TRANSPARENT ARTICLES MADE OF GLASS CERAMIC WITH HIGH SURFACE QUALITY AND METHODS PRODUCING

Abstract

A transparent article made of glass ceramic with high surface quality as well as a method producing are provided. The article is suitable for use as a viewing pane. The method includes the steps of: producing a melt with a raw material composition that is suitable for a ceramization; hot shaping a flat substrate made of ceramizable green glass having two oppositely arranged, essentially flat surfaces from the melt; processing of at least one of the surfaces of the substrate with a smoothing fine-treatment process; ceramizing the substrate to produce the article made of glass ceramic.

Claims

1. A method producing a flat, transparent articles made of glass ceramic, comprising: producing a melt with a raw material composition configured for ceramization; hot shaping a flat substrate made of ceramizable green glass having two oppositely arranged, flat surfaces from the melt; processing a first surface of the two flat surfaces with a smoothing fine-treatment process to a waviness of at most 500 m; and ceramizing the flat substrate.

2. The method of claim 1, wherein the step of processing further comprises processing a second surface of the two flat surfaces with the smoothing fine-treatment process to the waviness of at most 500 m.

3. The method of claim 1, wherein the ceramizable green glass is a lithium aluminosilicate system and/or comprises a nucleating agent selected from a group consisting of TiO.sub.2, ZrO.sub.2, SnO.sub.2, and combinations thereof and/or has a crystalline fraction that is less than 20 vol %.

4. The method of claim 1, wherein the raw material composition comprises a composition range (in wt. %) of: TABLE-US-00004 50-75.0.sup. SiO.sub.2, 15-28.0 Al.sub.2O.sub.3, 0-3.0 B.sub.2O.sub.3, 0-1.0 F, 2.0-6.0.sup. Li.sub.2O, 0-6.5 CaO + SrO + BaO, 0-7.0 TiO.sub.2, 0-5.0 ZrO.sub.2, 0-5.0 ZnO, 0-3.0 Sb.sub.2O.sub.3, 0-3.0 MgO, 0-3.0 SnO.sub.2, 2.0-7.0.sup. TiO.sub.2 + ZrO.sub.2 + SnO.sub.2 0-9.0 P.sub.2O.sub.5, 0-2.0 As.sub.2O.sub.3, 0-4.0 Na.sub.2O, 0-4.0 K.sub.2O, and 0-4.0 Na.sub.2O + K.sub.2O.

5. The method of claim 4, wherein the raw material composition further comprises, for a water content of 0.01-0.08 wt. %, conventional refining agents selected from a group consisting of Sb.sub.2O.sub.3, As.sub.2O.sub.3, SnO.sub.2, Ce.sub.2O.sub.3, fluorine, bromine, and sulfate.

6. The method of claim 1, wherein the step of hot shaping comprises rolling the melt between a pair of rollers that rotate in opposite directions relative to each other.

7. The method of claim 1, wherein the step of processing the first surface with the smoothing fine-treatment process comprises: rotating a material removal tool on the first surface around an axis perpendicular to the first surface; and guiding the material removal tool along predetermined tracks over the first surface, wherein the predetermined tracks overlap one another.

8. The method of claim 7, further comprising feeding a material selected from a group consisting of bonded abrasive, loose abrasive, cooling agent, and combinations thereof.

9. The method of claim 1, wherein the step of processing the first surface with the smoothing fine-treatment process comprises removing material from wave inclines on the first surface, while retaining material in wave valleys on the first surface.

10. The method of claim 1, further comprising supporting the flat substrate on a smooth base support during the step of ceramizing, and wherein the step of ceramizing comprises a batch kiln ceramizing or a continuous kiln ceramizing.

11. The method of claim 10, wherein the first surface is supported on the smooth base support during the step of ceramizing.

12. The method of claim 10, wherein the first surface is not supported on the smooth base support during the step of ceramizing.

13. The method of claim 1, wherein the step of processing the first surface with the smoothing fine-treatment process comprises a step selected from a group consisting of: removing material from an entire area of the first surface to a depth from about 0.1 m to 5 m; processing the first surface so that the waviness has a wavelength between 50 and 500 mm; processing the first surface so that only wave inclines on the first surface are processed; processing the first surface to a maximum roughness of at most 0.5 m; processing an entire area of the first surface to a roughness of less than 0.010 m; and any combinations thereof.

14. An article comprising: a flat substrate made of glass ceramic having two oppositely arranged, flat surfaces; a first surface of the two flat surfaces having a waviness of at most 500 m prior to ceramizing; and a transmittance in a visible wavelength region, in relation to a wall thickness of 4 mm, of greater than 0.75 at 400 nm, greater than 0.845 at 450 nm, greater than 0.893 at 550 nm, greater than 0.90 at 600 nm, and greater than 0.90 at 700 nm wavelength.

15. The article of claim 14, wherein the first surface has a chemical resistance against attack of materials selected from a group consisting of sulfur-containing exhaust gases, sulfur-containing acid, sulfuric acid, and combinations thereof.

16. The article of claim 14, wherein the waviness is at most 200 m and a waviness wavelength between 50 and 500 mm.

17. The article of claim 14, wherein the glass ceramic comprises a lithium-poor, glassy surface zone.

18. The article of claim 17, wherein the lithium-poor, glassy surface zone has a thickness, measured from the first surface, of between 200 nm and 2,000 nm.

19. The article of claim 14, wherein the flat substrate is configured for a use selected from a group consisting of a viewing pane, a bullet-proof viewing pane, a view pane for oven doors, a transparent baking oven plate, a fire-protection plate, a microwave turntable plate, a cooktop, and a fireplace viewing pane.

20. The article of claim 19, wherein the first surface comprises a surface area of at least 0.7 m.sup.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0159] FIG. 1 schematically, a curved fireplace viewing pane in an oblique view,

[0160] FIG. 2 schematically, a straight fireplace viewing pane in an oblique view,

[0161] FIGS. 3a and 3b by way of example in each case, a view from the top onto a substrate prior to the ceramization during the removal of material,

[0162] FIG. 4 a comparison of the impact strength of unpolished and polished material,

[0163] FIG. 5 schematically, a transverse view of a hot shaping process by means of shaping rollers,

[0164] FIGS. 6a and 6b comparison of the transparency through two different articles made of glass ceramic,

[0165] FIG. 7 important method steps, starting from the hot shaping until the article made of glass ceramic is obtained,

[0166] FIGS. 8a-8c transverse views of an excerpt from a substrate made of ceramizable green glass in various processing steps,

[0167] FIGS. 9a and 9b schematic illustrations of different polishing heads,

[0168] FIGS. 10 and 11 exemplary embodiments of the material removing faces of flexible polishing heads, and

[0169] FIGS. 12 and 13 views from the top onto a transverse polished section of two articles made of glass ceramic with and without a glassy surface zone.

DETAILED DESCRIPTION

[0170] In the following detailed description of preferred embodiments, for reasons of clarity, identical reference numbers refer to essentially identical parts in or on these embodiments. For better clarification of the invention, however, the preferred embodiments illustrated in the figures are not always drawn to scale.

[0171] With the method according to the invention, it is possible to produce a flat, transparent article made of glass ceramic, in particular for use as a viewing pane.

[0172] FIG. 1 shows, solely by way of example, schematically, a curved fireplace viewing pane 40 made of glass ceramic in an oblique view. Shown in FIG. 2, likewise solely by way of example, schematically, is a straight fireplace viewing pane 50 in an oblique view. Without any limitation to the illustrated exemplary embodiments, the fireplace viewing panes 40, 50 are designed with holding means 41, 51, which make it possible to mount the fireplace viewing pane 40, 50 in a fireplace in an especially simple manner. The illustrated fireplace viewing panes 40, 50 represent, solely by way of example, selected articles made of glass ceramic according to the invention and are produced with the method according to the invention.

[0173] In this case, the production comprises the preparation of a glass melt, whereby a batch that has a composition of a glass suitable for later ceramization and production of a glass ceramic is fed to a melting facility and is melted. By means of a rolling process for the hot shaping, a glass body made of ceramizable green glass is produced therefrom.

[0174] The ceramizable green glass here is based on the lithium aluminosilicate system and comprises nucleating agents, preferably TiO.sub.2 and/or ZrO.sub.2 or also SnO.sub.2. In this case, the composition range (in wt. %) given below is especially suitable: [0175] 50-75 SiO.sub.2, preferably 58-74 SiO.sub.2, especially preferred 60-73 SiO.sub.2, [0176] 15-28 Al.sub.2O.sub.3, preferably 15-25 Al.sub.2O.sub.3, [0177] 0-3.0 B.sub.2O.sub.3, preferably 0-2.0 B.sub.2O.sub.3, [0178] 0-1.0 F, [0179] 2.0-6.0 Li.sub.2O, preferably 2.0-5.5 Li.sub.2O, especially preferred 2.5-5.0 Li.sub.2O, [0180] 0-6.5 CaO+SrO+BaO, preferably 0-6 CaO+SrO+BaO, especially preferred 0-5 CaO+SrO+BaO, [0181] 0-7.0 TiO.sub.2, preferably 0-6.0TiO.sub.2, especially preferred 0-5.0 TiO.sub.2, [0182] 0-5.0 ZrO.sub.2, [0183] 0-5.0 ZnO, [0184] 0-3.0 Sb.sub.2O.sub.3, [0185] 0-3.0 MgO, [0186] 0-3.0 SnO.sub.2, [0187] 2.0-7.0 TiO.sub.2+ZrO.sub.2+SnO.sub.2, [0188] 0-9.0 P.sub.2O.sub.5, [0189] 0-2.0 As.sub.2O.sub.3, preferably 0-1.5 As.sub.2O.sub.3, [0190] 0-4.0 Na.sub.2O+K.sub.2O, wherein the respective proportions lie within the ranges given below: [0191] 0-4.0 Na.sub.2O, [0192] 0-4.0 K.sub.2O, [0193] preferably 0-3 Na.sub.2O+K.sub.2O, wherein the respective proportions lie within the ranges given below: [0194] 0-2.0 Na.sub.2O, [0195] 0-2.0 K.sub.2O; [0196] especially preferred 0-1.2 Na.sub.2O+K.sub.2O, wherein the respective proportions lie within the ranges given below: [0197] 0-1.0 Na.sub.2O, [0198] 0-0.5 K.sub.2O; [0199] as well as conventional refining agents such as Sb.sub.2O.sub.3, As.sub.2O.sub.3, SnO.sub.2, Ce.sub.2O.sub.3, fluorine, bromine, and sulfate, for a water content of 0.01-0.08 wt. %.

[0200] The green glass that is produced in the rolling process is suitable for being ceramicized in a ceramization process and for being transformed into a glass ceramic. Accordingly, in the ceramizable green glass, the crystal formation has not yet taken place or at least not yet notably taken place. The ceramizable green glass produced in this way thus contains nucleating agents for crystal formation, but the crystalline fraction is still very low. In particular, the crystalline fraction of the ceramizable green glass is less than 20 vol %, preferably less than 10 vol %, and especially preferred less than 5 vol %.

[0201] The ceramizable green glass coming from the glass melt is fed continuously to at least one pair of shaping rollers in order to obtain the desired thickness. This can take place in a single rolling step, but can also occur in a plurality of steps with a plurality of shaping rollers that are arranged in succession. After the rolling, the ceramizable green glass rests on transport rollers, which serve for the transport. After the intended thickness has been achieved, the rolled ceramizable green glass is separated into pieces by scoring it and breaking it along these scores in order to obtain substrates made of ceramizable green glass.

[0202] In the sense of the invention, at least one surface of the ceramizable green glass produced in the rolling process is subjected to a smoothing fine treatment, in particular to a polishing or to a lapping process, whereby the crystalline proportion of the ceramizable green glass is less than 20 vol %, preferably less than 10 vol %, and especially preferred less than 5 vol %.

[0203] In accordance with the invention, in this case according to a first embodiment of the invention, the wave inclines on the surface of the ceramizable green glass are removed in the course of the smoothing fine treatment, whereas, according to a second embodiment of the invention, material is removed over the entire surface of the ceramizable green glass.

[0204] In accordance with the second embodiment, in the case of the viewing panes shown in FIGS. 1 and 2, the material has been removed from the surface 42, 52 of the substrate made of ceramizable green glass prior to the ceramization by the smoothing fine treatment using at least one material removal tool with at least one material removing face. In other words, the smoothing fine treatment was carried out on the side of the ceramizable green glass that, in operation, later represents the side with the required high chemical resistance. The surfaces 53 and 43 were additionally smoothed after the ceramization. Because, for the impact strength, the structure of the bottom side of the substrate or the back side 42, 52 facing the combustion space is relevant for use as a viewing pane, this additional processing does not negatively affect the impact strength.

[0205] For this purpose, directly after the scoring and breaking or the cutting into corresponding large formats at the melting tank, the flat substrate made of ceramizable green glass is polished with cerium oxide or other known polishing agents for glass.

[0206] In accordance with the first embodiment of the invention, the smoothing fine treatment in this case is conducted in such a manner that only the fine waviness is reduced to a predetermined extent. The removal of material here takes place down to a depth, measured from the surface, that represents about half of the amplitude of the waves. In accordance herewith, the surface in the region of wave valleys is retained in terms of its earlier nature.

[0207] In accordance with the second embodiment of the invention, the smoothing fine treatment in this case is conducted in such a manner that a removal of material, albeit to a lesser extent, takes place over the entire surface. Material is removed here not only in the region of wave inclines, but also in the region of wave valleys, so that, ultimately, a new surface is created by the smoothing fine treatment. In accordance with an especially preferred design of this embodiment, only the surface that, during the hot shaping, has stood in contact with at least one shaping roller and/or with transport rollers and is consequently the side that can most likely be contaminated with particles, is subjected to a smoothing fine treatment prior to the ceramization.

[0208] The first embodiment is characterized in that, by means of the smoothing fine treatment, the fine waviness of the surface of the substrate made of ceramizable green glass is reduced. The remaining, low degree of fine waviness leads to the fact that air pockets are strongly reduced in size upon contact with the very smooth base supports or support plates during the ceramization and, in part or for the most part, are even no longer present.

[0209] This embodiment offers the great advantage that not only is the fine waviness reduced, but also the interfering air pockets are hardly created or not at all created during the ceramization and, therefore, no further damage is brought about during the ceramization as a consequence of relative movements between the substrate and the base support. This embodiment is especially suitable for colored ceramizable green glass for the production of colored articles made of glass ceramic for which no high requirements are placed on the transparency.

[0210] The achievable waviness of the ceramizable green glass or of the substrate made of ceramizable green glass is at most 500 preferably at most 50 and most especially preferred at most 10 m, measured as the difference in height between a wave valley and an adjacent wave peak. Furthermore, a low degree of waviness can be retained and, after the fine treatment, can be preferably at least 0.1 mm or at least 0.2 mm. The waviness of the wavelengths is between 50 and 500 mm, preferably between 60 and 200 mm, and especially preferred less than 135 mm.

[0211] The roughness of the surface of the substrate treated by the smoothing fine-treatment process in accordance with the first embodiment, in which predominantly wave inclines are removed, does not essentially change in this case. The maximum roughness Ra is at most 0.5 m and preferably is 0.2 m-0.5 m, preferably at most 0.4 m, especially preferred at most 0.3 m.

[0212] In contrast, the roughness of the surface of the substrate treated by the smoothing fine-treatment process in accordance with the second embodiment, in which the material removal takes place over the entire surface, is less than the original roughness and is Ra<0.02 m, preferably Ra<0.010 m.

[0213] The following two TABLES 1 and 2 show, by way of example, measured roughness values of a surface of a ceramizable green glass prior to the smoothing fine treatment and afterwards.

TABLE-US-00001 TABLE 1 Measured roughness values of a surface of a ceramizable green glass prior to the smoothing fine treatment Comment Top Ra 0.090 Mean side Rz 0.489 value Bottom Ra 0.114 side Rz 0.660

TABLE-US-00002 TABLE 2 Measured roughness values of a surface of an article made of glass ceramic in accordance with the invention after the smoothing fine treatment. Polishing Polishing agent 1 agent 2 Comment Top Ra 0.018 0.017 Mean side Rz 0.161 0.153 value Bottom Ra 0.018 0.017 side Rz 0.148 0.142

[0214] The values of the mean roughness Ra and of the mean roughness depth Rz, given in TABLE 2, show, on the basis of two examples, the qualities that can be achieved for articles made of glass ceramic in accordance with the invention. The articles were subjected here to a smoothing fine treatment using different polishing agents 1 and 2. In this case, the surface referred to as the top side was treated over the entire surface area prior to the ceramization in accordance with the second embodiment of the invention, whereas the surface referred to as the bottom side was treated after the ceramization.

TABLE-US-00003 TABLE 3 Grain size distribution of the polishing agents 1 and 2. RARE EARTH OXIDE BASIS D50 Laser m 2.0 3.0 CeO.sub.2 % 55 min 15 m Oversize % <= 0.5 REO % >= 85

[0215] The polishing agents 1 and 2 that were utilized are based on a slurry with a suspension of cerium oxide (CeO.sub.2) in water. The two polishing agents 1 and 2 differ in terms of the mean particle size D50 (D50 laser), wherein the respective values are D50=2.0 m or D50=3.0 m. Mean particle sizes up to about D50=5.0 m can also be regarded as being suitable.

[0216] The smoothing fine treatment here takes place using a material removal tool. The material removal tool in this case has at least one material removing face that rotates around an axis perpendicular to the material removing face. The material removal tool is guided here along predetermined tracks, for example, with constant advance, but different process parameters, such as pressure and speed of rotation, over the post-processed surface, whereby the tracks overlap one another. In addition, bonded and/or loose abrasive is or are added as a grinding agent, and/or a cooling agent is added.

[0217] FIG. 3a shows, solely by way of example, a device 1a of a possible embodiment for the smoothing fine treatment of the substrate 3 made of ceramizable green glass. The method and the device 1a configured for carrying out this method are based on the fact that with a plurality of material removal tools 6, 10, material is simultaneously removed from the surface 31 of a glass or glass ceramic plate 3 that is to be post-processed, whereby the material removal tools 6, 10 have material removing faces 7, 11 that each rotate around an axis perpendicular to the material removing face 7, 11, whereby the material removal tools 6, 10 are guided along predetermined tracks 8, 12 over the surface 31 that is to be post-processed, whereby the tracks 8, 12 overlap one another, and whereby at least one first material removal tool 6 grinds the surface 31 that is to be post-processed with a grinding agent having a first grit and at least one second material removal tool 10 polishes the surface that has been ground by the at least one first material removal tool 6 with a second grinding agent, which, in comparison to the first grinding agent, has a finer grit, whereby the second grinding agent comprises a slurry, and whereby this slurry is also introduced during the grinding between the material removing face 7 of the at least one first material removal tool 6 and the surface 31 that is to be post-processed.

[0218] In FIG. 3a, only one of tracks 8, 12 that are traversed by the tools 6, 10 is illustrated as a cross-hatched area in each case. Overall, for the processing of the entire surface 31, the material removal tools 6, 10 are guided along tracks that are parallel to one another over the surface.

[0219] For this purpose, as illustrated on the basis of the paths 9, 13, a meandering movement is made by the tools 6, 10 against the surface 31. In order to carry out such a movement, the material removal tools 6, 10 are moved forward and backward along the tracks 8 and 12 and, in each case, after a forward and backward movement, the substrate is moved a bit further along the direction of advance 15. In order for both the tracks 8 and also the tracks 12 to each overlap one another, the advance is less than the diameter of the material removing faces 7 and 10. The overlap of the tracks 8, just like that of the tracks 12, here is preferably at least 20%, especially preferred at least a third of the diameter of the material removing face or, corresponding to it, the respective track width.

[0220] Obviously, it is also possible to design the device in such a way that only one material removal tool is employed. For this purpose, FIG. 3b shows, solely by way of example, a device 1b of a possible further embodiment for the smoothing fine treatment of the substrate 3 made of ceramizable green glass, whereby the device 1b is based on a single material removal tool 6.

[0221] FIG. 4 shows a comparison of the impact strength of unpolished and polished material, which was produced in accordance with the method. It can be clearly seen that there is an increase in strength, which, in the example, is at least 20%, for polished material in accordance with the invention, which was polished on the side relevant to the impact strength prior to the ceramization.

[0222] The determination of the impact strength in this case is produced according to the following method on the bases of the ball drop test in accordance with DIN 52306. In this test, a steel ball weighing 200 g is allowed to drop from a defined height in free fall onto the middle of the sample, that is, onto the article made of glass ceramic, in a format of 1001004 mm. The drop height is increased in steps until breakage occurs. The impact strength is a statistical quantity and is determined on a series of about 20 sample specimens. The analysis is produced according to the Weibull model in accordance with DIN EN 61649.

[0223] The exemplary embodiment shown in FIG. 4 shows that, in accordance with the invention, it is possible to increase the strength markedly. In other examples, an increase in strength of the article made of glass ceramic of 10% or even of more than 20% could be determined.

[0224] After the smoothing fine treatment of the at least one surface of the substrate made of ceramizable green glass, there occurs a ceramization for the production of the flat, transparent article made of glass ceramic.

[0225] In accordance with the first embodiment of the invention, after the smoothing fine treatment, the substrate made of ceramizable green glass is applied, with the treated surface resting on the base support or support plate. In this embodiment, owing to the reduction in the fine waviness, the relative movement of the resting substrate is strongly reduced or, in the ideal case, no longer takes place when it travels through the roller kiln.

[0226] In accordance with the second embodiment of the invention, after the smoothing fine treatment, the substrate made of ceramizable green glass is applied on the base support or support plate, with the treated surface facing upward. This surface, which, on the basis of the fine treatment, is free of microstructures and/or particles, thus faces upward during the ceramization and thus is more strongly exposed to the prevailing kiln atmosphere, so that the creation of the desired glassy surface zone is achieved especially well.

[0227] In this way, an article made of glass ceramic in accordance with the invention is obtained. This is followed by the usual post-processing treatments of the article, such as, for instance, cutting into pieces or the introduction of a facet, or also the imprinting.

[0228] It is noted at this point that the curved construction of the fireplace viewing pane 40 in accordance with FIG. 1 is based on a reshaping of the article made of glass ceramic subsequent to the ceramization. In order to produce a curved fireplace viewing pane 40, such as shown in FIG. 1, therefore, a gravity sagging of the article made of glass ceramic into a corresponding mold follows.

[0229] The exemplary embodiments depicted in FIGS. 1 and 2 for the article made of glass ceramic produced in accordance with the invention comprise front sides 43, 53, as well as an opposite-lying back sides 42, 52, which, in the exemplary embodiments, are facing the combustion space.

[0230] During a smoothing fine treatment and ceramization in accordance with the second embodiment, this side represents the side that was subjected to fine treatment and, during the ceramization, did not rest on the base support. FIG. 7 further below clarifies important method steps.

[0231] FIG. 5 first shows, in a schematic manner, in a transverse view of one example, an excerpt of a hot shaping process by means of two shaping rollers 90 operating in opposite rotation, which, in operation, have a rotational movement indicated by R and which roll ceramizable green glass. Moving between these shaping rollers 90 is ceramizable green glass 80, coming from the glass melt (not shown) in the form of a flat, continuous glass ribbon. Obviously, it is also possible for more than one set of shaping rollers 90 to be employed in order to roll the ceramizable green glass to the predetermined thickness. The ceramizable green glass 80 is guided here over transport rollers 92, which likewise are only drawn solely by way of example. In the illustration, four transport rollers 92 are drawn.

[0232] Solely by way of example, the rolled ceramizable green glass 80 is illustrated with characteristic waves or fine waves, whereby, in this case, the reference number 81 indicates a wave peak and the reference sign 82 indicates a wave valley.

[0233] The rolled surfaces of the ceramizable green glass 80 further have a microstructuring in the form of open pores 83, which represent the so-called orange skin.

[0234] Furthermore, the rolled surfaces of the ceramizable green glass 80 have particles 84, which can be manufacturing relics or other contaminants. They can rest on or adhere to the surface, but preferably appear on the bottom side, because this side is in contact with the glass.

[0235] It is obvious that the depicted waves, microstructures, and particles are just illustrated solely by way of example and, consequently, can be present in different shape, size, and appearance. Thus, for example, the particles 84 can also be present only on one surface, in particular on the bottom side, and not on both sides.

[0236] FIGS. 6a and 6b show, solely by way of example, the problem posed by transparency through two different articles made of glass ceramic. In FIG. 6a, an article 97 made of glass ceramic is illustrated in which the above-described surface flaws result in a marked deterioration in the transparency and lead to the fact that, in the case of transparency, a blurred impression is created for the human eye with fuzzy, not clearly defined contours of an object lying on the other side of the article. In contrast, the article 87 made of glass ceramic in FIG. 6b, which was produced by use of the method according to the invention, behaves differently. In this case, a clear transparency is possible without the contours being fuzzy or not clearly defined.

[0237] FIG. 7 illustrates, in a schematic manner, important method steps for the example of the second embodiment of the invention, starting from the hot shaping and ending when the article made of glass ceramic is obtained

[0238] Frequently, most contaminants of the ceramizable green glass 80 having particles 84 are found on the side that stands in contact with the transport rollers 92 during the hot shaping. In the example, this is the bottom side 86.

[0239] In a most highly advantageous manner, therefore, this side 86 of the ceramizable green glass 80 is subjected to the smoothing fine treatment, that is, is processed and preferably polished in accordance with the invention, whereas the opposite-lying side 85, which had no contact with the transport rollers during the hot shaping, remains unchanged in its surface nature. Because this surface only had contact with the shaping roller, hardly any contamination with particles 84 from the transport rollers is to be feared.

[0240] Against this background, the ceramizable green glass 80 is separated into pieces and cut into sections and the substrates made of ceramizable green glass 100 produced in this way are subjected to a smoothing fine treatment with the bottom side 86 facing upward.

[0241] Prior to the fine treatment, therefore, the substrate 100 with the bottom side 86, which stood in contact with the transport rollers 92 during the hot shaping, is rotated upward and then subjected to smoothing fine treatment. After the fine treatment, a substrate 101 with a fine-treated surface 86a is obtained, as can be seen in FIG. 7.

[0242] Accordingly, this surface 86a, which has been subjected to a smoothing fine treatment, has a slightly reduced waviness in comparison to the original surface 86 of the ceramizable green glass. By way of the smoothing fine treatment of this surface 86a of the substrate made of ceramizable green glass 80, the microstructures, in particular the open pores 83 and/or the particles 84 on this surface 86a, are removed completely.

[0243] For the ceramization in the subsequent method step, this substrate made of ceramizable green glass 101 is laid on a base support, which, in the example, is a support plate 95, for the ceramization preparation. It thereby becomes evident that what is now the surface 85, with which the substrate 101 rests on the support plate 95, was formerly the surface 85 of the ceramizable green glass 80 during the hot shaping and now represents the bottom side. Accordingly, this surface 85 can still comprise, at least partially, microstructures and/or also particles.

[0244] During the ceramization (not depicted), it is possible for the glassy, lithium-poor zone to form on the top side of the substrate 101, that is, on the surface 86a, in a very advantageous manner, and, later, when the article is used, to offer an especially high chemical resistance. This surface 86a is characterized in that, as a result of the smoothing fine treatment, it is, in addition, pore-free and particle-free and needs no further post-processing.

[0245] Obtained in this way is an article 87 made of glass ceramic according to the invention.

[0246] Optionally, a further method step can follow, in which the bottom side 85 of this article 87 made of glass ceramic is subjected once again to a fine treatment in order to produce, after the ceramization, a surface 85a that has undergone a smoothing fine treatment. A smoothing fine treatment after the ceramization makes it possible to remove particles that, for instance, arise during the ceramization and adhere to the surface 85.

[0247] The embodiment of the article 87 made of glass ceramic shown in FIG. 7 shows a processing of this kind, whereby, prior to the ceramization, the one surface 86a and, after the ceramization, the opposite-lying surface 85a have been subjected to smoothing fine treatment.

[0248] FIGS. 8a, 8b, and 8c show the difference in the smoothing fine treatment according to the two embodiments in accordance with the invention for a section of a ceramizable green glass 100, by way of example.

[0249] For this purpose, by way of example, an excerpt of a substrate made of ceramizable green glass 100 is illustrated in FIG. 8a. For simplicity, in the excerpt shown, only one surface 85, 86 is illustrated, and, in principle, may have been either the top side or the bottom side of the ceramizable green glass in the hot shaping. Furthermore, for reasons of clarity, only the waves are illustrated; the illustration of microstructures and particles has been dispensed with.

[0250] FIG. 8b shows the same excerpt of a substrate 101a made of ceramizable green glass, in which a surface 85, 86 in accordance with the first embodiment has been subjected to a smoothing fine treatment. The material 88 of the wave inclines was removed, whereas, in contrast, the wave valleys 82 were retained. In accordance therewith, the surface in the region of wave valleys 82 continues to have its former nature, whereas the waviness has decreased owing to removal of the material of the wave inclines and the creation of correspondingly flattened areas.

[0251] FIG. 8c shows the excerpt of a substrate 101b made of ceramizable green glass shown in FIG. 8a. In the illustration, the region 89 shown with cross-hatching is the material that is removed in accordance with the second embodiment of the invention. Accordingly, for the substrate 101b subjected to a smoothing fine treatment, material is removed over the entire area of the surface. Accordingly, in this case, material is removed both in the region of wave inclines 81 and in the region of wave valleys 82 and a new surface 110 is created, which replaces the former surface 85, 86. This new surface further comprises wave valleys 82a and wave inclines 81a.

[0252] FIGS. 9a and 9b show, in a schematic illustration in a transverse view, two polishing heads 60, 70, which can be used as material removal tools 6, 10 for the smoothing fine treatment. Whereas the polishing head 60 has a rigid material removing face 62, the polishing head 70 is designed as a flexible material removing head and, for this purpose, has a flexible intermediate layer 71, which affords the material removing face 72 a better pliability over the entire material removing face. The force arrows 63, 73 show qualitatively, solely by way of example, the course of the contact pressure when the polishing heads 60, 70 are utilized.

[0253] The flexible intermediate layer 71 can comprise a pliable material, such as, for instance, a flexible polyurethane. For the smoothing fine treatment in accordance with the second embodiment, the flexible polishing head 70 is especially well-suited, because it can remove material in a uniform and distributed manner over the surface area. The utilization of the rigid polishing head 60, in contrast, leads to the fact that initially material in the region of wave inclines is removed and is thus suitable for a smoothing fine treatment in accordance with the first embodiment.

[0254] FIGS. 10 and 11 show exemplary embodiments of the material removing faces of flexible polishing heads 70. Shown in the depiction are so-called polishing pads 120, which are designed as quarter circles. Between this total of four segments of polishing pads 120 in each case, small grooves 121 are formed and assist the material transport during the removal of material. The polishing pads 120 can comprise a felt or an elastomer, preferably polyurethane.

[0255] FIGS. 12 and 13 show a view from the top of a transverse polished section of two articles made of glass ceramic. FIG. 12 shows a transverse polished section of an excerpt of an article made of glass ceramic for which material was removed from the surface down to a depth of about 6 m. There is no longer any glassy surface zone; the glass ceramic 132 extends to the surface 133.

[0256] In the transverse polished section shown in FIG. 13, what is involved is an excerpt of an article made of glass ceramic, which was produced in accordance with the invention. In this case, a glassy surface zone 131 can be seen, which has a thickness of about 100 nm and which then transitions into the actual glass ceramic 132. The transition region of the glassy surface zone 131 to the glass ceramic 132 is indicated by the dotted line 134, whereby the transition is seamless. This glassy surface zone 131 leads to the high chemical resistance of the article made of glass ceramic produced in accordance with the invention.