TECHNICAL FIELD The present invention relates in a general sense to the manufacture of ceramic products for wall facings or flooring. More particularly, the present invention relates to a method and plant for decorating ceramic tiles.

PRIOR ART Such decoration is known to be implemented by subjecting the ceramic tile, generally still unfired, to one or more printing operations by which shapes and/or figures of various types are produced on the exposed face of the tile. To carry out said printing operations, plants are known using contact printing systems, i.e. systems using a suitable matrix to impress the decoration on contact with the exposed face of the tile. The most widespread of such contact systems are: silk-screen printing using flat or cylindrical screens which selectively allow a colouring ceramic paste to pass; flexographic printing using elastomer plates which receive a suitable ceramic ink on the top of reliefs provided on their surface; and rotary- recess printing using silicone rollers, the outer surface of which is provided with small recesses for receiving the ceramic ink. Although said printing systems are the result of lengthy technological refinement in the ceramics sector, they present certain limitations which prejudice the decorating quality. In particular, said systems leave an undecorated outer border on the tile, due to the fact that although the matrices deform elastically on contact, they do not enable the exposed face of the tile to be printed up to its edge, as this latter is generally radiused. Moreover, contact printing systems, which are particularly suitable for pronounced decorations, have difficulty in representing light shading or faint tones, which would tend to display the weave of the screen (silk- screen printing), the individual recesses (rotary-recess printing), or the individual reliefs (flexographic printing). Another unwelcome limitation of contact printing systems is the fact that the decoration is repeated always totally or partially identical, without any possibility of modification; this characteristic, which is essential if the decoration to be printed is geometrical or intentionally repetitive, becomes unacceptable if the decoration is required to imitate the appearance of natural materials (stone, marble, granite). In an attempt to solve some of the aforesaid problems, and in particular decoration repetitiveness, digital printing systems are currently under development, i.e. printing systems able to produce on the tile exposed face, either consecutively or continuously, a plurality of identical elemental graphic units(pixels), the distribution, density and arrangement off which enable any type of shape or complex figure to be recreated. Generally, these digital printing systems use print heads able, without using contact members, to directly sprinkle the exposed face of the tiles with metered and controlled quantities of ceramic inks in the form of small droplets, said droplets forming said elemental graphic units. As these print heads are usually controlled by a processor, they usually provide digital systems with high flexibility in use, the resultant decorations being able to be varied at will for each tile processed. However, digital printing systems still currently present decoration reliability and print quality problems; in particular, the maximum resolution of figures obtainable with ceramic inks perfectly compatible with the materials usually used is still insufficient.

DISCLOSURE OF THE INVENTION The object of the present invention is to overcome said drawbacks related to contact printing systems (silk-screen, flexographic, rotary-recess printing) and to digital printing systems, in order to achieve a variegated non-repetitive decoration on ceramic tiles, in particular a decoration which imitates the appearance of natural materials (stone, marble, granite). This object is attained by a method for decorating ceramic tiles, comprising the steps of: A) decorating at least one first portion of the exposed face of the ceramic tile by means of at least one printing operation by a digital printing system; B) decorating at least one second portion of the exposed face of the ceramic tile by means of at least one printing operation by a contact printing system. With this solution, the decoration step by means of the digital system enables faint tone and light shading shapes and figures to be produced, variable at will for each tile, while the next decoration step by means of the contact printing system enables more pronounced parts to be produced. Moreover, the digital printing system, which does not use contact members, enables the most outer edges of ceramic tiles to be decorated. According to a preferred embodiment of the invention, that portion of the exposed face decorated by the contact printing system is at least partly superimposed on the portion decorated by the digital system. According to said embodiment, a background decoration is produced on the entire exposed face of the tile using a digital printing system, and on which a completing decoration is superimposed using a contact printing system, which does not occupy more than 5% of said exposed face. My means of this solution, decorations can be achieved simultaneously presenting faint tones and light shading, and precise well defined edges. In particular, the appearance of natural materials can be imitated by superimposing prominent veining typical of marble or hard stone slabs on a variegated background decoration. In a further preferred embodiment of the invention, the step of decorating by a digital printing system is implemented by executing a plurality of printing operations, each of said printing operations involving the use of ceramic ink of a different colour. In this manner, shapes or figures of different colours can be printed superimposed or inserted, with reproduction of the entire range of colour visible to the human eye, or a significant portion thereof, if the primary colours (yellow, red, blue and black) are used. Likewise the step of decorating by a contact system can be implemented by executing a plurality of printing operations. In another embodiment of the invention, said decoration steps A and B can be preceded by a step in which the exposed face of the tile is covered by a preparation base layer, preferably of ceramic glaze; this step is necessary if the ceramic tile mass is of inexpensive material (for example simple clay), in order to improve its appearance, render it impermeable to soiling, and enable the ceramic inks to correctly adhere. Finally, said decoration steps A and B can be repeated until the required final decoration quality is achieved. The dependent claims define particularly advantageous preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Further characteristics and advantages of the invention will become apparent on reading the ensuing description provided by way of non- limiting example, with the aid of the figures shown in the accompanying drawing, in which: Figure 1 is a schematic view of a decoration plant applying the method of the invention; Figure 2 is a perspective view of a first preferred embodiment of the plant of Figure 1 ; Figure 3 is a plan view of a second preferred embodiment of the plant of Figure 1 ; Figure 4 is a perspective view of a further embodiment of the plant of Figure 1.

BEST MODE FOR CARRYING OUT THE INVENTION Said figures show a plant 1 for decorating ceramic tiles 2. Said plant 1 comprises an automatic feed system 10 for ceramic tiles 2, and an assembly of print units disposed in succession along the path of said feed system 10, each of which is arranged to carry out at least one printing operation on the exposed face 20 of the tiles 2, to form part of the final decoration.- As the number, the arrangement and type of each print unit pertaining to a plant 1 generally depends on the final decoration to be obtained, Figure 1 shows only those print units strictly necessary to implement the invention, but without affecting the generality of the description. Specifically, said plant 1 comprises two print units of which the first 12 is a digital printing unit, i.e. a unit using a digital printing system, while the second 13 is a rotary-recess printing unit using a rotary-recess printing system. It should however be noted that for the purposes of the invention, said rotary-recess printing unit 13 could be replaced by any other print unit using a contact printing system, for example flexographic or silk-screen. The digital printing unit 12 comprises a print head 15 provided with a reservoir 16 for containing the ceramic ink and at least one dispensing device 17 (see Figure 1 ); said dispensing device 17 is arranged to dispense ceramic ink droplets G which, by expanding and adhering to the exposed face 20 of the tile 2, provide the elemental graphic units (pixels) which form the decoration. In detail (see Figure 1 ), the dispensing device 17 comprises: a tubular conduit 18 having an angularly cut first end open to the outside, and a second end associated with the reservoir 16; and piezoelectric means 19 associated with said tubular conduit 18 to vibrate this latter. By virtue of this solution, the ceramic ink contained in the reservoir 16 reaches by capillarity the free end of the tubular conduit 18, from which it cannot escape because of its surface tension; when the piezoelectric means 19 are activated, the tubular conduit 18 vibrates so that said ceramic ink emerges in the form of small droplets G. Preferably (see Figures 2-4), the print head 15 is provided with a plurality of close-together parallel dispensing devices 17 disposed transversely to the advancement direction A of the ceramic tile 2. Specifically (see Figure 2), the number of said dispensing devices 17 is such as to occupy the entire length of the tile 2, so that the ceramic ink can be delivered to each point of the exposed face 20 by a single printing operation; alternatively (see Figure 4), the same result can be achieved by installing the print head 15 on suitable drive means arranged to move it transversely to said ceramic tile advancement direction A. According to a particular embodiment of the invention, the digital printing unit 12 can comprise a plurality of identical print heads 15 disposed in succession along the advancement direction A, so that it is able to carry out a plurality of consecutive digital printing operations. In particular, as dispensing devices 17 associated with a fixed print head 15 (see Figure 2) are inevitably spaced apart and consequently unsuitable for achieving a uniform colour on the exposed face 20 of the tile 2, the digital printing unit 12 is provided with a succession of identical print heads 15 mutually offset in a direction transverse to the advancement direction of the tile 2 (see Figure 3). Instead, the rotary-recess printing unit 13 comprises a silicone roller 14 which receives the ceramic ink in small recesses provided in its outer surface and which, in the manner of a matrix, is able to transfer it onto the tile 2 on contact (see Figures 1-4). According to a particular embodiment of the invention, said rotary-recess printing unit 13 can also comprise a plurality of rollers 14 with which to carry out several consecutive printing operations. When in use, the tile 2, possibly subjected to a preliminary step of glazing its exposed face 20, is positioned on the feed system 10 of the plant 1 which conveys it automatically towards the digital printing units 12 and rotary-recess printing units 13 (see Figures 1-4). When the tile 2 reaches the digital printing unit 12, the print head 15 dispenses ceramic ink while the tile 2 translates along the advancement direction A, enabling a printing operation to be carried out on the exposed face 20, in order to decorate at least a first portion thereof; said portion preferably comprises the entire exposed face 20, including the most outer edges which are unreachable by other known printing systems. If said digital printing unit 2 is provided with a plurality of print heads 15, the tile 2 is subjected to successive digital printing operations by which, by filling the reservoirs 16 of each print head 15 with ink of a different colour, polychrome decorations can be obtained. After the digital printing step at 12 (see Figures 1-4), the tile 2 is conveyed by the feed system 10 to the print unit 13 where at least one second portion of the exposed face 20 is decorated by the roller 14; said second portion is preferably superimposed on the first portion previously decorated by the digital system, in order to obtain a final decoration presenting a variegated background plus more pronounced regions. Finally, said decorating steps using a digital printing system and then a contact printing system can be repeated until the final required result is achieved, by simply providing the plant with further print units 12 and/or 13, or by configuring the feed system 10 so that it returns the processed ceramic tile 2 to the commencement of the plant 1.