Cooking surface composed of LAS glass ceramic plates

Abstract

To provide a cooking surface composed of an LAS glass ceramic plate containing TiO2 which can be perceived by an observer without a substantial change in a color of a lower surface coating through a glass ceramic plate.SOLUTION: A cooking surface 1 composed of a transparent lithium aluminum silicate glass ceramic plate 2 has an upper surface 4 and a lower surface 6, in which the lower surface 6 includes at least partially a lower surface coating 8. In the cooking surface 1, the glass ceramic plate 2 contains a high temperature quartz mixed crystal as a main crystal phase, the glass ceramic plate 2 contains TiO2 as a nucleus formation agent, the glass ceramic plate 2 has brightness LT* and color saturation c* in a CIELAB colorimetric system after passage of light of a standard light source D65 in 4 mm thickness, and the brightness LT* satisfies relational expression: LT*≥a cT*+b (Condition B1). In the expression, expressions of a=0.765, b≥93.5 and 0≤cT*≤3 are satisfied.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a cooking surface composed of a highly transparent lithium aluminum silicate glass ceramic plate, having an upper surface and a lower surface, and the lower surface being at least partially provided with a lower surface coating.

[0002] The term cooking surface, unlike a cooktop, refers to a glass ceramic plate for the cooking process. A cooktop includes, in addition to the cooking surface, a heating device as well as operating means and display means.

[0003] In some established applications of glass ceramics, heat-resistant special glasses such as aluminosilicate glass or borosilicate glass are increasingly being used. Additional technical measures such as air cooling in a fireplace glass window or electronic temperature control in an induction heating cooking surface can reduce the use temperature, and in some cases, enable the use of these special glasses.

[0004] These special glasses are described, for example, in the following documents: WO 2018 / 225627, EP 3228601 A1, WO 2015 / 009483.

[0005] In an alternative solution using borosilicate glass, although this glass has high color neutrality, it has low thermal durability, and thus has obvious disadvantages regarding the scope of application compared to a cooking surface using a glass ceramic plate. Furthermore, borosilicate glass has only a low transmittance in the wavelength range exceeding 3300 nm, which is a disadvantage for the use of an optical temperature sensor of a cooktop.

[0006] Compared with these glasses, the color of transparent glass ceramics is a significant disadvantage. Therefore, it is necessary to develop a transparent glass ceramic with less color.

[0007] Approaches to avoid or limit the nucleating agent TiO2, which is part of the cause of colored complexes in glass ceramics (WO 2008 / 065167, US Patent No. 3,252,811), have not hitherto led to a technical realization. Higher contents are required with alternative nucleating agents ZrO2 and / or SnO2, which leads to disadvantages during melting and shaping, such as an increase in the melting temperature and shaping temperature, and insufficient devitrification resistance during shaping.

[0008] DE 10 2010 035 544 A1 and DE 10 2011 107 831 A1 disclose transparent glass ceramics with a color value c * of less than 3. The high transparency in DE 10 2010 035 544 A1 is due to a ZnO content of more than 4%, in which case the light transmittance exceeds 88%. In DE 10 2011 107 831 A1, the high transparency is due to the sum of MgO + ZnO being less than 2.2% by weight. The disadvantage of these glass ceramics is that higher contents of the alternative nucleating agents ZrO2 and / or SnO2 are also required in this case. The low proportion or absence of the effective nucleating agent TiO2 is disadvantageous for a high nucleation rate and thus for the desired short ceramization time. Furthermore, the high contents of the nucleating agents ZrO2 and / or SnO2 lead to disadvantages during melting and shaping. The melting of the raw materials of these components requires more time and is economically disadvantageous with regard to the tank throughput and the energy requirement.

[0009] WO 2013 / 124373 describes the physical decolorization of a transparent glass-ceramic containing high-temperature quartz mixed crystals as the main crystal phase, excluding inevitable raw material impurities and containing no arsenic and antimony, by the addition of 0.005 wt% to 0.15 wt% of Nd2O3. The principle of this physical decolorization is based on the existing absorption bands being canceled out by the complementary absorption bands of the decolorizing agent. Of course, this increases the light absorption and thus reduces the lightness. To achieve favorable manufacturing conditions, i.e., low melting and low forming temperatures, the exemplary glasses of this document contain MgO, a component that reduces viscosity, in a high content of 0.44 wt% to 0.93 wt%. In addition to the high MgO content, the relatively high SnO2 content of the exemplary glass-ceramic is disadvantageous for color c * and is disadvantageous for color c

[0010] An induction cooking surface consisting of a transparent colored glass-ceramic plate usually has a colored bottom coating. For the bottom coating, light color tones, and in particular white, are increasingly being used. Conventional glass-ceramics have the disadvantage that the color tone of the bottom coating shifts towards a yellowish color tone. An undesirable increase in the CIELAB color component b occurs.

[0011] In transparent colorless glass-ceramics, no coloring compounds are added during production. In other classes of transparent colored glass-ceramics, V2O5 is usually added to color the whole (in volume) in order to reduce the lightness and achieve a black appearance. This is typical for cooking surfaces with a black appearance.

[0012] In transparent colorless glass-ceramics, high transparency, i.e., high lightness and little color, is desired. Since the absorption bands not only reduce the lightness depending on their position within the visible spectrum but also enhance the color, both mean low absorption.

[0013] EP 1 837 314 A1 discloses a transparent, colorless LAS glass ceramic having on one side a coating of a substantially or almost completely opaque, colored temperature stability. In the composition of this glass ceramic, Nd2O3 with a content of 0.01 wt% to 0.4 wt% is used for overcoloring. However, this LAS glass ceramic exhibits a slightly grayish color tone, which also impairs the perception of the color of the bottom coating.

[0014] The object of the present invention is to present a cooking surface composed of a LAS glass ceramic plate containing TiO2, in which the color of the bottom coating can be perceived by an observer with little change through the glass ceramic plate.

[0015] By "unchanged" it is understood that the difference in the color of the bottom coating due to the presence or absence of the glass ceramic plate cannot be recognized by an observer, i.e., a colorimetric standard observer, and is a perception by the human eye.

[0016] The colorimetric standard observer is defined in CIE 1931.

[0017] This object is solved according to a first alternative form (so-called transmission deformation form) having the features of claim 1 and a second alternative form (so-called diffuse reflection deformation form) having the features of claim 3.

[0018] The cooking surface according to the first alternative form is such that the glass ceramic plate contains high-temperature quartz mixed crystals as the main crystal phase, the glass ceramic plate contains TiO2 as a nucleating agent, and after the passage of light from a standard light source D65 at a thickness of 4 mm, the lightness L and chroma c in the CIELAB color system of the glass ceramic plate T * and chroma c * are such that the lightness L T * satisfies the following relational expression: L T * ≧a·c T * +b (condition B1) [where a = 0.765, b ≥ 93.5 and 0 ≤ c T * ≤ 3] It is characterized by satisfying.

[0019] The subscript T represents transmission.

[0020] Preferred values are a = 0.765 and b = 93.5. The preferred range of b is 93.5 ≤ b ≤ 94.4.

[0021] The lightness or light transmittance is represented by the L T * value in the CIELAB color space system or the luminance value Y (luminance) according to the CIE standard color space system. The CIELAB color model is standardized in DIN EN ISO 11664-4 "Colorimetry - Part 4: CIE 1976 L * a * b * color space". The national standardization of the international CIE standard in Germany is specified in DIN 5033.

[0022] Spectrophotometric measurements are performed in the spectral region of 380 to 780 nm for a sample polished on both sides within the scope of the present invention. From the spectral values measured in this range, which is the visible light spectrum, the standard light source and the observer angle for the thickness in this case are selected, and the light transmittance is calculated.

[0023] For glass ceramics, according to the following calculation, the value c * ,a * ,b * using the CIELAB color space system with * (chroma) as the color scale is generally adopted:

Equation

[0024] The coordinates of the CIELAB color space system can be calculated by a known method from the color coordinates x, y and the luminance Y of the CIE color space system. The c of the sample * from.T * The value is determined by performing spectrophotometric measurement of transmittance using parameters selected for the standard light source and the observer angle.

[0025] A further essential feature of LAS glass-ceramics is the scattering determined by the crystal size, birefringence, and the refractive index difference between the crystal and the residual glass. With a sufficient amount of nucleating agent and sufficient nucleation time during ceramization, a high nucleation density is achieved, and as a result, the growing high-temperature quartz mixed crystals become smaller than the wavelength range of visible light. Usually, the average crystallite size of the high-temperature quartz mixed crystals is in the range of 20 nm to 50 nm. Low scattering further requires low birefringence of the crystals and a good match of the refractive index between the crystals and the residual glass phase. These conditions for the high transparency of LAS glass-ceramics are described in the paper "Nanophase Glass-Ceramics" by Beall and Pickney, Journal of the American Ceramic Society, Vol. 82, No. 1, pp. 5-16; 1999.

[0026] The refractive index of the residual glass phase is adjusted by its composition and the cooling rate during ceramization.

[0027] Low scattering is desirable so that the appearance when the bottom coating is seen through does not change and the display part can be seen clearly and distinctly. The scattering of the glass-ceramic is determined by measurement of the haze. The haze in accordance with ASTM D1003-13 is the percentage of transmitted light that has deviated by more than 2.5° on average from the incident light beam.

[0028] Based on the CIE color system, the following condition B1a corresponding to condition 1: Y * ≧d·c * +e (condition B1a) [where d = 1.83, e ≧ 84.4 and 0 ≦ c * ≦ 3] applies to the luminance Y * to be applied.

[0029] Both color systems can be converted into each other. Hereinafter, the present invention will be described based only on the CIELAB system.

[0030] Preferably, the lightness L T * satisfies the following relational expression: 0.765·c * +94.4 ≥ L T * ≥ 0.765·c T * +93.5 (Condition B2) and is satisfied.

[0031] For the TiO2-containing glass ceramic, it has been found that when Condition B1, particularly Condition B2, is satisfied, the color of the lower surface coating of the cooking surface is perceived without change.

[0032] c T * For the chroma value of c ≤ 3, the inherent color of the LAS glass ceramic has already become very small. As the value of c T * increases and the accompanying increase in the inherent color occur, it has been shown that the inherent color can be compensated by the increase in the lightness value L T * and the color of the lower surface coating can be perceived without change. Conversely, as the value of c T * decreases, a lower lightness value L T * can be tolerated.

[0033] When Condition B1 is not satisfied, the values of c * and L * cannot be compensated until the change in the color of the lower surface coating is no longer perceived by the human eye.

[0034] Glass ceramics that do not contain TiO2 or have a very low TiO2 content, that is, a TiO2 content of less than 1.6% by weight, have a low c T * value and L T *have values, but these glass ceramics have disadvantages with respect to manufacturing properties, such as higher melting temperatures and forming temperatures, etc. Therefore, in order to achieve favorable manufacturing properties, a proportion of TiO2 of more than 1.6% by weight is desirable. It is preferable to comply with a corridor having an upper limit line defined by condition B2.

[0035] This corridor shows the optimized (L * ;c * ) range within the scope of the present invention. The lower limit line is advantageous for vision in that the color of the bottom coating does not change as compared to other known transparent glass ceramics containing TiO2 as a nucleating agent. Transparent LAS glass ceramics that do not contain TiO2 or have a very low content can achieve values exceeding the upper limit line, but have the described technical and economic disadvantages with respect to melting and forming.

[0036] According to a second alternative form, the cooking surface is such that a glass ceramic plate with a thickness of 4 mm has a maximum chroma c T * of 3 when light of the standard light source D65 passes through, the glass ceramic plate contains TiO2 as a nucleating agent, the cooking surface has a color position A with color coordinates (L R * , a R * , b R * ) in the CIELAB color space measured by diffuse reflection by light of the standard light source D65, the bottom coating has a color position B with color coordinates (L Rc * , a Rc * , b Rc * ) in the CIELAB color space, and the color positions A and B have a distance ΔE Rc,R * , where

Equation

[0037] The subscript R represents diffuse reflection, and the subscript c represents coating.

[0038] The color coordinates of color position A are preferably determined experimentally. The measured values are the spectral reflectance ρ(λ) and the spectral transmittance τ(λ). These are determined in accordance with DIN 5036-3 1979-11.

[0039] The color coordinates of color position B are preferably calculated as follows: The calculation of the color values is carried out in accordance with DIN EN ISO 11664-3 (August 2013). Here, in the standard colorimetric system (Normvalenzsystem) CIE 1931, X, Y, and Z are the following equations as integrals over the spectral region from 360 nm to 830 nm:

Equation

[0040] In this case, φ λ (λ) is the color stimulus function to be evaluated,

Equation

[0041] k is a normalization constant defined below.

[0042] The standard method for evaluating these integrals is the equation:

Number

[0043] The normalization constant k is chosen such that Y = 100 when the color stimulus function φ λ (λ) is 1 for all wavelengths:

Number

[0044] Furthermore, the following variables in accordance with DIN EN ISO 11664-4 (June 2012): L * CIELAB lightness a * , b * CIELAB coordinate a * , b * C ab * CIELAB chroma, or briefly: C * ΔE ab * CIELAB color distance, or briefly: ΔE * are used below.

[0045] At this time, it becomes as follows: L n * Color stimulus function φ λie (λ) is used for CIELAB lightness a n * , b n * Color stimulus function φ λe (λ)n is used for CIELAB coordinates C ab,n * = Cn * Color stimulus function φ λie CIELAB chroma using φ(λ) ΔE ab,nm * = ΔE nm * Two color stimulus functions φ λ,n (λ) and φ λun CIELAB color distance between (λ) For the characteristic evaluation of the described examples, the following color stimulus functions are used for the calculation of color parameters: (1) Glass-ceramic plate in transmission (subscript T = "transmission"): φ λ,T,S (λ) = τ(λ)S(λ) resulting in CIELAB: C T * (2) Cooking surface in diffuse reflection = glass-ceramic plate + bottom coating (subscript R = "diffuse reflection"): φ λ,R,S (λ) = ρ(λ)S(λ) resulting in CIELAB: L R * (3) Bottom coating in diffuse reflection using the calculation rule "diffuse reflection (cooking surface) / transmission (glass-ceramic plate) / transmission (glass-ceramic plate)" (subscript RC = "diffuse reflection coating"): φ λ,RC,S (λ) = ρc(λ)S(λ) resulting in CIELAB: L RC * As a calculation rule for the characteristic evaluation of the spectral reflectance of the bottom coating, ρc(λ) = ρ(λ) / τ(λ) / τ(λ) gives = ρ(λ) / τ(λ) / τ(λ)·S(λ) (4) The influence of the glass-ceramic plate is represented by the ΔE value of the diffuse reflection between the cooking surface (subscript R) and the bottom coating (subscript RC): φ λ,R,S (λ) and φ λ,RC,S (λ) S(λ) resulting in CIELAB: ΔE RC,R * In this case, ρ(λ) is the spectral reflectance, τ(λ) is the spectral transmittance (in accordance with DIN 5036-3 November 1979), and S(λ) is the spectral distribution of the standard light source D65.

[0046] Distance ΔE Rc,R * ≦0.09·L R * In the case of +3.4, it has been shown that the glass-ceramic according to the invention is clearly improved compared to the prior art (for example numbers 12, 14, 15) and clearly approaches the ideal state of the (light-colored) layer below that of a glass (number 18) having almost ideal color neutrality.

[0047] Preferably, ΔE Rc,c * ≦0.09·L R * +3.05 is applied, In particular, ΔE Rc,c * ≦0.09·L R * +2.7. Preferably, ΔE Rc,c * ≧0.07·L R * +1.8 is applied, Particularly preferably, ΔE Rc,c * ≧0.07·L R * +2.2, In particular, ΔE Rc,c * ≧0.07·L R * +2.6.

[0048] Preferably, 50≦L R * ≦98, particularly preferably 50≦L R * ≦93, particularly 50≦L R * ≦88 is applied.

[0049] Nucleating agent TiO2 The component TiO2 is essential for the transparency of glass-ceramics as an effective nucleating agent. TiO2 brings about a high nucleation rate and thus, even with a short ceramization time, sufficient nucleation and hence a small average crystallite size are achieved. As a result, it is possible to obtain glass-ceramics without visually problematic scattering even with a short ceramization time.

[0050] However, since Fe / Ti and Sn / Ti colored complexes are formed, a higher TiO2 content has a significant impact. Therefore, the proportion of TiO2 is preferably more than 1.6 wt% and up to 2.8 wt%.

[0051] A minimum TiO2 content of 1.8 wt% is preferred. This minimum content is advantageous for shortening the ceramization time and avoiding scattering. A TiO2 content exceeding 2 wt% is particularly preferred. To limit the color effect, it is preferably up to 2.8 wt% of TiO2 is included.

[0052] The glass-ceramic plate for the cooking surface preferably contains the following components (by weight% of the oxide base): Li2O 3 - 4.5 Al2O3 19 - 24 SiO2 62 - 70 TiO2 more than 1.6 up to 2.8 ZrO2 1 - 2.5 Fe2O3 0.005 - 0.025 and contains.

[0053] Li2O, Al2O3 and SiO2 The oxide Li2O, Al2O3 and SiO2 are essential components of the high-temperature quartz mixed crystal phase and / or the keatite mixed crystal phase within the above-mentioned ranges or will become essential components.

[0054] Regarding the crystalline glass and the glass ceramic produced therefrom, the content of Li2O is preferably 3 to 4.5% by weight. A minimum content is required to achieve the desired low processing temperature of the glass. It has been shown that it is difficult to achieve zero thermal expansion of the desired glass ceramic at a content higher than 4.5% by weight. Also, a content higher than 4.5% by weight is disadvantageous regarding the low color c * of the desired glass ceramic. Preferably, the Li2O content is less than 4.5% by weight, more preferably less than 4.2% by weight, and particularly preferably less than 4% by weight. The minimum content is preferably 3.2% by weight, and particularly preferably 3.4% by weight.

[0055] The content of Al2O3 is preferably 19 to 24% by weight. A content higher than 24% by weight is disadvantageous because mullite tends to devitrify during forming. Furthermore, it has been shown that the higher the content, the greater the scattering at a shorter ceramization time. Therefore, a proportion of less than 23% by weight is preferred. Al2O3 is advantageous for the low processing temperature and low color c * of the glass ceramic, so the minimum content is 19% by weight. The minimum content of Al2O3 is preferably at least 20% by weight.

[0056] The content of the main component SiO2 is desirably at least 62% by weight because it is advantageous for the required properties of the glass ceramic, such as low thermal expansion and chemical resistance. Furthermore, the scattering at a short ceramization time is reduced. A minimum content of 64% by weight is particularly advantageous. The SiO2 content is desirably at most 70% by weight because this component increases the processing temperature and melting temperature of the glass. The SiO2 content is preferably at most 68% by weight.

[0057] Nucleating agent ZrO2 As a further nucleating agent, ZrO2 is preferably contemplated. The ZrO2 content is preferably 1 to 2.5% by weight. Preferably, the ZrO2 content is limited to less than 2.2% by weight, more preferably less than 2% by weight. This is because when the content is higher, the melting behavior of the batch during glass production deteriorates, and devitrification may occur during molding due to the formation of Zr-containing crystals. This component helps to avoid an increase in the content of the alternative nucleating agent TiO2, so it is advantageous for providing a glass ceramic with a low color c * is advantageous for providing a glass ceramic with a low color c. The minimum ZrO2 content is preferably 1.6% by weight.

[0058] Fe2O3 Since the cost of low-iron batch raw materials is high, it is uneconomical to limit the Fe2O3 content of the crystalline glass to a value of less than 0.005% by weight, i.e., less than 50 ppm. On the other hand, the concentration of the Fe / Ti colored complex in the glass ceramic also increases with the Fe2O3 content. The color c * increases, and the lightness L T * decreases due to absorption. Therefore, the Fe2O3 contained in the crystalline glass and the glass ceramic produced therefrom is desirably up to 0.025% by weight, preferably 0.02% by weight.

[0059] Finishing agent Finishing can be supported by adding chemical finishing agents such as arsenic oxide, antimony oxide or cerium oxide, and finishing additives such as manganese oxide, sulfate compounds, halide compounds, etc., preferably at a total content of up to 2.0% by weight.

[0060] Preferably, the glass ceramic plate contains 0.1 to 2.0% by weight, particularly 0.4 to 2.0% by weight of As2O3.

[0061] Alkali Alkali Na2O and K2O lower the melting temperature and processing temperature during glass forming. The melting of the poorly soluble raw materials of ZrO2 and SiO2 is accelerated. Since these components are not incorporated into the crystal phase and remain in the residual glass phase of the glass-ceramic, the content of each needs to be limited to a maximum of 1.5 wt%. An overly high content impairs the crystallization behavior when the crystalline starting glass transforms into the glass-ceramic and has an adverse effect on the time / temperature stability of the glass-ceramic.

[0062] Preferably, the Na2O content is 0 wt% or more than 0 wt%, and particularly preferably, the glass contains at least 0.05 wt% of Na2O. The maximum ratio is preferably 1.5 wt%, particularly 1 wt%. Preferably, the ratio of Na2O is 0.1 wt% to 1.5 wt%.

[0063] Preferably, the K2O content is 0 wt% or more than 0 wt%, and particularly preferably, the glass contains at least 0.05 wt% of K2O. The maximum ratio is preferably 1.5 wt%, particularly 1 wt%. Preferably, the ratio of K2O is 0.1 wt% to 1.5 wt%.

[0064] In a preferred embodiment, 0.2 wt% ≤ Na2O + K2O ≤ 1.5 wt% (Condition B7a) is applied. The total alkali Na2O + K2O is more preferably at most 1.2 wt%. In order to further improve the meltability and lower the processing temperature, it is particularly preferred that the total alkali Na2O + K2O is at least 0.4 wt%. Particularly preferably, 0.2 wt% ≤ Na2O + K2O ≤ 1.2 wt% (Condition B7b) is applied.

[0065] MgO The component MgO is a constituent of both the mixed crystal and the residual glass phase, thus having a strong influence on many properties. This component promotes economical production by reducing the melting temperature and processing temperature of the glass. The MgO content is at least 0.1% by weight, particularly preferably at least 0.15% by weight. In glass ceramics, this component increases the thermal expansion and causes an especially unfavorable enhancement of color. This is due to the promotion of the formation of Fe / Ti color species (Farbspezies) and Sn / Ti color species.

[0066] The MgO content is desirably less than 0.5% by weight, preferably at most 0.4% by weight, particularly preferably at most 0.35% by weight. In the preferred range of MgO values from 0.1% to 0.4% by weight, the requirements for less color and low processing temperature of the glass ceramic can be particularly well reconciled. In this case, the thermal expansion of the glass ceramic can be adjusted close to zero expansion.

[0067] Alkaline earths Similar to the alkalis Na2O and K2O, the alkaline earths CaO, SrO, and BaO are not incorporated into the mixed crystal phase and remain in the residual glass phase of the glass ceramic. These are advantageous for lowering the melting temperature and processing temperature. An overly high content impairs the nucleation behavior and crystallization behavior when the crystalline glass transforms into the glass ceramic, and has an adverse effect on the time / temperature stability of the glass ceramic.

[0068] The component CaO has been found to be advantageous for lowering the processing temperature V A and improving the devitrification resistance. However, CaO causes an increase in color and scattering with a short ceramization time. CaO is contained at a maximum content of 2% by weight, preferably at most 1% by weight. To minimize scattering with a short ceramization time, an upper limit of CaO content of less than 0.8% by weight is particularly preferred. Preferably, CaO of 0% by weight or more than 0% by weight, particularly preferably 0.05% by weight or more, more preferably at least 0.1% by weight, particularly preferably at least 0.3% by weight is applied. The preferred range is from 0.04% to 2% by weight.

[0069] The BaO content is preferably up to 4% by weight. The SrO content is preferably up to 2% by weight. The BaO content is preferably up to 2.5% by weight. The SrO content is preferably up to 1.5% by weight. A BaO content of 0% or more than 0% and at least 0.1% by weight is particularly preferred.

[0070] The preferred range of BaO is 0.1% to 4% by weight.

[0071] The proportion of SrO is preferably 0%, particularly preferably more than 0%, especially 0.01% or more.

[0072] The preferred range of SrO is 0.01% to 2% by weight.

[0073] In order to lower the melting temperature and the processing temperature, it is preferable that the total of the SrO content and the BaO content is 0.5 to 2.5% by weight (condition B5). Higher contents are disadvantageous for the time / temperature stability of the glass ceramic.

[0074] Alkalis Na2O, K2O and alkaline earths CaO, SrO, BaO accumulate not only in the residual glass phase between the crystals but also on the surface of the glass ceramic. During crystallization, a glassy surface layer with a thickness of about 50 to 1000 nm that contains almost no crystals and is enriched with these elements and depleted in Li2O is formed. This glassy surface layer has a beneficial effect on the acid resistance of the glass ceramic surface. A sufficient thickness of the glassy layer of at least 50 nm requires components of both classes at a minimum content, namely alkalis and alkaline earths. Contents higher than the upper limit may increase the thickness of the glassy layer, which is disadvantageous for the strength of the glass ceramic.

[0075] It has been found to be advantageous to adjust the content of the residual glass former as defined. In addition to the adjustment of the glassy surface layer, good refractive index matching between the crystals and the residual glass phase is required for low scattering. Since the Fe / Ti colored complex is formed in the residual glass phase, this is also crucial in its formation kinetics. However, it is necessary to limit the content of the residual glass former. Higher contents have an adverse effect on the time / temperature stability of the glass-ceramic. The crystallization of high-temperature quartz solid solution is also impaired, causing an increase in scattering due to the formation of larger crystallites. The total of the residual glass formers is crucial for harmonizing color, lightness, scattering, and economic production, and can reduce disadvantages compared to special glasses with high lightness values and low color.

[0076] The glass according to the invention preferably has a total of the components forming the residual glass, Na2O + K2O + CaO + SrO + BaO (wt.%), preferably less than 3 wt.%, preferably less than 2.6 wt.%, particularly preferably less than 2.2 wt.%. The total of the components is preferably more than 1.5 wt.%, particularly preferably more than 1.7 wt.%.

[0077] Scattering (haze) is measured on a 4 mm thick polished sample of the LAS glass-ceramic in accordance with ASTM D1003-13. The haze value is preferably less than 2.5%, more preferably less than 2%, particularly preferably less than 1.8%. Above 2.5%, the turbidity is usually visually problematic. A value less than 2% is preferred because scattering may be more noticeable in the case of a dark, i.e., for example, black bottom coating.

[0078] For this reason, the transparent glass-ceramic according to the invention does not have visually problematic light scattering. As a result, the appearance when objects and the bottom coating are seen through is not distorted. The light-emitting display part such as a display or a screen under the glass-ceramic plate can thus be clearly seen with distinct contours without scattering.

[0079] ZnO The component ZnO is advantageous for reducing the melting temperature and processing temperature of the glass and for reducing scattering with a short ceramization time. ZnO is incorporated into the mixed crystal phase and part of it also remains in the residual glass phase. This component, like the component Li2O, results in a decrease in the thermal expansion of the glass ceramic. The ZnO content is limited to a maximum value of 3 wt% due to the evaporation tendency from the molten glass and the zero expansion required for the glass ceramic. A ZnO content of at most 2.5 wt%, particularly preferably at most 2.0 wt%, is preferred. Preferably, ZnO of 0 wt% or more than 0 wt% is applied. A minimum content of 0.5 wt%, particularly preferably more than 1 wt%, is preferred. The ZnO content is preferably 0.5 wt% to 3 wt%.

[0080] SnO2 In the first embodiment with arsenic clarification, SnO2 is not intentionally added to the glass ceramic plate because it is significant for the color of the glass ceramic due to the formation of Sn / Ti colored species during crystallization (the proportion of SnO2 is 0%). In this case, it only exists at a content of less than 50 ppm derived from impurities.

[0081] In particular, the manufacturing characteristics during normal forming by a water-cooled roll have recently attracted attention. Industrial tests using LAS glass clarified with SnO2 containing 0.18 wt% SnO2, disclosed in WO 2013 / 124373, have shown that an Sn-containing roll coating (Walzbelag) accumulates during forming. Due to the effect of tin oxide as a nucleating agent, this roll coating comes into contact with the molten glass and induces an increase in surface crystals on the glass ribbon being cooled. These are visually prominent and may reduce the strength. The removal of the roll coating involves manufacturing losses. Using the present invention, this economic disadvantage due to the roll coating is reduced.

[0082] To avoid these disadvantages, the SnO₂ content is limited compared to the prior art, preferably less than 0.10% by weight, more preferably less than 0.08% by weight, and particularly preferably less than 0.05% by weight. As the SnO₂ content decreases, the color and roll coating continuously decrease. A distinct improvement in the formation of surface crystals during shaping appears with SnO₂ of less than 0.10% by weight.

[0083] In the second embodiment, especially when not using arsenic oxide or antimony oxide as the fining agent, a minimum content of 0.01% by weight of SnO₂ is preferred for SnO₂, as the polyvalent tin oxide prevents bubble formation (reboil) in the precious metal parts (Edelmetalleinbaut) in the melting tank. Even at a low content, SnO₂ acts as a fining agent and, in combination with technical measures in the melting tank, guarantees the required bubble quality. Preferably, the glass and the glass ceramic produced therefrom contain at least 0.03% by weight, particularly preferably at least 0.04% by weight of SnO₂.

[0084] A particularly preferred range considering all aspects regarding less color and economic manufacturing characteristics is SnO₂ of 0.03 to less than 0.10% by weight.

[0085] P2O5 To improve the meltability and devitrification resistance during shaping, up to 4% by weight of P₂O₅ can be incorporated. Higher contents are disadvantageous for chemical resistance. Preferably, 0% by weight or more than 0% by weight is applied to P₂O₅. The preferred lower limit is 0.01% by weight, particularly preferably 0.02% by weight. The preferred upper limit is 2% by weight of P₂O₅. The preferred range is 0.01% by weight to 2% by weight.

[0086] B2O3, PbO and fluorine The addition of up to 1 wt% of B2O3 improves the fusibility and devitrification resistance, but this is disadvantageous for the time / temperature stability of the glass-ceramic. The same applies to the addition of fluorine. What is common to both components is that they reduce the stability of the high-temperature quartz mixed crystal, which is an important main crystal phase for transparent glass-ceramics, and promote the transition to the keatite mixed crystal phase. Preferably, the glass-ceramic contains less than 0.5 wt%, particularly preferably less than 0.2 wt% of B2O3. Particularly preferably, the glass-ceramic does not technically contain B2O3, i.e., the content is less than 1000 ppm, preferably less than 500 ppm, particularly preferably less than 100 ppm.

[0087] The preferred upper limit for the addition of fluorine is 0.5 wt%. It is more preferably less than 0.2 wt%, even more preferably less than 0.1 wt%, even more preferably less than 500 ppm, and particularly preferably less than 100 ppm.

[0088] The addition of PbO improves the fusibility and devitrification resistance, but this is disadvantageous for the time / temperature stability of the glass-ceramic and is not desirable for environmentally friendly compositions. The content is preferably less than 0.1 wt%, even more preferably less than 100 ppm. Particularly preferably, PbO is not added and is only present in a content of usually less than 5 ppm, which is derived from impurities.

[0089] Nd2O3 The proportion of Nd2O3 can be 0 wt% or more than 0 wt%.

[0090] In the case of a transparent glass ceramic produced from lithium aluminum silicate glass according to the present invention, the problematic color due to Fe / Ti colored complexes and / or Sn / Ti colored complexes is reduced, in a preferred embodiment, by the addition of Nd2O3 at a content of 0.005 wt% to 0.25 wt%. If it is less than 0.005 wt%, the decolorizing effect is slight, and the preferred lower limit of Nd2O3 is 0.01 wt%, particularly 0.03 wt%. If it exceeds 0.5 wt%, the brightness is unnecessarily reduced due to the absorption of the Nd band in the visible light range. Therefore, it is preferably added up to 0.25 wt%. The preferred range of Nd2O3 is 0.01 wt% to 0.25 wt%.

[0091] CoO Addition of CoO in an amount up to 30 ppm, preferably up to 20 ppm, can support decolorization. A CoO content of 0.1 ppm to 20 ppm of CoO is preferred. If it exceeds 30 ppm, CoO causes a reddish tint in the transparent glass ceramic.

[0092] MgO / As2O3 Preferably, the components MgO and As2O3 are subject to the following conditions: MgO / As2O3 < 0.8 (Condition 6) [where As2O3 > 0 wt%] is applied.

[0093] The ratio of the component MgO / As2O3 is critically important for reconciling the low color, high brightness of the glass ceramic and the good fining potential of the glass as well as the low melting temperature and forming temperature. Therefore, by economical production, the chroma c of the glass ceramic * can be further reduced, the luminance Y can be further improved, and the disadvantages can be reduced compared to a special glass with a high brightness value and little color.

[0094] In the glass according to the present invention, the ratio of the components MgO / As2O3 (both in wt%) is preferably less than 1 (Condition B2a), more preferably less than 0.8, particularly less than 0.7, and particularly preferably less than 0.5.

[0095] This is an essential condition for reconciling the desired favorable manufacturing characteristics of the glass with the low color and high lightness of the transparent glass ceramic produced therefrom.

[0096] Advantageously, the following ratios of components apply: MgO / As2O3 < 0.7 (Condition B6a) are applied.

[0097] In a series of tests using various MgO contents and As2O3 contents, it was found that both components have opposite effects on the Fe / Ti colored complex. As explained, increasing the MgO content increases the formation of the colored complex, but this formation decreases by increasing the As2O3 content. At the preferred upper limit of the ratio of both, these effects advantageously compensate for each other.

[0098] The ratio of these components is preferably more than 0.05, more preferably more than 0.08, and particularly preferably more than 0.1. This is because when the MgO content is low despite a higher As2O3 content, clarification deteriorates due to an increase in the melting temperature.

[0099] When a proportion of 0 wt% is specified for a component, it means that the corresponding component is not included in the raw material mixture. However, these components may be present as unavoidable impurities.

[0100] In a preferred embodiment, the cooking surface is such that the glass ceramic plate has the following components (in wt% based on oxides): Li2O 3.2 - 4.5 Al2O3 19 - 24 SiO2 62 - 68 Na2O 0 - 1 K2O 0 - 1 Na2O + K2O 0.2 - 1.5 (Condition B7a) MgO 0.05 - less than 0.5 TiO2 1.8 - 2.8 ZrO2 1 - less than 2.2 TiO2 + ZrO2 + SnO2 3.8 - 4.8 (Condition B8) Fe2O3 0.007 - 0.02 CaO 0 - 1.5 SrO 0 - 1.5 BaO 0 - 2.5 ZnO 0 - 2.5 P2O5 0 - 4 Characterized by containing

[0101] According to a further embodiment, the cooking surface is a glass - ceramic plate having (by weight % of the oxide - based) the following components: Li2O 3.2 - less than 4.2 Al2O3 20 - less than 23 SiO2 62 - 68 Na2O 0.1 - 1 K2O 0 - 1 Na2O + K2O 0.2 - 1.2 (Condition B7b) MgO 0.1 - 0.4 CaO 0.05 - 1 SrO 0 - 1.5 BaO 0 - 2.5 SrO + BaO 0.5 - 2.5 TiO2 1.8 - 2.8 ZrO2 1 - less than 2.2 SnO2 0.01 - less than 0.10 TiO2 + ZrO2 + SnO2 3.8 - 4.8 (Condition B8) Fe2O3 0.008 - 0.02 ZnO 0 - 2.5 B2O3 0 - 1 P2O5 0 - 2 Characterized by containing

[0102] In the above composition, it should be understood that the described components are at least 98% by weight, usually 99% by weight of the total composition. Compounds of a number of elements, such as F, Cl, alkali metals Rb, Cs, or elements such as Mn, Hf, etc., are common impurities in batch raw materials used industrially. Other compounds, such as compounds of elements W, Nb, Ta, Y, Mo, rare earths, Bi, V, Cr, Ni, may be contained in a small proportion, usually in the ppm range.

[0103] The water content of the crystalline glass for the production of glass ceramics is preferably 0.015 to 0.06 mol / l, depending on the selection of batch raw materials and process conditions during melting. This corresponds to a β-OH value of 0.16 to 0.64 mm -1 When transferring to glass ceramics, the IR band changes, which is used to determine the water content. Thus, the β-OH value of the glass ceramics changes without changing the water content. The method for determining the β-OH value is described, for example, in European Patent Application Publication No. 1074520.

[0104] Glass ceramics Crystalline lithium aluminum silicate glass is transferred to glass ceramics by a multi-stage temperature process.

[0105] The glass ceramics have the same composition as the lithium aluminum silicate glass.

[0106] Preferably, the glass ceramics contain high-temperature quartz mixed crystals as the main crystal phase.

[0107] The thermal expansion of the glass ceramics is preferably adjusted to a value of around 0 ± 0.5·10 -6 / K in the temperature range from room temperature to 700 °C.

[0108] The bottom coating preferably consists of at least one layer.

[0109] The following coating preferably consists of two layers, with the first colored layer applied to the underside of the glass-ceramic plate and the second layer applied to the first layer.

[0110] The first colored layer is preferably applied directly to the underside of the glass-ceramic plate.

[0111] The first layer preferably contains a crosslinked polysiloxane. Preferably, the first layer contains a pigment and / or a flaky filler.

[0112] The second layer preferably contains a non-crosslinked polysiloxane and may additionally contain talc or another layered silicate.

[0113] If the following coating consists of one layer, this layer preferably consists of the first layer.

[0114] For the production of the colored layer, a solid or liquid polysiloxane having a methyl group or a phenyl group as an organic group and a hydroxy group, an alkoxy group or a vinyl group as a functional group is contemplated as the silicone resin of the first layer. Thermal crosslinking by the functional group is carried out by firing at over 180°C.

[0115] The layer thickness of each layer is preferably 10 - 50 μm, particularly 15 - 30 μm. The total layer thickness of this two-layer system is desirably at least 20 μm so as to achieve a scratch resistance of 500 g. To achieve an even higher scratch resistance, the total layer thickness is preferably 25 - 65 μm, particularly 30 - 50 μm.

[0116] As pigments, inorganic coloring pigments and black pigments, such as iron oxide pigments, chromium oxide pigments, or oxide mixed-phase pigments having a rutile-type structure or a spinel-type structure, and inorganic white pigments (oxides, carbonates, sulfides) are preferred. Examples of suitable pigments include iron oxide red pigments composed of hematite (α-Fe2O3), iron oxide black pigments containing Fe3O4, and mixed-phase pigments such as cobalt blue CoAlO4, zinc iron brown (Zn,Fe)FeO4, chromium iron brown (Fe,Cr)2O4, iron manganese black (Fe,Mn)(Fe,Mn)2O4, spinel black Cu(Cr,Fe)2O4, and TiO2 and ZrO2 as white pigments.

[0117] In order to achieve special effects during coloring, inorganic bright pigments (metal effect pigments, nacreous pigments and interference pigments) can also be used.

[0118] As metal effect pigments, flaky particles composed of aluminum alloy, copper alloy or copper-zinc alloy are suitable, especially when coated with silicon oxide, for example, to enhance the color stability under thermal stress. Floating types (aufschwimmende Typen) (leafing pigments) can be used to enhance the impermeability of the layer to water, oil, adhesives, especially liquids. As nacreous pigments and interference pigments, mica coated with, for example, TiO2, SiO2 or Fe2O3 is suitable.

[0119] In order to obtain a color paste suitable for screen printing from silicone resin, pigments and fillers, it is necessary to dissolve the silicone resin and further add a solvent that can disperse the pigments and fillers. In order to optimize the wetting of the glass substrate or glass-ceramic substrate, an antifoaming agent, a wetting agent and a leveling agent may be added to the color paste.

[0120] The thickness of the glass-ceramic plate is preferably 2 mm to 20 mm.

[0121] Exemplary embodiments will be described below with reference to the drawings.

Brief Description of the Drawings

[0122]

Figure 1

Figure 2

Figure 3

Figure 4

[0123] FIG. 1 shows a cross-section of a cooking surface 1 having a glass-ceramic plate 2 with an upper surface 4 and a lower surface 6. A lower surface coating 8 composed of a first layer 9a and a second layer 9b is provided on the lower surface 6.

[0124] As an example, incident light ray 10 and light ray 12 reflected by the lower surface coating 8 used for diffuse reflection measurement, for example, measurement of L R * are shown.

[0125] Table 1 summarizes the glass compositions, relationships between components, and properties of 11 glasses according to the present invention (glass numbers 1 to 11) and 6 comparative glasses (glass numbers 12 to 18). Glass number 18 relates to Borofloatglas sold under the name BOROFLOAT 40 by SCHOTT AG.

[0126] Table 1 shows the water content measured by IR spectroscopy for some of the crystalline glasses.

[0127] Transition temperature Tg [°C], processing temperature V A [°C], 10 2 dPas temperature [°C], devitrification upper limit OEG [°C], and devitrification resistance V AThe characteristics of the glassy state such as -OEG are also described in Table 1. For the measurement of OEG, the glass is melted in a Pt / Rh10 crucible. Subsequently, the crucible is held for 5 hours at various temperatures within the range of the treatment temperature. The OEG temperature is determined by the highest temperature at which the first crystals appear on the contact surface between the molten glass and the crucible wall.

[0128] The crystalline LAS glasses 1 to 11 according to the present invention and the comparative glasses 12 to 17 were melted from batch raw materials common in the glass industry at a temperature of 1620 °C for 4 hours.

[0129] After melting the batch in a crucible made of sintered quartz glass, the experimental melt was transferred to a Pt / Rh crucible equipped with an inner crucible made of quartz glass and homogenized by stirring at a temperature of 1600 °C for 60 minutes. After this homogenization, the glass was clarified at 1640 °C for 3 hours.

[0130] Subsequently, a mass of about 120×140×30 mm 3 in size was cast and cooled from 660 °C to room temperature in a slow cooling furnace to relieve stress. The castings were divided into the sizes required for testing and ceramization.

[0131] Table 2 shows the characteristics of the crystalline glass after it has transformed into a transparent glass-ceramic containing high-temperature quartz mixed crystals as the main crystal phase.

[0132] The characteristics of the glass-ceramic, namely the spectral transmittance [%] at 400 nm, the infrared transmittance [%] at 1600 nm, the thermal expansion [10 -6 / K] from 20 °C to 700 °C, and the phase content [% by weight] of the main crystal phase consisting of high-temperature quartz mixed crystals, the phase content of the residual glass phase, and further the average crystallite size [nm] measured using X-ray diffraction are shown in Table 2. The luminance Y according to the CIE colorimetric system measured by transmission, and the lightness L * in the CIELAB system using the value c * as a measure of color (chroma), and the color coordinates a * , b * , and further the haze value as a measure of scattering are also shown in Table 2.

[0133] The transmission measurement was carried out using a Perkin-Elmer Lambda 900 instrument at a standard light D65, 2° with a polished plate having a thickness of 4 mm. From the measurement spectral values in the range of 380 nm to 780 nm, which is the visible light spectrum, the luminance Y compliant with DIN 5033 for the selected standard light source D65 and the observer angle of 2° is calculated. From these measurement values, the lightness L * and the color coordinates a * , b * , as well as the chroma c * are also calculated.

[0134] The diffuse reflection measurement was also carried out in accordance with DIN 5033 using these parameters. The value L * in diffuse reflection correlates with the perceived whiteness (Weisseindruck) of the glass ceramic.

[0135] The haze value is measured in accordance with ASTM D1003-13 using a BYK Gardner measuring device haze-guard at a standard light C, 2° with a polished sample of 4 mm thick LAS glass ceramic.

[0136] Table 2 further includes the L T * value and the c T * value for the glass ceramics according to the present invention produced from glasses 1 to 11 using the ceramization program 1. For the comparative glasses 12 to 17, in addition to the ceramization program 1, the ceramization program 2 was also used.

[0137] Ceramization program In the ceramization program 1, it is heated to 720 °C for 24 minutes in an experimental furnace enabling a high heating rate. In the temperature range of 720 °C to 800 °C, the heating rate is reduced for sufficient nucleation so that visually problematic scattering does not occur.

[0138] From above 800 °C to the maximum temperature of 890 °C, since the crystallization of high-temperature quartz mixed crystals occurs in this range, the heating rate is further decreased. The accompanying shrinkage process should not proceed too rapidly, because otherwise the article may develop unevenness. In this temperature range, the formation of the problematic Fe / Ti colored complex and Sn / Ti colored complex also increases. The total time from 800 °C to reaching the maximum temperature of 890 °C is 53 minutes, and the subsequent holding time is 10 minutes. At the maximum temperature, the composition of the crystal and the residual glass is determined, and the microstructure is homogenized. At that time, the chemical and physical properties of the glass-ceramic are determined. After controlling the cooling to 600 °C, the furnace door is opened and the sample is rapidly cooled to room temperature.

[0139] Ceramization program 1 (ceramization time 136 minutes): a) Rapidly heat from room temperature to T a = 720 °C in 24 minutes, b) Raise the temperature from T a = 720 °C to 800 °C in 20 minutes (heating rate 4 °C / min), c) Raise the temperature from 800 °C to 890 °C in 53 minutes (heating rate 1.7 °C / min), d) Hold at the maximum temperature of 890 °C for 10 minutes, e) Cool from 890 °C to 600 °C (at 10 °C / min) within 29 minutes and then rapidly cool to room temperature.

[0140] In the ceramization program 2, the ceramization time is shortened overall and the nucleation time is extended. In the ceramization program 2, the temperature in the ceramization furnace is heated to 720 °C in 24 minutes. In the temperature range of 720 °C to 800 °C, the heating rate is further reduced for sufficient nucleation so that visually problematic scattering does not occur. The total time from 720 °C to 800 °C is 33 minutes. When the temperature exceeds 800 °C, crystallization of the desired high-temperature quartz mixed crystal phase occurs. The total time from 800 °C to the maximum temperature of 885 °C is 28 minutes. At the maximum temperature of 885 °C and a holding time of 10 minutes, the composition of the crystal and the residual glass is determined and the microstructure is homogenized. After controlling the cooling at a cooling rate of 8.5 °C / min for 10 minutes until 800 °C, the furnace door is opened and the sample is rapidly cooled to room temperature. That is, in summary, Ceramization program 2 (ceramization time 105 minutes): a) Rapidly heat from room temperature to T a = 720 °C in 24 minutes (heating rate 30 °C / min), b) Heat from T a = 720 °C to 800 °C in 33 minutes (heating rate 2.4 °C / min), c) Heat from 800 °C to 885 °C within 28 minutes, but heat to 820 °C at 1 °C / min and then heat to 885 °C at 8 °C / min, d) Hold at the maximum temperature of 885 °C for 10 minutes, e) Cool to 800 °C within 10 minutes and then rapidly cool to room temperature.

[0141] Comparative Example 16 corresponds to Example 1 in Table 2 of European Patent Application Publication No. 1837314, which is a document considered in the prior art, and was crystallized as shown therein.

[0142] The results in Table 2 are shown in FIG. 2 for L T * as a function of c * and in FIG. 3 for Y T * as a function of c * .

[0143] The following applies to the straight line G1: G1 = 0.765·c T * + 93.5

[0144] G1 represents the preferred limit line within the range of condition B1

[0145] The following applies to the straight line G2: G2 = 0.765·c T * + 94.4

[0146] G2 represents the upper limit line within the range of condition B1. Both the limit lines G1 and G2 represent the preferred corridors of the lightness L of the glass-ceramic plate 2 T * Table 3 includes the ΔE values of the glass-ceramics 1 to 11 according to the invention, and the comparative examples 12 to 17 or the comparative glass 18

[0147] The bottom coatings are designated as A, B, C, and D. These four bottom coatings differ in the first colored layer 9a. The materials of the first layer are summarized in Table 4 Rc,R include

[0148] The advantages of the transparent glass-ceramics according to the invention with respect to a low chroma c

[0149] and a high lightness L T * in accordance with the preferred value ranges are particularly evident on a cooking surface having a bottom coating, especially when it is a white, generally light-colored bottom coating. A light-colored bottom coating having a lightness L T * of more than 50, more preferably more than 60, and particularly preferably more than 70 in accordance with the above-described calculation rules for diffuse reflection is preferred. The chroma c of the bottom coating Rc * is preferably less than 10, preferably less than 8. The lightness L of the bottom coating * as well as the color coordinates a Rc * and b * and *The values are shown in Table 3. Therefore, the bottom coatings B, C, and D are preferred examples. In the case of a bottom coating having several layers, as shown in FIG. 1, the values relate to the colored layer (the first layer 9a in FIG. 1) in contact with the glass-ceramic plate.

[0150] A preferred cooktop is one in which the glass-ceramic plate has a chroma c T * and a lightness L T * , or Y (see also FIGS. 2 or 3) having the values according to the invention, and L Rc * > 50 and a chroma c * < 10, and is a cooking surface by induction heating having a bottom coating.

[0151] The cooking surface has a preferred lightness L of more than 50, more preferably more than 55, and particularly preferably more than 60 when observed by diffuse reflection through the glass-ceramic plate. R * The examples and values of L R * can be found in Table 3 and FIG. 4.

[0152]

Table 1-1

[0153]

Table 1-2

[0154]

Table 1-3

[0155]

Table 2-1

[0156]

Table 2-2

[0157]

Table 3-1

[0158]

Table 3-2

[0159]

Table 4

Explanation of Symbols

[0160] 1 Cooking surface 2 Glass ceramic plate 4 Upper surface 6 Lower surface 8 Lower surface coating 9a First layer 9b Second layer 10 Incident light ray 12 Reflected light ray (diffuse reflection) List of conditions L T * ≧a·c T * +b (Condition B1) [Here, a≧0.765, b≧93.5 and 0≦c T * ≦3] Y * ≧d·c * +e (Condition B1a) 0.765·c * +94.4≧L T * ≧0.765·c T * +93.5 (Condition B2) 0.07·L R * +1.8≦ΔE Rc,R * ≦0.09L R* +3.4 (Condition B3) 50% ≦ L R * ≦ 100% (Condition B4) 0.5 wt% ≦ SrO + BaO ≦ 2.5 wt% (Condition B5) MgO / As2O3 < 0.8 (Condition B6) MgO / As2O3 < 0.7 (Condition B6a) 0.2 wt% ≦ Na2O + K2O ≦ 1.5 wt% (Condition B7a) 0.2 wt% ≦ Na2O + K2O ≦ 1.2 wt% (Condition B7b) 3.8 wt% ≦ TiO2 + ZrO2 + SnO2 ≦ 4.8 wt% (Condition B8)

Claims

1. A cooking surface (1) composed of a transparent lithium aluminum silicate glass ceramic plate (2), having an upper surface (4) and a lower surface (6), wherein the lower surface (6) is at least partially provided with a lower coating (8). In the cooking surface (1), The glass ceramic plate (2) with a thickness of 4 mm has a maximum chroma c of 3 when light from the standard light source D65 passes through it. Ｔ ＊ and has The glass ceramic plate contains TiO as a nucleating agent ２ and The cooked surface (1) has a color position A having color coordinates (L Ｒ ＊ , a Ｒ ＊ , b Ｒ ＊ ) in the CIELAB color space, which is measured by diffuse reflection with light from a standard light source D65, The bottom coating (8) has a color position B having color coordinates (L Ｒｃ ＊ , a Ｒｃ ＊ , b Ｒｃ ＊ ) in the CIELAB color space, and The color positions A and B have a distance ΔE Ｒｃ，Ｒ ＊ where 【Number 1】 and the distance ΔE Ｒｃ，Ｒ ＊ to 0.07·L Ｒ ＊ +1.8 ≤ ΔE Ｒｃ，Ｒ ＊ ≤ 0.09·L Ｒ ＊ +3.4 (Condition B3) is applied, where the color coordinate L in % Ｒ ＊ represents the lightness of the cooking surface (1) measured by diffuse reflection with light from the standard light source D65, where 50% ≤ L Ｒ ＊ ≤ 100% (Condition B4) It is characterized in that it becomes the cooking surface (1). The TiO ２ proportion is more than 1.6% by weight and up to 2.8% by weight, and the cooked noodle (1) according to claim 1 is characterized by this.

2.

3. Li ２ O 3 - 4.5 Al ２ O ３ 19 - 24 SiO ２ 62 - 70 TiO ２ 1.6 to 2.8 ZrO ２ 1 to 2.5 Fe ２ O ３ 0.005 to 0.025 The glass ceramic plate contains the following components (in weight % based on the oxide basis): The cooking surface (1) according to claim 1 or 2, characterized in that it contains The glass-ceramic plate contains Na in a proportion of 0.05 to 1.5% by weight ２ The cooking surface (1) according to claim 3, characterized in that it contains O

4. The glass ceramic plate contains K in a proportion of 0.05 to 1.5% by weight ２ The cooking surface (1) according to claim 3 or 4, characterized in that it contains O

5.

6. The glass ceramic plate contains MgO in a proportion of 0.1 to less than 0.5 wt%, and the cooking surface (1) according to any one of claims 3 to 5 is characterized in that it contains

7. The glass ceramic plate contains CaO in a proportion of 0.04 to 2 wt%, and the cooking surface (1) according to any one of claims 3 to 6 is characterized in that it contains

8. The glass ceramic plate contains SrO in a proportion of 0.01 to 2 wt%, and the cooking surface (1) according to any one of claims 3 to 7 is characterized in that it contains

9. The glass ceramic plate contains BaO in a proportion of 0.1 to 4 wt%, and the cooking surface (1) according to any one of claims 3 to 8 is characterized in that it contains

10. To the components BaO and SrO, the condition 0.5 ≦ SrO + BaO ≦ 2.5 (Condition B5) is applied, and the cooking surface (1) according to claim 8 or 9 is characterized in that

11. The glass ceramic plate contains ZnO in a proportion of 0.5 to 3 wt%, and the cooking surface (1) according to any one of claims 3 to 10 is characterized in that it contains The glass ceramic plate contains As in a proportion of 0.1 to 2% by weight, particularly 0.4 to 2% by weight ２ O ３ The cooking surface (1) according to any one of claims 3 to 11, characterized in that it contains the same.

12. The glass ceramic plate contains SnO at a ratio of less than 0.1% by weight ２ The cooking surface (1) according to any one of claims 3 to 12, characterized by containing the same.

13. The glass ceramic plate contains P in a proportion of 0.01% to 2% by weight ２ O ５ The cooking surface (1) according to any one of claims 3 to 13, characterized in that it contains the above.

14. The glass ceramic plate contains Nd in a proportion of 0.01% by weight to 0.25% by weight ２ O ３ The cooking surface (1) according to any one of claims 3 to 14, characterized by containing the same.

15. The glass ceramic plate contains Fe in a proportion of 0.005% by weight to 0.02% by weight ２ O ３ The cooking surface (1) according to any one of claims 3 to 15, characterized in that it contains the above.

16. The components MgO and As ２ O ３ are subject to the condition MgO / As ２ O ３ <0.8 (Condition B6) [Here, As ２ O ３ is more than 0% by weight] is applied, characterized in that the cooked noodle (1) according to any one of claims 6 to 16.

17.

18. Li ２ O 3.2 to 4.5 Al ２ O ３ 19 - 24 SiO ２ 62 to 68 Na ２ O 0 to 1 K ２ O 0 to 1 Na ２ O + K ２ O 0.2 to 1.5 (Condition B7a) The glass ceramic plate contains the following components (in weight % based on the oxide basis): TiO ２ 1.8 to 2.8 ZrO ２ 1 to less than 2.2 TiO ２ +ZrO ２ +SnO ２ 3.8 to 4.8 (Condition B8) Fe ２ O ３ 0.007 to 0.02 MgO 0.05 to less than 0.5 CaO 0 to 1.5 SrO 0 to 1.5 BaO 0 to 2.5 P ２ O ５ 0 to 4 ZnO 0 to 2.5 and the cooking surface (1) according to claim 3 is characterized in that it contains

19. Li ２ O 3.2 to less than 4.2 Al ２ O ３ 20 to less than 23 SiO ２ 62 to 68 Na ２ O 0.1 to 1 K ２ O 0 to 1 Na ２ O + K ２ O 0.2 to 1.2 (Condition B7b) The glass ceramic plate contains the following components (in weight % based on the oxide basis): MgO 0.1 to 0.4 CaO 0.05 to 1 SrO 0 to 1.5 BaO 0 to 2.5 TiO ２ 1.8 to 2.8 ZrO ２ 1 to less than 2.2 SnO ２ 0.01 to less than 0.10 TiO ２ +ZrO ２ +SnO ２ 3.8 - 4.8 (Condition B8) Fe ２ O ３ 0.008 to 0.02 SrO + BaO 0.5 to 2.5 B ２ O ３ 0 to 1 P ２ O ５ 0 to 2 ZnO 0 to 2.5 and the cooking surface (1) according to claim 3 is characterized in that it contains

20. The bottom coating (8) is composed of two layers (9a, 9b), a first colored layer (9a) is applied to the lower side of the glass ceramic plate (8), and a second layer (9b) is applied to the first layer (9a). The cooking surface (1) according to any one of claims 1 to 19, characterized in that.

21. The cooking surface (1) according to any one of claims 1 to 20, characterized in that the thickness of the glass ceramic plate is 2 mm to 20 mm.

Images