Glazed ceramic floor tiles are becoming fashionable world-wide.
But glossy finishes are not very hard-wearing and shiny tiles show
wear more easily than rough ones. So a consortium of tile makers and
distributors got together to commission research on how to make glossy
but hard-wearing glazes for floor tiles.
There was a need in the ceramic tile industry
to develop floor tiles with a very white and glossy surface
and with very good resistance to wear by abrasion.
Shops and restaurants like glazed floor tiles
because the smooth, shiny surface does not retain dirt and is easier
to clean. But until now glossy tiles have not been as hard-wearing
as rougher tiles, and they scratch easily. In areas of high traffic
the surfaces rapidly deteriorate, losing the gloss and getting dirty.
"There was a need in the ceramic tile industry to develop floor tiles
with a very white and glossy surface and with very good resistance
to wear by abrasion," says Ana Gozalbo, of the Instituto de Tecnología
Cerámica (ITC) in Castellón, a research institute dedicated to ceramics.
"The problem is that where you have a smooth and white or very clear
surface you can see wear more easily than with very rough and decorated
ITC were approached by Keraben SA, a well-known Spanish manufacturer
of wall and floor tiles, to see if they could devise a new kind
of glaze that would not be so vulnerable to wear. Together with
tile manufacturers Gres de Nules and other international partners
they put together a CRAFT project that started in 1993. Other partners
included distributors of tiles in Germany, Ireland and the United
Kingdom, and an Italian company - Esmalglass - who supply the materials
for making glazes for tiles.
Melting the frit
Tile glazes are made of a kind of glassy coating. "The tile has
a ceramic body with a very thin layer of vitreous material on top
and this is made of 'frit', which is a sort of glass," explains
Ana Gozalbo. "You have a raw material like quartz, kaolin, or feldspar,
then you mix them finely and then they are melted in a furnace at
about 1500°C and then quenched in water. So you obtain fine particles
of vitreous material, a glass, which is applied to the surface of
the ceramic body then fired to obtain the final product. This surface
layer is what gives the product its decoration."
When the tile is fired, the glassy layer "devitrifies" forming
a tough, crystalline material that gives the surface its hard-wearing
properties. "You need those crystalline faces to be as hard as possible,"
says Mrs Gozalbo. The ITC researchers chose as a starting material
a mixture of silica, alumina and zinc oxide which was used to make
a range of frits. By using different mixtures of the basic components
and adding extra ingredients the team made a wide range of different
frits of varying properties. "With these frits you can mix them
with other constituents in order to give different characteristics
to the surface. And every different mixture is treated in a different
heating cycle to give a different glaze. There were hundreds of
them because not only were we making products here, but also Esmalglass
and Gres de Nules were doing experimental work in pilot and industrial
Cross-section of one of the glazes developed
in the project, viewed with a scanning electronic microscope.
The white zones correspond to gahnite crystals that have formed
surrounding the surface of the frit particles..
But hardness is not the only desirable property of a glaze. "The
whole crystalline and glassy mass has to have an expansion coefficient
similar to that of the ceramic body. As you fire the product the
glass sticks to the ceramic body. When you cool the product down
you get problems if they have a difference in their expansion coefficients."
The glaze can crack and come away if it does not contract at the
same rate as the tile itself.
Another important characteristic is porosity, because porous surfaces
will hold dirt. But because the final glaze contains crystalline
particles, it is impossible to make a glaze that is not porous to
some degree. "The most important thing is that it isn't an open
porosity," Mrs Gozalbo points out. "It is just in the inner part
of the glaze so it is not open at the surface. But if it is very
porous then it will retain dirt and in that case it is not useful
even if it is very hard."
Of the hundreds of glazes tested, a few were selected to be developed
on a pilot scale at the premises of Keraben and Gres de Nules. CRIBC
in Belgium tested the mechanical properties. The new glazes were
excellent in most respects, but although they were hard they were
not as hard as the researchers had hoped.
Glazes in production
"Some of the goals were too high to attain," admits Mrs Gozalbo.
"It's very difficult if you have a smooth glassy product to raise
the Mohs' hardness. But the glazes did have characteristics that
are needed in a ceramic style floor: they weren't attacked by acids
and bases, they didn't stain, they didn't retain dirt, and the most
important thing was their resistance to wear by abrasion. Also they
were quite glossy for a floor tile."
The project finished early in 1995, and Keraben and Gres de Nules
then had exclusive use of the new glazes for two years. Now being
commercialised more widely, the glazes can be employed in normal
manufacturing processes and are no more costly than conventional
glazes. The methods developed to identify the mechanical properties
of the glazes in the project can now be used to characterise glaze
properties in a systematic way that was not possible before.