Porous, vitrified and semi-vitrified are some types of ceramic bodies whose formulation and design criteria are described in this article. There are four essential components for body formulations for ceramic tiles:
Clay works as a flux (e.g., illite) or mullite precursor during firing, stabilizing suspensions (in the wet process), ensuring the plasticity required for shaping, and providing mechanical strength to unfired tiles (e.g., kaolinite).
– Flux; feldspars are the traditional flux, but sericite, feldspathoids, and other raw materials are also used. Flux forms a liquid phase at high temperatures that permits the ceramic body to be densified through a viscous flow.
– Filler, which offers a coarse-grained framework (a “skeleton”) to contrast deformations during drying and fire; quartz sand is the most frequent filler, though chamotte or other sand types may also be employed sometimes.
– Additions, such as pore-forming agents that enhance sintering by surface diffusion and generate porosity during firing (carbonates typically play this role); additional additives include pigments, opacifiers, and sintering promoters.
Technology advancement has made tilemaking processes more adaptable, enabling the creation of ever-more complicated batches, which often contain up to a dozen different raw materials.
Utilizing many sources of clay, flux, and filler is a common industrial procedure to dump any variations in composition and technological qualities.
Body formulation is tightly related to the essential characteristics of each ceramic tile typology since it is focused on both the technological behavior and technical performance of completed goods.
Particular emphasis is placed on the following fundamental characteristics about conduct during the tile-making process:
bending strength of unfired tiles must be high enough (typically N4 MPa) to withstand wet glazing and mechanical stress of screen printing (this requirement can be relaxed in case of non-contact decoration, such as by inkjet printing);
porosity and linear shrinkage that must match the target after a proper firing schedule; absence of bulk; and rheological properties of slips, in the wet route, that have a significant impact on the efficiency of milling and spray drying (black core, planarity, glaze appearance, etc.).
For each tile typology, the technical performances are in accordance with the requirements of international standard ISO 13006; the following are particularly pertinent: – Coefficient of thermal expansion (CTE), which ensures that the body-glaze thermodilatometric matching, preventing crazing defects;
– Water absorption to fit the window of acceptance for each class; – Modulus of rupture and resistance to deep abrasion according to each class;
– Resistance to moisture expansion to prevent damages to flooring and wallings. The body color after firing is another crucial commercial factor.
For ease of explanation, the criteria for body formulation will be broken down into three major categories: vitrified light-firing bodies (GWS, GPS, UPS), semi-vitrified and vitrified red bodies (GRS), and porous bodies (WBR, WMP, RBR, and RMP typologies).
These three categories roughly correspond to the BIII, BII, and BI classes of the ISO 13006 standard, respectively.
However, it must be taken into consideration that the industry occasionally develops multifunctional bodies; a single batch can be created to produce two distinct typologies (for example, BIII or BIIb; BIIa or BIb) by simply altering the processing parameters.
Ceramic Porous Bodies
For both colored bodies (RMP and RBR) and white bodies, wall tiles with ceramic porous bodies (water absorption N10%) are produced using either the monoporosa technology (quick single fire) or the birapida technology (fast double firing) (WMP and WBR).
Industrial firing schedules typically operate at maximum temperatures between 1000 and 1140 °C with cold-to-cold times of 30 to 45 minutes.
Porous bodies are made to obtain net-shaped tiles, whose final dimensions match those of the pressing mold. As a result, shrinkages during drying and fire must be extremely minimal, never exceeding 1 cm/m.
The required minimum bending strength of 15 MPa limits the amount of porosity. Since porous bodies are always glazed, extra care must be taken to ensure that the body and glaze’s.
thermodilatometry match by regulating both CTE (generally between 6.5 and 8.0 MK-1 degrees) and moisture expansion (recommended at 0.1% moisture).
This overall goal is more easily attained with calcium-rich bodies because they produce Ca-Mg silicates (plagioclase, pyroxene, melilite, etc.) that are stronger and more resistant to moisture during burning than low calcium bodies, even those with significant porosities.
The literature provides a thorough explanation of the phase changes that take place when carbonatic clays and porous materials are heated.
Designing appropriate porous bodies can be done in one of three ways: P1 involves employing carbonate-bearing clays (MC and CC); P2 involves adding calcite.
or dolomite to the batch; and P3 involves using raw materials other than carbonates, such as calcium or calcium-magnesium.
Red bodies are typically produced using route P1; RMP and RBR tiles are produced using route P2 only when a suitable carbonate-bearing clay is not readily accessible.
Route P2 is always used to generate white bodies, but if neither calcite nor dolomite are available (or large size, high quality WMP tiles have to be produced without carbonates).
In these situations, route 3 presents a variety of options for raw materials containing alkaline earth oxides, including talc, wollastonite, diopside, and synthetic silicates (including frits and glass–ceramic systems).
In any case, for WMP and particularly WBR, the intended color of the burned body is white (the batch’s Fe2O3 content is typically between 1% and 2% wt dry basis).
Although color is not very important for RMP and RBR, burned bodies typically have a Fe2O3 composition of 4 to 6% (and a maximum of 8% wt).
The wet route’s porous tiles (especially white bodies and numerous red monoporosa plants) forbid the use of raw materials rich in expandable clay minerals (BC3 and some RC3 and MC cannot be utilized as main components).
With the help of monoporosa technology, it is possible to simultaneously fire the body and glaze, which has obvious advantages.
in terms of cost and production. But because of the quick firing cycle, calcite thermal decomposition occurs at fairly high temperatures and may coincide with glaze melting; if the CO2 emitted from.
CaCO3 breakdown goes through a melted glaze, it will result in flaws (pinholes). The kinetics of calcite (or dolomite) breakdown must be sped up in order to prevent pinholes.
This can be done by: I reducing the carbonate particle size; ii) promoting gas permeability (by lowering the bulk density of unfired tile by lowering the load at the press); and iii) restricting the batch’s total carbonate content.
Carbonates in monoporosa bodies are limited to 7–12 wt% (at most 15%) for this reason
. It suggests that in order to limit gas permeability, MC must be within 60% of RMP batches (much less in the case of CC) and contain some RC along with filler or RL.
While RL is favoured in Spanish-style batches, Filler is given preference in Italian-style bodies. Due to the fact that WMP bodies.
are made from batches of stoneware, their composition may include low-grade kaolins, kaolinitic loams, ball clays, or pyrophyllitic clays, as well as considerable amounts of filler and fluxes.
With the exception of the increased cost of a second fire, the manufacture of porous tiles is now more straightforward thanks to the development of Birapida technology, which overcame the technological limitations of monoporosa.
The proportion of carbonates in the batch can be more than 12-15% because the body is fired without a glaze at first, but percentages above 20% (which were typical in the traditional majolica and calcareous earthenware industries) are not advised.
As there is no requirement for gas permeability, RBR batches are based on 60–80% marly clays adjusted by some red clays, with minimal filler or RL. A greater quantity of RC is required if CC are used.
The fundamental difference between RBR and RMP bodies is really the higher MC content and the frequently lower filler and flux percentages.
WBR, on the other hand, uses kaolins, a lot of filler, and occasionally fluxes because it is made from batches of calcareous earthenware.
In comparison to WMP batches, WBR bodies show both a higher kaolin-to-ball clay ratio and more filler. Wollastonite without carbonates is frequently used to flux pyrophyllite-containing batches.
Vitrified and Semi-vitrified Bodies
For floor tiles in the BIb and BIIa classes (water absorption of 0.5-3% and 3-6%, respectively), vitrified and semi-vitrified red-firing bodies are used; semi-porous bodies with water absorption of 6-10% (BIIb) are directed to wall coverings in addition to flooring.
Regardless of whether they fall under the BIb, BIIa, or BIIb categorization, this substantial group of items goes under the trade term red stoneware with its variations.
(glazed, unglazed, rustic). Red stoneware is produced by quick single firing at a maximum temperature of 1120–1180 °C in 20–50 minutes from cold to cold.
Sometimes heavy-clay industry processes rustic tiles through a slow, single firing. Nowadays, the dry approach is used to manufacture red stoneware more commonly.
than the wet method. Only rustic tiles and a few low porosity tiles for outdoor applications (BIb) are unglazed; the others are nearly always glazed.
Mineralogical changes that take place during the fire of low porosity red bodies are often less complicated than those that occur in batches that contain carbonate.
In actuality, they primarily involve the disintegration of feldspars and clay minerals, which result in the production of new crystalline phases such as mullite and, less frequently, sanidine, spinel, and olivine (with development of a more or less abundant vitreous phase).
One distinguishing characteristic of these entities is the crystallization of hematite from Fe-oxyhydroxides. Designing red stoneware bodies.
with firing schedules that must be as quick (best 40 min or less) and as cold (best 1170 °C or less) as feasible can help meet the standard aim.
(water absorption, mechanical strength, etc.). As firing deformations must be kept under careful control, firing shrinkage must be within 6 cm/m (BII) or 9 cm/m (BIb).
It is helpful to have a specific quartzous skeleton for this purpose (usually 20–30%), which also aids in obtaining the right CTE (7-8MK1).
Beyond some convergence of requirements, different properties are anticipated for bodies processed by the wet or dry approach.
For example, highly plastic clays are tolerated in small amounts during both the wet milling and dry cycle (as they enhance the slip viscosity) (they are difficult to moist by granulation).
Generally speaking, coarse-grained clay materials should not be used in the dry route, and expandable clay minerals should be avoided in the wet way (recommended b5% in the batch) (limiting particularly the silty fraction that is little affected by dry milling).
Because the proportion of Fe2O3 is typically between 3 and 7 percent (or up to 9 weight percent), the body color ranges from light red to dark brown (when MnO2 is added to prevent a black core).
Rustic to glazed tiles use different formulation techniques, and these latter use semi-vitrified or vitrified materials. Rustic tile.
batches are straightforward and frequently composed entirely of red loam that is pliable enough for extrusion; alternatively, red clays are adjusted with a filler.
Vitrified red stoneware, or 5.2.1 The classic unglazed stoneware was transformed into vitrified red stoneware by modifying the technological behavior of red clays to meet the demands of quick single fire.
To enable the rapid firing rates, batches were modified by adding some flux (to enhance sintering kinetics) and some filler (to contrast black core and pyrodeformation).
As a result, the following standards are used in the sector to create red stoneware bodies: – Red clays that are fine-grained, frequently plastic, and refractory (such kaolinitic kinds RC1).
can be utilized up to 30% since they require the addition of both flux and filler, which are typically added at a rate of 20% each; – raw materials, such as various.
RC2 and RC3 clays, that naturally contain sufficient feldspars, illite, and/or chlorite to be “self-sintering,” can be used in significant proportions (35–65%), with filler,
often below 20%, to compensate; – Red loams (RL) typically have enough coarse-grained fraction (when used, their amount is 25–40%) to replace both filler and clay at the same time.
Due to the versatility of body formation, it became possible—and frequently necessary—to combine two or more of the aforementioned criteria. This included combining various red clay types and even various fillers and fluxes.
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