A Process for Imitating Irregular and Eroded Edges of Natural Rock and Archaeological Remains in Ceramic Tiles and Slabs.

Technical Field The invention relates to a process for modifying the edges of ceramic tiles and slabs so that they become irregular and worn, like natural rocks and archaeological remains they seek to imitate. Background Art The prior art includes processes which simulate aging of ceramic products for use in the construction field, which use compressed air to subject the entire surface which will be on view to sand-blasting and other abrasive agents which are generally used for cleaning and finishing metal elements but which also give a final effect of fading and wear. Also known and effective, but less used, are chemical treatments which do not, however, guarantee that the acids used to corrode the surfaces do not reach the bottom of the ceramic products, thus considerably reducing their original mechanical resistance properties. The prior art also teaches a heat treatment, known as flaming, which is exclusively used for granite, as granite is an aggregate of several minerals all having different heat dilation coefficients. During this process, when the surface is subjected to sharp heating the various crystals of the different materials making up the surface of the granite all react with a differentiated thermal dilation, which though limited to the most external layer of the material leads to micro-fractures, dis-jointing and roughness with a consequent detachment of crystals and tiny portions which are sufficient to give an aged/rough effect to the previously-sanded surface. Flaming, the effects of which vary unpredictably each time it is used, due to the varied physical-chemical characteristics of the treated rock, is limited to use with granites either for aesthetic finishing, such as alternating shiny zones with opaque zones, or for anti-chipping in staircases or floorings exposed to rain or ice. The process of the present invention uses a heat treatment, which is differentiated from the ones described above both in its process and its aims, as it is limited to edges or in any case to the perimeter zone of the ceramic tiles and slabs, where it causes a bursting and detachment of irregularly-shaped fragments, at first large and then increasingly small, up until it gradually erodes the sharpest points of the fragmentation, in a way which is similar to the crumbling caused in nature due to the effects of wear over time and long exposure to atmospheric agents. To obtain a similar result with a homogeneous ceramic mixture, i.e. without being able to count on a differential in heat dilation of the components in a heterogeneous aggregation such as the granular structure of granites, the idea of a solution on the basis of the process of the invention consists in the creation of heat peaks between contiguous zones of a ceramic element though rapid heat treatments, at high temperatures and, especially, localised. While with flaming the different coefficients of heat dilation of the different minerals composing a granite determine the detachment of some crystals from a uniformly heated surface, with the process of the invention it is the heat peaks at contiguous homogeneous zones of a same ceramic element which determine heat dilations which then cause a breakage of peripheral portions, specially heated so that they autonomously break away from the rest of the element, which is protected from the jet of heat source. It is also important in this process that the heat, however obtained and selected on the basis of practical and economical questions, be rapid and concentrated, as otherwise the tiles, instead of being subjected to a heat shock breaking them as expected, might fuse and deform softly. Also, since the aim of the invention is to modify the edges of the ceramic tiles and slabs to make them irregular and worn-looking like natural rocks and archaeological remains, in the process of the invention the localised heat jet, which can be obtained with flames issuing from single tubes or multiple tubes, but in any case supplied in such a way as to supply temperatures which are indicatively comprised between 9000C and 25000C and beyond, is aimed such as to hit bordering portions of at least two corner surfaces so that the heat radiations absorbed simultaneously thereby, before propagating by conduction towards the central zone of the ceramic element, mostly protected on both faces, flow and are concentrated at the edge comprised between the portions of corner surface, causing a dilation up to point of explosion and detachment of the ceramic element. Disclosure of Invention Some preferred embodiments of the invention are described herein below, with the help of five tables of drawings which, for reasons of non-limiting description, illustrate the following: figures 1, 2, 3, 4 show, at a beginning and end of the heat treatment, plan views and corresponding vertical elevation views of the pile of ceramic tiles rotating between two vertical lines of burners; figure 5 is a plan view, at start of heat process, of the pile of ceramic tiles rotation between vertical lines of burners arranged in a sem-circle; figures 6 and 7 show, at start of heat process, a plan view and a vertical elevation of a pile of ceramic tiles translating vertically between burners or vertical lines of burners converging in couples on each of the corners of the tiles; figures 8 and 9 show, at mid-point of the heat treatment, a plan view and a vertical elevation of a pile of quadrangular ceramic tiles translating horizontally between opposite vertical lines of burners, which at each passage expose two parallel edges of each tile. Though the figures illustrate the use of quadrangular tiles, it is pointed out that this does not exclude the suitability and the use of the process with any shape of tile; indeed this is probable given that the area of production envisaged is that relating to the production of variously-shaped tiles made through breakage or cuts, however achieved. In the embodiment of figures 1, 2, 3 and 4, the best exposition of the quadrangular ceramic tiles 1 to the localised heat treatment is obtained by making a pile in which tiles oriented as in IA and tiles oriented as in IB are alternated, i.e. with a 45° stagger between one tile and a next. In this way, during the rotation of the pile of tiles between the two vertical rows of burners 3, each of the four corners of each tile undergoes a localised and simultaneous heating of the four surfaces which converge there: the four corner portions of the two opposite surfaces and the two border edges delimiting the corners. This takes place while the pile protects a large part of each tile from exposure to the heat source, and thus creates the heat peaks and differing heat dilations which cause the desired fractures. In order that the first and last tile in the pile are exposed to the heat in the same way that the intermediate tiles are, the pile is topped and bottomed at the ends by two sheets 2 of suitable refractory material, having the same dimensions and angular stagger as the ceramic tiles. In the embodiment of figure 5, the best exposure of the quadrangular ceramic tiles 1 to the localised heat source of the invention is obtained by creating a pile of tiles composed with the tiles in orientations IA, IB and 1C, which correspond to angular staggers of about 30° between one tile and a next. Thus formed, by rotating the pile in proximity of the vertical lines of burners 3 arranged, in this case, in a semicircle, the four corners of each tile are exposed to the heat treatment, i.e. each tile is subjected to the localised and simultaneous heating of four surfaces which converge in the corners, namely: the portions of corner of the two opposite surfaces and those of the two tracts of edge delimiting them. In this case too, in order that the first and last tiles in the pile are exposed to the heat as all the other intermediate tiles, the pile is completed at the ends by two slabs 2, made of a suitable refractory material, having the same dimensions and same angular stagger as the ceramic tiles. In the embodiment of figures 6 and 7, the optimal exposure of the quadrangular tiles 1 to the localised heat treatment according to the process of the invention is obtained by realising a pile which has the tiles 1 in perfect alignment to one another and alternated with slabs 4 which are made of a suitable refractory material and which are of an adequate thickness as well as being octagonal or square in shape, or at least being of a shape which will leave the four corners of each tile free. This shape enables localised and simultaneous heating of each corner of the four surface which converge: the portions of corner of the two opposite surfaces and those of the two tracts of edge delimiting them. The heating takes place during the vertical translation of the pile between the burners 3 or vertical lines of burners 3 which converge in pairs on each of the four corners of the tiles. In the embodiment of figures 8 and 9 the best exposure of the quadrangular tiles 1 to the localised heat treatment of the process of the invention is obtained by making a pile with the tiles 1 in perfect alignment and alternated with slabs 5 which are of a suitable refractory material, preferably thicker but smaller than the tiles in relation to the width of the perimeter part to be subjected to heat treatment during the horizontal translation of the pile, alternated with 90° rotations between opposite vertical lines of burners 3 acting at each passage of the tiles on two parallel edges of each tile and up until completion of operations on the perimeter thereof. Though the embodiments described and illustrated refer to simultaneous treatment of a plurality of quadrangular ceramic tiles, superposed one on top of another or alternated with refractory slabs, the process can be extended to ceramic elements of any shape, and can be adapted to treat tiles singly, whatever the shape thereof, using flame screens or other guards and protections which adequately insulate the rest of the tile from the peripheral zone thereof which is exposed to the localised heat treatment, all with the intent of producing heat peaks and consequent differentiated dilations which are necessary for causing the desired fractures. Obviously, whatever the embodiment used for actuation of the process, the heat treatment of the process should be carried out in a transparent tunnel or in another similarly protective environment in order to remove the risk of flying splinters of ceramic material for the operators. Finally, though the overall characteristics as illustrated and described remain basically the same, the process is susceptible to modifications which, though comprised in the scope of the invention, might regard the use of laser beams instead of the flame burners described, or might involve heat treatment of single tiles sandwiched between pairs of specially-conformed and arranged refractory elements.