Background of the Invention Ceramic tiles covered by a glaze are routinely provided with a decor. The decor is printed on said tile prior to providing this tile with a glaze. Another known method is to provide the glaze with a printing or to provide said printed glaze with another glaze layer. But all the mentioned methods can also be combined.

It is known to provide plate-like facing materials, so as ceiling or wall panels, tiles, advertisement boards and like elements, with decors using special or unusual light effects, light reflected back by matt or glossy surfaces, light reflected back after the light passage through the material surface layers by objects provided with reflective surfaces provided in the interior of said material or by light reflected after the light passage through the whole material by means of mirrors provided on the back side of said material.

It is also known to produce colour decors showing a rainbow effect caused by a li-ght passage through a system of transparent materials and optical layers, where the resulting decor varies in accordance with the illumination and vies angles.

It is also known to produce hidden decors giving a colour effect by illuminating the respective element provided with-a surface relief which relief is comprised of a series of traces formed by planar or relief structures, whereby, said structures exhibiting a light diffraction effect, when, at given illumination and a view angles one or more images are generated.

Another already known solution is represented by a decor of an object, which decor is evoked in dependency on a variable angle of view, which object comprises a translucent plate-like element and, further, two mutually crossing rasters, which rasters are placed adjacently on both opposite sides of said plate, whereby, based on light interference phenomenon, a certain system of lines, the so called moire effect is produced.

Patent CZ 274,488 discloses a decor in the form of a relief pattern on a sheet of organic or inorganic glass. On the back surface, said glass sheet is decorated by a raster, which raster is comprised of optional elements and the face surface of said glass is decorated either by a negative of said raster, which raster is enlarged or diminished within the range of 0 to 15 % or by another raster which raster is differing from said back surface raster by plus minus 15 %, whereby, by mutual optical overlaying of both rasters a three dimensional pattern is created which pattern forms a three dimensional glass decor. The elements can consist of lines or circles, etc. To enhance the effect, it is possible to provide the back surface of said glass e. g. with a paint layer or with a milky glass. Optionally, it is also possible to combine the three dimensional decor with a normal flat printing or with another traditional decorative technique.

None of the above mentioned solutions makes possible to create in one arrangement of the optical environment a whole number of patterns accompanied by light effects that would be of a satisfactory quality and that would be created in a simple manner and at moderate costs.

Summary of the Invention It is an object of present invention to create various decors and colour effects on at least partially transparent and hard materials so as on a glaze of ceramic tiles in a simple manner by means of optical phenomena.

The subject matter of the present invention is a ceramic tile provided with a decor. The decor is produced by means of light effects. The light effects are obtained by light passage through an at least partially transparent material which material is provided with a system of arranged formations, whereby, the formations consist in that they comprise a face printing on the face side of said at least partially transparent material which face side is formed by a tile glaze and further a back printing situated behind the back side of said partially transparent material, whereby, said printing is formed by a plane raster containing parallel lines and slits or characters grouped into formations having slits between them, whereby, the number of said formations of said back surface printing and of said slits of said face printing is different and the prevailing direction of said back printing formations is to more than 50 % identical with the direction of said slits of said face printing.

Preferably, the tile glaze thickness is at least 0,1 mm.

Further, it is preferable if said face printing is formed by a line raster which line raster comprises a system of parallel lines and slits and if said back printing is formed by a system of characters which characters are grouped into lines, whereby, said lines are separated by slits from each other.

Optionally, said face printing is also formed by a line raster which raster comprises a system of parallel lines and slits and said back printing comprises a system of characters which characters are grouped into formations which formations have the form of curves with slits between said curves.

Preferably, the direction of said back printing formations locally diverges by an angle up to 30 degrees away from the to more than 50 % identical prevailing direction of said formations.

In one preferable embodiment of said ceramic tile, the tile glaze is provided by a face printing on the front face side of said glaze, whereby, said back printing, situated behind the back side of said tile glaze, is formed on the front face side of a bottom glaze which bottom glaze is provided between said tile glaze and a base ceramic body of said tile, whereby, the face side of said tile glaze is covered by a top glaze.

Said top glaze can also be provided with a printing.

According to another preferable embodiment, said tile glaze is provided by a face printing on said front face side, whereby, said back printing situated behind the back side of said tile glaze is provided on said front face side of said bottom glaze which bottom glaze is situated between said tile glaze and said basic ceramic body of said tile, whereby, said printing is made by an abrasion resistant ink.

The subject matter of this invention makes possible to create decors on hard materials, e. g. on ceramic tiles or glazes of ceramic tiles which decors are of such a quality that could not be reached by the usual decorative techniques. The invention is relatively simple to practice and the products are of moderate costs and, therefore, suitable for a broad use. In case that fired inks and crushed glass are used, the printed materials are very abrasion resistant. Therefore, such materials provide a possibility to use the resulting product e. g. as facing tiles. This technical solution makes also possible to imitate easily also the so called metallic pigments having the colours of copper, silver or gold, etc.

The desired colour depends only on choosing the appropriate combination of optical methods used in the decor production.

Further, it is preferable if the direction of the printing formations on the rear back side of said tile declines locally by up to 30 degrees away of the for more then 50 % identical prevailing direction of formations. By this measure, it is then possible to produce locally more or less differentiated interference maxima and minima that are manifested by a substantial brightening of the face decor or in reverse by its darkening what can be used to express shadow and light in the objects depicted.

Brief Description of the Drawings An embodiment of a ceramic tile with a decor according to this invention is described below with reference to the drawing attached showing a sectional view of a ceramic tile with a decor.

Detailed Description of the Invention A special pattern and a colour effect of a ceramic tile according to this invention are obtained on the basis of such optical phenomena as are the diffraction of a light ray, interference, dispersion, reflection and by mutual exposing or covering of the corresponding points of a face and a back printings on an at least partially transparent and hard material of said ceramic tile glaze in relation to the angle of view.

The material that is to be used to obtain the effect according to the present invention is an at least partially transparent and hard material, a tile glaze 3, a plate material of certain thickness. The tile glaze 3 is provided with printed patterns both at its face side and behind its back side, the pattern having the form of a plane line raster or a raster with characters grouped into formations, the formations being in the form of lines and slits, whereby, the prevailing direction of said slits of said printing of said back printing 6 is, in an extent being more than 50 %, identical with the direction of slits of said face printing 5. The face side of said material is understood to be the front face side facing away of said ceramic basic body 1 of said tile and the back side is understood to be the back inner side of said material which is turned to said ceramic base body 1 of said tile.

It is apparent that so as the angle of view is changed and depending on the thickness of the transparent material the mutually co-operating points of both printings 5,6 change. So e. g. in the view direction at an angle of 90 degrees to the front face side of said material, the light conditions of the points situated at intersections of a line perpendicular to said front face side with the face printing 5 and to that with the back printing 6 mutually co-operate. Then, when the view angle to the same point on the front face side is changed by an angle a so that the view angle to this point on the front face side of said material is 90 degrees + a, the light conditions of the points situated at the intersections of both printings 5,6 with a line situated at the changed angle with regard to the front face side of said material co-operate, and thereby, the view is changed in comparison to the previous view direction because the intersection point of the changed line with the back printing 6 differs from the intersection point of said perpendicular line with said back printing 6.

The above mentioned optical phenomena can be used, individually or mutually combined, to provide a desired decor exhibiting various patterns and colour effects, For example, it is possible to use diffraction, dispersion and interference in such a way that in said printing 5 of said face side 3 of said at least partially transparent material, tile glaze 3, diffraction openings or gaps are formed, what produces interfaces between different optical environments, whereby, the passing light rays bend at interfaces of different optical densities and partially decompose to individual spectral colours and along their further optical path they then interfere depending on their mutual vicinity.

The bright or the dark interference points on said back printing 6 than influence the resulting brightness of said decor. The diffraction openings can be made e. g. by a laser gun, which gun removes some parts of the previously printed surface or by a special printing procedure or in the most easy way by admixing transparent grains of a size corresponding to the thickness of the printings 5,6, e. g. of grains of crushed glass or of Iceland spar, into the printing inks in accordance with a graphic design. The method of making diffraction openings can exhibit an influence on break-up of light spectrum to some extent. The strongest effect is obtained by using crushed glass as it brings about multiple refraction and thereby contributes to a higher spectral break-up.

In accordance with the present invention light ray reflection can be accomplished e. g. by a printing applied to the back printing 6 using inks of various ability to reflect light, whereby, small mirror areas are formed. But it is also possible to use a composition of inks and crushed transparent material, e. g. of crushed glass. Such a composition forms both mirror surfaces and refractory interfaces in its crystal system. Such refractory interfaces can both break up and partially interfere the incoming light rays. The above mentioned reflexive or combined reflexive effect then varies both with the angle of view and with the transparency of individual points of said face printing 5.

The mutual exposing or hiding of points can be reached e. g. as follows. If the plate is provided with a printing 5 on its face side and a printing 6 on its back side and the front face side of said material is transparent at least partially at various places according to the intended graphic design, its colour always depends on light conditions of individual points of the back printing 6, i. e. the effects of individual colour points of both printings 5,6 interact at the same time. At places where there are different light conditions in the back printing 6, e. g. with regard to colour or brightness, etc., for individual points situated next to each other, the colour interactions of the colours of the front and the rear printings 5,6 change simultaneously with the change of the angle of view. This can be used e. g. in such a way that a certain configuration of points, e. g. a pattern, is visible only if viewed from a specific angle and in a pre-defined colour. If the viewing angle is changed, the pattern disappears or changes its colour.

The above mentioned solutions influence the pattern and the colour scheme of the printed materials in such a way which cannot be realised on hard, at least partially transparent materials by the commonplace methods in the same final quality and in case that fired inks and crushed glass are used the printed materials are very abrasion resistant. It is an advantage of this solution that it is stable and rather simple and provides products at moderate costs. Tiles according to this invention can e. g. easily imitate colour of the so called metallic pigments, so as copper, silver, gold, etc.

A specific application product of this invention is represented by a ceramic tile provided with a glaze.

Such ceramic tile can have shape of a rectangular, square, polygon or other. A cross-section of such ceramic tile is shown in the attached drawing.

The tile consists of a basic ceramic body 1. The tile glaze 3 is provided with a face printing 5 on its front face side.

Behind the rear back side of said tile glaze 3 a back printing 6 is provided on a bottom glaze 2, which bottom glaze 2 is situated between the tile glaze 3 and the base ceramic body 1 of said tile. The face side of said tile glaze 3 is protected by a top glaze 4.

The face printing 5 consists of a system of narrow slits, in the simplest case formed by substantially parallel lines printed by an abrasion resistant ink. Thickness of such lines is e. g. 0.1 to 2 mm, their pitch is 0.1 to 1.5 mm. There is a general rule that the smaller dimensions provide results of a better quality. But the dimensions specified are sufficient to evoke the desired effect. The back printing 6 is formed by a chaotic formation of characters, whereby the formation is made so that more than 50 % of said formations has the same direction (forms lines) so as the formation of slits of the face printing 5. That means that the prevailing direction of the back printing 6 is substantially parallel to the direction of slits of the face printing 5. In this way a state is obtained when in the narrow slits of the face printing 5 light rays are diffracted and at their incidence on the back printing 6 after their passage through said at least partially transparent tile glaze additional diffraction and interference take place at the interfaces of different optical environments. By mutual light interactions of said slits of said face printing 5 and of the set of features of said back printing 6 a colour shift and interference take place what results in the resultant unusual brightening of the face side printing which printing layer exhibits a colour shift which colour shift cannot be obtained by other technology.

Locally, it is possible to produce different characteristics by non-keeping the above mentioned condition regarding the parallelity of both directions to more than 50% and by locally declining the direction of said slits by an angle which angle is up to 30 degrees, whereby, locally, more or less differentiated interference minima or maxima are obtained. The minima and maxima are manifested in said places by a substantial brightening of the face printing or by darkening of said face printing. This can be used effectively to express light and shadow in the patterns depicted.

For example, it is so possible to create printings 5,6 arranged in four quadrants of a basic circle is such a way, which is described later, both on the back and the face printing of said quadrant of said basic circle with co- ordinates +y,-x, i. e. of the left upper quadrant. Diameter of the basic circle, as well as the number of slits are proportional only to the tile size. The back printing 6 consists of a system of characters in the form of a certain number of curves, whereby, their form nears to the form of a line in the third of the quadrant which is next to the centre of said circle. The face printing 5 is formed by a system of line-like slits of a smaller number then as is the number of characters in the back printing 6. When the printings 5 and 6 are put together, they create a mutually shifted interference network having its maxima at intersections of said characters and slits, whereby said maxima are less dense in the third of said quadrant which is adjacent to the centre of said basic circle and their connecting lines are nearing to the form of a line deflected by several degrees away from the axis of said basic circle. On the other hand, in the third which is adjacent to the circumference of said basic circle the interference maxima are dense and their connecting lines form a circle of a radius greater than the radius of said basic circle. When such systems are then adapted as printings 5,6, interference phenomenon is produced resulting from the difference in the number of characters and slits and in the effect of a narrow slit used for light ray interference. The result of such an effect is a colour effect of a pattern, especially if the remaining quadrants of the basic circle are also arranged in this way.

Similar other arrangement of interference maxima and minima are also possible to obtain the required pattern and colour effects. For example, it is possible to make a back printing 6 consisting of a system of characters in the form of inclined curves of a wave form and a face printing 5 in the form of a formation of parallel slits. According to the form of such of printing formations 5,6, it is then possible to obtain both geometric patterns having areas of different brightness and of one or more colours, and virtually 3-dimensional patterns or ornamental patterns or patterns with pictorial motives.

In case of a ceramic tile embodiment according to the example the top glaze 4 can also be provided with a printing. If abrasion resistant inks are used a top glaze 4 is not necessary at all.

Industrial Use The present invention will find broad use in producing printed decors on hard, at least partially transparent materials, so as on glazes of ceramic tiles in a wide variety of patterns and colour in a simple way and at acceptable costs.