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Ceramic tile is very often decorated with a glaze coating that seals the body and provides the tile’s outer surface with the aesthetic qualities (color, gloss, opacity) and technical characteristics (hardness, chemical resistance, scratch resistance, etc.) required for its intended use. Since single-deck kilns first appeared in the 1970s, and particularly in the last 15 years, the single-fast-firing process (simultaneous thermal treatment of the ceramic body and the glaze) has become prevalent in the manufacture of glazed ceramic tiles.
This firing process has necessitated the adaption of the materials used (e.g., glaze compositions and ceramic bodies) to meet the new conditions and demands. Thus, it has become necessary to use frits in glaze formulations in order to obtain fired glazes that display the appropriate technical and aesthetic characteristics in the short firing times (30-40 minutes) of the single-fast-firing process.
Traditional ceramic frits are materials of a glassy nature, prepared by fusion of a mixture of crystalline raw materials at high temperatures (about 1500ºC). The melt is quenched in air or water, yielding a glassy mass known as a frit. The structure of a traditional frit resembles that of glass, and may be described as a glassy network made up of network-forming cations (Si4+, B3+), network-modifying cations (R+, R2+, etc.), and the O2- anion.
Boron, a typical glaze constituent in ceramic tile manufacturing, is added in fritted form because this type of raw material is less soluble than other boron-contributing raw materials. In fact, the high solubility of natural and processed boron-containing raw materials makes it impossible to control the rheological characteristics of typical glaze suspensions.
New Semi-Processed BorateThe growing presence on the international market of low-cost ceramic tiles from Asian countries has led the ceramic floor and wall tile sector to set increasingly ambitious objectives with regard to end-product properties, cost reductions, energy savings, and the minimization of environmental impact. In this context, a new borate raw material* has been developed that allows glaze compositions to be formulated without using conventional frits. The new material’s suitability for use has been confirmed by several years of research.**
This borate, resulting from the calcination of a series of raw materials at a temperature far below that used in conventional ceramic frit manufacturing, is characterized by low solubility, which enables it to be used as a raw material in glaze formulations. It has been verified that the use of this borate allows glaze compositions with different characteristics to be obtained without the use of a traditional frit.
The five oxides that this raw material mainly contributes (SiO2, Al2O3, B2O3, CaO, and Na2O) are typical frit and glaze constituents, as are the material’s minor constituents or impurities and the proportions in which they appear. This material has a high amorphous component, while quartz is the major crystalline phase.
*Developed by Rio Tinto Minerals(patent pending WO 2007/148101).
**Research conducted by Rio Tinto Minerals in collaboration with the Instituto de Tecnología Cerámica (ITC) in Castellón, Spain.
SolubilitySolubility was determined through a method that simulates glaze composition preparation conditions. The method consists of three phases: preparation of a suspension, separation of the liquid fraction, and determination of the cation content in the liquid fraction. The suspensions were prepared by wet milling the raw materials in distilled water in a fast laboratory alumina ball mill. Sodium tripolyphosphate was used as a deflocculant, and sodium carboxymethylcellulose was used as a binder in the glaze preparation.
Solubility was determined by extracting aliquots of the suspensions after preparation. The liquid fraction was then separated in each aliquot by centrifugation and subsequent vacuum filtration using a 0.2-µm-pore-size cellulose nitrate filter. Calcium, magnesium, potassium and boron were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) in the liquid fraction obtained after filtration.
The solubility of the new boron-containing raw material (calcined borate, referred to as CB) was compared with that of a traditional ceramic frit containing 3.3% B2O3 by weight, and with that of a calcium borate (VitriborTM, CaO•3B2O3•4H2O, referred to as V), a zinc borate (Firebrake®, 2ZnO•3B2O3•3.5H2O, or F), and colemanite (Ca2B6O11•5(H2O), or CL). Table 1 details the compositions tested and the results of the determination of the quantity of boron present in the solution. Composition G-STD has been used as a reference composition and corresponds to a glaze formulation prepared with a conventional frit that contains 3.3% B2O3 by weight. The other compositions were prepared with the different test borates and provided the same quantity of B2O3 as the frit in the G-STD composition. Quartz was used as an inert filler.
As Table 1 shows, the quantity of boron solubilized by the suspension prepared with the new borate is similar to that solubilized by the standard composition. However, it is smaller than that of the other borates that might be used in ceramic glaze formulation, particularly V and F.
Glaze FormulationsGlaze compositions were prepared for earthenware wall tiles (WG), vitrified floor tiles (FG) and porcelain tiles (PG). The formulations all yielded fired glazes with a matte appearance, which is the most widespread ceramic flooring finish. A matte appearance was also chosen for the earthenware wall tile glaze (owing to the difficulty of obtaining glossy finishes with an appropriate texture), though this type of glaze is less commonly used. The glazes were characterized as follows:
- A fusion test with a hot stage microscope, in which the characteristic temperatures-TST(shrinkage start), TSE (shrinkage end), TSF (softening), TSP (sphere), T1/2 (half-sphere), and TF (fusion)-were determined.
- Dilatometric analysis by determining the linear coefficients of expansion in two temperature ranges, α50-300 and α300-500, in addition to the glass transformation temperature (TG) and dilatometric softening temperature (TDSF).
- Sealing temperature, or the temperature range in which the glaze seals completely (meaning that gas coming from the tile body cannot escape). It is important that this temperature not be too low in order to avoid surface defects (pinholes).
- Evaluation of glazed test pieces. The gloss of the glazed test pieces obtained at different peak firing temperatures (1100ºC for the earthenware wall tile glaze, 1140ºC for the vitrified floor tile glaze, and 1180ºC for the porcelain tile glaze) was determined. Also measured was glaze resistance to acids (hydrochloric and lactic acid) and to alkalis (potassium hydroxide) according to UNE-EN ISO 10545-13:1998 “Ceramic Tiles–Part 13: Determination of Chemical Resistance.”
Function of the New Material in the GlazeThe fired glaze coatings obtained with the new borate were microstructurally characterized. The crystalline structures present were identified by X-ray diffraction, while glaze cross-sections were observed and analyzed with a scanning electron microscope coupled with an energy-dispersive X-ray microanalysis (EDAX) instrument. Figures 1 to 3 show the cross-sections of the fired glazes, displaying the identified crystalline structures.
A Colorful FutureThe quantity of boron solubilized by the new borate raw material analyzed in this study was found to be low. It was similar to that of a conventional ceramic frit and smaller than that of other borates that were tested.
For additional information regarding the new borate material, contact Simon Cook, Ph.D., Director-Ceramics & Glass Development, Rio Tinto, 2 Eastbourne Terrace, London, W2 6LG, UK; (44) 020-7781-1450; fax (44) 020-7781-1851; e-mail email@example.com; or visit www.borax.com.