Method for obtaining a fine porcelain tile reproducing the natural stone

Abstract

A method for obtaining a fine porcelain tile (10) reproducing a natural stone and having a polished surface with natural finish provides for a usual step for producing the tile comprising obtaining a tile from pressing ground and atomised raw materials, its drying, enamelling and lapping. Such lapping is carried out with a crest lapping method, but using polish and matt finish abrasives associated with a lapping member, such member having a comb-like arrangement of said abrasives. A tile thus obtained is also claimed herein.

Description

[0001] METHOD FOR OBTAINING A FINE PORCELAIN TILE REPRODUCING THE

[0002] NATURAL STONE

[0003] The present invention relates to a method for producing a fine porcelain tile and a tile thus obtained, according to the preamble of the respective independent claims .

[0004] As known, a fine porcelain tile is usually obtained starting from clays , feldspars , kaolins and silica (that is , sand) . Such raw materials are suitably treated (ground and atomised to obtain a homogeneous fine powder) and then conveyed to a press to form a ceramic substrate or "unfinished" tile . The latter is then usually dried using hot air (with temperature at approximately 200 ° C) so as to remove the moisture present in the compacted body of the tile .

[0005] Such drying is required to confer to the unfinished or "raw" tile a suitable mechanical strength modulus of rupture and, in particular, an appropriate modulus of rupture so that the tile can withstand, without breaking, subsequent mechanical stresses during the production process steps that precede a firing and therefore the final manufacturing of the ceramic tile .

[0006] Simultaneously with the above there is prepared an enamel adapted to be deposited on a surface (which will subsequently become a walkable or exposed surface of the laid tile ) of the ceramic substrate or raw tile . The enamel has specifications and known vitrifying and hardening characteristics : it contains various products including, for example, glass , quartz , nepheline, alumina and other .

[0007] Therefore, after the drying step, on the tile there is deposited a first enamel layer called slipware which has the function of providing a support on the exposed surface of the tile for a subsequent decoration step . The slipware also has the purpose of adjusting the colour tone on the exposed surface of the tile and softening such surface .

[0008] Once such slipware has been deposited ( in a special structure of a plant where the tile is produced) , the tile is subjected to a decoration in a special "station" of the plant . In such decoration station, the tile is carried using belts or rollers which connect the deposition structure of the slipware and the decoration station . In the decoration station, an operation which is for example advantageously carried out using one or more digital decoration machines for example of the ceramic ink-jet type, on the exposed surface of the tile there are positioned organic pigments through digital printer heads which define the "graphic design" that the exposed surface of the tile should have . For example, such surface may take the appearance of wood or of natural stone, such as marble .

[0009] Obviously, the decoration may also be carried out using other known methods .

[0010] There subsequently follows an actual enamelling step in which a liquid enamel, containing vitreous components (known in the technical jargon as " frits" ) at a suitable density and weight , is deposited on the decorated surface and the entire exposed surface of the tile . Preferably, the ( final ) enamelling step ( subsequent to the first enamelling with slipware ) is carried out using several enamels ( for example ) in separate implementation steps of the final enamelling step . Such enamels have different characteristics and functions : for example a first enamel ( for example an enamel known under the trade name KR1 ) has the purpose of improving the spreading of the subsequent enamel and conferring brilliance to the surface of the tile, the second enamel ( for example the one known under the trade name MIX SF 01 ) has the actual vitrifying function of protecting the surface which will be subjected to vitrification in the subsequent tile firing step .

[0011] Advantageously, within the enamel there are dispersed known micro-grits which will contribute, after firing, to determining the final "micro-connotated" finish of the exposed surface of the tile and simultaneously reduce the amount of enamel required for the final result . Each of such micro-grits have a particle size smaller than 100 m and preferably smaller than 60 pm .

[0012] Subsequently to the enamelling, the raw tile moves to the firing step in appropriate roller furnaces in which the tile is subjected to vitrifying or firing . This occurs at a temperature of approximately 1200 ° C-1250 ° C and for a dwell time in the furnace which varies depending on the format of the tile and its thickness .

[0013] Upon exiting from the furnace, the tile is subjected to the lapping step through which the tile takes the final surface finish . Therefore, the tile moves to the grinding step, in which it is squared and dimensioned to the appropriate size and then it is packaged and readied for sale on the market .

[0014] However, the process for producing lapped ceramic tiles currently involves significant waste of abrasive raw materials , with clear repercussions both on the production costs and on the environment . As a matter of fact , the lapping is carried out by removing a significant layer of surface enamel (up to a few hundreds of microns ) and at the same time using abrasive members which have high wear .

[0015] Therefore, given that the lapping process is very aggressive on the tile, it is necessary that the latter has at the beginning a sufficiently thick layer of protection enamel so as to protect the decoration during the processing . Therefore, on the decoration there is usually applied an amount comprised between 5g and 15g of a first enamel having density comprised between 1200g / l and 1300g / l and, subsequently, an amount comprised between 70g and 90g of a second enamel having density comprised between 1450g / l and 1550g / l, the latter actually acting as a surface vitrifying protection for the tile . Obviously, the amounts of enamel mentioned are provided by way of example referring to a tile portion having a dimension surface measuring ( 30x30 ) cm2.

[0016] Otherwise said, the thickness applied before the firing of the first enamel mentioned above is usually comprised between 45 m and 140 pm, while the thickness applied before the firing of the second enamel mentioned above is comprised between 500 pm and 690 pm .

[0017] Therefore, generally, the thickness of the enamel applied before the firing is comprised between 545 pm and 830 pm .

[0018] This also allows to obtain a particularly thick final layer (that is after lapping) of protection enamel for the lapped ceramic tiles .

[0019] Currently, the main lapped finishes are the full field gloss lapping (mirror finish) and polished lapping ( semigloss finish) .

[0020] As mentioned, the tiles have various aesthetic surface traits , as a function of the graphic design present on the exposed surface, the use or other commercial characteristics . A particular trait is the degree of reflectance of the light or "gloss" (measured on a scale 0 to 100 ) , intended as the capacity to reflect the light that hits the surface in a more or less mirror-like fashion . Normally, depending on this value, a tile is indicated as : matt if it has a reflectance value comprised between 1 and 10 ; matt with gloss if the reflectance value is between 11 and 30 ; medium matt if the reflectance value is between 31 and 40 ; semi-matt if the value mentioned above is comprised between 41 and 50 ; semi-gloss if such value ranges between 52 and 80 ; and shiny if the reflectance value is greater than 80 .

[0021] Currently, the lapped tiles have a gloss comprised between 40 and 80 . These characteristics reflect on the visual and touch appearance of the tiles .

[0022] Together with the appearance of the walkable surface, such characteristic is very important for a tile .

[0023] W02022190062 discloses an antibacterial preparation for amorphous or crystalline surfaces like that of a ceramic product such as a tile . This prior art document discloses that on the surface of the product mentioned above, before being subjected to the firing step, there are deposited various compositions comprising one or more inorganic compounds and containing components selected from various materials such as Cerium, Iron, Praseodymium, Chromium, Aluminium, Cobalt , Titanium Copper, Barium, Lithium, Sodium or others . Together with additives such as silicon oxides , dolomite, aluminium oxides and others are capable of offering an antibacterial activity to the preparation or composition thus obtained which is then deposited on the surface of the ceramic product .

[0024] This prior art document discloses that after firing, a tile is subjected to lapping so as to remove the initial 150 m of vitreous material which forms the upper part of the ceramic item .

[0025] Considering that the surface of a tile has a very variable roughness comprising crests (or peaks ) and depressions with maximum values , respectively, with height or depth above 100 pm and at 60 pm with respect to an average height of the roughness profile, it is clear that the lapping disclosed in the prior art document is such to limit as much as possible the height of the peaks that is the roughness crests mentioned above .

[0026] Therefore, the lapping action disclosed in the prior art document involves a high removal of material from the surface of the tile and it necessarily leads to having thereon, prior to carrying out the lapping, a layer of usual highly thick protection enamel ( in the light of the presence of the antibacterial compound) . This with the resulting costs and waste of product which is removed by the lapping .

[0027] Therefore, if from an antibacterial effect point of view W02022190062 discloses a method which allows to create an effective protection layer, such solution is rather demanding in terms of overall production costs of a ceramic product ( such as a tile ) .

[0028] Furthermore, the prior art document does not in any manner disclose a method for obtaining an exclusive aesthetic trait on the (walkable ) surface of a tile, and at the same time adapted to obtain a tile with a walkable surface having a particular reflectance value, which resembles the natural stone .

[0029] Lastly, such prior art document discloses a lapping operation that leads to a high removal of material from the exposed surface of the tile, with the consequences mentioned above .

[0030] An object of the present invention is to provide a method for manufacturing a fine porcelain tile and a tile thus obtained which has an exclusive aesthetic trait and which has a lapped surface, but with a degree of reflectance or gloss with a reflectance value between matt and medium matt .

[0031] A further object is to provide a method of the type mentioned above to obtain a fine porcelain tile which has a substantially smooth surface like the one of polished marble .

[0032] A further object is to provide a method of the type mentioned above which, specifically due to the method used to carry out the lapping, can be implemented with a significant saving in raw materials used in the tile enamelling step, such as enamels and grits .

[0033] Another object is to provide a method which, in the lapping step, allows to save abrasive material, with resulting lower production costs for the tiles .

[0034] Another object is to provide a fine porcelain tile that is smooth and " soft to touch" , and which are with microdetails and with different opacity visible in backlight similar to those of natural tile, which increase the naturalness and the realistic degree of the product .

[0035] A further object is to provide a tile of the type mentioned above whose walkable surface, after the final process steps , does not need further production surface treatments .

[0036] These and other objects which shall be more apparent to the person skilled in the art are attained by a method for obtaining a fine porcelain tile and a fine porcelain tile thus obtained according to the claims attached hereto .

[0037] For a better understanding of the present invention, the following drawings are attached hereto, purely by way of non-limiting example, wherein : figure 1 schematically shows a method for producing a lapped ceramic tile according to the invention; figure 2 is a perspective view of a plant for lapping tiles used to implement the method according to the invention; figure 3 is a top view of an abrasive tool used in a ceramic tile lapping step; figure 4 is a lateral view of the abrasive tool of figure 2 ; figure 5 is a top view of a different abrasive tool used in a second ceramic tile lapping step; figure 6 is a lateral view of the abrasive tool of figure 5 ; figure 7 schematically shows the interaction of the abrasive tool of figure 5 with a surface of the tile to be lapped; figures 8A and 8B show a comparison between a vertical section of a lapped ceramic tile according to the invention (Figure 8A) and a section of a lapped ceramic tile according to the prior art ( figure 8B) ; figure 9 schematically shows a vertical section of a portion of a lapped ceramic tile according to the invention .

[0038] The present invention relates to a method for manufacturing a fine porcelain tile that has a lapped surface finish which faithfully reproduces both the three- dimensional and graphic design microdetails of the natural stone . Such tile is therefore moderately reflecting, smooth and soft to touch and, similarly to natural stone, it has on its exposed surface, for example a walkable surface , areas with different opacity visible in backlight , which increase the naturalness and the realistic degree of the product .

[0039] The tile in question provides for a production method which, similarly to the prior art methods for producing fine porcelain tiles , comprises the following known operating steps : firstly ( step SI of figure 1 ) , a preparation of the raw materials to be used for the production (and that is clays , feldspars , kaolins and silica) , their grinding and possible atomisation and the conveying thereof to a usual press . The latter presses said raw materials and produces an "unfinished" or " raw" tile which is dried ( step S2 ) using known methods . Subsequently, such " raw" tile is conveyed to a first enamelling step ( step S3 ) in which a slipware coating is deposited on a surface of the raw tile . Purely by way of example (and with reference to figure 9) , a slipware coating 13 is arranged on an exposed surface 12 of a raw tile or ceramic substrate 11 . The slipware coating 13 may comprise, for example, a slipware having a density comprised between 1330 g / 1 and 1450 g / 1 . More particularly, there may be applied an amount of the slipware mentioned above comprised between 10g and 30g, preferably between 15g and 25g .

[0040] Otherwise said, prior to the firing, the thickness of the slipware coating 13 mentioned above is comprised between 75 m and 250 pm, preferably between 115 pm and 210 pm . Like mentioned in the introductory part of the present document , such grammage refers to a tile portion having a surface area measuring ( 30x30 ) cm2.

[0041] The deposition in step S3 of the slipware coating 13 allows both to harmonise and standardise the colour of the exposed surface 12 and soften it ; such step is implemented by applying slipware known to the person skilled in the art .

[0042] In detail, the means for applying slipware comprise, for example, an airbrush provided with a plurality of airless brushes adapted to deposit the slipware evenly on the entire exposed surface 12 of the tile 11 or a first digital jet printer adapted to deposit the slipware on the exposed surface 12 according to a predefined conformation . Such predefined configuration pattern may be such to entail a uniform coating of the entire exposed surface 12 of the raw tile 11 .

[0043] Subsequently, the method comprises a step S4 for decorating the slipware coating 13 , in which there is created a decoration which can represent a wooden surface or a natural stone, for example marble .

[0044] The decoration step S4 is carried out using means for applying the decoration known to the person skilled in the art which comprise, for example, a ceramic ink jet digital printer adapted to print one or more pigmented ceramic inks according to a predefined model . As mentioned above, the predefined model may represent a natural element such as wood or stone .

[0045] Should a ceramic ink jet digital printer be used, the decoration step S4 may be carried out after a time interval At such to allow the slipware coating 13 to cool sufficiently so as to prevent the digital heads of the digital printer from being damaged . Usually, the cooling of the slipware coating 13 is accelerated using special cooling means known to the person skilled in the art .

[0046] Subsequently, the method comprises an enamelling step S5 in which there is deposited an enamelling coating 15 on the slipware coating 13 . Such step is carried out using enamelling coating deposition means known to the person skilled in the art . For example, they comprise an airbrush provided with a plurality of airless brushes adapted to deposit the enamelling coating 15 evenly on the entire slipware coating 13 .

[0047] Advantageously and in a known manner, the enamelling coating 15 comprises a mixture of micro-grits 16 which has a particle size smaller than 100 pm .

[0048] The mixture of micro-grits preferably comprises AI2O3, CaO, K2O, MgO, Na2O, SiO2, SrO and ZnO .

[0049] In particular, the mixture of micro-grits comprises : AI2O3 between 25% and 30% by weight on the total, CaO between 5% and 15% by weight on the total, K2O between 1% and 5% by weight on the total, MgO between 1% and 5% by weight on the total, Na2O between 1% and 5% by weight on the total, SiO2 between 50% and 60% by weight on the total, SrO between 1% and 5% by weight and ZnO between 1% and 5% by weight on the total . In a non-limiting embodiment of the invention, the mixture of micro-grits 16 comprises a first micro-grit 16 ' and a second micro-grit 16" .

[0050] The first micro-grit 16 ' comprises : AI2O3 between 10% and 20% by weight on the total, CaO between 10% and 20% by weight on the total, K2O between 1% and 5% by weight on the total, MgO between 1% and 5% by weight on the total , Na2O between 1% and 5% by weight on the total, SiC>2 between 50% and 60% by weight on the total, SrO between 1% and 5% by weight on the total and ZnO between 1% and 5% by weight on the total .

[0051] Otherwise, the second grit 16" comprises : AI2O3 between 20% and 30% by weight on the total, CaO between 5% and 15% by weight on the total, K2O between 1% and 5% by weight on the total, MgO between 1% and 5% by weight on the total, Na2O between 1% and 5% by weight on the total, SiO2 between 50% and 60% by weight on the total and ZnO between 1% and 5% by weight on the total .

[0052] In detail, the first micro-grit 16 ' has a particle size in a percentage comprised between 40% and 50% , smaller than 60 m, while the second micro-grit 16" , instead, has a particle size in a percentage comprised between 50% and 70%, smaller than 60 pm .

[0053] In particular, by way of example, the mixture of microgrits 16 comprises the first micro-grit 16 ' in a percentage comprised between 70% and 30% and the second micro-grit 16" in a percentage comprised between 30% and 70% . Even in greater detail, the mixture of micro-grits 16 has : 0%-10% by weight of components with particle size comprised between 75 pm and 100 pm, 50 %-70% by weight of components with particle size comprised between 60 pm and 75 pm, 15%-30% by weight of components with particle size comprised between 45 pm and 60 pm, 5%-15% by weight of components with particle size comprised between 38 pm and 45 pm, and 5%-15% of components with particle size smaller than 38 pm .

[0054] In the non-limiting embodiment described herein, the enamelling step S5 comprises a first enamelling step S5a and a second enamelling step S5b, said steps being implemented in any known manner . Therefore, the enamelling coating 15 , comprises a first enamelling coating 15a (deposited in step S5a) and a second enamelling coating 15b (deposited in step S5b) .

[0055] The first coating 15a is adapted to facilitate the spreading of the second enamelling coating 15b and it confers gloss to the exposed surface 12 . Advantageously, by way of example, the first enamelling coating 15a comprises an enamel having a density comprised between 1000g / l and 1200g / l and even more advantageously, there is applied an amount of the enamel mentioned above comprised between 5g and 15 g, preferably comprised between 5g and 10g .

[0056] It should be observed that the deposited thickness of the first enamelling coating 15a mentioned above is advantageously comprised between 45 pm and 165 pm, preferably comprised between 45 pm and 110 pm . Like mentioned above, the grammage always refers to a tile portion having a surface area measuring ( 30x30 ) cm2.

[0057] The second enamelling coating 15b, instead, has the function of creating a surface vitrifying protection layer on the tile and it comprises an enamel added with the mixture of micro-grits 16 described above . Advantageously, by way of example the second enamelling coating 15b may comprise an enamel having a density comprised between 1300g / l and 1450g / l and there is applied a suitable amount of the enamel mentioned above for example comprised between 10g and 30g, preferably comprised between 15g and 25g, in any case smaller than the one provided for in the process for producing lapped ceramic tiles of the prior art . The mixture of micro-grits 16 is added to the enamel without using a further application made of aqueous suspension and, therefore, the mixture of micro-grits 16 is "embedded" in the enamel and it does not remain free on the surface of the second enamelling coating 15b after the deposition . In other words , the mixture of micro-grits 16 is surrounded by an enamel layer .

[0058] The deposited thickness of the second enamelling coating 15b mentioned above is comprised between 75 m and 260 pm, preferably it is comprised between 115 pm and 210 pm .

[0059] By way of example, the mixture of micro-grits 16 is present in a percentage smaller than 25% by weight with respect to the total dry weight of the enamelling coating 15 ; even more particularly, the mixture of micro-grits 16 is present in a percentage smaller than 25% by weight with respect to the total dry weight of the second enamelling coating 15b, preferably comprised between 15% and 25% by weight and advantageously in a percentage comprised between 17% and 23% by weight . Even more advantageously in a percentage comprised between 19% and 21% by weight .

[0060] Preferably, the second enamelling coating 15b is made up of , by weight with respect to the total dry weight , 20% of the mixture of micro-grits 16, the remaining percentage of second enamelling coating 15b mainly being defined by the enamel inside which the mixture of micro-grits 16 is embedded .

[0061] In a further embodiment of the invention not shown in the attached figures , the enamelling step S5 comprises an intermediate deposition step, between step S5a and step S5b, in which there is deposited a vitreous coating on the first enamelling coating 15a .

[0062] Such step is carried out using vitreous coating deposition means known to the person skilled in the art which, for example, comprise an airbrush provided with a plurality of airless brushes adapted to deposit the vitreous coating evenly on the entire first enamelling coating 15a .

[0063] In detail, the vitreous coating comprises water, a suspending agent ( for example the one known under the trade name CERADD 600 ) and the mixture of micro-grits 16 described above .

[0064] Even in greater detail, there is applied an amount of the vitreous coating mentioned above comprised between 10g and 30g, preferably comprised between 15g and 25g .

[0065] In particular, the mixture of micro-grits 16 is present in a percentage smaller than or equal to 40% by weight with respect to the weight of the vitreous coating 15c, preferably comprised between 25% and 35% , the suspending agent being present in a percentage smaller than or equal to 15% and the remaining percentage of the vitreous coating being defined by water .

[0066] In the light of the above, it is clear that the enamelling coating 15 of a ceramic tile 10 produced according to the further embodiment of the invention mentioned above comprises a first enamelling coating 15a (deposited in step S5a) , a vitreous coating (deposited in the intermediate step) and a second enamelling coating 15b (deposited in step S5b) .

[0067] Subsequently, according to the invention, the method comprises a step S6 for firing the raw ceramic tile which is carried out according to the known methods .

[0068] After the firing step, upon exiting from the furnace, the tile is transferred to a lapping step S7 which is followed by per se known surface treatment steps (washing) S8 , brushing S9 and final grinding S10 .

[0069] According to the invention, such lapping is carried out so as to confer to the exposed surface of the tile a degree of reflectance or gloss comprised between 9 and 25 (on a scale ranging from 0 to 100 ) , preferably between 12 and 23 , advantageously comprised between 15 and 19 . Such reflectance value is significantly smaller than the one that is usually obtained in a process for the polished lapping of a surface of a tile, a value that is conveniently comprised between 40 and 50 and which provides high shininess to the tile .

[0070] In this manner, the tile reproduces the reflectance of the natural stone having a gloss comprised in the range which makes the exposed surface of the tile matt or medium matt (and not gloss ) . This confers to the product (tile ) which may resemble the natural stone, a very realistic appearance .

[0071] In order to achieve such result , the lapping is carried out using the " crest" lapping method .

[0072] Before clarifying this type of lapping in greater detail, the following should be observed .

[0073] Usually, the lapping is carried out to obtain a surface finish of the tile such to make it smooth and mirror-like or semi-gloss . Such operation is carried out by mechanically removing part of the previously enamelled surface of a tile and changing the roughness thereof . The latter is a surface characteristic defined by an unevenness of the surface of the tile due to the succession of peaks or crests and ridges thereon . In particular, the crests (or peaks ) may also have "heights" greater than 150-200 m with respect to the ridges .

[0074] A usual lapping, carried out using the methods known in the prior art , involves the removal of a high surface vitreous layer, an operation which can also result in the full polishing of the surface of the tile . However, given that the exposed surface of the tile is defined by the enamel already deposited on the ceramic substrate, so as to avoid the full removal of the enamel during the lapping, in the prior art method for producing a tile, it is provided for that the thickness of the enamel should be sufficiently high to withstand the full removal thereof with the lapping operation .

[0075] Therefore, this requires the deposition of a lot of enamel on the ceramic substrate, and this impacts the production costs of the tile .

[0076] Therefore, in order to obtain the tile with the degree of reflectance mentioned above, in order to obtain the aesthetic appearance of natural stone, and in order to avoid the excessive use of enamel, the present invention provides for that the lapping be carried out by mainly acting only on the crests or peaks of the rough surface of the tile, that is it be carried out by removing a small layer ( from 10 m to 50 pm and in any case smaller than 100 pm) of material from the surface of the enamelled tile .

[0077] Therefore, such lapping affects the crests of the reliefs defining the roughness of the tile, without proceeding into depth on such surface ( like it happens in the prior art ) ; therefore, such lapping is commonly referred to as "crest lapping" .

[0078] Therefore, this operation allows to abrase only the outermost area of the surface of the tile, basically lapping the crest (the most superficial areas ) that is the protrusions present on such surface alone and leaving the innermost areas of the (enamelled) surface of the tile intact , therefore generating a contrast between the gloss of the lapped surface and the matt of the tile beneath the enamelled surface .

[0079] However, such "crest lapping" is not carried out only using, like it usually occurs in crest lapping, abrasives containing solid resin in which there are dispersed diamond ground materials (materials which allow to obtain the usual "gloss" effect of the surface of the lapped tile ) ; in the method subject of the invention, crest lapping is carried out also using comb-like-shaped abrasion heads and where the abrasive material comprises polishing and matting abrasives with compound that is softer than that of the usual lapping abrasion heads used to create the "gloss" effect of the surface of the tiles according to the prior art method .

[0080] The abrasive material used ("polishing and matting" or "opacifying" ) is such to allow, with a reduced pres sure (up to 1 bar approximately) exerted on the surface of the tile, to matt such enamelled surface (with gloss above 40 ) . Such material contains known resins , fine grain diamond ground materials .

[0081] In particular, the fine grain diamond ground materials comprise silicon carbide diamond particles with particle size comprised between 50 m and 95 pm .

[0082] Preferably, the diamond particles have a particle size comprised between 60 pm and 85 pm .

[0083] Besides this , while the prior art lapping is carried out with a pressure exerted on the surface of the upper tile at 1 . 5 bar, usually comprised between 2 and 3 bar, the present invention provides for that the lapping be carried out by applying a pressure lower than or equal to 1 bar on the tile .

[0084] Figure 2 shows a lapping system 1 used for implementing the method according to the invention . Such system 1 comprises a base 2 on which the tiles 10 (only one shown in figure 2 ) slide movable on conveyor means 3 , such as belts or rollers , present on a work surface 4 of the base 2 .

[0085] At such work surface 4 there is present a plurality of abrasion heads 5 : at least half , preferably two-thirds , advantageously the total of such abrasion heads , use "comblike" abrasion or abrasive tools like the one shown in figures 5 and 6 . In the event where not all the abrasion heads 5 are provided with a "comb-like" tool, the remaining abrasion or abrasive tools are tools with conventional abrasives and they are obtained as shown in figures 3 and 4 . In particular, the abrasion heads 5 equipped with conventional tools are arranged upstream of the abrasion heads 5 provided with "comb-like" tools with respect to the sliding direction of the tiles 10 along the base 2 .

[0086] According to the invention, contrary to the state-of- the-art , the lapping step S7 is particularly "mildly- aggressive" , that is it entails a reduced abrasion of the enamelling coating 15 which remains limited to the most superficial part thereof and, otherwise said, in a preferred embodiment described below, the lapping step S7 solely involves the second enamelling coating 15b .

[0087] In greater detail, according to an embodiment of the invention, the lapping step S7 comprises a first lapping step S7a and a second lapping step S7b . The first lapping step S7a is adapted to reduce the surface roughness of the enamelling coating 15 while the second lapping step S7b is adapted to smooth and opacify (and not level ) the undulation profile of the enamelling coating 15 and, therefore, to even the roughness thereof (also making the surface soft to touch) .

[0088] The first lapping step S7a is carried out using first and known abrasive tools 100 , schematically shown in figures 3 and 4 , which have a hard compound, a very fine particle size and a rigid geometry given that they are designed to reduce surface roughness .

[0089] In particular, the first abrasive tools 100 are arranged in contact with the enamelling coating 15 at a pressure of approximately 1 bar .

[0090] By way of example, as shown in figures 3 and 4 , the first abrasive tools 100 comprise a base 101 and a plurality of abrasive bearings 102 , having a substantially parallelepiped rectangle shape, adapted to come into contact with the enamelling coating 15 during the lapping . In particular, the geometry of the first abrasive tools 100 is called rigid given that the abrasive bearings 102 contain solid resin into which there are dispersed diamond ground materials and their height H is smaller than the sides LI , L2 of the bases .

[0091] Such first abrasive tools are, for example, currently used in producing gloss full field lapped ceramic tiles (mirror finish with gloss 80-90 ) and polished lapped ( semigloss finish with gloss 40-50 ) respectively at a pressure of 2-3 bar and approximately 1 . 5 bar .

[0092] Otherwise, the second lapping step S7b is carried out using second abrasive tools 200 which have a softer compound and a "comb-like" geometry as schematised in figures 5 and 6.

[0093] In particular, the second abrasive tools 200 are arranged in contact with the enamelling coating 15 at a pressure of approximately 1 bar .

[0094] By way of example, with reference to figures 5 and 6, second abrasive tools 200 comprise a base 201 and a plurality of abrasive bearings 202 , defining "bristles" of the yieldable type that are not rigid and adapted to come into contact with the enamelling coating 15 during the lapping . Such "bristles" or abrasive bearings are arranged evenly, on parallel rows and polish the enamelling coating 15 but following the profile thereof as a whole . Such bristles or bearings do not fully flatten the surface, but leave it with some micro-unevenness , despite being smooth to touch .

[0095] The interaction between an abrasive having a "comb-like" geometry and a surface to be lapped SL, is schematically shown in figure 7 . However, it should be observed that the abrasive tools 100 , 200 , if both are present , operate at lower pressures with respect to the pressures normally used in the production of tiles .

[0096] The abrasion tools 200 are obtained through per se known polishing and matting abrasive means and interfere with the surface of the tile through a movement transversal to the movement of the tile along the workbench .

[0097] In this manner, also thanks to the low pressure on the surface of the tile, there is a low consumption of abrasing material, with obvious advantages in terms of costeffectiveness and the production of dust and proces sing residues which remain on the tile . Such residues , in the production methods are removed from the surface of the tile by treating the latter with phosphoric acid and then washing with water and drying the tile so as to remove excess acid .

[0098] In an advantageous embodiment of the invention, the surface treatment step S8 comprises , following the washing described above, the application of a protection layer (not shown in the attached figures ) so as to obtain a ceramic tile 10 that is difficult to dirty and that is easy to clean .

[0099] The protection layer comprises a per se known compound, for example the one known under the trade name FABER 550 .

[0100] The use of a method as described (with low pressure exerted on the surface of the tile ) and of the abrasion members described with resulting lower consumption thereof (and smaller amount of produced lapping "sludge" in need of disposal ) therefore involves lesser use of acid for cleaning the tile, less water for washing and less energy consumed for lapping the tile . This with the advantages both in terms of environmental impact of the processing of the tiles according to the invention and from an energy standpoint reducing the time and energy used for lapping the tile . Thanks to the different type of lapping (which is carried out "in crest" fashion, that is by acting on the crests or peaks of the surface roughness of the tile ) with respect to the usual process currently used to confer gloss finish to the tiles , even the enamel layer on the exposed surface of the tile may be small and reduced with respect to the usual enamel layers which are deposited on the tiles to be lapped (of any type : a gloss full field to obtain a mirror finish with gloss 80-90 ; polished to obtain semi-gloss finish with gloss 40-50 ) .

[0101] As a matter of fact , as observable in figures 8A and 8B the thickness of the enamelling coating 15 deposited on the tile according to the invention is comprised between 80 m and 105 pm, preferably between 85 pm and 95 pm; while the one conventionally present on the tiles to be lapped is at least twice as much ( for example the one of tiles subjected to polished lapping is comprised between 250 and 270 pm) . The deposition of the layer mentioned above is specifically directly allowed by the method used to carry out the lapping : such operation, carried out so as to act on the crests or peaks of the surface roughness of the tile, allows to remove a reduced layer of enamel from such surface . As a result , in order to have the desired aesthetic effect of the exposed surface of the tile, there can be deposited thereon a reduced thickness layer, more limited than the thickness usually deposited on tiles subjected to a usual lapping according to the operating modes of the prior art (as shown by the comparison reported above between the values of the thicknesses of the enamel layer deposited according to the present invention and the prior art ) .

[0102] The method according to the invention therefore allows a significant cost-effectiveness and saving raw materials when carrying out the surface enamelling step . The use of the production method described herein allowed to obtain a tile with an exposed surface that is compact and surface roughness that is even and reduced .

[0103] Last but not least , there is shown a further aspect of the invention : the tile obtained according to the method described above has a high planarity . This is important so as to have a thin layer of enamel and enable the lapping thereon . Such planarity is smaller than 1 . 2 mm so as to allow for the lapping step to be carried out homogeneously and excellently, with high aesthetic appeal .

[0104] In the event of tiles obtained according to prior art production methods , it should be observed that such planarity can reach 1 . 8 mm while still being within required standards .

[0105] The invention allows to provide a fine porcelain tile with an exclusive aesthetic trait thereof , that is substantially matt or moderately reflecting, with a gloss value comprised between 9 and 25 as indicated above , which has a surface that is smooth and soft to touch and which faithfully reproduces the microdetails of natural stone . The fine porcelain tile therefore takes a natural stone-like appearance .

[0106] The method further consists of having a reduced use of abrasion heads with respect to those used in the lapping method of the prior art , a lower consumption of abrasives with resulting decrease in processing waste . Removing a reduced amount of material of the tiles allows to use less water for the purification of processing waste, with clear benefits for the environment too .

[0107] There is used even less enamel with respect to the surface processing operations of the prior art , therefore even further reducing the consumption of material for the production of tiles and the process costs .

Claims

CLAIMS1. Method for manufacturing a fine porcelain tile (10) for conferring to such tile a lapped surface finish which reproduces the natural stone, said method comprising:- preparing raw materials to obtain tiles, their grinding and conveying them to a press (step SI) ,- obtaining a ceramic substrate or raw tile (11) from such press,- drying the ceramic substrate or raw tile (step S2),- creating a decoration (step S4) and depositing an enamel (step S5; S5a, S5b) on an exposed surface (12) of the tile;- firing (step S6) the tile in a furnace and obtaining a fired tile in ceramic form (10) ,- lapping (step S7; S7a, S7b) the exposed surface of the fired tile,- dimensional grinding (step S10) of the fired tile or ceramic (10) , characterised in that the lapping (step S7) is at least partially obtained by operating on the crests or peaks of the surface roughness of the fired or ceramic tile (10) by removing a layer of material comprised between 10 and 50 m from such surface that is through a lapping operation at the crest (step S7b) generating a pressure on the surface of the ceramic tile (10) smaller than or equal to 1 bar, using abrasion tools (200) shaped like a comb or brush.

2. Method according to claim 1, characterised in that the abrasive material of the abrasion tool (200) for crest lapping (step 7b) is arranged on parallel lines and it protrudes from a support (201) of the abrasion tool (200) to cooperate with a surface layer of the tiles.

3. Method according to claim 1, characterised in that in the lapping step (step S7) there is provided for lapping (step S7a) the ceramic tile (10) with an abrasion tool (100)provided with abrasive elements (102) having hardness greater than that of the abrasive material of the abrasion tool (200) used in the crest lapping step (step S7b) .

4. Method according to claim 1, characterised in that, in the enamel deposition step (step S5) , it is envisaged to deposit on a surface of the raw tile (11) an enamel comprised between 120 m and 425 pm, preferably between 160 pm and 320 pm.

5. Method according to claim 4, characterised in that the enamel deposition step (step S5) comprises a first step (S5a) in which there is deposited an enamelling coating (15a) and a second step (S5b) in which there is deposited a second enamelling coating (15b) .

6. Method according to claim 5, characterised in that before the enamel deposition step (step S5) there is created a decoration (step S4) on an exposed surface (12) of the raw tile (11) , said first enamelling coating (15a) being adapted to confer shininess to said decoration and being obtained with an enamel having density comprised between 1000g / l and 1200 g / 1.

7. Method according to claim 5, characterised in that said second enamelling coating (15b) comprises a mixture of micro-grits (16) .

8. Method according to claim 7, characterised in that said mixture of micro-grits (16) is in a by weight percentage smaller than or equal to 25% on the total dry weight of said second enamelling coating (15b) , the remaining percentage being defined by enamels having a density between 1300 g / 1 and 1450 g / 1.

9. Method according to claim 7, characterised in that said mixture of micro-grits (16) has a particle size smaller than 100 pm.

10. Method according to claim 9, characterised in thatthe mixture of micro-grits ( 16 ) comprises A12O3 between 25% and 30% by weight on the total, CaO between 5% and 15% by weight on the total, K2O between 1% and 5% by weight on the total, MgO between 1% and 5% by weight on the total, Na2O between 1% and 5% by weight on the total, SiO2 between 50% and 60% by weight on the total, SrO between 1% and 5% by weight on the total and ZnO between 1% and 5% by weight on the total .11 . Method according to claim 9, characterised in that said mixture of micro-grits ( 16 ) comprises a first microgrit ( 16 ' ) and a second micro-grit ( 16" ) , said first microgrit ( 16 ' ) comprising A12O3 between 10% and 20% by weight on the total, CaO between 10% and 20% by weight on the total, K2O between 1% and 5% by weight on the total, MgO between 1% and 5% by weight on the total, Na2O between 1% and 5% by weight on the total, SiO2 between 50% and 60% by weight on the total, SrO between 1% and 5% by weight on the total and ZnO between 1% and 5% by weight on the total, while said second micro-grit ( 16" ) comprises A12O3 between 20% and 30% by weight on the total, CaO between 5% and 15% by weight on the total, K2O between 1% and 5% by weight on the total, SiO2 between 50% and 60% by weight on the total and ZnO between 1% and 5% by weight on the total .12 . Method according to claim 5 , characterised in that the enamel deposition step ( step S5 ) comprises an intermediate deposition step in which there is deposited the vitreous coating on the first enamelling coating ( 15a) .13 . Ceramic tile obtained according to the method according to claim 1 , comprising a ceramic substrate ( 11 ) on whose lapped exposed surface ( 12 ) there is obtained a decoration ( 14 ) coated with enamel ( 15 ) , characterised in that , after lapping, the thickness of the enamel is comprised between 80 m and 105 pm, advantageously between 85 pm and95 pm.

14. Ceramic tile according to claim 13, characterised in that said enamel is defined by a first enamelling coating (15a) and by a second enamelling coating (15b) superimposed on the first, only said second enamelling coating (15b) containing a mixture of micro-grits (16) .

15. Ceramic tile (10) according to claim 14, characterised in that the first enamelling coating (15a) comprises an enamel having a density comprised between 1000 g / 1 and 1200 g / 1, said second enamelling coating (15b) comprising said mixture of micro-grits (16) in a percentage smaller than or equal to 25% by weight with respect to the total dry weight of said second enamelling coating (15b) , the remaining percentage being mainly defined by enamel having a density comprised between 1300 g / 1 and 1450 g / 1.

16. Tile according to claim 14, characterised in that said mixture of micro-grits (16) has a particle size smaller than 100 m and it comprises A12O3 between 25% and 30% by weight on total weight, CaO between 5% and 15% by weight on total weight, K2O between 1% and 5% by total weight, MgO between 1% and 5% by weight on total weight, Na2O between 1% and 5% by weight on total weight, SiO2 between 50% and 60% by weight on total weight, SrO between 1% and 5% by weight on total weight and ZnO between 1% and 5% by weight on total weight .

17. Ceramic tile according to claim 14, characterised in that it comprises a vitreous coating interposed between said first enamelling coating (15a) and said second enamelling coating (15b) .

18. Ceramic tile according to claim 13, characterised in that said decoration (14) represents a natural stone or wood.

19. Tile according to claim 13, characterised in that it has a planarity smaller than 1.2 mm.

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