pinholes or blisters. Since matt glazes always exhibit this kind of behaviour anyway, we never detect defects on surfaces.
Because we maintain calcium and zinc oxides levels at around 10% in glossy frits (opaque or transparent) they crystallize during heating and these new crystals (wollastonite and willemite) keep glazes in the solid state delaying the sintering process so gases can pass through. When the temperature is high enough, usually for normal monoporosa glazes above 1050 C, we observe the formation of a liquid glassy phase which subsequently takes the crystals into solution to form a mature glassy glaze. In roller kilns the cooling gradient is very quick and tiles pass from temperatures around 1100C to around 600C within a few minutes, so we cannot observe any new crystallization of calcium and zinc compounds to marr the glossy surface (this rapid transition denies the conditions necessary to form the crystals that would otherwise grow). In this way we also obtain frits maturing in the range of firing of monoporosa bodies (the so called “high temperature frits” that are suitable even for higher temperatures and floor bodies).
Usually monoporosa frits contain low percentages sodium, potassium and boron oxides because they actively generate a glassy phase during heating (which is obviously dangerous for the above reasons). Usually boron oxide is in the range 0-3% (in addition it quickly reduces viscosity of the melt during soaking and this could generate pinholes on the surface). Glossy glazes are made using high percentages of frits (90-95%) plus kaolin, clays and micronized zircon. Both engobes and glazes contain about 0.2-0.3% of sodium CMC and 0.1-0.2% of STPP: Usually glazing is by bell or suitable devices to obtain the smooth laydown needed for wall tiles.
Compared to normal firing cycles for floor tiles, monoporosa firing cycles show a longer preheating. This is a direct consequence of the reactions of carbonates present in bodies (the kinetic of reactions corresponding to carbon dioxide elimination is quick only in the range 800-950C). Practically all tiles are quickly heated to the range 800-1000C for enough time to guarantee organic matter combustion, if present, and carbon dioxide evolution from carbonates. For this reason the kiln must have sufficient burner capacity in the preheating zone. Once carbon dioxide is expelled tiles are quickly heated to the soaking temperature of the glaze (by design that is also the same temperature that guarantees a good mechanical strength for ceramic body). Soaking time is normally less than 5 minutes, which is enough time to completely melt the glaze but not enough to allow bubbles to grow.
There have been recent developments in glazes for monoporosa: they are porous and solid until above 1100. For them preheating can be shorter and soaking temperature is set near half sphere temperature.
Right now both processes cohabit for the production of wall tiles, each one with its negative and positive aspects. Monoporosa requires a covered plant area that is smaller than double firing and a smaller investment with a direct production cost that is also lower for similar products. On the other hand monoporosa shows a lower quality in cases where raw materials for the body are not perfectly suitable or where there is not a tight control of the process.
During the last decade the monoporosa process became more popular because companies producing porcelain tiles for floor can also produce monoporosa wall tiles by simply adding calcite or dolomite to the ceramic body (the one they use for floor tiles). Using this method we wet-mill a mixture of calcite or dolomite with a small amount of clay or kaolin and afterward mix the carbonate slurry with the floor tile body composition slurry (in the required proportions). This can be done easily using mass-flow meters. Once the mixture is spray-dried we have the powder for press. This method enables us to obtain body powders for floor and wall tiles by just preparing one basic composition and using the same equipment with the addition of a single discontinuous mill.
This comment was later made by Mr. Tozzi: I don't have cost comparison data with me (from Italian companies). For what I remiember energy savings for monoporosa are about 25% in terms of specific consumptions of natural gas and electrical power. Usually total manufacturing cost is about 15% lower for monoporosa but this value depends on local cost of energy. As noted, double firing needs a bigger investment, a bigger covered area and more manpower. In general the quality of glossy tiles is better for double firing.
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The term Terra Cotta can refer to a process or a kind of clay. Terra cotta clays are high in iron and available almost everywhere. While they vitrify at low temperatures, they are typically fired much lower than that and covered with colorful glazes.
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Glaze Types, Formulation and Application in the Tile Industry
An technical overview of various glaze type used in the tile industry along with consideration of the materials, processes and firing.
By Nilo Tozzi