PIECE CERAMIQUE A L'EPREUVE DES TACHES, ET SA PRODUCTION

STAIN-PROOF CERAMIC BODY AND METHOD OF PRODUCTION

Abrégé anglais

ABSTRACT OF THE DISCLOSURE
A formed ceramic body is produced by a
method which includes the steps of producing a batch
of ceramic material, forming a tile from a first
portion of that batch, slipping a second portion of
that batch of ceramic material to render that slipped
second portion flowable, applying a coating of the
slipped second portion of the batch to the tile, and
firing the tile and coating thereon. The tile is
further made stain-resistant by grinding up the
second portion of the batch of ceramic material to
reduce the grain size of that second portion relative
to the grain size of the first portion of the batch
forming a tile, prior to slipping that second portion
and applying it to the tile.

Revendications

The embodiments of the invention, in which an exclu-
sive property or privilege is claimed are defined
as follows:
1. A method of producing a ceramic tile which
has a stain-proof surface that has an appearance like
that of unglazed ceramic, which comprises the steps
of:
a) producing a batch of ceramic material;
b) forming a tile from a first portion of
said batch;
c) grinding up a second portion of said
batch of ceramic material such that the coarsest
grain therein is many times finer than the coarsest
grain in the first portion of said batch of ceramic
material, the second portion of said batch of ceramic
material having the same composition as the first
portion of said batch of ceramic material;
d) slipping the second portion of said
batch of ceramic material, to render the second
portion flowable;
e) applying a coating of the second
portion of said batch to a surface of the tile formed
from the first portion of said batch; and
f) firing the coated tile.
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2. A method according to claim 1, wherein the
slipping step includes adding a liquefying agent to
the second portion of said batch of ceramic material.
3. A method according to claim 1 or 2, where-
in the coating is applied in a layer thickness of
approximately 60 to 70 µm.
4. A ceramic tile which has a stain-proof
surface having an appearance of unglazed ceramic, the
tile comprising:
- a ceramic body formed from a first
portion of a batch of ceramic material; and
- a surface coating on a one side of the
ceramic body, with the coating being formed from a
second portion of the same batch of ceramic material,
the second portion of the batch of ceramic material
having the same composition as the first portion of
the batch of ceramic material, the second portion
ground up to an extent such that the coarsest grain
therein is many times finer than the coarsest grain
in the first portion of the batch of ceramic material
and which, after being fired, presents the appearance
of noncoated ceramic,
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whereby the tile has a stain-proof visible
surface and uniformity of color throughout such that
abrasion of the surface does not alter the color
appearance of the tile.
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Description

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The present invention relates to a method
for producing a tile-shaped formed body of fired
ceramic material which is stain-proof on its visible
side, and to a formed body produced by this method.
It is known to provide ceramic formed
bodies, in particular tile-shaped ceramic formed
bodies, with an engobe to protect them against
abrasion and wear. Engobes are special clays,
possibly with dyes added, for dyeing ceramics and
roof tiles by firing. They are connected in a
weatherproof and firm manner to the ceramic base
material and should have the same coefficient of
expansion since otherwise cracking must be feared.
The engobes must therefore be selected extremely
carefully.
It is also known to use glazes, i.e. sub-
stantially glass-like substances consisting of
quartz, alum earth, alkalis, alkaline earths and
low-melting oxides as the flux, as a coating material
for ceramic formed bodies, in particular tile-shaped
ceramic formed bodies. The problem also arises here
that one must be very careful when selecting the
appropriate glaze materials in order to avoid
subsequent cracking.
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It is common to both types of coatings
that the appearance of the coated ceramic formed body
changes with abrasion, so that sooner or later a
relatively unattractive stained appearance comes
about, depending on the degree of wear.
This appearance problem is heightened
because it is virtually impossible in practice to
obtain an engobe or glaze color identical to the
color of the ceramic foundation covered thereby.
According to one aspect of the invention,
there is provided a method of producing a ceramic
tile which has a stain-proof surface that has an
appearance like that of unglazed ceramic, which
method comprises the steps of:
a) producing a batch of ceramic material;
b) forming a tile from a first portion of
the batch;
c) grinding up a second portion of the
batch of ceramic material such that the coarsest
grain therein is many times finer than the coarsest
grain in the first portion of the batch of ceramic
material, the second portion of the batch of ceramic
material having the same composition as the first
portion of the batch of ceramic material;

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d) slipping the second portion of the
batch of ceramic material, to render the second
portion flowable;
e) applying a coating of the second
portion of the batch to a surface of the tile formed
from the first portion of the batch; and
f) firing the coated tile.
The present invention also provides, in
another aspect thereof, a ceramic tile which has a
stain-proof surface having an appearance of unglazed
ceramic, the tile comprising a ceramic body formed
from a first portion of a batch of ceramic material,
and a surface coating on a one side of the ceramic
body, with the coating being formed from a second
portion of the same batch of ceramic material, the
second portion of the batch of ceramic material
having the same composition as the first portion of
the batch of ceramic material, the second portion
ground up to an extent such that the coarsest grain
therein is many times finer than the coarsest grain
in the first portion of the batch of ceramic material
and which, after being fired, presents the appearance
of noncoated ceramic, whereby the tile has a
stain-proof visible surface and uniformity of color
throughout such that abrasion of the surface does not
alter the color appearance of the tile.
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The invention is intended to remedy the
problems mentioned above, and proposes a method for
producing formed bodies, in particular tiles, made of
fired ceramic material, which basically retain their
appearance in spite of abrasion on the surface,
thereby avoiding a stained appearance. This invention
is particularly useful when an appearance like that
of unglazed ceramic formed bodies is desired but
surface protection is still required.
This problem on which the invention is
based is solved by applying a coating to the visible
side of the formed body before the formed body is
fired. The coating, which is in flowable form from
being slipped in a mill, is made of the batch mate-
rial used to form the formed body.
This material can be applied by a conven-
tional glaze centrifuge. In the production of tile-
shaped formed bodies by the extrusion molding method,
for example, a glaze centrifuge coating step follows
the extruding die relief but comes at the latest
before the tile-shaped material enters the baking
oven.
If striped or blotchy differences in color
appear on the surface of the tile-shaped ceramic
formed bodies when they are produced by extrusion
moiding, the fact that the coating is made of the
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batch material of the formed body (as provided
according to the invention) ensures a sufficient
covering which prevents irregularities from coming
about in its appearance. This is so even if a surface
of the ceramic body formed in this way is affected
accordingly by abrasion.
In connection with a method for producing
unglazed tiles by the dry pressing method with a
layer of decorative compound on the surface, a sus-
pension of the decorative compound is applied before
the end of the pressing process and then pressed as
well. German Patent No. 32 012 45 also reports a
method disclosed in German Patent No. 25 50 474 in
which "porphyry" is used as the decorative compound,
i.e., a continuous coating having substantially the
same mineralogical composition as that of the ground-
mass but a coarse grain, so that the different grains
are visible alongside each other.
However, these two known methods and the
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quite different problem. The objective is not, as in
the case of the present invention, to make it
possible for the appearance of the surface of the
formed body to remain unchanged in spite of the
inevitable abrasion of the surface, but rather to add
a further method to decorative technology which
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allows for new surface designs. It was not investi-
gated, and is not recognizable, whether the formed
bodies with surfaces decorated in this manner acquire
an unattractive appearance in the case of abrasion.
According to a preferred embodiment of the
invention, a liquefying agent is added to the portion
of the batch forming the coating in the course of
slipping.
According to another preferred embodiment,
the coating is applied in a layer thickness of
approximately 60 to 70 lum.
As the portion of the batch to be slipped
is ground up to an extent such that the coarsest
grain therein is many times finer than the coarsest
grain in the portion of the batch forming the tile,
the stain-resistance qualities of the coating on the
tile after firing are improved, i.e., even unintended
stains of inks or similar dyeing substances can be
removed without leaving any traces.
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