DESCRIPTION

FIELD OF THE INVENTION

The present invention is related to a method for manufacturing slabs, tiles or sheets of artificial agglomerated stone with a novel chromatic effect obtained from at least two different mixtures, with at least one cavity by means of, among others, a tool configured to stir a portion of the at least one cavity. Also, the present invention is related to the slabs, tiles or sheets of artificial agglomerated stone directly obtained by the above- mentioned method.

The invention falls in the field of materials for construction, decoration and architecture, made of artificial stone, as well as to their manufacture and fabrication. Particularly, the invention is done within the technological area of artificial agglomerated stone slabs having chromatic effects and decorations, used as surfaces for counters, kitchen tops, sinks, shower trays, wall cladding, flooring, or the like.

BACKGROUND OF THE INVENTION

Artificial agglomerated stone slabs, tiles or sheets frequently simulate natural stones and are common in the construction, decoration, architecture and design sectors. The habitual processes for manufacturing theses slabs, tiles or sheets at industrial scale are well established nowadays.

One of the most popular artificial stone materials and highly appreciated for their aesthetic, hardness and resistance to staining and wear, are the so-called quartz agglomerated surfaces or engineered stone surfaces. These stone materials are extensively used for countertops, claddings, floorings, sinks and shower trays, to name a few applications. Also, they can be more generally called artificial agglomerated stones. Their applications partially overlap with the applications of natural stones such as marble or granite. Artificial agglomerated stones can be made simulating colors and patterns in natural stone or they might also have a totally artificial appearance, e.g. with plain bright red or fuchsia colors. The basis of their composition and the technology currently implemented for manufacturing said artificial agglomerated stones dates back from the late 1970s, as developed by the company Breton SpA which is nowadays commercially known in the sector under the name Bretonstone®.

General concepts of the manufacturing of artificial agglomerated stones are disclosed, for example, in the patent document US4204820A. In the disclosed production process, inorganic (stone, mineral or synthetic) granulates and powders - commonly quartz, synthetic cristobalite and/or other mineral granulates - having varied particle sizes, are firstly mixed with a hardenable binder, frequently a liquid organic resin. The resulting mixture is homogenized and distributed into a temporary mold or alternatively onto a sheet of paper, where it is then compacted by vibro-compaction under vacuum and subsequently hardened.

Recently, there has been an increasing demand for this type of products, in particular the versions which are imitations of natural marble slabs. Some of those marble slabs, such as Calacatta or Statuario, are characterized by having relatively wider veins of a material of different color than the background material, showing veins which extend through different distances in the length and/or the width of the slab, and which are visible partially or fully through the thickness of the slab.

In order to simulate the appearance of these natural materials, the common approach has been to use different types of pigmented (unhardened) mixtures, which before compaction and hardening steps, are distributed forming separately either the vein or the background component of the slab. Most relevant publications in this field describing different strategies for achieving simulation of natural material appearance are the following:

WO 2014108582 A1 relates to articles that comprise an agglomerate of artificial stone and polymerisable resin, such as tiles or slabs, to be used as a construction material, characterized in that they comprise various large strati or veins having a heterogeneous variable adjustable directions and dimensions, which provide a specific aesthetic appearance similar to natural stone products. WO 2016123433 A1 describes systems and processes for forming synthetic mold slab products, for example, a synthetic mold slab that is compacted to a selected slab shape from a plurality of different mixtures including particulate mineral material, resin binder, and pigments so that the synthetic molded slab has a veined appearance and is suitable for use in living or working spaces.

WO 2016189377 A1 discloses manufacturing artificial stone slabs comprising veins, for example imitating marble slabs, by means of combining at least two moldable hardenable fluid mixtures of a first material and a second material having different colors, said two materials being combined following a predefined precise pattern coinciding with a pattern of thin veins to be obtained.

WO 2019101823 A1 relates to a method and a system for producing slabs, tiles or sheets of artificial stone, with a wide vein effect, which comprise at least two mixtures of inorganic particles of different sizes and hardened binders, and which simulate the veined appearance that some types of natural stone have.

WO 2020115644 A1 describes the formation of veining in slabs, preferably made of conglomerate stone and/ or ceramic material. According to the disclosure of this document, it relates firstly to a method for manufacturing slabs of conglomerate stone and/ or ceramic material with a veined effect. Secondly, it relates to a robot island for the formation of veining in a base mix for manufacturing slabs of conglomerate stone and/or ceramic material. Thirdly, it relates to an apparatus for manufacturing slabs of conglomerate stone and/ or ceramic material with a veined effect.

WO 2021070030 A1 and WO 2021064627 A1 both relates to a process and equipment for the manufacture of slabs of ceramic and/or stone material presenting veins of different color compared to the base material.

Consumers have different preferences regarding the final appearance of artificial agglomerated stone slabs. While some of them would appreciate artificial stones which are as close imitation of natural materials as possible, others would prefer other chromatic combinations or effects.

Starting from the known art, the invention seeks to propose an original method to produce artificial agglomerated stone slabs, in an industrial efficient and highly reproducible way, resulting in new types of chromatic effects that, without being imitations of natural stones, still maintain some resemblance to, and therefore also the appeal of, the patterns found in natural marble and other veined stone materials.

Document US 2021/10268686 A1 discloses a process and equipment for the realization of slabs of ceramic and/or stone material.

DESCRIPTION OF THE INVENTION

The present invention disclosed herein is based on the inventors surprising finding, departing from the typical veined effects of natural stones and their known methods of manufacture, and aiming to achieve chromatic effects not found in nature, that products of high appeal can be obtained incorporating veining with some degree of controlled randomness and intentional visual asymmetry of color gradients. These effects can be created extending at least partially through the thickness of the slab and are thus also visible in the edges of the slab, and in the new edges created when the slab is cut-to- size for the intended application.

The present invention provides a solution for the above mentioned issues by a method for manufacturing slabs, tiles or sheets of artificial agglomerated stone according to independent claim 1. In dependent claims, preferred embodiments of the invention are defined.

In an inventive aspect, the present invention provides a method for manufacturing a slab of artificial agglomerated stone which comprises the following steps: a) depositing a first layer of a first mixture onto a surface, the first layer having a first thickness h _lt b) creating at least one cavity, having a width w; and a length L _h in the first layer of first mixture wherein the at least one cavity is defined by at least one cavity wall extending at least partially through the first thickness h _± of the first layer and comprising a first wall portion and a second wall portion in opposition through the at least one cavity width w c) depositing a second mixture into the at least one cavity of the first layer, forming a second layer by the combination of first and second mixtures, and the second layer having a second thickness h ₂ , d) compacting and hardening the second layer, wherein the method further comprises after step c) and before step d), - inserting a first tool at least partially into the second thickness h ₂ of the second layer, and

- actuating the first tool wherein the first tool is configured to stir the first wall portion while not stirring the second wall portion.

The index i of the terms w; and Lj respectively referring to the width of the cavity i of the at least one cavity and the length of the cavity i of the at least one cavity can take any whole number from 1 upwards. Consequently, w _± would refer to the width of the cavity 1 and the length of the cavity 1 , w ₂ and L ₂ would refer to the width of the cavity 2 and the length of the cavity 2 and so on for the following values of the index i.

Throughout the whole document, the term agglomerated stone or artificial agglomerated stone refers at least to all materials included in the definition contained in European standard EN-14618:2009.

Throughout the whole document the length of the at least one cavity is the line corresponding to the average line or direction of the at least one cavity, that is to say, the average line which points are equidistant to opposed wall portions of the at least one cavity walls.

Further, throughout the whole document the width of the at least one cavity shall be the distance between the first wall portion and the second wall portion along a line perpendicular to the average line or direction of the at least one cavity. The width of the at least one cavity is variable depending on the point of the average line or direction selected.

Thus, both first and second wall portions are wall portions of the at least one cavity opposed along a width of the at least one cavity .

Also throughout the whole document, by the first wall portion and the second wall portion being in opposition through the at least one cavity width w _{i t} the present invention refers to any type of cavity shape which is defined by two portions distant one from the other by a distance being the at least one cavity width Wj. In case of preferred cavities having a vein shape, the first wall portion and the second wall portion extend through the length Lj along the line of the vein, where the first wall portion and the second wall portion may converge into two points, each one at an end of the length Lj of the at least one cavity, or they might end at edges of the first or second layers.

In case of elliptical shaped cavities, as shown in Figure 1a, it is understood that the major axis of said elliptical shaped cavity crosses the perimeter of said cavity in two points distant along the length Lj of the cavity. One region of the perimeter, represented as a circled area in Figure 1a, defines a first wall portion and its corresponding opposite circled area is symmetrical with respect to the major axis which defines a second wall portion. Preferably, the first wall portion is the wall which is about to be stirred as claimed by the present invention. The distance in between the first wall portion and its opposite wall portion, the second wall portion, defines the at least one cavity width w; which is perpendicular to the major axis of the elliptical shaped cavity, that is the length Lj. Particularly, said cavity width w; is perpendicular to the length Lj of said cavity and such cavity width w; is variable along the length Lj up to w _imax where w _imax is the minor axis of the elliptical shaped cavity.

In case of a circle shaped cavity, as shown in Figure 1 b, it is understood that a diameter of said circle shaped cavity defines the length Lj of the cavity. One region of the perimeter of the circle shape cavity, represented as a circled area in Figure 1 b, defines a first wall portion and its corresponding opposite circled area is symmetrical with respect to the length Lj which defines a second wall portion. As above mentioned, preferably, the first wall portion is the wall which is about to be stirred as claimed by the present invention. The distance in between the first wall portion and its opposite wall portion, the second wall portion, defines the at least one cavity width Wj. Particularly, said cavity width wt is perpendicular to the length Lj of said cavity and such cavity width w; is variable along the length Lj up to w _imax where w _imax is equal to another diameter of the circle shaped cavity also perpendicular to the length Lj.

The artificial stone is preferably in the form of a slab and the term slab shall be understood to cover also tile, sheet, plank, board, or plate. Moreover, the term slab covers artificial stone blocks which have a major dimension of at least 20 times its thickness.

In a preferred embodiment, the dimensions of the agglomerated stone slab is at least 1500 mm in length, at least 1000 mm in width and 4 to 40 in thickness; preferably 200 to 3500 mm in length, 1000 to 1800 mm in width and 4 to 40 mm in thickness. The method of the first aspect of the invention comprises the step of depositing a first layer of a first mixture onto a surface and another step of depositing a second mixture into cavities previously created in the first layer of the first mixture to form a second layer as the combination of the first and second mixtures.

At least one cavity is created in the first layer of the mixture. More preferably, a plurality of cavities are created in the first layer of first mixture. The at least one cavity being defined by at least one cavity wall as the boundary between the first mixture and the at least one cavity space. Also, the at least one cavity is defined by its width w; and its length Lj where the width w; is the shortest distance between a first wall portion and a second wall portion, both first and second wall portions being opposed wall portions of the at least one cavity wall.

The first and/or the second wall portion of the at least one cavity extends at least partially through the first thickness h _± of the first layer. In some preferred embodiments, the first and/or the second wall portion of the at least one cavity extends fully through the first thickness h _± of the first layer. The wall portions of the at least one cavity might have a general planar direction parallel or relatively inclined with respect to the first layer thickness h _± direction.

The first mixture forms a first layer onto the surface, having a first thickness h _± . This first thickness h _± shall be understood as the maximal thickness of the first layer, and it is defined as the dimension of the layer thickness h _± measured from the surface where it is deposited, in the direction perpendicular to it, until the part of the first layer most distal from said surface.

Particularly, the first thickness might range from 8 cm to 0.5 cm, or from 6.0 cm to 0.5 cm.

The at least one cavity comprises two opposed wall portions, understood as portions of the at least one cavity wall opposed through the cavity width Wj. These two wall portions might be from the same cavity wall, or they could be from different cavity walls defining the same cavity. The at least one cavity in the first layer comprises a first and a second wall portions which are in opposition through the cavity width Wj. The first tool, once it is inserted into the second layer, is actuated to stir one of the two opposing wall portions - the first opposing wall portion - while not stirring the other wall portion - the second opposing wall portion, that is to say, the first tool is actuated to stir only one of the two opposing wall portions. In this manner, the first mixture of the first layer and the second mixture of the second layer are intermingled in this portion or area.

The second opposing wall portion remains unaffected while the stirring of the first opposing wall portion occurs. The action of the first tool results in the intermingling of the first and second mixtures in areas where the first opposing wall portion is present. This is in contrast with the second opposing wall portion, which is not stirred, and which maintains a higher definition of the boundary between the first and second mixtures.

The first opposing wall portion being stirred might have any length or dimension, and it preferably comprises a portion of the at least one cavity wall length. The first and/or second opposing wall portions may comprise the whole length of the at least one cavity wall, or only a portion of it. Preferably, the first wall portion being stirred have a length of 0.05 - 2.0 meters, or 0.10 - 1.0 meters. The second opposing wall portion, which is opposed through the cavity width to the first opposing wall portion, has a substantially equal length or dimension to the first opposing cavity width.

The second thickness h ₂ might be similar to the first thickness h _lt or it might be different, such as in cases wherein for instance, the second mixture is deposited in excess into the at least on cavity, or it is deposited onto parts of the first mixture.

The present invention provides a first tool actuated to stir one of the two opposing wall portions, in particular the area of the second layer, where the first opposing wall portion is located so that the first mixture of the first layer and the second mixture of the second layer are intermingled in this portion or area of the second layer.

The first tool is inserted at least partially into the second thickness h ₂ of the second layer. The first tool is preferably inserted in areas of the second layer where it contacts simultaneously the first mixture and the second mixture deposited into the at least one cavity of the first layer, or, in areas of the second layer where the first tool contacts the first opposing cavity wall portion, but not the second opposing wall portion.

According to embodiments in this disclosure, in between step c) and d), the stirring of the first opposing wall is produced while leaving the second wall portion unaffected resulting in a desired intermingling of one mixture by the other in order to incorporate some degree of controlled randomness and intentional visual asymmetry.

The first tool might be in the form of a blade, rake, fork, trident, stirrer, impeller, paddle, whisk, or beater.

The second layer is then compacted and hardened. For this, in illustrative embodiments, the second layer comprising the first and second mixtures, that include the areas where the first and second mixtures have been stirred, is preferably covered with a protective sheet on its top surface and subjected to vacuum vibrocompaction.

Preferably, the second layer, once compacted, goes to a hardening or curing stage. In this stage, depending on the type of binder, as well as the use or not of any optional catalysts or accelerants, the compacted second layer may be subjected to the effect of temperature in a curing oven, or it might be left to cure at room temperature.

In preferred embodiments, the obtained agglomerated stone is allowed to cool down after step d) of compacting and hardening.

After hardening and, given the case, cooling, the artificial agglomerated stone obtained, which can be shaped as blocks, slabs, boards, plates, tiles or sheets, can be cut and/or calibrated to the desired final dimensions. Further, or in addition to the hardening and cooling, the artificial agglomerated stone obtained may be finished (polished, honed, etc.) on one or both of its larger surfaces, depending on the intended application.

In a particular embodiment, the first mixture and/or the second mixture comprises between 80 to 95 weight percent of inorganic filler and 5 to 20 weight percent of hardenable binder.

This high filler content contributes to make the first mixture and/or second mixture of a self-supporting consistence, so that the at least one cavity created does not close or change its shape during the time need to deposit the second mixture in them. Also, the high content of filler in relation to the binder results, among other characteristics, in an elevated superficial hardness and resistance to abrasion and scratching, as well as in a close appearance to natural stone.

Preferably, for both the first mixture and the second mixture, the inorganic filler comprises 70 to 95 weight percent of grain particles of a size ranging from 0.1 to 2.0 mm and 5 to 25 weight percent of micronized powders with particle sizes smaller than 100 micrometers.

Also, the inorganic filler, grain particles and/or micronized powders, may be selected from stone, stone-like and ceramic materials, such as quartz, silicate glass (virgin, frit or recycled), silica sand, feldspar, feldspathic sand, mirror, granite, basalt, (synthetic) cristobalite, dolomite, clay or porcelain ceramic, and mixtures thereof; or quartz, silicate glass (virgin, frit or recycled), silica sand, feldspar, feldspathic sand, (synthetic) cristobalite, clay or porcelain ceramic and mixtures thereof.

Preferably, the inorganic fillers might also comprise synthetic silicate granulates according to those described in WO 2021019020 A1 or WO 2021018996 A1.

In a preferred embodiment, the first mixture of the invention preferably comprises 5 to 20 weight percent of a hardenable binder, or a hardenable organic resin, based on the total weight of the mixture.

In certain embodiments, the amount of hardenable binder or organic resin is 5 to 15 weight percent, or 5 to 12 weight percent, based on the weight of the agglomerated stone slab.

The hardenable binder of the first mixture and/or the second mixture is/are preferably liquid or at least shall have some capacity to flow during the mixing with inorganic filler, during deposition of the mixtures, and/or during the compaction steps. The binder, after hardening, functions holding the inorganic filler particles together in the form of a slab.

The hardening of the binder, and thus, of the first and second mixtures after compaction, can ultimately be accelerated by raising the temperature, depending on the binder used, and/or by using suitable catalysts and accelerators as known in the art.

Additionally, additives, such as colorants or pigments, curing catalysts, curing accelerators, adhesion promoters (e.g. silanes), antimicrobial agents, ultraviolet stabilizers, rheology modifiers, or mixtures thereof, can be included in the first mixture. These types of additives and the proportion used thereof, are known in the state of the art. Preferably, these additives may be present in the first mixture in an amount of 0.01 - 5.0 weight percent, based on the weight of the composition.

In another preferred embodiment, the first and/or second mixture/s can be achieved, for example, by adding the different components to conventional industrial planetary mixers, followed by the step of stirring, or while stirring, in a manner known in the art.

In a particular embodiment, the inorganic filler of the first mixture and/or the second mixture comprises 5 to 50 weight percent of silicate glass, translucent synthetic silicate, or mixtures thereof.

In a preferred embodiment, the silicate glass, the translucent synthetic silicate, or both, are white highly translucent silicates.

Thanks to their translucency, the silicate glass, the translucent synthetic silicate, or the mixture of both provides to the manufactured slab of the invention a chromatic effect which is more attractive and a higher color depth while opaquer particles only rely on the appreciation of effects present on the surface of the slab.

In particular, the translucent or highly translucent silicate content allow that the chromatic effects generated by the invention are visible not only on the surface of the slabs, but they can be appreciated from the surface also into some depth of the slab material, advantageously enhancing the optical appeal of the material.

Additionally, the sum of the added amounts of crystalline silicate (quartz, cristobalite, tridymite) comprised in the first mixture preferably ranges 0 to 50 weight percent, or 0 to 40 weight percent, or O to 30 weight percent, or 0 to 15 weight percent, of the total weight of the first mixture, as measured e.g. by X-Ray Diffraction techniques disclosed below. In addition to this embodiment, or in an alternative embodiment, the sum of the added amounts of crystalline silicate (quartz, cristobalite, tridymite) in the second mixture preferably ranges 0-50 weight percent, or 0-40 weight percent, or 0-30 weight percent, or 0-15 weight percent, of the total weight of the second mixture.

In a particular embodiment, the hardenable binder is an organic resin, hydraulic cement based or a geopolymer based material.

Preferably, the hardenable binder is an organic resin. More preferably, the binder is a translucent organic resin.

The term hardened organic resin and hardenable organic resin are understood as a material of predominantly organic nature formed by a compound or a mixture of compounds, optionally including a diluent.

The compound or the compounds in the mixture of compounds in the resin might be monomeric, oligomeric or polymeric, optionally with variable molecular weights and crosslinking degrees.

At least some of the compounds in the hardenable organic resin, and optionally also the diluent, have functional reactive groups capable of undergoing curing by a crosslinking or curing reaction which hardens the organic resin, resulting in a hardened organic resin (or hardened binder) when the curing is concluded.

Also preferably, the hardened (reacted or polymerized) organic resin is a hardened organic thermosetting resin, suitably liquid when not hardened, and may be selected from unsaturated polyester resins, acrylate and methacrylate-based resins, vinyl resins and epoxy resins.

The hardenable organic resins are preferably reactive and can be hardened in a curing (or cross-linking) reaction, for instance, when energy, such as heat or radiation, is applied to them. The hardened organic resin is preferably translucent and or transparent, to favor the visual appreciation of the chromatic effects sought by the invention.

In a particular embodiment, the organic resin is a translucent organic resin, preferably a translucent unsaturated polyester resin.

In an embodiment, the unsaturated polyester resin comprises a prepolymer obtained by polymerization of unsaturated dicarboxylic acids (or anhydrides) with diols. For example, by the condensation of an acid or anhydride such as maleic acid or anhydride, fumaric acid, (ortho)phthalic acid, isophthalic acid, terephthalic acid, adipic acid, succinic acid, sebacic acid, or mixtures thereof, with a diol such as ethylene glycol, diethylene glycol, propylene glycol, butylene glycol, neopentyl glycol, hydrogenated bisphenol A, or mixtures thereof.

Additionally, the unsaturated polyester resin also comprises an ethylenically unsaturated monomer as a diluent, such as styrene, preferably in an amount of 25 to 45 weight percent based on the weight of the prepolymer.

The first and the second mixture might have the same composition and/or the same inorganic filler particle size distribution. In a particular embodiment, the first mixture and the second mixture have different composition or different inorganic filler particle size distribution or both.

In embodiments herein, the first mixture and the second mixture have different colors, e.g. by comprising different types and/or concentrations of pigmenting compounds or dyes, or by having differently colored inorganic fillers, or both.

In a particular embodiment, in between steps b) and c), a first pigment composition is applied to the at least one cavity wall of the at least one cavity.

The first pigment composition is preferably applied to the two opposed wall portions comprised in the at least one cavity wall. Also, the first pigment composition is preferably colored with a tone or hue that creates a strong contrast with the color of the first and/or second mixtures.

Additionally, the first pigment composition is preferably sprayed or projected into the at least one cavity wall and/or opposed cavity wall portions. The first pigment composition might be solid or liquid. Preferably, the first pigment composition comprises a dye or pigment dispersed, dissolved or diluted in a liquid solvent compatible with the dye or pigment, and with the binder. Moreover, the liquid solvent is preferably reactive with the binder of the first and/or second mixtures.

Also, the application of the pigment composition to the opposed cavity wall portions, particularly when the pigment composition is selected to provide some visual contrast, reinforces the visual appreciation of the intermingling between the first and second mixtures caused in steps performed between steps c) and d) of the method of the invention.

In a particular embodiment, during step a), the first mixture is deposited onto the surface by means of a first distributing means and/or, during step c), the second mixture is deposited into the at least one cavity by means of a second distributing means.

Preferably, the first mixture is deposited onto the surface by using a first distributing means and the second mixture is deposited onto the cavities of the first mixture and optionally partially also onto the surface of the first layer by using a second distributing means.

In particular, distributing means known for being suitable for depositing these mixtures are, for instance, those distributor devices used for the distribution of the (unhardened) agglomerated mixtures in the manufacture of quartz agglomerated surfaces. The distributor devices suitably consist of a feeding hopper that receives the first mixture from the mixers in the top opening thereof and a conveyor belt positioned below the bottom outlet opening of the hopper, which collects and/or extracts the first mixture from the hopper and deposits it to the surface. However, other distributor devices are possible within the general concept of the invention.

Preferably, the first distributing means are movable along the length (this is, the longest dimension) of the surface, which suitably corresponds to the length (or longest dimension) of the slab being manufactured. Also preferably, the first distributor deposits the first mixture covering the full width of the slab being manufactured, as it moves along the length of the surface. The method of the present invention envisages the deposition of a second mixture into the at least one cavity created in the first layer. This deposition can be done by different means without departing from the scope of the general concept of the invention.

In preferred embodiments, the second distributing means are moved over the first layer to the locations where the at least one cavity is present. This can be achieved, for instance, if the second distributing means are mounted on a robotic device, such as an anthropomorphic robotic arm or a cartesian robot. Additionally, the movement of the second distributing means and the amount of second mixture discharged in any location over the first layer are synchronized so that the second mixture is predominantly deposited into the at least one cavity.

The second distributing means for filling the cavities with second mixture can be, for example, any of the distributors mounted on a robotic device disclosed in the document EP3713729A1.

In less preferred embodiments, the second distributing means are a hose moved by a robotic device, and with a pneumatic second mixture discharging system, optionally provided with one or several shutters, endless screws, valves, and the like, to regulate the amount of mixture being discharged.

Other methods for depositing the second mixture into the at least one cavity formed in the first layer are possible, and inside the scope of the invention.

Alternatively, and although not practical in an industrial setting, the second mixture could be deposited manually into the at least one cavity.

In a particular embodiment, the surface is a sheet of Kraft paper, a sheet or film of a polymeric material, a flat sheet of film or a temporary mold.

In a preferred embodiment, the first mixture is deposited onto a surface to form a first layer having the first thickness h _± . The surface can be a sheet of a material compatible with the first mixture, such as a sheet of Kraft paper or a sheet or film of a polymeric material - e.g. a rubber sheet. In non-limiting embodiments, the surface is part of a temporary mold, for instance, the bottom of a tray-shaped mold, which is used to retain the first mixture during the manufacturing of the slab.

In additional embodiments, the surface is a flat sheet or film which supports and carries the self-supporting first mixture with the shape of a slab on top of it through the different slab manufacturing steps.

As previously mentioned, in the step b) of the method of the invention, at least one cavity having a width w; and a length Lj is created in the first layer.

In a particular embodiment, the at least one cavity is defined by a width w; varying along the length Lj of the at least one cavity or a width w; varying along the first thickness h _± of the first layer or varying along both the length Lj and the first thickness h _± of the first layer.

The at least one cavity width w; might vary along the extension of the at least one cavity, or along the first thickness, or it might be constant.

Preferably, the at least one cavity define a path with a length, the longest dimension of the path, and a width, the shortest dimension of the path.

In another embodiment, the at least one cavity comprises two or more intersecting or overlapping cavities.

In a particular embodiment, the at least one cavity width w; is in the range 0.03 - 1.00 m, more preferably 0.05 - 0.50 m.

The at least one cavity may extend longitudinally, either lengthwise, diagonally or widthwise, across the length and width of the slab shaped first layer.

The method for manufacturing a slab of the present invention is not limited by the width of the created cavities in order to enhance the possibilities of simulating different types of natural stones. In a particular embodiment, the at least one cavity length L; is in the range 0.01 - 3.5m, more preferably 0.1 - 3.3m.

The method for manufacturing a slab of the present invention is not limited by the length of the created cavities in order to enhance the possibilities of simulating different types of natural stones.

In a particular embodiment, during step b), the at least one cavity in the first layer is created by means of a template.

In a preferred embodiment, a first template is used for creating the at least one cavity in the first layer. However, the general concept of the present invention is not limited to this particular embodiment.

Particularly, the first template preferably comprises one or a several forms - or shapes - defining a pattern corresponding to the at least one cavity to be obtained in the first layer. The form or forms are preferably mounted in a framed structure comprised in the first template. The form or forms might define a contour, and might be solid or hollow bodies. Then, the first template is positioned over the surface before the first mixture is deposited onto it.

In certain embodiments, by removing the first template after the first mixture has been deposited, and before the second mixture is deposited in step c), at least one cavity is created in the first layer where the form or forms were located.

The template, and the form/forms it might comprise, can have any desired shape or combination of shapes, and may define any desired pattern. Preferably, the form or forms are shaped following longitudinal paths, following regular or irregular general directions, and extending partially or fully through the whole length and/or width of the slab being manufactured.

The form or forms might have a length from 1 cm to 3.5 m, or 10 cm to 3.3 m, and a width from 1 cm to 100 cm, or from 3 cm to 50 cm, and a height of from 15.0 cm to 0.5 cm, or 10.0 cm to 1.0 cm. Additionally, the forms can be designed to partially or fully block the deposition of the first mixture onto the parts of the surface they cover, or alternatively, instead of blocking, they can divert the first mixture being discharged by the distributor away from the areas covered by the forms, and into the areas not covered by the forms, for instance, by having a crest-shaped upper part.

The at least one cavity wall defining the at least one cavity created by the first template, can be made having a parallel or inclined general planar direction in relation to the direction of the first thickness, by a corresponding design of the forms in the first template.

Different templates for creating cavities in the first layer are possible and inside the broadest scope of the invention. Also, more than one template can be used. Non-limiting examples of templates useful for the method of the invention are described in document EP3713729A1.

In a particular embodiment, during step b), the at least one cavity in the first layer is created by means of a wheel-shaped indenter.

In this particular embodiment, the at least one cavity is preferably created after the first mixture has been deposited onto the surface. Then, the wheel-shaped indenter is inserted from the upper side at least partially into the first thickness h _± of the deposited first layer, and moved (e.g. by being coupled to the end of an automatic robotic arm), while being inserted, through the first layer.

Preferably, the wheel-shaped indenter might be shaped as two frustums of right cones joined by their bases, having a rotation axis coinciding with their frustum axes, with a maximal wheel width at the location of the rotation axis.

The maximal width size of the wheel-shaped blade can be suitably selected corresponding to the maximal width of the at least one cavity to be achieved in the first layer, and may range from 20 to 120 mm, or 30 to 110 mm. The maximal outer diameter of the wheel-shaped indenter may range from 100 mm to 300 mm, or 150 to 250 mm.

By moving the wheel-shaped indenter inserted and through the first layer, following certain trajectories, cavities defining at least one cavity wall are created as result of the indenter pressing the first mixture with its slanting laterals. The depth and width of the cavities can be controlled by controlling the depth to which the wheel-shaped indenter is inserted into the first layer, the width of the wheel-shaped indenter and/or the slope of the wheel slanting laterals. Other shapes and designs of the indenter are possible and well inside the general scope of the invention.

Other methods of the present invention for creating at least one cavity in the first layer are possible and well inside the general concept of the invention. Thus, the at least one cavity might be created by at least one (non-rotating) insert, or a combination of inserts, which is introduced from the upper side into the deposited first layer of first mixture. This can be achieved by arranging the at least one insert or combination of inserts over the first layer, as it rests onto the surface, and then moving it down until the insert or combination of inserts penetrated the desired depth into the first layer.

By raising the at least one insert or combination of inserts to its original position (or by lowering the position of the first layer), at least one cavity defined by at least one cavity wall is created into the first layer, with shapes and paths corresponding to the shapes and paths of the at least one insert or combination of inserts. As with the form or forms in the embodiment using a first template, the insert or combination of inserts may have any desired shape, and they can define any desired pattern.

The insert or inserts may follow regular or irregular general directions, and they might extend partially or fully through the whole length and/or width of the slab being manufactured. As well, the insert or inserts might be inserted partially or fully through the first thickness of the first layer.

In other embodiments, the at least one cavity might be created removing the first mixture from specific locations in the first layer, for instance, as a suctioning or aspirating device is moved (e.g. by a robotic device) over the first layer and the first mixture under the device gets suctioned or aspirated.

The at least one cavity, in any of the embodiments herein, is defined by at least one cavity wall. As mentioned above, the at least one cavity wall may extend partially or fully through the first thickness of the first layer. In other words, the at least one cavity created by any of the methods covered by the invention does not need to be completely free of first mixture. In some embodiments, it is preferred that the first mixture is partially allocated in the areas over the surface where the at least one cavity is created. Preferably, the first mixture fills less than 75% and more than 5 % of the cavity volume, or less than 50% of the cavity volume and more than 10% of the cavity volume, wherein the cavity volume is understood as the volume defined by the projection in the direction of the first thickness of the upper cavity walls edges.

In a particular embodiment, during step c), a portion of the second mixture is deposited also outside of the at least one cavity, preferably proximal to the at least one cavity.

Preferably, the second mixture shall at least partially fill the at least one cavity, preferably at least 75% or 80% of the volume of the at least one cavity, and it is possible that it is deposed in excess and some of the second mixture overflows or forms a bulge reaching out of the cavity. Alternatively, the second distributing means can be made to deposit the second mixture in the at least one cavity, but also a part outside the at last one cavity.

In particular, it is preferred that from 3 - 20 % of the second mixture is deposited outside the at least one cavity. Advantageously, this enhances the intermingling effect sought by the method of the invention.

Also preferably, the second mixture is deposited at most at 10cm from the first wall portion of the at least one cavity.

After the deposition of the second mixture, a second layer is formed comprising the combination of the first mixture and the second mixture deposited into the at least one cavity of the first layer. The second layer has a second thickness h ₂ , understood as the maximal thickness of the second layer, following the same definition as the first thickness h _± in relation to the first layer. This second thickness h ₂ might be equal or different to the first thickness h _± .

As above mentioned, after the second mixture is deposited and the second layer is formed, the method of the invention comprises inserting a first tool at least partially into the second thickness h ₂ of the second layer. In a particular embodiment, the first tool is rotatable around an axis X-X’ essentially parallel to the second thickness h ₂ of the second layer.

The first tool might be in the form of a blade, rake, fork, trident, stirrer, impeller, paddle, whisk, or beater. The first tool is preferably mounted on a robotic anthropomorphic arm or a XYZ Cartesian robot, so that it can be programmed to automatically move through the second layer following trajectories, stirring it, as well as close and away from the surface holding the second layer.

Also, the rotational movement suitably is produced around an axis X-X’ which is substantially parallel to the direction of the second thickness h ₂ of the second layer. This is particularly preferred, when the first tool is configured to comprise a number of separated prongs, such as 2 to 8 prongs, distributed, homogeneously or not, around the rotation axis. The speed of rotation might range from 1 - 30 rpm, or 5 - 25 rpm. The rotation might be done in any clockwise or anticlockwise direction, or the rotation might be alternated between periods of clockwise rotation and periods of anticlockwise rotation.

Advantageously, the rotation of the first tool while inserted in the second layer, stirs (or breaks) the first opposing wall portion.

In preferred embodiments, the first tool comprises a number of separated prongs, such as 2 to 8 prongs, distributed, homogeneously or not, around the rotation axis X-X’.

The first tool might have a length and/or a width of 1 to 30 cm, or more preferably 2 to 20 cm. The first tool may also have a larger outer diameter, for example, when the first tool is shaped as a closed curve, such as a circle, an ellipse, an oval or an ovoid, in the range of 1 to 30 cm, or more preferably 2 to 20 cm.

In certain embodiments, the first tool is provided with a projecting head for a second pigment composition. The projecting head, when in use, projects or sprays the second pigment composition to the areas of the second layer being stirred by the first tool.

In another particular embodiment, the first tool is preferably moved following at least partially the path of the first wall portion being stirred. This can be achieved by the first tool being mounted in a programmable automatic robotic device that moves the first tool and being inserted through the second layer. The first tool might also be moved in programmed trajectories approaching or departing from the surface where the second layer rests, in the direction of the second thickness h ₂ , producing, advantageously, additional intermingling and randomness effects.

In another particular embodiment, the stirring of the first opposing wall portion, suitably by the rotational movement of the first tool, is done preferably so as to produce a visually noticeable intermingling of the first and second mixtures. The stirring preferably produces the transferring respectively of the first mixture outside, and of the second mixture inside, the areas where the cavities where created

The present invention also provides an artificial agglomerated stone slab comprising at least a first and a second mixtures defining chromatic effects, being manufactured according to the method of any of the embodiments of the inventive aspect of the invention.

The artificial agglomerated stone slab dimensions are at least 1500 mm in length, at least 1000 mm in width and 4-40 mm in thickness; preferably 2000-3500 mm in length, 1000- 1800 mm in width and 4-40 mm in thickness.

The artificial agglomerated stone slab has preferably an apparent density in the range 2000 - 2600 kg/m ³ , or from 2100 - 2500 kg/m ³ . Apparent density can be measured according to EN 14617-1 :2013-08.

The water absorption of the artificial agglomerated stone slab preferably does not exceed 0.10 %, or 0.07%, or 0.05%, according to EN-14617-1 :2013.

The artificial agglomerated stone material might be used for construction or decoration, for manufacturing counters, kitchen countertops, sinks, shower trays, wall or floor coverings, stairs or similar.

Artificial agglomerate stone slabs having an essentially reproducible appearance are manufactured in an industrial setting according to the method of the first aspect of the invention. The inventive concept also covers a plant for the manufacture of the mentioned artificial agglomerated slabs, where the plant is configured for applying the method of any of the embodiments described herein. Such plant might be designed comprising any of the embodiments of the means and devices disclosed herein for the different embodiments of the method of the invention.

DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will be seen more clearly from the following detailed description of a preferred embodiment provided only by way of illustrative and non-limiting example in reference to the attached drawings.

Figure 1a, 1 b These figures show two embodiments of the shape of the cavity according to the invention.

Figure 2 This figure shows a perspective view of a first layer of a first mixture comprising 3 cavities according to an embodiment of the invention.

Figure 3 This figure shows a view from above according to the same embodiment shown in Figure 2

Figure 4 This figure shows a perspective view of a second layer according to an embodiment of the invention, comprising a combination of a first mixture and a second mixture deposited filling the 3 cavities of the first layer of Figure 1 .

Figure 5 This figure shows a perspective view of a second layer according to an embodiment of the invention, where three cavity wall portions of each cavity of Figure 1 , are stirred.

Figure 6 This figure shows a schematic front view of an embodiment of the first tool according to certain embodiments, configured to stir the second layer.

Figure 7 This figure shows a schematic front view of an embodiment of a wheelshaped indenter for creating a cavity in the first layer of first mixture according to certain embodiments.

DETAILED DESCRIPTION OF THE INVENTION

As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as method or product directly obtained from the method for manufacturing said product.

Figure 1a and Figure 1 b depict two embodiments of cavity shapes that can be created. Particularly, Figure 1a shows a specific embodiment of a cavity being shaped as an ellipse and Figure 1b shows a specific embodiment of a cavity being circular. In both cases, these figures define the dimensions, length Lj and width w _{i t} and their positions in order to characterize each one of the cavities. In the particular case of an elliptical shaped cavity, the length Lt is equal to the major axis of the cavity. In the particular case of a circular shaped cavity, the length Lj is equal to one diameter of the cavity. Additionally, and also in both cases, the width w; is perpendicular to the length Lj and said cavity width w, is variable up to a maximum w, _mor .

Figure 2 shows an exemplary embodiment of a first layer (1.1) of a first mixture (Mi) after performing a first a) and second b) step of the method for the manufacture of slabs of artificial agglomerated stone. Previously to the creation of the cavities 3 in the embodiment shown in Figure 2, a first layer (1 .1) of first mixture (Mi) is deposited onto a surface (2), wherein the first layer (1.1) has a shape corresponding to the shape of the slab to be manufactured, and presents a first thickness h _± . In embodiments, the first layer (1.1) has a first thickness h _± of 4.5 cm.

In preferred embodiments, the surface (2) is a sheet of Kraft paper, a sheet or film of a polymeric material, or a temporary mold.

As shown in Figure 2, at least one cavity (3) is comprised in the first layer (1.1) of first mixture (Mi). In particular embodiments depicted in Figure 2, the slab-shaped first layer (1.1) presents 3 different cavities (3) of different shapes.

The two cavities respectively on the left and right side of the first layer (1.1) shown in Figure 2 might have a cavity width at its widest part of 15-30 cm, and might be created with a template (not shown) as disclosed in embodiments herein. In this embodiment of the invention, the template (not shown) is designed having two different solid forms mounted in a framed structure of 3.4 m length and 1.7 m width. The template is positioned over and in contact with a surface (2). In this non-limiting example, the forms had an oval and a wavy shape, respectively, corresponding to the shapes of the cavities on the left and right side of the first layer (1.1) shown in Figure 2.

In the same embodiment, the first mixture (Mi) is deposited from a first distributor onto the surface, and over the template. The deposition of the first mixture (Mi) is performed by moving the distributor over and along the length of the template, while discharging the first mixture (Mi) all through the template width. Once the discharging of the first mixture is concluded, the template is removed leaving a first layer (1.1) of the first mixture (Mi) on top of the surface, having the two cavities (3) with shapes corresponding to the forms in the template. Although the first mixture is self-supporting, and the cavities (3) are retained without fully collapsing, about 10% of the volume of the cavity might be filled with the first mixture (not shown in Figure 2) upon removal of the template.

The central cavity crossing the first layer (1.1) from lateral edge to lateral edge might be created using a wheel-shaped indenter, as disclosed in embodiments above. At the top left of Figure 2, the cavity (3) is essentially elliptical. Then, at the right side of the slabshaped first layer (1.1), it is shown a cavity (3) having a vein shape extending over the whole thickness h _± of the first layer (1.1) and showing substantially vertical walls (4). Finally, the central cavity is shown having a vein shape extending over the whole width of the slab-shaped first layer (1.1), crossing it laterally, and presenting inclined walls (4).

In preferred embodiment, each cavity (3) is defined by a width w; varying along the length Lt of said cavity (3) or a width varying along the first thickness h _± of the first layer (1.1) or varying along both the length Lj and the first thickness h _± of the first layer (1.1).

In the same embodiment, each cavity (3) presents a first wall portion (4.1) and a second wall portion (4.2) in opposition through the at least one cavity width and extending at least partially along the cavity length Lj, and throughout the first thickness h _± of the first layer (1.1) of first mixture (Mi).

In more preferred embodiments, at least one cavity (3) in the first layer (1.1) is created by means of a wheel-shaped indenter, for example, the wheel-shaped indenter (7)further shown in Figure 7. Said at least one cavity (3) might be created by moving the wheelshaped indenter (7) through the first layer (1.1) when inserted in said first layer (1.1). Thus, in the embodiment of Figure 2, by following certain trajectories with the indenter (not shown), the central cavity (3) defining at least one cavity wall (4.1 , 4.2) is created as a result of the indenter pressing and/or displacing the first mixture (Mi). The depth and width of each cavity can be controlled by controlling the depth to which the wheel-shaped indenter (not shown in Figure 2) is inserted into the first layer (1.1). Other shapes and designs of the indenter and of the cavity created are contemplated in other embodiments of the invention.

Also in preferred embodiment, the first layer (1.1) of first mixture (Mi) is deposited onto the surface (2) by means of a first distributing means (not shown).

Figure 3 is a view from above of the embodiment also shown in Figure 2. This embodiment depicts a first layer (1.1) of a first mixture (Mi) after performing a first a) and second b) step of the method for the manufacture of slabs of artificial agglomerated stone. As also shown in Figure 2, the first layer (1.1) of first mixture (Mi) is deposited onto a surface (2), wherein the first layer (1.1) has a shape corresponding to the shape of the slab to be manufactured.

In Figure 3, the 3 cavities (3) show a first wall portion (4.1) and a second wall portion (4.2) which are 2 portions (4.1 , 4.2) of the perimeter of each cavity (3) and where said portions (4.1 , 4.2) are wall portions opposed through the width of each cavity (3). Also in Figure 3, the wall portions (4.1 , 4.2) are represented as circled and are small segments or sections of the perimeter of each cavity (3). That is, the cavity walls (partially shown in this Figure) can be divided in a multitude of small segments where one of said multitude of small segments can be selected as a first wall portion (4.1). The second wall portion (4.2) is always defined as the wall portion in opposition to the first wall portion (4.1) through the at least one cavity width Wj.

Furthermore, each wall portions (4.1 , 4.2) defines a segment of the upper surface of the cavity walls (partially shown in this Figure) of each cavity (3), said upper surface being the top of the first layer (1.1) of first mixture (Mi). Both of these wall portions, the first wall portion (4.1) and the second wall portion (4.2), are separated by a distance equal to the width wt crossing one unique point of the cavity length Lj. Each cavity width w _{i t} defined by two opposite wall portions (4.1 , 4.2) is crossing one unique point of the length Lt of each cavity (3) following the shape of the cavity (3). Also, the first wall portion (4.1) always defines the side of the cavity wall or wall portion to be stirred.

Figure 4 shows the following step of the manufacture of a slab of artificial agglomerated stone where a second mixture (M2) has been deposited into each previously created cavity (3) of the first layer (1), to form jointly with the first mixture (Mi) a second layer (1 .2). The second layer (1 .2) presents a second thickness h ₂ .

In certain embodiments, the first and/or second walls (4), or first and/or second wall portions (4.1 , 4.2) of the cavities (3) created in the first layer (1.1), before the second mixture (M2) is deposited into them, are sprayed using a nozzle mounted on a robotic device (not shown in the Figures) with a dark black pigment composition comprising styrene and a dye.

Subsequently, a second distributor (not shown) mounted on a robotic device deposits the second mixture (M2) into the cavities (3), creating a second layer (1 .2) on top of the surface by the combination of the first and second mixtures (Mi and M2). The second mixture (M2) is discharged in controlled amounts as the robotic device is moved over the first layer (1.1). The depositing of the second mixture (M2) is predominantly made into the cavities (3), filling them to more than 75 weight percent of their volume, and with 1 to 5 weight percent of the second mixture (M2) being deposited outside the cavities (3) (not shown) in areas proximal to or not farther than 10 cm from the cavity walls (4). In the embodiment shown in Figure 4, the second layer (1.2) has a thickness h ₂ practically identical to the thickness Ti^of the first layer (1.1).

In preferred embodiments, the second mixture (M2) at least partially fills each of the cavities (3), preferably it fills 75 to 100% of the volume of each of the cavities (3).

In another preferred embodiment the second mixture (M2) is deposited in excess and some of the second mixture (M2) overflows or forms a bulge reaching out of each cavity (3).

In particular embodiments of the invention, the first mixture (Mi) and/or the second mixture (M2) comprises between 80 to 95 weight percent of inorganic filler and 5 to 20 weight percent of hardenable binder. Additionally, the inorganic filler of the first mixture (Mi) and/or the second mixture (M2) comprises 5 to 50 weight percent of silicate glass, translucent synthetic silicate, or mixtures thereof. Finally, the hardenable binder is an organic resin, hydraulic cement based or a geopolymer based material. Even more preferably, the organic resin is a translucent organic resin and/or an unsaturated polyester resin.

In a preferred embodiment, the first mixture (Mi) and the second mixture (M2) have different composition or different inorganic filler particle size distribution or both.

In further embodiments of the invention, the two mixtures (Mi and M2) are previously prepared in separated planetary mixers comprising different amounts of granulates and powders, having different particle sizes, of quartz, synthetic cristobalite, feldspar and recycled silicate glass, and a liquid transparent unsaturated polyester resin. Two different pigment mixtures are added to each of the mixtures (Mi and M2) so that they have a different coloration. Preferably, the amount of granulates and powders make is 85 to 90 weight percent of the total weight of the mixtures (Mi and M2), while the remaining 12 to 7 weight percent is made up by the resin, with the remaining 0.5 to 3.0 weight percent being colorants, catalysts, accelerants and other additives. Both mixtures (Mi and M2) comprise 15 to 30 weight percent of silicate glass, in relation to the total weight of each mixture (Mi and M2).

Figure 5 shows the step after forming the second layer (1.2) by depositing the second mixture (M2) into the cavities (3) of the first layer (not shown). In that particular step, a first tool (5) in inserted, at least partially, into the second thickness h ₂ of the second layer (1 .2) and then the first tool (5) is actuated wherein the first tool (5) is configured to stir a first wall portion (4.1) of each cavity (3) while not stirring a second wall portion (4.2) opposed to the first wall portion (4.1) along the cavity width Wj. Said stirring of the first wall portion (4.1) of each cavity (3) is responsible for the asymmetrical intermingling effect sought by the method of the invention.

In preferred embodiment, the first tool (5) is in the form of a blade, rake, fork, trident, stirrer, impeller, paddle, whisk or beater. Figure 6 shows an embodiment of the first tool (5) responsible for stirring one of the two opposing wall portions, in particular the area of the second layer (1.2) where the first opposing wall portion (4.1.) is located (not shown in this Figure 6, but in Figure 5).

In preferred embodiments, the first tool (5) presents a plurality of separated prongs (5.2). In more preferred embodiments, the first tool (5) presents from 2 to 8 separated prongs

(5.2). In the embodiment shown in Figure 4, the first tool (5) presents three separated prongs (5.2). Said separated prongs (5.2) are distributed homogeneously, or not, around the rotation axis X-X’.

In a preferred embodiment, the first tool (5) presents a plate (5.1), preferably round, which is rotatable around the axis X-X’, which might be essentially parallel to the second thickness h ₂ of the slab-shaped second layer (1.2) as shown in Figure 3. Preferably, the plate in the first tool (5) is shaped as a closed curve, such as a circle, an ellipse, an oval or an ovoid, in the range of 1 to 30 cm, or more preferably 2 to 20 cm.

Also in preferred embodiment, the first tool (5) has a larger outer diameter and a length and/or a width of 1 to 30 cm, more preferably 2 to 20 cm.

In certain embodiments, the first tool (5) is provided with a projecting head (not shown) for a second pigment composition. The projecting head, when in use, projects or sprays the second pigment composition to the areas of the second layer being stirred by the first tool (5).

In the particular embodiment shown in Figure 6, the first tool (5) is configured comprising a round ‘fork’, formed by three-prongs (5.2) homogeneously distributed around a rotation axis X-X’ and fixed to a round plate (5.1) with an outer diameter of e.g. 10 cm. This tool can be coupled to the end (or hand) of a robotic arm, to be inserted into the second layer

(1 .2), more preferably in an area of the second layer (1 .2), where a first cavity wall portion (4.1) of said cavity (3) is present. When in use in preferred embodiments of the method of the invention, the first tool (5) is inserted into the second layer (1 .2) so that it contacts the first cavity wall portion (4.1), but it does not contact the opposed cavity wall portion, or second cavity wall portion (4.2), which is opposed to the first cavity wall portion (4.1) through the cavity width Wj. When in use in preferred embodiments of the inventive method, the first tool (5), for instance, the first tool (5) shown in Figure 6, is firstly inserted into the second thickness h ₂ of the second layer (1 .2) until the separation of the end-parts of the prongs with the surface (2) is approximately 1 cm. Then, the first tool (5) is actuated (in this example, by a computer program) to produce the rotation of the three prongs (5.2) around the rotation axis X-X’, and simultaneously moved following the longitudinal path along the second layer (1 .2) of the first cavity wall portion (4.1), e.g. for 5 to 30 centimeters, while avoiding contacting the opposed second cavity wall portion (4.2) along the path. This is schematically shown in Figure 5. The rotation of the three prongs (5.2) produces the intermingling of the first and second mixtures (Mi and M2) in the areas where the first wall portion (4.1) is present. The second opposed cavity wall portion (4.2) is not stirred by the round fork (5). The same process is repeated in the other two filled cavities (3) present in the second layer (1 .2) of the present embodiment, along different portion lengths along the longitudinal paths of their respective cavity wall sections (4).

In another preferred embodiment, the second opposing wall portion (4.2) is not stirred by the first tool (5), or by any other tool, during the manufacture of the slabs of artificial agglomerated stone.

Once the stirring with the first tool (5) is completed, in subsequent steps not shown in the Figures, the second layer (1 .2) is covered on its exposed surface with a protective sheet, e.g of Kraft paper, before it is transferred to a vacuum vibrocompaction press. In a particular and preferred example, after compaction, the second layer (1 .2) is hardened in a kiln for about 40-60 minutes at 70-110°C.

The compacted and hardened slab of artificial agglomerated stone obtained according to these embodiments, are trimmed and calibrated to the final dimensions, e.g. of 3.3 x 1.6 cm, and one of the two major surfaces is polished to enhance the appreciation of the chromatic effects created.

Before use in the final application, the slabs of artificial agglomerated stone are cut-to- size, whereby the chromatic effects created are visible through the slab thickness in the edges of the cut pieces.

Figure 7 shows an embodiment of the indenter (7) for creating a cavity in a first layer (1.1) of first mixture, e.g. as the central cavity shown in Figure 2. In particular, as shown in Figure 7, the indenter (7) is a wheel-shaped indenter shaped as two frustums (7.1) of right cones joined by their bases, such shape provides to the indenter (7) a maximal wheel outer diameter D at the location of the junction of said frustums (7.1). In this configuration, the rotation axes Y-Y’ coincides with the frustum (7.1) axes.

In preferred embodiments, the maximal width size T of the wheel-shaped indenter (7) is suitably selected corresponding to the maximal width of the at least one cavity to be achieved in the first layer.

In particular embodiments, the width T of the wheel-shaped indenter (7) might range from 20 to 120 mm, or 30 to 110 mm. Also, the maximal outer diameter D of the wheelshaped indenter (7) might range from 100 mm to 300 mm, or 150 to 250 mm.