Tile glaze compositions can be formulated with a new semi-processed borate raw material without using traditional frits.
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
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
, etc.), and the O2-
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 Borate
The 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
, CaO, and
O) 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).
by Rio Tinto Minerals in collaboration with the Instituto de Tecnología
Cerámica (ITC) in Castellón,
Solubility 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% B2
and with that of a calcium borate (VitriborTM
referred to as V), a zinc borate (Firebrake®
or F), and colemanite (Ca2
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% B2
by weight. The other compositions were prepared with the different test borates
and provided the same quantity of B2
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 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
- 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
- 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).
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
- 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.”
The characterization of the three ceramic glaze compositions (see Table
2) shows that glazes can be formulated for different types of ceramic tiles
without needing to use traditional frits as glaze materials. These glazes
exhibit characteristics similar to those of traditional frit-containing glaze
compositions. The borate content in the fritless glazes was larger than 25% by
Figure 1. Cross-section of the WG-fired glaze coating.
Identified crystalline structures: Q = quartz (SiO2); W
= wollastonite (CaSiO3); An = anorthite
and Ab = albite (NaAlSi3O8).
Function of the New Material in the Glaze
The 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.
2. Cross-section of the FG-fired glaze coating. Identified crystalline
structures: Q = quartz (SiO2); Co = corundum
(Al2O3); ZrSi = zircon
(ZrSiO4); and Geh = gehlenite (Ca2Al2SiO7).
of these structures correspond to starting raw materials that were not
incorporated into the glassy matrix, like quartz, albite, zircon and corundum.
The remaining crystalline structures formed during the glaze firing process.
Figure 3. Cross-section of the PG-fired glaze coating.
Identified crystalline structures: ZrSi = zircon
(ZrSiO4); Co = corundum
(Al2O3); Geh = gehlenite
and Gah = gahnite (ZnAl2O4).
main borate function is thus shown to be glassy-phase formation, though the
borate may also encourage the formation during firing of crystalline structures
that contain some elements that originate from the borate (Si, Al and Ca),
namely anorthite (CaAl2
In these glazes, the calcined borate is also the source of the identified
semi-processed borate material.
A Colorful Future
The 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.
new borate readily forms glassy phase and can be used as a fluxing raw material
in ceramic glaze compositions. This new semi-processed borate has enabled the
successful development of fritless glaze compositions for earthenware wall and
floor tile glazes with good aesthetic and technical characteristics.
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 firstname.lastname@example.org;
or visit www.borax.com.