Abstract:
A coated glass or glass ceramic article is provided that has a so-called “dead front” effect, where display features or icons are not visible in the off state. In some embodiments, the glass or glass ceramic article includes a sheet-like glass or glass ceramic substrate having two opposite faces, where the substrate exhibits, in a visible spectral range from 380 nm to 780 nm, light transmittance τ vis  of at least 1% for visible light that passes through from one face to the opposite face. An opaque coating on one face that exhibits, in the visible spectral range from 380 nm to 780 nm, light transmittance τ vis  of not more than 5%. An opening is provided in the opaque coating. The opening allowing light that is incident on the surface of the opaque coating to pass through the coating and through the glass or glass ceramic substrate. The opening has a width of not more than 80 μm at the glass or glass ceramic substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit under 35 U.S.C. §119(a) of German Patent Application No. 102015102743.7 filed Feb. 26, 2015, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to glass or glass ceramic articles which are provided with an opaque coating. More particularly, the invention relates to such articles which are provided with light-emitting display elements or intended to be equipped with light-emitting display elements. 
         [0004]    2. Description of Related Art 
         [0005]    From the prior art, glass ceramic cooktops are known which are coated on their lower surface in order to modify the appearance and to conceal parts of the cooktop installed below the glass ceramic. 
         [0006]    One option for this purpose are sol-gel coatings which are quite heat resistant and are distinguished by good adhesion to the glass ceramic plate. For concealing internal parts of a cooktop, opaque coatings are typically used. 
         [0007]    For some applications it is desirable that the coating does not cover the entire surface but has windows. Such windows are in particular arranged in front of light-emitting display elements, so that these display elements shine through the glass ceramic plate to be visible to an operator which looks at the utilization side of the cooktop. Partly, these windows are covered by translucent coatings to improve aesthetic appearance. With the same hue, a homogeneous surface is created in this manner. 
         [0008]    Nowadays, icons, characters, or other logos and designs are printed on cooktops by screen printing. However, it is difficult in this case to produce very fine patterns such as thin lines, for example. 
         [0009]    Moreover, concerning screen printing, reproducibility is difficult in areas of very fine resolution or runouts of lines due to the screen printing mesh. Furthermore, for every new product request or design change a new screen needs to be created, so that set-up costs are very high, which is especially noticeable in small series. Manufacturing of individual designs for each end user is therefore expensive. 
         [0010]    Furthermore, as to multilayer coatings, in case of a printing technique such as screen printing the problem arises that congruent patterning is difficult. Therefore, in case of multilayer coatings usually a larger window is omitted, to allow to pattern a further coating layer with exactly the desired pattern in the area of the window. However, especially in combination with light-emitting display elements the window might be visible if the more precisely patterned coating layer is not completely opaque. 
         [0011]    EP 0 868 960 B1 discloses a method for manufacturing control panels, in particular for electrical household appliances, wherein at least one personalized laser engraving is produced in at least one screen printed layer which has previously been applied to a basic panel blank, the engraving consisting in material removal so as to form decorative features, icons, or similar signs in the screen printed layer, and then these engravings are covered by manually or automatically applying a layer of different color, which may be effected immediately after the engraving step or in a separate operation. 
         [0012]    With regard to design aspects, it would moreover be especially desirable that the windows in the coating, which allow the light from the display elements to pass therethrough are as visually inconspicuous as possible. A seemingly homogeneous surface in which the display features are invisible in the off state would be ideal. 
       SUMMARY 
       [0013]    Therefore, an object of the invention is to provide a glass or glass ceramic article which appears opaque to an observer or at least comprises opaque areas, but yet allows light from display elements to pass and to be clearly visible. In other words, the invention intends to provide a so-called “dead front” effect in which the display features or icons to be displayed are not visible for an operator in the off state. 
         [0014]    Accordingly, the invention provides a glass or glass ceramic articles, comprising: a sheet-like glass or glass ceramic substrate having two opposite faces, which in the visible spectral range from 380 nm to 780 nm exhibits light transmittance τ vis  of at least 1%, preferably at least 30% for visible light which passes through the glass or glass ceramic substrate from one face to the opposite face; wherein one face of the glass or glass ceramic substrate has a coating which in the visible spectral range from 380 nm to 780 nm exhibits light transmittance τ vis  of not more than 5%, and wherein an opening is provided in the opaque coating, which allows light that is incident on the surface of the coating to pass through the coating and through the glass or glass ceramic substrate; wherein the opening has a width of not more than 80 μm, preferably not more than 40 μm. Due to the low transmittance of not more than 5% the coating is opaque. 
         [0015]    Such a glass or glass ceramic article is produced by providing a sheet-like glass or glass ceramic substrate having two opposite faces, which in the visible spectral range from 380 nm to 780 nm exhibits light transmittance τ vis  of at least 1%, preferably at least 30% for visible light which passes through the glass or glass ceramic substrate from one face to the opposite face; wherein one face of the glass or glass ceramic substrate is provided with an opaque coating which in the visible spectral range from 380 nm to 780 nm exhibits light transmittance τ vis  of not more than 5%; wherein a pulsed laser beam is directed to the opaque coating and the coating is locally removed by ablation down to the surface of the glass or glass ceramic article thereby producing an opening in the opaque coating, which opening has a width of not more than 80 μm, preferably not more than 40 μm, more preferably not more than 25 μm toward the glass or glass ceramic substrate or at its substrate end. Preferably, the opaque coating is directly applied to the substrate. However, it is also conceivable that an intermediate layer is provided between the substrate and the coating, for instance to improve adhesion of the opaque coating to the substrate. 
         [0016]    Light transmittance τ vis  can be determined with a standard illuminant D65 according to DIN EN  410 . 
         [0017]    The width of the opening is so small that the opening is visually inconspicuous to an observer as long as no light source is switched on at the side opposite the viewed side. Should the opening still be recognizable with the naked eye at all, reflections of ambient light at the substrate surface and at the interface to the opaque coating are predominating, because of the width and the upper limit of transmittance of the coating mentioned above. On the other hand, light transmission through the opening decreases with decreasing width thereof. For creating backlit display features that are still well perceptible, it is therefore preferable that the width of the opening at the glass or glass ceramic substrate is at least 0.5 μm. 
         [0018]    However, when a light source is turned on, which is otherwise hidden for a viewer by the opaque coating, the light passing through the opening will be clearly visible for the viewer. Moreover, surprisingly, due to the high contrast between the penetrating light and the rest of the surface appearing darker because of the opacity, the contours of the opening are outshined and the feature created by the opening seemingly has a greater width than the actual width of not more than 80 μm, preferably 40 μm or less, more preferably not more than 25 μm. The fact that the features appear larger or wider in the transmitted light may as well be attributed to diffraction effects occurring due to the small width. According to yet another embodiment, brightness control may additionally be provided for the light-emitting element intended for illumination. Such brightness control may serve to change the apparent width of the display feature, but also to signal a specific value such as the heating stage of a cooking zone. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention will now be described in more detail with reference to the accompanying figures. In the figures, the same reference numerals designate the same or equivalent elements. In the drawings: 
           [0020]      FIG. 1  shows a coated glass or glass ceramic article; 
           [0021]      FIG. 2  shows an apparatus for laser ablation for producing a glass or glass ceramic article according to the invention; 
           [0022]      FIG. 3  shows a glass or glass ceramic article produced by the method; 
           [0023]      FIG. 4  shows a glass ceramic cooktop comprising a glass ceramic article according to the invention as a cooking plate; 
           [0024]      FIG. 5  shows an embodiment with a side-emitting light guide for illuminating the opening; and 
           [0025]      FIG. 6  shows a refinement of the embodiment illustrated in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    For producing a glass or glass ceramic article according to the invention, initially a planar or sheet-like glass or glass ceramic substrate  2  is provided. Accordingly, the glass or glass ceramic substrate  2  has two opposite faces  20 ,  21 . One of the faces is provided with an opaque or light non-transmissive layer  5 , in the example shown in  FIG. 1  this is face  20 . 
         [0027]    Particularly preferred coatings  5  are inorganic and/or inorganic-organic sol-gel coatings which form an oxidic network and contain decorative pigments, or which form a matrix of such an oxidic network with embedded decorative pigments after curing. Most preferred oxidic networks are SiO 2  networks, or a SiO 2  matrix. Optionally, the network may still contain organic radicals. However, the coating  5  is yet inorganic or at least predominantly inorganic. The decorative pigments are preferably inorganic as well. 
         [0028]    Such coatings are highly durable and temperature resistant and can be produced in an almost unlimited number of different visual appearances, depending on the choice of the decorative pigments. However, the patterning of such coatings is a problem, especially if they contain a high proportion of pigments, or if the individual pigment particles are rather large. The latter is for instance the case when platelet-shaped decorative pigments are used to produce metallic or glitter effects. The inventive method even permits to sever the individual pigment particles and to cut them exactly. 
         [0029]    The decorative pigments and their content in the coating composition are selected so that with the intended layer thickness of the coating  5  the latter has a transmittance in the visible spectral range of less than 5%. Optionally, such low transmittance may be achieved by a multi-layer coating as well. 
         [0030]    Suitable coating compositions and coatings prepared therefrom are known, inter alia, from DE 10 2008 031 426 A1, and from DE 10 2008 031 428 A1, and the content disclosed therein concerning such coating compositions and coatings is hereby fully incorporated into the subject matter of the present application. Accordingly, in one embodiment of the invention an opaque coating is produced in the form of a sealing layer for decorative layers, wherein in a first step the decorative layer is produced by a sol-gel process, which layer is applied on the glass or glass ceramic substrate and cured by baking, and in a second step the decorative layer is covered by a sealing layer which is also produced by a sol-gel process, in which inorganic decorative pigments and fillers are mixed with a sol, wherein the inorganic decorative pigments comprise platelet-shaped pigment particles and inorganic solid lubricant particles which are added in a ratio ranging from 10:1 to 1:1 wt %, preferably from 5:1 to 1:1 wt %, and more preferably from 3:1 to 1.5:1 wt %, and wherein the prepared mixture is applied to the glass ceramic substrate provided with the cured decorative layer and is then cured at elevated temperatures. The cured sealing layer may have the same composition as the cured decorative layer, with the difference that in terms of the number of organic radicals the metal oxide network of the sealing layer has more organic radicals than the metal oxide network of the decorative layer, preferably at least 5% more organic radicals. Metal oxide network herein also refers to an oxidic network including elements which are semiconducting in elemental form (i.e. in particular the SiO 2  network already mentioned, inter alia). 
         [0031]    Unlike described before, other sealing layers may likewise be used. In addition to the sol-gel sealing layers described above, silicone layers or silicone-based layers are for instance suitable to seal an underlying coating. Optionally, plastics may be used as well. 
         [0032]    Also, ceramic inks may be used on the face, which are specifically adapted to the requirements of a ceramic lower surface coating. A preferred embodiment of this invention are hybrid layers, as described in DE 10 2012 103 507 A1. 
         [0033]    After producing the coating  5 , an opening  9  is created by means of an apparatus for laser ablation. An example of such an apparatus  11  is shown in  FIG. 2 . 
         [0034]    The apparatus for laser ablation  11  comprises a laser  71  and a device for guiding the laser beam  7  produced by the laser  71  across the surface  20  of the glass or glass ceramic substrate  2  coated with a coating  5 . The device employed for guiding the laser beam  7  over the surface may for example be a galvanometer scanner  15 . 
         [0035]    For some applications it is desired to produce recesses with a shape of long straight lines, for example in cooking plates in which such lines are intended to delineate a cooking area or to mark a cooking zone. For long straight lines it is advantageous to use a polygon scanner, because in stitching long lines a small offset may quickly occur. Due to the offset the line becomes wider at the interface and therefore would appear much brighter at this point in the transmitted light. 
         [0036]    As illustrated in  FIG. 2 , means for displacing the glass or glass ceramic substrate  2  may be provided alternatively or in addition to a galvanometer scanner. Particularly suitable for this purpose is an X-Y table  16 , also referred to as a cross table. In such an embodiment, the laser beam  7  can be fixed and the opening  9  with the desired shape may be introduced into the coating  5  by moving the X-Y table with the glass or glass ceramic substrate  2  arranged thereon. For producing long homogeneous linear openings  9  and at the same time maintain high accuracy, it is also possible to use a synchronized scanning and displacing apparatus which synchronizes the movement of table  16  or another means for displacing substrate  2  with the deflection of the scanner, such as galvanometer scanner  15 . 
         [0037]    For focusing the laser beam  7  on the surface in order to achieve the highest possible intensity, an appropriate focusing optical system  14  may be provided. In the example shown in  FIG. 2 , this focusing optical system is arranged downstream of galvanometer scanner  15 . However, it will be apparent to those skilled in the art that other configurations are likewise possible, which are suitable to focus the laser beam  7  onto the glass or glass ceramic substrate  2 . In order to achieve short focal lengths, it is favorable to arrange the focusing optical system behind the galvanometer as seen in the beam direction. Generally, regardless of the specific configuration of the optical system and the displacement mechanism as shown in the example of  FIG. 2 , a focusing optical system, in particular a lens or group of lenses or a focusing mirror with a focal length of less than 300 mm is preferred. 
         [0038]    For locally removing the coating  5  to create an opening  9  which extends through coating  5 , the device for guiding the laser beam moves the laser beam  7  over the surface, and the laser  7  is adjusted so that the ablation threshold of the material of coating  5  is exceeded and thus the coating is removed at the point of impingement. However, the output power of the laser is adjusted so that the ablation threshold of the substrate material, that means the material of the glass or glass ceramic of substrate  2 , is not reached so that only the coating is removed. The glass ceramics sold under tradenames ROBAX and CERAN CLEARTRANS may be mentioned as an example here. For these glass ceramics the ablation threshold for a laser wavelength of 1064 nm is approximately 5.2*10 17  W/m 2 . 
         [0039]    Generally, without limitation to the specific exemplary embodiment discussed above, it is therefore advantageous when the materials of the glass or glass ceramic material and of the opaque coating are selected so that the ablation threshold of the material of the glass or glass ceramic substrate  2  is higher than the ablation threshold of the opaque coating  5 , in particular in the infrared spectral range, more particularly at a wavelength of 1064 nm. 
         [0040]    Particularly preferably the opening  9  has a linear shape, without being limited to the illustrated example. In other words, the opening  9  has the shape of a trench in this case. Such a line or trench may have a closed form, that means it may generally be annular or have the shape of a frame. 
         [0041]    Furthermore, it is generally advantageous if the layer thickness of the opaque coating is not too high. This facilitates the removal by laser ablation on the one hand. On the other hand, this is advantageous for light transmission through the opening  9  in the coating. If the coating is too thick, the walls of the opening will be correspondingly long and will swallow an unnecessary amount of light. Therefore, it is generally preferred that the opaque coating  5  has a layer thickness of not more than 300 μm, most preferably not more than 160 μm. 
         [0042]    On the other hand, however, coatings that are too thin are also unfavorable, in particular in order to ensure sufficient light blocking. Preference is given to layer thicknesses of more than 10 μm, preferably at least 50 μm. The minimum and maximum thicknesses specified above are average values of layer thickness. 
         [0043]    The laser beam guiding means are controlled by a control device  13  which may for instance execute a program that translates the shape and location of the pattern feature into control signals by means of which the laser beam  7  is moved over the surface by the laser beam guiding means. Preferably, the control device also controls the laser  7 , in particular with regard to switching on and off and laser intensity. 
         [0044]    In one exemplary embodiment, a pulsed laser  71  was selected which can be sufficiently well focused to ablate tracks with a width of 0.02 mm. This was achieved with a neodymium-YAG laser of a wavelength of 1064 nm and a pulse length of 10 ps. A scanner with an optical system with a focal length of 255 mm was employed. The M 2  factor is less than 1.4. The beam had a diameter of 12 mm. Average output power W50 at 20% was about 10 W. Other lasers may also be used. However, it is advantageous in any case that the width of the ablated features is less than 0.025 mm. 
         [0045]      FIG. 3  is a schematic cross-sectional view of a glass or glass ceramic article  1  produced by the method according to the invention. An opening  9  has been introduced in the opaque coating  5 . This opening has a width  91  of not more than 80 μm at the bottom of coating  5  or at the substrate surface exposed in the opening, preferably of not more than 40 μm, most preferably not more than 30 μm or even not more than 25 μm. 
         [0046]    In the example shown on the left in  FIG. 3 , the wall  92  of opening  9  is substantially perpendicular. According to a further embodiment, opening  9  may taper from the surface  50  of the coating toward the glass or glass ceramic substrate  2 . One exemplary embodiment of this case is illustrated by the opening  9  on the right side in  FIG. 3 . Such an embodiment may be advantageous for introducing an opening even into rather thick coatings  5  by multiple or stepwise ablation. Preferably, however, the angle  93  between the wall  92  of opening  9  and the surface normal of the substrate is smaller than 20°, preferably smaller than 15°. This angle is the average angle of the wall which can be easily determined trigonometrically from the ratio of the width  91  of the opening at the substrate and the width  95  at the surface of coating  5 , and from the thickness of the coating  5 . Accordingly, the following applies to the thickness D of the coating  5  and the difference B of widths  95 ,  91  of this embodiment: B/2D≦tan(20°), preferably B/2D≦tan(15°). 
         [0047]    According to yet another embodiment, with the preferred layer thicknesses and the maximum width  91  of the opening at the substrate according to the invention, a condition is generally met in accordance to which the width  91  of opening  9  is smaller than the layer thickness of the opaque coating  5 . 
         [0048]    Both the smaller width of the opening compared to the thickness and the small angle of the wall  92  to the vertical, if any, imply that already when looking at the opening  9  slightly obliquely, the visual axis will not pass through the opening  9  but will end at wall  92 , which in conjunction with the small width  91  of the opening results in the fact that the opening remains invisible to a viewer. It can only be perceived when light from a light source on the side of the glass or glass ceramic article  1  that is hidden to the viewer due to the light blocking layer  5  passes through the opening. 
         [0049]    However, laser ablation may cause a dark discoloration of the coating. If the coating itself is dark, such discoloration and hence the opening  9  will remain invisible. However, for coatings having a light color this is different. In this case, the dark discoloration may be visible at the edges of the opening  9 . This can be counteracted by adjusting the pulse frequency of the laser and the speed at which the laser beam is directed over the coating such that the points of incidence of the laser pulses do not overlap each other. However, it is not possible to create a trench-shaped opening  9  in this manner, since first only consecutively stringed pits are ablated. To create a trench, the portion of the coating to be removed is then passed several times. According to this embodiment of the invention, it is thus even possible to produce openings invisible for a viewer in an opaque coating that has a color with an L value in the L*a*b color space of at least 20, preferably at least 40, more preferably at least 50. The L value of the color of the opaque coating may for example be determined using a spectrophotometer. The value refers to an exposed surface of the coating  5 , that means it is not a color value measured through the glass or the glass ceramic. 
         [0050]    According to one refinement of the invention, a top-hat profile of the laser beam  7  is used in order to minimize the thermal impact in the edge region of the opening to be produced so as to avoid the darkening effect. In this case, the edge regions of the initially Gaussian beam which have not enough energy for ablating the coating but yet have enough energy to heat the coating to an extent to cause discoloration thereof, are eliminated. Another advantage of a top-hat profile is better contour sharpness, since a Gaussian profile does not permit to remove multilayer systems with sharp contours, although this effect causes blurs in the micrometer range that are hardly visible or not visible at all to the eye. 
         [0051]    Particularly preferably, the invention is applied so that the coating is deposited on the surface  20  of the glass or glass ceramic substrate  2  that faces away from the user. Accordingly, the light from a light source will therefore first pass the coating through opening  9 , then the glass or glass ceramic substrate and will then exit from the opposite surface  21 . 
         [0052]    The invention can be employed in many ways for backlit glass or glass ceramic articles of household appliances. The invention is particularly suitable for cooktops. A control panel, for example on a stove or oven, may also be implemented using a glass or glass ceramic article according to the invention. In case of a household appliance, the opaque coating serves to create a certain visual appearance on the one hand, on the other to hide the components of the household appliance. 
         [0053]    Generally, without being limited to the exemplary embodiments described below, the invention relates to a household appliance which has an operating surface that is defined by the glass or glass ceramic article, and which comprises at least one light-emitting element that is arranged in the interior of the household appliance so that light emitted from the light-emitting element impinges to the opening  9  in the opaque coating  5  and can pass through the opening  9  and the substrate  2 . 
         [0054]      FIG. 4  shows an example of a preferred embodiment of such a household appliance  3  in the form of a glass ceramic cooktop  30 , the cooking plate of which is formed by a glass ceramic article  1  according to the invention. 
         [0055]    Regardless of the type of household appliance  3 , the opaque coating  5  is preferably applied on the inside surface  20  of the glass or glass ceramic substrate  2 . In the example of glass ceramic cooktop  30 , the opaque coating  5  is accordingly provided on the lower surface of the substrate, which accordingly is a glass ceramic substrate  2  in this case. One or more heating elements  23  are arranged below the glass ceramic substrate  2 , for heating food to be cooked or cookware placed on the cooking plate or surface  21 . For example, the heating elements  23  may comprise induction coils for an induction cooker. 
         [0056]    Without limitation to the illustrated exemplary embodiment, a light-emitting diode is used as the light-emitting element  18 . Depending on the design of the opening, an assembly with a plurality of light-emitting diodes  18  may be used as well. The latter is favorable, for example, when the opening  9  is elongated and is to be illuminated as uniformly as possible. According to yet another embodiment of the invention, it is also possible to use a laser diode as the light-emitting element, in order to provide for much light to pass through the small opening. 
         [0057]    Generally, without being limited to the illustrated example, it may moreover be favorable that a diffusing element  19  is arranged in front of light-emitting element  18 . Diffusing element  19  extends along a trench-shaped opening  9  and ensures that the light from light-emitting element  18  is distributed more evenly along trench-shaped opening  9 . In this manner, a more uniform illumination of the linear display feature created with such a trench-shaped opening  9  is achieved. 
         [0058]    The display feature created by the illuminated opening  9  may for instance serve to mark a cooking zone. Such marking is used to indicate which one of the cooking zones is currently enabled and heated. For this purpose, the trench-shaped opening  9  may for instance extend annularly around the area heated by heating element  23 . 
         [0059]    Besides a diffusing element  19 , a side-emitting light guide is suitable as well for distributing the light emitted by light-emitting element  18  along opening  9 .  FIG. 5  shows an example. Again, opening  9  has a trench-like shape, so that when light passes through the opening  9 , the impression of a line-shaped display feature is created. A side-emitting light guide  25  extends along opening  9 , which is optically coupled to light-emitting element  18  so that the light from light-emitting element  18  is injected into light guide  25 . Light guide  25  emits the injected light in distributed manner along its longitudinal extension and therefore also in distributed manner along the trench-shaped opening  9 , so that opening  9  is uniformly illuminated. Besides a light-emitting diode as the light-emitting element, a laser diode is suitable for this embodiment as well. With such a laser diode, high light intensity can be injected even into a thin light guide. The latter can be arranged close to the opening so that the light can be efficiently directed to the opening. Regardless of the type of light source, the embodiment with a side-emitting fiber is also suitable for guiding light into regions that are strongly heated during operation of the household appliance, since in this case the light-emitting element itself need not be located in the heated region. In this way it is possible, among other things, to provide light-emitting display features within a cooking zone. 
         [0060]    Generally, a coating on a glass or glass ceramic substrate may not only serve to prevent transparency. In addition, a coating may also be advantageous for sealing the coated side of the substrate. In the region of opening  9 , such a sealing layer would however be interrupted. According to one embodiment of the invention, if a transparent sealing layer is used, it can as well be applied after introducing the opening  9 , so as to cover or close the opening. The light from the light-emitting element will still be able to pass through opening  9  across the transparent sealing layer. 
         [0061]    Such a refinement of the invention, in which opening  9  is sealed by a transparent sealing layer  6  is shown in  FIG. 6 . The sealing layer may cover the opening  9  and/or even fill the opening, as illustrated. Suitable for sealing layer  6  are for instance transparent silicone layers, silicone-based layers and transparent sol-gel layers. Furthermore, it is also possible with such a sealing layer to fix a diffusing element  18  or a side-emitting light guide or even the light-emitting element in the vicinity of the opening. 
         [0062]    A sealing layer as represented by layer  6  refers to a coating which protects the glass or glass ceramic material and/or the opaque coating  5  from environmental influences. Such environmental influences may for instance include condensation products. Therefore, the sealing layer should be impermeable to liquid- and oil-containing substances as included in food, for example. If such substances should penetrate into coating  5 , this might cause visible, unattractive changes in visual appearance. 
         [0063]    Moreover, the opaque coating  5  itself may constitute a sealing layer for protecting the surface of the glass or glass ceramic covered by coating  5 . 
         [0064]    Besides lighting that is not visible in the off state, another application is to create an invisible label which serves as a flag against counterfeits. If it is desired to determine if a particular glass or glass ceramic article is a genuine product, this can then be easily verified by examining the article under back lighting. Therefore, according to one aspect of the invention, it is contemplated to use a marking in the form of a preferably linear opening  9  in the opaque coating  5  as produced according to the invention for labeling the origin of the glass or glass ceramic article. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           1  Glass or glass ceramic article 
           2  Sheet-like glass or glass ceramic substrate 
           3  Household appliance 
           5  Opaque coating 
           6  Sealing layer 
           7  Pulsed laser beam 
           9  Opening in  5   
           11  Apparatus for laser ablation 
           13  Control device 
           14  Focusing optical system 
           15  Galvanometer scanner 
           16  X-Y table 
           18  Light-emitting element 
           19  Diffusing element 
           20 ,  21  Face of  2   
           23  Heating element 
           25  Side-emitting light guide 
           30  Glass ceramic cooktop 
           50  Surface of  5   
           71  Laser 
           91  Width of  9  on substrate  2   
           92  Wall of  9   
           93  Angle of wall  92  relative to the surface normal of the substrate 
           95  Width of opening  9  at surface  50  of  5