COOKING SURFACE MADE FROM A LAS GLASS CERAMIC PLATE

Description

[0001] The invention relates to a cooking surface made of a lithium aluminum silicate glass ceramic plate with high transparency, which has a top surface and a bottom surface, wherein the bottom surface is at least partially provided with a bottom coating.

[0002] The term "cooking surface" refers to the glass ceramic plate used for cooking, unlike the "hob." In addition to the cooking surface, the hob includes the heating element, as well as the controls and display.

[0003] Heat-resistant specialty glasses, such as alumino or borosilicate glasses, are increasingly being used in some established glass-ceramic applications. Additional technical measures such as air cooling for fireplace glass or electronic temperature control for induction-heated cooktops can lower operating temperatures and, in some cases, enable the use of these specialty glasses.

[0004] These special glasses are described, for example, in the following documents: WO 2018 / 225627 A, EP 3 228 601 A1, WO 2015 / 009483 A1.

[0005] Alternative solutions using borosilicate glasses, while highly color-neutral, have low thermal stability, making these glasses significantly less suitable for use in comparison to glass-ceramic cooktops. Furthermore, borosilicate glasses have low transmission in the wavelength range > 3300 nm, which is a disadvantage for the use of optical temperature sensors in cooktops.

[0006] Compared to these glasses, the color of transparent glass-ceramics represents a significant disadvantage. There is therefore a need to develop transparent glass-ceramics with low color.

[0007] The attempts to avoid (WO 2008 / 065167 A1, US 3,252,811) or to limit (WO 2008 / 065166 A1) the nucleating agent responsible for the color complexes in glass-ceramics have not yet led to industrial implementation. The required higher contents of the alternative nucleating agents ZrO 2 and / or SnO 2 lead to disadvantages during melting and forming, such as higher melting and forming temperatures and insufficient devitrification resistance during forming.

[0008] DE 10 2010 035 544 A1 and DE 10 2011 107 831 A1 disclose transparent glass-ceramics with a color value c* < 3. The high transparency in DE 10 2010 035 544 A1 is attributed to a ZnO content of > 4%, with light transmission of > 88%. DE 10 2011 107 831 A1 attributes the high transparency to small total values ​​for MgO + ZnO < 2.2 wt.%. A disadvantage of these glass-ceramics is that higher contents of the alternative nucleating agents ZrO 2 and / or SnO 2 are required. The low or missing proportions of the effective nucleating agent TiO 2 are detrimental to high nucleation rates and thus to the desired short ceramization times. Furthermore, the high contents of the nucleating agents ZrO 2 and / or SnO 2 lead to disadvantages in melting and forming. Melting the raw materials for these components requires longer times, which is economically disadvantageous in terms of furnace throughput and energy consumption.

[0009] WO 2013 / 124373 A1 describes the physical decolorization of transparent glass-ceramics with high-quartz solid solutions as the main crystal phase, which are free of arsenic and antimony except for unavoidable raw material impurities, by adding 0.005 wt.% - 0.15 wt.% Nd 2 O 3 . The principle of physical decolorization is based on the neutralization of existing absorption bands by complementary absorption bands of the decolorizing agent. This naturally leads to greater light absorption and thus reduces brightness. To achieve favorable manufacturing conditions, i.e., low melting and low forming temperatures, the example glasses in this document contain high contents of the viscosity-reducing component MgO, ranging from 0.44 wt.% to 0.93 wt.%. In addition to the high MgO contents, the comparatively high SnO 2 contents of the example glass ceramics are detrimental to the color c*.

[0010] Induction-heated cooktops consisting of a transparent, colored glass-ceramic plate are typically coated with a colored underside. Light colors, especially white, are increasingly being used for the underside coating. Current glass-ceramics have the disadvantage of shifting the color of the underside coating toward yellowish tones. This results in an undesirable increase in the color component b of the CIELAB system.

[0011] For transparent, uncolored glass-ceramics, no coloring compounds are added during production. For the other class, transparent colored glass-ceramics, V2O5 is usually added to the volume for coloring to reduce brightness and achieve a black appearance. This is typical for cooktops with a black appearance.

[0012] For transparent, uncolored glass-ceramics, high transparency, meaning high brightness and low color, is desired. Both of these characteristics mean low absorption, since the absorption bands, depending on their position in the visible spectrum, both reduce brightness and increase color; see EP 3 018 107, DE 10 2016 208300, or EP 1 837 313.

[0013] EP 1 837 314 A1 discloses a transparent, colorless LAS glass-ceramic coated on one side with a full-surface or largely full-surface opaque, colored, temperature-stable coating. This glass-ceramic composition also uses Nd 2 O 3 at a concentration of 0.01 wt.% to 0.4 wt.% for overcoloring. However, this LAS glass-ceramic exhibits a slight gray tint, which also impairs the perception of the color of the underside coating.

[0014] The object of the invention is to provide a cooking surface made of a TiO 2 -containing LAS glass ceramic plate, in which the color of the underside coating can be perceived by an observer almost unadulterated through the glass ceramic plate.

[0015] Unadulterated is understood to mean a perception by the human eye in which an observer, i.e. a colorimetric standard observer, detects no difference in the color of the underside coating with and without the glass ceramic plate.

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

[0017] This problem is solved according to the so-called remission variant with the features of claim 1.

[0018] According to the so-called transmission variant, which is not part of the invention, the cooking surface is characterized in that the glass ceramic plate contains high quartz mixed crystals as the main crystal phase, that the glass ceramic plate contains TiO 2 as a nucleating agent, and that the glass ceramic plate, after passage of light of standard illuminant D65 at a thickness of 4 mm, has a brightness L T * and has a chroma c* in the CIELAB color system, where the brightness L T * satisfies the following relationship: L T * ≥ a ⋅ c T * + b with a = 0.765, with b ≥ 93.5 and with 0 ≤ c T * ≤ 3 .

[0019] The index T stands for transmission.

[0020] Preferred values ​​are a = 0.765 and b = 93.5. A preferred range for b is 93.5 ≤ b ≤ 94.4.

[0021] The brightness or light transmission is determined by the L T * Value in the CIELAB color system or the brightness value Y (brightness) according to the CIE standard color system. The CIELAB color model is standardized in DIN EN ISO 11664-4 "Colorimetry - Part 4: CIE 1976 L*a*b* Color space." The German implementation of the international CIE standard is defined in DIN 5033.

[0022] Within the scope of the invention, the spectrophotometric measurements are carried out on samples polished on both sides in a spectral range between 380 and 780 nm. From the measured spectral values ​​in the range representing the visible light spectrum, the light transmission is calculated with the choice of standard illuminant and observer angle for the given thickness.

[0023] For glass ceramics, it has become common practice to use the value c* (chroma) from the CIELAB color system with the coordinates L*, a*, b* as a measure of color, according to the calculation: c T * = a * 2 + b * 2

[0024] The coordinates of the CIELAB color system can be calculated in a known way from the color coordinates x, y and the lightness Y* of the CIE color system. The determination of the c T * -value of the samples is determined from the spectrophotometric measurements of the transmission with the selected parameters for standard illuminant and observer angle.

[0025] Another key feature of LAS glass-ceramics is scattering, which is determined by the size of the crystals, their birefringence, and the difference in the refractive indices between the crystals and the residual glass. Sufficient amounts of nucleating agents and sufficient nucleation times during ceramization achieve a high nucleation density, resulting in the growing high-quartz solid solutions with sizes below the wavelength range of visible light. Typically, the average crystallite size of high-quartz solid solutions is in the range of 20 nm to 50 nm. Low crystal birefringence and a good match between the refractive indices of the crystals and the residual glass phase are also required for low scattering. These conditions for high transparency of LAS glass-ceramics are presented in the article "Nanophase Glass-Ceramics," Journal of the American Ceramic Society, Vol. 82, No. 1, pp.5-16; 1999 by the authors Beall and Pickney.

[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 to ensure that the view of the underside coating is not distorted and that the displays are clearly visible. The scattering of glass-ceramics is determined by measuring haze. According to ASTM D1003-13, haze is the percentage of transmitted light that deviates from the incident light beam by an average of more than 2.5°.

[0028] If the CIE color system is used as a basis, the following condition B1a, corresponding to condition 1, applies to the brightness Y*: Y * ≥ d ⋅ c * + e with d = 1.83, with e ≥ 84.4 and with 0 ≤ c* ≤ 3.

[0029] Both color systems can be converted into each other. In the following, the invention will be described only on the basis of the CIELAB system.

[0030] Preferably the brightness is sufficient L T * the following relationship: 0 , 765 ⋅ c * + 94 , 4 ≥ L T * ≥ 0 , 765 ⋅ c T * + 93 , 5

[0031] It has been found that the color of the underside coating of the cooking surface of TiO 2 -containing glass ceramics is perceived as authentic if condition B1, in particular condition B2, is met.

[0032] With the value for the chroma c T * ≤ 3 The inherent color of the LAS glass-ceramic is already very low. It has been shown that with increasing c T * -value and a concomitant increase in the intrinsic color, a compensation of the intrinsic color by increasing brightness values L T * so that the color of the underside coating can be perceived unadulterated. Conversely, with decreasing c T * -value lower brightness values L T * be tolerated.

[0033] If the condition B1 is not met, the values ​​c* and L* can no longer compensate each other to the extent that the human eye no longer perceives the falsification of the color of the underside coating.

[0034] Glass ceramics without TiO 2 or with very low contents of TiO 2 , ie with TiO 2 contents below 1.6 wt.%, show low c T * - and L T * -values, however, these glass-ceramics have disadvantages in manufacturing properties, such as higher melting and forming temperatures. Therefore, TiO 2 contents > 1.6 wt.% are desirable to achieve favorable manufacturing properties. A corridor with an upper limit line, defined by condition B2, is preferably maintained.

[0035] This corridor shows the optimized range for (L*;c*) within the scope of the invention. Compared to other known transparent glass-ceramics with TiO2 as a nucleating agent, the lower limiting line is advantageous for the color-unadulterated visual perception of the underside coating. Transparent LAS glass-ceramics without TiO2 or with very low TiO2 contents can achieve values ​​above the upper limiting line, but have the described technical and economic disadvantages in melting and shaping.

[0036] According to the remission variant as part of the invention, the cooking surface is characterized in that the glass ceramic plate with a thickness of 4 mm has a chroma of c T * of at most 3, that the glass ceramic plate has TiO 2 as a nucleating agent, that the cooking surface has a color location A with the color coordinates ( L R * , a R * , b R * ) in the CIELAB color space, which is measured in remission with light of standard illuminant D65, that the underside coating has a color location B with the color coordinates ( L Rc * , a Rc * , b Rc * ) in the CIELAB color space has the color locations A and B at a distance ΔE Rc , R * have: ΔE Rc , R * = L Rc * − L R * 2 + a Rc * − a R * 2 + b Rc * − b R * 2 and that for the distance ΔE Rc , R * applies: 0 , 07 ⋅ L R * + 1 , 8 ≤ ΔE Rc , R * ≤ 0 , 09 ⋅ L R * + 3 , 4 where the color coordinate L R * in % is the brightness of the cooking surface, which is measured in remission with light of standard type D65, with 50 % ≤ L R * ≤ 100 %

[0037] The index R stands for remission and the index c for "coating".

[0038] The color coordinates of the color coordinate A are preferably determined experimentally. The measured values ​​are ρ(λ) the spectral reflectance and τ(λ) the spectral transmittance. They are determined according to DIN 5036-3 1979-11.

[0039] The color coordinates of the color location B are preferably calculated as follows: The color values ​​are calculated according to DIN EN ISO 11664-3 (Aug-2013). In the CIE 1931 colorimetric system, X, Y, and Z are defined as integrals over the spectral range from 360 nm to 830 nm according to the following equations: X = k ∫ λ φ λ λ x ¯ λ dn Y = k ∫ λ φ λ λ y ¯ λ dn Z = k ∫ λ φ λ λ z ¯ λ dn

[0040] Where φ λ (λ) is the color stimulus function to be evaluated, x(λ), y(λ) are the spectral value functions of the colorimetric standard observer CIE 1931 (also known as the colorimetric CIE 2° standard observer).

[0041] k is a normalization constant defined below.

[0042] The standard procedure for evaluating these integrals consists of the numerical summation from 360 nm to 830 nm at wavelength intervals Δλ equal to 1 nm according to the equations: X = k ∑ λ φ λ λ x ¯ λ Δλ Y = k ∑ λ φ λ λ y ¯ λ Δλ Z = k ∑ λ φ λ λ z ¯ λ Δλ

[0043] The normalization constant k is chosen so that Y = 100 when the color stimulus function φ λ (λ) is equal to 1 for all wavelengths: k = 100 ∑ λ S λ y ¯ λ Δλ

[0044] Furthermore, the following sizes are used according to DIN EN ISO 11664-4 (Jun-2012): L* the CIELAB brightness away* the CIELAB coordinates a*, b* C ab ∗ the CIELAB chroma, or in short: C* ΔE ab ∗ the CIELAB color difference, or in short: ΔE* These include:

[0045] L n ∗ the CIELAB brightness with a color stimulus function φ λie (λ) a n ∗ , b n ∗ the CIELAB coordinates with a color stimulus function φ λ e (λ)n C ab , n ∗ = C n ∗ the CIELAB chroma with a color stimulus function φ λie (λ) ΔE ab , nm ∗ = ΔE nm ∗ the CIELAB color distance between two color stimulus functions φ λ,n (λ) and φ λun (λ)

[0046] To characterize the described embodiments, the following color stimulus functions are used to calculate the color parameters for: (1) Glass ceramic plate, in transmission (index T = "transmission"): φ λ,T,S (λ)=τ(λ)S(λ) resulting in CIELAB: C T ∗ with S(λ) (2) Cooking surface = glass ceramic plate + underside coating, in remission (index R = "remission"): φ λ,R,S (λ)=ρ(λ)S(λ) resulting in CIELAB: L R * with S(λ) (3) Underside coating in remission (Index RC = "Remission Coating"), with the calculation rule "Remission (cooking surface) / Transmission (glass ceramic plate) / Transmission (glass ceramic plate)": φ λ,RC,S (λ) =ρc(λ)S(λ) resulting in CIELAB: L RC * with S(λ) =ρ(λ) / τ(λ) / τ(λ)·S(λ) with ρc(λ)=ρ(λ) / τ(λ) / τ(λ) as a calculation rule for characterizing the spectral underside coating reflectance (4) The influence of the glass ceramic plate is described by the ΔE values ​​between the remission of the cooking surface (index R) and the underside coating (index RC) with: φ λ,R,S (λ) and φ λ,RC,S (λ) resulting in CIELAB: ΔE RC , R * with S(λ)

[0047] Where ρ(λ) is the spectral reflectance, τ(λ) is the spectral transmittance (according to DIN 5036-3 1979-11) and S(λ) is the spectral distribution of the standard illuminant D65.

[0048] It has been shown that at a distance ΔE Rc , R * ≤ 0 , 09 ⋅ L R * + 3 , 4 the glass ceramics according to the invention are significantly improved compared to the prior art (e.g. Nos. 12, 14, 15) and have come significantly closer to the ideal state of a (bright) layer under almost ideally color-neutral glass (No. 18).

[0049] Preferably: ΔE Rc , c * ≤ 0 , 09 ⋅ L R * + 3 , 05 and in particular ΔE Rc , c * ≤ 0 , 09 ⋅ L R * + 2 , 7 .

[0050] Preferably: ΔE Rc , c * ≥ 0 , 07 ⋅ L R * + 1 , 8 , particularly preferred ΔE Rc , c * ≥ 0 , 07 ⋅ L R * + 2 , 2 and in particular ΔE Rc , c * ≥ 0 , 07 ⋅ L R * + 2 , 6 .

[0051] Preferably 50 ≤ L R * ≤ 98 , particularly preferred 50 ≤ L R * ≤ 93 and in particular 50 ≤ L R * ≤ 88 . Nucleating agent TiO 2

[0052] The component TiO2 is an effective nucleating agent, essential for the transparency of glass-ceramics. It leads to high nucleation rates and thus to sufficient nucleation and thus small average crystallite sizes, even with short ceramization times. This makes it possible to obtain glass-ceramics without visually disturbing scattering, even with short ceramization times.

[0053] However, higher TiO 2 contents are critical due to the formation of Fe / Ti and Sn / Ti color complexes. Therefore, the TiO 2 content should preferably be more than 1.6 wt.% and up to 2.8 wt.%.

[0054] A minimum TiO 2 content of 1.8 wt.% is preferred. This minimum content is advantageous for reducing ceramization times and avoiding scattering. A TiO 2 content of more than 2 wt.% is particularly preferred. A maximum of 2.8 wt.% TiO 2 is preferably present to limit the color effect.

[0055] The glass ceramic plate of the cooking surface preferably contains the following components (in % by weight on an oxide basis): Li2O 3 - 4,5 Al2O3 19 - 24 SiO2 62 - 70 TiO2 > 1,6 - 2,8 ZrO2 1 - 2,5 Fe2O3 0,005 - 0,025. Li 2 O, Al 2 O 3 and SiO 2

[0056] The oxides Li 2 O, Al 2 O 3 and SiO 2 are or become necessary components of the high quartz and / or keatite solid solution phases within the specified limits.

[0057] For the crystallizable glass and the glass-ceramic produced therefrom, the Li2O content is preferably 3 to 4.5 wt.%. This minimum content is necessary to achieve the desired low processing temperature of the glass. It has been shown that at contents higher than 4.5 wt.%, it is difficult to achieve the desired zero thermal expansion of the glass-ceramic. Contents higher than 4.5 wt.% are also disadvantageous with regard to the desired low color c* of the glass-ceramic. The Li2O content is preferably less than 4.5 wt.%, more preferably less than 4.2 wt.%, and particularly preferably less than 4 wt.%. The minimum content is preferably 3.2 and particularly preferably 3.4 wt.%.

[0058] The Al 2 O 3 content is preferably 19-24 wt.%. Contents higher than 24 wt.% are disadvantageous due to the tendency of mullite to devitrify during shaping. Furthermore, it has been shown that with higher contents, the scatter increases with short ceramization times. A proportion of less than 23 wt.% is therefore preferred. The minimum content is 19 wt.% because Al 2 O 3 is advantageous for a low processing temperature and a low color c* of the glass-ceramic. The minimum Al 2 O 3 content is preferably at least 20 wt.%.

[0059] The content of the main component, SiO 2 , should be at least 62 wt.% because this is advantageous for the required properties of the glass-ceramic, such as low thermal expansion and chemical resistance. Furthermore, scattering is reduced with short ceramization times. A minimum content of 64 wt.% is particularly advantageous. The SiO 2 content should not exceed 70 wt.% because this component increases the processing temperature of the glass and the melting temperature. The SiO 2 content is preferably not more than 68 wt.%. Nucleating agent ZrO 2

[0060] ZrO 2 is preferably provided as an additional nucleating agent. The ZrO 2 content is preferably 1 to 2.5 wt.%. The ZrO 2 content is preferably limited to less than 2.2 wt.% and more preferably to less than 2 wt.%, since higher contents impair the melting behavior of the batch during glass production and can lead to devitrification due to the formation of Zr-containing crystals during shaping. Since the component helps to avoid higher contents of the alternative nucleating agent TiO 2, it is advantageous for providing a glass-ceramic with a low color c*. The minimum ZrO 2 content is preferably 1.6 wt.%. Fe2O3

[0061] Due to the high cost of low-iron raw materials, it is uneconomical to limit the Fe 2 O 3 content of the crystallizable glass to values ​​below 0.005 wt.%, i.e., less than 50 ppm. On the other hand, as the Fe 2 O 3 content increases, so does the concentration of Fe / Ti color complexes in the glass-ceramic. The color c* is increased, and absorption increases the brightness. L T * reduced. The crystallizable glass and the glass-ceramic produced from it should therefore contain a maximum of 0.025 wt.%, preferably up to 0.02 wt.%, of Fe 2 O 3. refining agent

[0062] The refining can be supported by the addition of chemical refining agents such as arsenic, antimony or cerium oxide and refining additives such as manganese oxide, sulfate and halide compounds in total contents preferably up to 2.0 wt.%.

[0063] The glass ceramic plate preferably contains 0.1 - 2.0 wt.% As 2 O 3 , in particular 0.4 - 2.0 wt.%. Alkalis

[0064] The alkalis Na2O and K2O lower the melting temperature and processing temperature during glass forming. This accelerates the melting of the poorly soluble raw materials ZrO2 and SiO2. The contents of both must be limited to a maximum of 1.5 wt.% because these components are not incorporated into the crystal phases but remain in the residual glass phase of the glass-ceramic. Excessively high contents impair the crystallization behavior during the conversion of the crystallizable starting glass into the glass-ceramic and adversely affect the time / temperature resistance of the glass-ceramic.

[0065] Preferably, the Na 2 O content is 0 wt.% or > 0 wt.%, and particularly preferably the glass contains at least 0.05 wt.% Na 2 O. The maximum content is preferably 1.5 wt.%, in particular 1 wt.%. Preferably, the Na 2 O content is 0.1 wt.% - 1.5 wt.%.

[0066] The glass ceramic plate preferably contains Na 2 O in a proportion of 0.05 - 1.5 wt.%.

[0067] Preferably, the K2O content is 0 wt% or >0 wt%, and particularly preferably the glass contains at least 0.05 wt% K2O. The maximum content is preferably 1.5 wt%, in particular 1 wt%. Preferably, the K2O content is 0.1 wt% - 1.5 wt%.

[0068] The glass ceramic plate preferably contains K 2 O in a proportion of 0.05 - 1.5 wt.%.

[0069] In a preferred embodiment, 0.2 wt.% ≤ Na 2 O + K 2 O ≤ 1.5 wt.% (condition B7a). The sum of the alkalis Na 2 O + K 2 O is more preferably a maximum of 1.2 wt.%. The sum of the alkalis Na 2 O + K 2 O is particularly preferably at least 0.4 wt.% in order to further improve meltability and lower the processing temperature. Particularly preferably, 0.2 wt.% ≤ Na 2 O + K 2 O ≤ 1.2 wt.% (condition B7b). MgO

[0070] The MgO component is present in both the solid solutions and the residual glass phase and therefore has a strong influence on many properties. This component lowers the melting and processing temperature of the glass, thus promoting economical production. The MgO content is at least 0.1 wt.% and particularly preferably at least 0.15 wt.%. In glass-ceramics, this component increases thermal expansion and leads to a particularly detrimental color enhancement. This is attributed to the promotion of the formation of Fe / Ti and Sn / Ti color species.

[0071] The MgO content should be less than 0.5 wt.%, preferably at most 0.4 wt.%, and particularly preferably at most 0.35 wt.%. A preferred MgO value range of 0.1 wt.% to 0.4 wt.% allows the requirements of low color of the glass-ceramic and low processing temperature to be particularly well combined. The thermal expansion of the glass-ceramic can be adjusted to near zero expansion. alkaline earths

[0072] Like the alkalis Na2O and K2O, the alkaline earths CaO, SrO, and BaO are not incorporated into the solid solution phases, but remain in the residual glass phase of the glass-ceramic. They are advantageous for lowering melting and processing temperatures. Excessively high concentrations impair nucleation and crystallization behavior during the conversion of crystallizable glass into glass-ceramic and adversely affect the time / temperature resistance of the glass-ceramic.

[0073] The component CaO has proven advantageous for lowering the processing temperature VA and improving devitrification resistance. However, it leads to increased color and scattering with short ceramization times. CaO is included in amounts of a maximum of 2 wt.%, preferably a maximum of 1 wt.%. The upper limit for the CaO content is particularly preferably less than 0.8 wt.% in order to minimize scattering with short ceramization times. CaO is preferably 0 wt.% or > 0 wt.%, particularly preferably ≥ 0.05 wt.%, further preferably at least 0.1 wt.% and particularly preferably at least 0.3 wt.%. A preferred range is 0.04 wt.% to 2 wt.%.

[0074] The glass ceramic plate preferably contains CaO in a proportion of 0.04 - 2 wt.%.

[0075] The BaO content is preferably up to 4 wt.%. The SrO content is preferably up to 2 wt.%. The BaO content is preferably up to 2.5 wt.%. The SrO content is preferably up to 1.5 wt.%. A BaO content of 0 wt.% or > 0 wt.%, at least 0.1 wt.%, is particularly preferred.

[0076] A preferred range for BaO is 0.1 wt% to 4 wt%.

[0077] The SrO content is preferably 0 wt.%, particularly preferably > 0 wt.%, in particular ≥ 0.01 wt.%.

[0078] A preferred range for SrO is 0.01 wt% to 2 wt%.

[0079] To lower melting and processing temperatures, it is preferable for the sum of the SrO and BaO contents to be 0.5–2.5 wt.% (condition B5). Higher contents are detrimental to the time / temperature resistance of the glass-ceramic.

[0080] The alkalis Na2O, K2O and the alkaline earths CaO, SrO, and BaO accumulate not only in the residual glass phase between the crystals but also on the surface of the glass-ceramic. During ceramization, a glassy surface layer approximately 50 to 1000 nm thick is formed, which is almost free of crystals and enriched with these elements, with Li2O being depleted. This glassy surface layer has a positive effect on the acid resistance of the glass-ceramic surface. For a sufficient thickness of the glassy layer of at least 50 nm, minimum contents of both classes of components, i.e. alkalis and alkaline earths, are required. Contents higher than the upper limits can lead to greater thicknesses of the glass-ceramic layer, which is detrimental to the strength of the glass-ceramic.

[0081] It has proven advantageous to adjust the contents of the residual glass-forming agents in a defined manner. In addition to adjusting the glassy surface layer, a good match between the refractive indices of the crystals and the residual glass phase is necessary for low scattering. Since the Fe / Ti color complexes form in the residual glass phase, this phase also determines their formation kinetics. However, the contents of the residual glass-forming agents must be limited. Higher contents have an adverse effect on the time / temperature resistance of the glass-ceramic. The crystallization of the high-quartz solid solutions is also impaired, and the formation of larger crystallites leads to increased scattering. The sum of the residual glass-forming agents is crucial for reconciling color, brightness, scattering, and cost-effective production, and the disadvantage compared to specialty glasses with high brightness values ​​and low color can be reduced.

[0082] The glass according to the invention preferably has a sum of the residual glass-forming components Na 2 O + K 2 O + CaO + SrO + BaO (wt.%) of preferably less than 3, preferably less than 2.6, and particularly preferably less than 2.2 wt.%. The sum of the components should preferably be more than 1.5 and particularly preferably more than 1.7 wt.%.

[0083] Haze is measured according to ASTM D1003-13 on 4 mm thick polished samples of LAS glass-ceramic. The haze value is preferably less than 2.5%, more preferably less than 2%, and most preferably less than 1.8%. Above 2.5%, haze is generally visually disturbing. Values ​​below 2% are preferred because the haze can otherwise become noticeable on dark, for example, black, underside coatings.

[0084] The transparent glass-ceramic according to the invention therefore exhibits no visually disturbing light scattering. This prevents the view of objects and underside coatings from being distorted. Luminous displays such as displays or screens beneath the glass-ceramic plate are clearly visible with sharp contours and without scattering. ZnO

[0085] The ZnO component is advantageous for lowering the melting and processing temperature of the glass and for reduced scattering during short ceramization times. ZnO is incorporated into the solid solution phases, and a portion also remains in the residual glass phase. Similar to the Li2O component, this component leads to a reduction in the thermal expansion of the glass ceramic. The ZnO content is limited to values ​​of no more than 3 wt.% due to the tendency to evaporate from the glass melt and the required zero expansion of the glass ceramic. A ZnO content of no more than 2.5 wt.% is preferred, and no more than 2.0 wt.% is particularly preferred. ZnO is preferably 0 wt.% or > 0 wt.% A minimum content of 0.5 wt.% is preferred, and more than 1 wt.% is particularly preferred. The ZnO content should preferably be between 0.5 wt.% and 3 wt.%.

[0086] The glass ceramic plate preferably contains ZnO in a proportion of 0.5 - 3 wt.%. SnO2

[0087] In a first embodiment with arsenic refining, the glass-ceramic plate contains no intentionally added SnO 2 (SnO 2 content equal to 0%), because this component is critical for the color of the glass-ceramic due to the formation of the Sn / Ti color species during crystallization. Only the impurity contents of less than 50 ppm are present.

[0088] New, special attention is being paid to the manufacturing properties during conventional forming using water-cooled rolls. Large-scale trials with SnO 2 -refined LAS glasses containing 0.18 wt.% SnO 2 , disclosed in WO 2013 / 124373 A1, have shown that a Sn-containing roll coating builds up during forming. Due to the effect of tin oxide as a nucleating agent, this roll coating increasingly induces surface crystals on the cooling glass ribbon when it comes into contact with the glass melt. These crystals are visually noticeable and can reduce strength. Removing the roll coating is associated with production downtime. The invention mitigates this economic disadvantage caused by the roll coating.

[0089] To avoid these disadvantages, the SnO 2 content is limited compared to the prior art and is preferably less than 0.10 wt.%, more preferably less than 0.08 wt.%, and particularly preferably less than 0.05 wt.%. As the SnO 2 content decreases, the color and rolling deposits continuously decrease. A significant improvement in the formation of surface crystals during molding is observed below 0.10 wt.% SnO 2 .

[0090] In a second embodiment, a minimum content of 0.01 wt.% is preferred for SnO 2 . Particularly when the refining agents arsenic or antimony oxide are not used, the polyvalent tin oxide counteracts the formation of bubbles (reboil) on precious metal components in the melting tank. Even at low contents, the SnO 2 acts as a refining agent and, in combination with technical measures on the melting tank, ensures the required bubble quality. The glass and the glass-ceramic produced therefrom preferably contain at least 0.03 wt.% and particularly preferably at least 0.04 wt.% SnO 2 .

[0091] A particularly preferred range, taking into account all aspects of low color and economic manufacturing properties, is 0.03 to < 0.10 wt% SnO 2 . P2O5

[0092] To improve meltability and devitrification resistance during molding, up to 4 wt.% P2O5 may be included. Higher contents are detrimental to chemical resistance. P2O5 is preferably 0 wt.% or > 0 wt.%. A preferred lower limit is 0.01 wt.% and particularly preferably 0.02 wt.%. A preferred upper limit is 2 wt.% P2O5. A preferred range is 0.01 wt.% to 2 wt.%.

[0093] The glass ceramic plate preferably contains P 2 O 5 in a proportion of 0.01 wt.% to 2 wt.%. B 2 O 3 , PbO and fluorine

[0094] The addition of up to 1 wt.% B 2 O 3 also improves meltability and devitrification resistance, but is detrimental to the time / temperature resistance of the glass-ceramic. This also applies to the addition of fluorine. What both components have in common is that they reduce the stability of the high-quartz solid solution, the main crystal phase that is important for transparent glass-ceramics, and promote the transition to the keatite solid solution phase. The glass-ceramic preferably contains less than 0.5 and particularly preferably less than 0.2 wt.% B 2 O 3 . The glass-ceramic is particularly preferably technically free of B 2 O 3 , i.e. the contents are below 1000 ppm, preferably below 500 and particularly preferably below 100 ppm.

[0095] A preferred upper limit for the addition of fluorine is 0.5 wt.%. More preferably, the content should be less than 0.2 wt.%, more preferably less than 0.1 wt.%, more preferably less than 500 ppm, and most preferably less than 100 ppm.

[0096] The addition of PbO improves meltability and devitrification resistance, but is detrimental to the time / temperature resistance of the glass-ceramic and is undesirable for an environmentally friendly composition. The content should preferably be less than 0.1 wt% and more preferably less than 100 ppm. Particularly preferably, no PbO is added, and only the contents resulting from impurities, typically less than 5 ppm, are present. Nd2O3

[0097] The Nd 2 O 3 content can be 0 wt% or > 0 wt%.

[0098] In the transparent glass-ceramics produced from the lithium aluminum silicate glasses according to the invention, the disturbing color based on Fe / Ti and / or Sn / Ti color complexes is reduced in a preferred embodiment by adding Nd 2 O 3 in amounts of 0.005 wt.% to 0.25 wt.%. Below 0.005 wt.%, the decolorizing effect is slight and preferred lower limits for Nd 2 O 3 are 0.01 wt.%, in particular 0.03 wt.%. Above 0.5 wt.%, the brightness is undesirably impaired by the absorption of the Nd bands in the visible light range. Therefore, up to 0.25 wt.% is preferably added. A preferred range for Nd 2 O 3 is 0.01 wt.% to 0.25 wt.%. CoO

[0099] Additions of CoO in amounts up to 30 ppm, preferably up to 20 ppm, can assist decolorization. A CoO content of 0.1 ppm to 20 ppm CoO is preferred. Above 30 ppm, CoO produces a reddish tint in transparent glass-ceramics. MgO / As 2 O 3

[0100] Preferably, the condition applies to the components MgO and As 2 O 3 MgO / As 2 O 3 < 0,8 with As 2 O 3 > 0 wt%.

[0101] The ratio of the components MgO / As 2 O 3 is crucial for combining low color and high brightness of the glass-ceramic with good refining properties and low melting and forming temperatures. This allows for further reduction of the chroma c* and further improvement of the brightness Y of the glass-ceramic with economical production, thus reducing the disadvantage compared to specialty glasses with high brightness values ​​and low color.

[0102] The glass according to the invention has a ratio of the components MgO / As 2 O 3 (both in wt.%) of preferably less than 1 (condition B2a), more preferably less than 0.8 and in particular less than 0.7 and particularly preferably less than 0.5.

[0103] This is an essential condition to combine the desired favorable manufacturing properties of the glass with low color, high brightness of the transparent glass-ceramic produced from it.

[0104] Advantageously, the ratio of the components is: MgO / As 2 O 3 < 0,7

[0105] In series of experiments with varying MgO and As 2 O 3 contents, it was found that the two components have opposing effects on the Fe / Ti color complex. As described, higher MgO contents enhance the formation of the color complexes, while higher As 2 O 3 contents reduce this formation. By preferentially limiting the ratio of the two, the effects complement each other advantageously.

[0106] The ratio of the components should preferably be greater than 0.05, more preferably greater than 0.08, and most preferably greater than 0.1. This is because at low MgO contents, despite higher As 2 O 3 contents, the refining is impaired due to the higher melting temperature.

[0107] Where a component content of 0 wt.% is specified, this means that the component in question is omitted from the raw material mixture. However, these components may be present as unavoidable impurities.

[0108] In a preferred embodiment, the cooking surface is characterized in that the glass ceramic plate contains the following components (in wt.% on an oxide basis): 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 - < 0,5 TiO2 1,8 - 2,8 ZrO2 1 - < 2,2 TiO2 + ZrO2 + SnO 2 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

[0109] According to a further embodiment, the cooking surface is characterized in that the glass ceramic plate contains the following components (in wt.% on an oxide basis): Li2O 3,2 - < 4,2 Al2O3 20 - < 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 - < 2,2 SnO2 0,01 - < 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

[0110] The aforementioned compositions are to be understood as meaning that the listed components constitute at least 98 wt.%, generally 99 wt.%, of the total composition. Compounds of a variety of elements, such as F, Cl, the alkalis Rb, Cs, or elements such as Mn and Hf, are common impurities in the batch raw materials used on an industrial scale. Other compounds, such as those of the elements W, Nb, Ta, Y, Mo, rare earths, Bi, V, Cr, and Ni, may be present in small amounts, typically in the ppm range.

[0111] The water content of the crystallizable glasses used to produce glass-ceramics is preferably between 0.015 and 0.06 mol / l, depending on the choice of batch raw materials and the melting process conditions. This corresponds to β-OH values ​​of 0.16 to 0.64 mm -1< . During conversion to glass-ceramics, the IR band used to determine the water content changes. This changes the β-OH value for the glass-ceramic without changing the water content. The method for determining the β-OH values ​​is described, for example, in EP 1 074 520 A1. Glass ceramic

[0112] The crystallizable lithium aluminum silicate glasses are converted into a glass ceramic through a multi-stage temperature process.

[0113] The glass ceramic has the same composition as the lithium aluminum silicate glass.

[0114] The glass ceramic contains high quartz solid solutions as the main crystal phase.

[0115] The thermal expansion of the glass ceramic is preferably set to values ​​around 0 ± 0.5 • 10 -6< / K in a temperature range between room temperature and 700 °C.

[0116] The underside coating preferably consists of at least one layer.

[0117] The underside coating preferably consists of two layers, wherein a color-imparting first layer is applied to the underside of the glass ceramic plate and wherein a second layer is applied to the first layer.

[0118] The first coloring layer is preferably applied directly to the underside of the glass ceramic plate.

[0119] The first layer preferably comprises a crosslinked polysiloxane. Preferably, the first layer comprises pigments and / or platelet-shaped fillers.

[0120] The second layer preferably comprises an uncrosslinked polysiloxane, which may preferably additionally contain talc or another layered silicate.

[0121] If the underside coating consists of one layer, it preferably consists of the first layer.

[0122] Solid or liquid polysiloxanes with methyl or phenyl groups as organic residues and hydroxy, alkoxy, or vinyl residues as functional groups are used as silicone resins for the first layer to create a color-imparting layer. Thermal crosslinking occurs via the functional groups by baking at temperatures above 180 °C.

[0123] The layer thickness of each layer is preferably 10-50 µm, especially 15-30 µm. The total layer thickness of the two-layer system should be at least 20 µm to achieve a scratch resistance of 500 g. The total layer thickness is preferably 25-65 µm, especially 30-50 µm to achieve even higher scratch resistance.

[0124] Preferred pigments are inorganic colored pigments and black pigments such as iron oxide pigments, chromium oxide pigments, or mixed-phase oxidic pigments with a rutile or spinel structure, and inorganic white pigments (oxides, carbonates, sulfides). Examples of suitable pigments include iron oxide red pigments made from hematite (α-Fe 2 O 3 ), iron oxide black pigments with Fe 3 O 4 , and the mixed-phase pigments cobalt blue CoAlO 4 , zinc iron brown (Zn, Fe)FeO 4 , chromium iron brown (Fe, Cr) 2 O 4 , iron manganese black (Fe, Mn)(Fe, Mn) 2 O 4 , spinel black Cu(Cr, Fe) 2 O 4 , and, as white pigments, TiO 2 and ZrO 2 .

[0125] In order to achieve special coloring effects, inorganic luster pigments (metallic effect pigments, pearlescent pigments and interference pigments) can also be used.

[0126] Platelet-shaped particles made of aluminum, copper, or copper-zinc alloys are suitable as metallic effect pigments, especially when coated with, for example, silicon oxide to increase color stability under thermal stress. Floating pigments (leafing pigments) can be used to increase the impermeability of the layer to water, oil, adhesives, and other liquids. Suitable pearlescent and interference pigments include mica coated with TiO 2 , SiO 2 , or Fe 2 O 3 .

[0127] To obtain a color paste suitable for screen printing from the silicone resin, pigments, and fillers, a solvent must be added to dissolve the silicone resin and disperse the pigments and fillers. Defoamers, wetting agents, and leveling agents can be added to the color paste to optimize wetting of the glass or glass-ceramic substrate.

[0128] The thickness of the glass ceramic plate is preferably 2 mm to 20 mm.

[0129] Exemplary embodiments are explained below with reference to the figures.

[0130] They show: Fig.1 a section through a cooking surface, Fig.2 a diagram in the CIELAB color system to explain the first transmission variant (L* depending on c*), Fig.3 a diagram how Fig.2 but in the CIE color system to explain the transmission variant, and Fig.4 a diagram with ΔE Rc,R as a function of L R * to explain the remission variant.

[0131] In the Figure 1 1 shows a section through a cooking surface 1 comprising a glass ceramic plate 2 having a top side 4 and a bottom side 6. Applied to the bottom side 6 is a bottom coating 8 consisting of a first layer 9a and a second layer 9b.

[0132] By way of example, an incident light beam 10 and a light beam 12 reflected at the underside coating 8 are shown, as they are used for the remission measurements, for example for the measurement of L R * be used.

[0133] Table 1 lists the glass compositions, component relationships and properties of 11 glasses according to the invention (glass nos. 1 - 11) and of 6 comparison glasses (glass nos. 12 - 18), whereby glass no. 18 concerns a borofloat glass marketed under the name BOROFLOAT 40 by SCHOTT AG.

[0134] The water content measured by IR spectroscopy is given in Table 1 for some of the crystallizable glasses.

[0135] Table 1 also lists the properties in the glassy state, such as transformation temperature Tg [°C], processing temperature VA [°C], 10 2< dPas temperature [°C], upper devitrification limit UEL [°C], and devitrification strength VA - UEL. To measure the UEL, the glasses are melted in Pt / Rh10 crucibles. The crucibles are then held for 5 hours at various temperatures within the processing temperature range. The highest temperature at which the first crystals appear at the contact surface of the glass melt and the crucible wall determines the UEL temperature.

[0136] The crystallizable LAS glasses 1 to 11 according to the invention and the comparison glasses 12 to 17 were melted from batch raw materials commonly used in the glass industry at temperatures of 1620 °C for 4 hours.

[0137] After melting the mixture in sintered silica glass crucibles, the laboratory melts were poured into Pt / Rh crucibles with silica glass inner crucibles and homogenized by stirring at 1600 °C for 60 minutes. Following this homogenization, the glasses were refined for 3 hours at 1640 °C.

[0138] Subsequently, pieces measuring approximately 120 x 140 x 30 mm 3< were cast and cooled in a lehr, starting at 660 °C, to room temperature to relieve stresses. The castings were divided into the sizes required for testing and ceramization.

[0139] Table 2 shows the properties of the crystallizable glasses after conversion into the transparent glass-ceramic with high quartz solid solutions as the main crystal phase.

[0140] Table 2 shows the properties of the glass-ceramics, namely spectral transmission [%] at 400 nm, infrared transmission [%] at 1600 nm, thermal expansion between 20 °C and 700 °C [10 -6 < / K], and the phase content [wt. %] of the main crystal phase, consisting of high-quartz solid solutions, measured by X-ray diffraction, the phase content of the residual glass phase, and the average crystallite size [nm]. Table 2 also shows the brightness Y measured in transmission from the CIE color system and the brightness L*, as well as the color coordinates a*, b* from the CIELAB system with the value c* as a measure of color (chroma), as well as the haze value as a measure of scattering.

[0141] The transmission measurements were performed on polished panels with a thickness of 4 mm using standard illuminant D65, 2°, using a Perkin-Elmer Lambda 900. From the measured spectral values ​​in the range between 380 nm and 780 nm, which represents the visible light spectrum, the brightness Y was calculated according to DIN 5033 for the selected standard illuminant D65 and an observer angle of 2°. These measurements were also used to calculate the brightness L* and the color coordinates a*, b* from the CIELAB system, as well as the chrominance c*.

[0142] The remission was also measured according to DIN 5033 using these parameters. The value L* in remission correlates with the white impression of the glass-ceramic.

[0143] The haze value is measured according to ASTM D1003-13 with standard light C,2° on 4 mm thick polished samples of the LAS glass ceramic using the haze-guard measuring device from BYK Gardner.

[0144] Table 2 also contains the L T * - and c T * Values ​​for the glass ceramics according to the invention, which were produced from glasses 1 - 11 using a ceramization program 1. For the comparison glasses 12 - 17, a ceramization program 2 was used in addition to the ceramization program 1. Ceramicization programs

[0145] In a ceramization program 1, the sample is heated to 720 °C in 24 minutes in a laboratory furnace that allows high heating rates. In the temperature range from 720 °C to 800 °C, the heating rate is reduced to ensure sufficient nucleation and to prevent visually disturbing scattering.

[0146] Above 800 °C up to the maximum temperature of 890 °C, the heating rate is further reduced, as this is where the crystallization of the high-quartz solid solutions takes place. The associated shrinkage process must not proceed too quickly, as otherwise the article may become uneven. In this temperature range, the formation of the disruptive Fe / Ti and Sn / Ti color complexes is also increased. The total time from 800 °C until the maximum temperature of 890 °C is reached is 53 minutes, with a subsequent hold time of 10 minutes. At the maximum temperature, the composition of crystals and residual glass is adjusted and the microstructure is homogenized. In the process, the chemical and physical properties of the glass-ceramic are adjusted. Cooling is controlled up to 600 °C, after which the sample is quenched to room temperature by opening the furnace door.

[0147] Ceramicization program 1, (ceramization time 136 min): a) rapid heating from room temperature to T a = 720 °C in 24 min, b) temperature increase from T a = 720 °C to 800 °C in 20 min (heating rate 4 °C / min), c) temperature increase from 800 °C to 890 °C in 53 min (heating rate 1.7 °C / min), d) holding time of 10 min at a maximum temperature of 890 °C, e) cooling within 29 min from 890 °C to 600 °C (at 10 °C / min), then rapid cooling to room temperature.

[0148] With ceramization program 2, the overall ceramization time is shortened and the nucleation time is extended. With ceramization program 2, the temperature is heated up to 720 °C in the ceramization furnace in 24 minutes. In the temperature range from 720 °C to 800 °C, the heating rate is further reduced to ensure sufficient nucleation and to prevent visually disturbing scattering. The total time between 720 °C and 800 °C is 33 minutes. Above 800 °C, the desired high-quartz solid solution phase crystallizes. The total time from 800 °C until the maximum temperature of 885 °C is reached is 28 minutes. At the maximum temperature of 885 °C, with a holding time of 10 minutes, the composition of crystals and residual glass is adjusted and the microstructure is homogenized. Cooling is carried out to 800°C in 10 min at a cooling rate of 8.5 °C / min, then the sample is quenched to room temperature by opening the furnace door; in summary: Ceramicization program 2 (ceramization time 105 min):

[0149] a) rapid heating from room temperature to T a = 720 °C in 24 min (heating rate 30 °C / min), b) temperature increase from T a = 720 to 800 °C in 33 min (heating rate 2.4 °C / min), c) temperature increase from 800 to 885 °C within 28 min, heating at 1 °C / min to 820 °C, further heating at 8 °C / min to 885 °C; d) holding time 10 min at maximum temperature 885 °C, e) cooling to 800 °C within 10 min, then rapid cooling to room temperature.

[0150] Comparative Example 16 corresponds to Example 1 from Table 2 in the document EP 1 837 314 A discussed in the prior art and was crystallized as stated therein.

[0151] The results from Table 2 are shown in the Figure 2 for L* depending on c T * and in Figure 3 for Y* depending on c T * shown.

[0152] For the straight line G 1 the following applies: G 1 = 0 , 765 ⋅ c T * + 93 , 5 .

[0153] G 1 describes a preferred limit line for the region of condition B1.

[0154] For the straight line G 2 the following applies: G 2 = 0 , 765 ⋅ c T ∗ + 94 , 4 .

[0155] G 2 describes an upper limit line for the range of condition B1. Both limit lines G 1 and G 2 describe a preferred corridor for the brightness L T ∗ the glass ceramic plate 2.

[0156] Table 3 contains the ΔE Rc,R values ​​for the inventive glass-ceramics 1 - 11 and the comparative examples 12 - 17 or the comparative glass 18.

[0157] The underside coatings are designated A, B, C, and D. The four underside coatings differ in the color-imparting first layer 9a. The materials of the first layer are summarized in Table 4.

[0158] The advantages of the transparent glass-ceramic according to the invention with regard to its low chromaticity c T ∗ and high brightness L T ∗ According to the preferred value range, cooking surfaces with underside coatings are particularly important when the underside coatings are white and generally light. Light underside coatings are preferred, which, according to the specified calculation rule, have a brightness in remission L Rc ∗ of greater than 50, more preferably greater than 60 and particularly preferably greater than 70. The chroma c* of the underside coating is preferably less than 10, preferably less than 8. The values ​​for the brightness L Rc ∗ and the color coordinates a* and b* of the undercoating are shown in Table 3. Accordingly, undercoatings B, C, and D are preferred examples. For undercoatings with multiple layers, as in Figure 1 shown, the sizes refer to the coloring layer that is in contact with the glass ceramic plate (first layer 9a in Figure 1 ).

[0159] A preferred hob is a cooking surface with induction heating, where the glass ceramic plate has the values ​​according to the invention for chroma c T ∗ and brightness L T ∗ , or Y (see also Figure 2 or 3 ) and a bottom coating with L Rc ∗ > 50 and chroma c* < 10.

[0160] When viewed in remission through the glass ceramic plate, the cooking surface preferably has a brightness L R ∗ > 50 , more preferably greater than 55 and most preferably greater than 60. Examples and values ​​for L R * are Table 3 and the Figure 4 can be found. Table 1: Composition and properties of crystallizable glasses Glass No. 1 2 3 4 5 6 7 8 composition % by weight Li2O 3,60 3,66 3,68 3,62 3,63 3,60 3,53 3,64 Na2O 0,51 0,52 0,52 0,52 0,53 0,65 0,46 0,22 K2O 0,16 0,20 0,20 0,20 0,20 0,12 0,38 0,28 MgO 0,28 0,27 0,27 0,27 0,27 0,18 0,13 0,27 CaO 0,40 0,34 0,34 0,34 0,35 0,04 0,17 0,32 SrO 0,05 0,05 0,05 0,05 0,05 0,19 0,11 0,05 BaO 0,92 0,93 0,93 0,93 0,93 1,45 1,45 0,95 ZnO 1,77 1,83 1,84 1,81 1,80 1,85 1,71 1,80 Al2O3 21,45 21,28 21,23 21,25 21,28 21,67 21,59 21,23 SiO2 65,75 65,70 65,60 65,70 65,70 65,10 65,30 66,07 TiO2 2,35 2,39 2,40 2,39 2,40 2,45 2,47 2,41 ZrO2 1,81 1,82 1,82 1,82 1,82 1,75 1,75 1,83 P2O5 0,03 0,028 0,028 0,030 0,030 0,028 0,029 SnO2 Fe2O3 0,015 0,014 0,014 0,015 0,014 0,009 0,010 0,015 Nd2O3 0,048 0,053 0,16 0,160 0,110 0,067 0,055 As 2 O 3 0,85 0,86 0,86 0,88 0,87 0,88 0,81 0,82 CoO 0,0012 H 2 O content (β-OH) 0,33 0,34 0,30 0,29 0,37 MgO / As 2 O 3 0,33 0,31 0,31 0,31 0,31 0,20 0,16 0,33 Na2O + K2O 0,67 0,72 0,72 0,72 0,73 0,77 0,84 0,50 CaO+SrO 0,45 0,39 0,39 0,39 0,40 0,23 0,28 0,37 SrO + BaO 0,97 0,98 0,98 0,98 0,98 1,64 1,56 1,00 TiO2 + ZrO2 + SnO2 4,16 4,21 4,22 4,21 4,22 4,20 4,22 4,24 Na 2 O + K 2 O + CaO + SrO + BaO 2,04 2,04 2,04 2,04 2,06 2,45 2,57 1,82 Properties glassy Transformation temperature Tg °C 683 678 678 681 689 685 10 2< - Temperature °C 1741 1736 1732 1746 1749 1742 Processing temperature VA °C 1313 1309 1310 1312 1321 1316 UEG temperature °C 1285 1280 1300 1300 1275 Devitrification resistance VA - OEG °C 28 29 12 21 41 Continuation of Table 1: Composition and properties of crystallizable glasses Comparison examples Glass No. 9 10 11 12 13 14 composition wt.% Li2O 3,63 3,68 3,70 3,70 3,71 3,71 Na2O 0,15 0,14 0,14 0,16 0,37 0,53 K2O 0,35 0,33 0,33 0,21 0,29 0,16 MgO 0,32 0,32 0,32 1,07 0,71 0,28 CaO 0,33 0,30 0,31 0,03 0,02 0,40 SrO 0,05 0,05 0,05 0,01 0,01 0,05 BaO 0,98 0,98 0,98 0,84 1,17 0,93 ZnO 1,83 1,81 1,81 1,60 1,76 Al2O3 21,19 21,25 21,25 20,12 21,80 21,65 SiO2 66,00 65,90 66,00 67,12 65,34 66,16 TiO2 2,43 2,41 2,42 2,35 2,11 2,36 ZrO2 1,83 1,83 1,83 1,81 2,24 1,83 P2O5 0,029 0,030 0,029 0,026 1,47 0,03 SnO2 0,069 Fe2O3 0,016 0,015 0,014 0,018 0,015 0,013 Nd2O3 0,055 0,120 0,062 0,052 As 2 O 3 0,8 0,81 0,79 0,86 0,75 CoO H 2 O content (β-OH) 0,36 0,30 0,48 0,47 MgO / As 2 O 3 0,40 0,40 0,41 1,24 0,95 Na2O + K2O 0,50 0,47 0,47 0,37 0,66 0,69 CaO+SrO 0,38 0,35 0,36 0,05 0,03 0,45 SrO + BaO 1,03 1,03 1,03 0,85 1,18 0,98 TiO2 + ZrO2 + SnO2 4,26 4,24 4,25 4,16 4,35 4,26 Na 2 O + K 2 O + CaO + SrO + BaO 1,86 1,80 1,81 1,26 1,86 2,07 Properties glassy Transformation temperature Tg °C 686 682 676 674 698 683 10 2< - Temperature °C 1741 1741 1740 1760 1756 1747 Processing temperature VA °C 1311 1312 1313 1315 1332 1318 UEG temperature °C 1275 1290 1280 1290 Devitrification resistance VA - OEG °C 36 22 35 28 Continuation of Table 1: Composition and properties of crystallizable glasses Comparison examples Glass No. 15 16 17 composition wt.% Li2O 3,66 3,75 4,11 Na2O 0,56 0,35 K2O 0,10 0,13 MgO 0,61 0,65 1,33 CaO 0,24 SrO 0,49 BaO 0,52 ZnO 1,76 1,57 Al2O3 21,58 21,3 18,2 SiO2 66,19 67,6 72,16 TiO2 2,21 2,23 ZrO2 1,83 1,76 3,69 P2O5 0,02 SnO2 0,120 Fe2O3 0,015 0,020 Nd2O3 0,054 0,090 As 2 O 3 0,0052 0,40 0,31 CoO H2O - Content (β-OH) 0,45 MgO / As 2 O 3 117,3 1,63 4,29 Na2O + K2O 0,66 0,48 0,00 CaO+SrO 0,73 0,00 0,00 SrO + BaO 1,01 0,00 0,00 TiO2 + ZrO2 + SnO2 4,16 3,99 3,69 Na 2 O + K 2 O + CaO + SrO + BaO 1,91 0,48 0,00 Properties glassy Transformation temperature Tg °C 684 681 733 10 2< - Temperature °C 1736 1376 Processing temperature VA °C 1309 1320 1376 UEG temperature °C 1250 1240 > 1470 Devitrification resistance VA - OEG °C 59 80 > 96 Table 2: Ceramicization conditions and properties of glass ceramics with high quartz solid solutions as the main crystal phase Example No. 1 2 3 4 5 6 7 8 9 Glass No. 1 2 3 4 5 6 7 8 9 ceramization program 1 1 1 1 1 1 1 1 1 Transmission standard light D65, 2° 4 mm thickness 400 nm % 79,1 79,4 78,7 78,3 78,4 80,9 80,6 79,6 79,6 1600 nm % 90,5 90,1 89,9 89,6 90,1 90,9 90,7 89,6 89,6 Light transmission Y % 88,9 88,7 86,4 85,2 87,1 89,8 88,2 88,4 88,7 L T * 95,5 95,4 94,5 94,0 94,8 95,9 95,2 95,3 95,5 a T * -0,4 -0,3 -0,3 0,2 -0,3 -0,3 -0,3 -0,3 -0,3 b T * 2,1 1,9 0,6 0,2 1,4 2,4 1,5 1,7 1,8 c T * 2,1 1,9 0,7 0,3 1,4 2,4 1,5 1,7 1,8 Scattering standard light C, 2° 4 mm thickness haze % 0,69 0,51 1,02 0,65 0,56 0,37 0,43 0,68 0,55 thermal expansion α 20 / 700 10 -6< / K -0,30 -0,30 -0,27 -0,28 -0,31 -0,25 -0,20 -0,41 -0,37 X-ray diffraction HQMK phase content % 71 69 71 70 70 68 65 72 72 Phase content residual glass phase % 26 28 26 27 27 29 32 25 25 average crystallite size nm 41 40 41 40 41 38 37 37 38 Continuation of Table 2: Ceramicization conditions and properties of glass ceramics with high quartz solid solutions as the main crystal phase Comparison examples Example No. 10 11 12 13 14 15 16 17 18 Glass No. 10 11 12 13 14 15 16 17 18 ceramization program 1 1 1 1 1 1 In the text 2 Glass for comparison Transmission standard light D65, 2° 4 mm thickness 400 nm % 79,2 79,7 67,9 77,4 76,4 65,7 84,7 91,0 1600 nm % 89,8 90,4 89,6 88,7 89.7 89,8 89,0 91,0 Light transmission Y % 87,1 90,0 87,2 89,5 86,0 84,3 87,0 90,0 92,1 L* 94,8 96,0 94,8 95,8 94,3 93,6 94,6 96,0 96,9 a* -0,3 -0,3 -0,5 -0,5 0,0 -0,2 -0,5 -0,2 -0,3 b* 0,9 2,6 3,8 3,4 2,4 4,2 1,8 1,1 0,4 c* 1,0 2,6 3,8 3,4 2,4 4,2 1,8 1,1 0,5 Scattering standard light C, 2° 4 mm thickness haze % 0,75 1,07 0,39 0,54 0,51 0,62 0,28 1,12 thermal expansion α 20 / 700 10 -6< / K -0,38 -0,41 -0,01 -0,26 -0,11 -0,26 0,40 X-ray diffraction HQMK phase content % 70 76 68 72 80 Phase content residual glass phase % 27 21 29 25 17 average crystallite size nm 37 40 37 41 73 Table 3: Glass No. 1 1 1 1 2 2 2 2 3 3 3 3 Underside coating A B C D A B C D A B C D CT * Glass ceramic 2,1 2,1 2,1 2,1 1,9 1,9 1,9 1,9 0,7 0,7 0,7 0,7 L RC * Underside coating 26,6 73,3 77,9 94,2 26,6 73,3 77,9 94,2 26,6 73,3 77,9 94,2 a* Underside coating 0,26 -1,1 -0,6 -0,6 0,26 -1,1 -0,6 -0,6 0,26 -1,1 -0,6 -0,6 b* Underside coating -0,5 -4,1 -0,6 3,1 -0,5 -4,1 -0,6 3,1 -0,5 -4,1 -0,6 3,1 LR * Cooking surface 26,1 66,8 71,4 86,0 23,3 66,5 70,7 85,7 22,7 65,2 69,3 84,1 a* Cooking surface 0,15 -1,49 -1,00 -0,88 0,04 -1,47 -1,03 -0,86 0,04 -1,43 -1,05 -0,89 b* Cooking surface -0,98 -0,88 2,36 6,46 0,87 -0,91 2,39 6,12 0,02 -2,64 0,50 3,89 ΔE Rc,R 3,9 7,6 7,9 9,0 3,6 7,6 7,8 9,0 3,9 8,3 8,7 10,1 Glass No. 10 10 10 10 12 12 12 12 14 14 14 14 Underside coating A B C D A B C D A B C D CT * Glass ceramic 1,0 1,0 1,0 1,0 3,8 3,8 3,8 3,8 2,4 2,4 2,4 2,4 L RC * Underside coating 26,6 73,3 77,9 94,2 26,6 73,3 77,9 94,2 26,6 73,3 77,9 94,2 a* Underside coating 0,26 -1,1 -0,6 -0,6 0,26 -1,1 -0,6 -0,6 0,26 -1,1 -0,6 -0,6 b* Underside coating -0,5 -4,1 -0,6 3,1 -0,5 -4,1 -0,6 3,1 -0,5 -4,1 -0,6 3,1 LR * Cooking surface 22,9 65,5 69,7 84,5 25,6 66,1 70,4 85,1 25,7 64,9 69,6 83,6 a* Cooking surface 0,06 -1,40 -1,01 -0,84 0,01 -1,63 -1,06 -0,90 0,14 -0,88 -0,39 -0,12 b* Cooking surface 0,22 -2,25 0,94 4,42 -0,51 1,61 4,95 8,73 -0,47 0,01 3,00 6,89 ΔE Rc,R 3,8 8,0 8,4 9,7 5,3 9,9 10,2 11,6 4,9 9,4 9,8 11,2 Continuation of Table 3: Glass No. 15 15 15 15 17 17 17 17 18 18 18 18 Underside coating A B C D A B C D A B C D CT * Glass ceramic 4,2 4,2 4,2 4,2 1,1 1,1 1,1 1,1 0,5 0,5 0,5 0,5 L RC * Underside coating 26,6 73,3 77,9 94,2 26,6 73,3 77,9 94,2 26,6 73,3 77,9 94,2 a* Underside coating 0,26 -1,1 -0,6 -0,6 0,26 -1,1 -0,6 -0,6 0,26 -1,1 -0,6 -0,6 b* Underside coating -0,5 -4,1 -0,6 3,1 -0,5 -4,1 -0,6 3,1 -0,5 -4,1 -0,6 3,1 LR * Cooking surface 25,8 64,9 68,9 83,1 23,7 67,3 71,5 86,7 24,3 68,5 72,9 88,3 a* Cooking surface 0,19 -1,09 -0,57 -0,21 0,13 -1,24 -0,82 -0,70 0,04 -1,45 -1,06 -1,04 b* Cooking surface -0,90 1,53 4,87 9,08 0,32 -2,12 1,20 4,80 -0,19 -3,22 0,10 3,62 ΔE Rc,R 6,4 11,9 12,2 13,8 3,0 6,4 6,6 7,7 2,3 4,9 5,1 5,9 Table 4: Materials of the first layer Recipe of the color pastes AC, e.g. applied using the sol-gel process. Silicone matrix Fillers solvent Color pigments e.g. methyl silicone resin E.g. SiO 2 Mica pigment coated with TiO 2 + SnO 2 (white) Mica pigment coated with TiO 2 + SnO 2 (silver) TiO 2 (white) Absorption pigment violet Absorption pigment blue Black Pigment Graphite pigment A 50,0% 10,0% 2,0% 30,0% 8,0% B 58,0% 4,0% 10,0% 10,0% 10,0% 3,0% 1,5% 1,5% 2,0% C 65,0% 2,0% 10,0% 22,0% 1,0% D Color, RAL 9003 (white) List of reference symbols

[0161] 1Cooking surface 2Glass ceramic plate 4Top 6Bottom 8Underside coating 9aFirst layer 9bSecond layer 10Incident light beam 12Reflected light beam (remission) List of conditions

[0162] L T * ≥ a ⋅ c T * + b with a ≥ 0,765 , mit b ≥ 93,5 und mit 0 ≤ c T * ≤ 3 Y * ≥ d ⋅ c * + e 0 , 765 ⋅ c * + 94 , 4 ≥ L T * ≥ 0 , 765 ⋅ c T * + 93 , 5 0 , 07 ⋅ L R * + 1 , 8 ≤ ΔE Rc , R * ≤ 0 , 09 L R * + 3 , 4 50 % ≤ L R * ≤ 100 % 0 , 5 Gew . − % ≤ SrO + BaO ≤ 2,5 Gew . − % MgO / As 2 O 3 < 0 , 8 MgO / As 2 O 3 < 0 , 7 0 , 2 Gew . − % ≤ Na 2 O + K 2 O ≤ 1,5 Gew . − % 0 , 2 Gew . − % ≤ Na 2 O + K 2 O ≤ 1,2 Gew . − % 3,8 Gew . − % ≤ TiO 2 + ZrO 2 + SnO 2 ≤ 4 , 8 Gew . − %

Claims

1. Cooking surface (1) made of a transparent lithium aluminum silicate glass ceramic plate (2) which has a top surface (4) and a bottom surface (6), wherein the bottom surface (6) is at least partially provided with a bottom coating (8), characterized in that the glass ceramic includes high quartz mixed crystals as the main crystal phase, the glass ceramic plate (2) with a thickness of 4 mm has a chromaticity c T * of at most 3 when light of standard illuminant D65 passes through, the glass ceramic plate has TiO2 as nucleating agent, the cooking surface (1) has a color locus A with the color coordinates L R * a R * b R * in the CIELAB color space, which is measured in remission with light of standard illuminant D65, the bottom coating (8) has a color locus B with the color coordinates L Rc ∗ a Rc ∗ b Rc ∗ in the CIELAB color space, the color loci A and B have a distance ΔE Rc , R ∗ with: ΔE Rc , R ∗ = L Rc ∗ − L R ∗ 2 + a Rc ∗ − a R ∗ 2 + b Rc ∗ − b R ∗ 2 and the following is applicable to the distance ΔE Rc , R ∗ : 0.07 ⋅ L R ∗ + 1.8 ≤ ΔE Rc , R ∗ ≤ 0.09 L R ∗ + 3.4 wherein the color coordinate L R ∗ in % denotes the brightness of the cooking surface (1), which is measured in remission with light of standard illuminant D65, with 50 % ≤ L R ∗ ≤ 100 %2. Cooking surface (1) according to claim 1, characterized in that the TiO2 proportion is > 1.6 - 2.8% in weight.

3. Cooking surface (1) according to at least one of the preceding claims, characterized in that the glass ceramic plate includes the following components (in % in weight on oxide basis): Li2O3 - 4.5Al2O319 - 24SiO262 - 70TiO2> 1.6 - 2.8ZrO21 - 2.5Fe2O30.005 - 0.0254. Cooking surface (1) according to claim 3, characterized in that the glass ceramic plate includes MgO with a proportion of 0.1 - < 0.5% in weight.

5. Cooking surface (1) according to at least one of claims 3 or 4, characterized in that the glass ceramic plate includes SrO with a proportion of 0.01 - 2% in weight.

6. Cooking surface (1) according to one of claims 3 to 5, characterized in that the glass ceramic plate includes BaO with a proportion of 0.1 - 4% in weight.

7. Cooking surface (1) according to claims 5 and 6, characterized in that for the components BaO and SrO the condition 0.5 ≤ SrO + BaO ≤ 2.5 is applicable.

8. Cooking surface (1) according to one of claims 3 to 7, characterized in that the glass ceramic plate includes As2O3 with a proportion of 0.1 - 2% in weight, in particular 0.4 - 2% in weight.

9. Cooking surface (1) according to one of claims 3 to 8, characterized in that the glass ceramic plate includes SnO2 with a proportion of less than 0.1% in weight.

10. Cooking surface (1) according to one of claims 3 to 9, characterized in that the glass ceramic plate includes Nd2O3 with a proportion of 0.01% in weight - 0.25% in weight.

11. Cooking surface (1) according to one of claims 3 to 10, characterized in that the glass ceramic plate includes Fe2O3 with a proportion of 0.005% in weight to 0.02% in weight.

12. Cooking surface (1) according to one of claims 4 to 11, characterized in that for the components MgO and As2O3 the condition MgO / As 2 O 3 < 0.8 with AS2O3 > 0% in weight is applicable.

13. Cooking surface (1) according to claim 3, characterized in that the glass ceramic plate includes the following components (in % in weight on oxide basis): Li2O3.2 - 4.5Al2O319 - 24SiO262 - 68Na2O0 - 1K2O0 - 1Na2O + K2O0.2 - 1.5 (condition B7a)MgO0.05 - < 0.5TiO21.8 - 2.8ZrO21 - < 2.2TiO2 + ZrO2 + SnO23.8 - 4.8 (condition B8)Fe2O30.007 - 0.02CaO0 - 1.5SrO0 - 1.5BaO0 - 2.5ZnO0 - 2.5P2O50 - 4.

14. Cooking surface (1) according to claim 3, characterized in that the glass ceramic plate includes the following components (in % in weight on oxide basis): Li2O3.2 - < 4.2Al2O320 - < 23SiO262 - 68Na2O0.1 - 1K2O0 - 1Na2O + K2O0.2 - 1.2 (condition B7b)MgO0.1 - 0.4CaO0.05 - 1SrO0 - 1.5BaO0 - 2.5SrO + BaO0.5 - 2.5TiO21.8 - 2.8ZrO21 - < 2.2SnO20.01 - < 0.10TiO2 + ZrO2 + SnO23.8 - 4.8 (condition B8)Fe2O30.008 - 0.02ZnO0 - 2.5B2O30 - 1P2O50 - 2.

15. Cooking surface (1) according to any of the preceding claims, characterized in that the bottom coating (8) is made up of two layers (9a, 9b), wherein a first coloring layer (9a) is applied to the bottom of the glass ceramic plate (8) and wherein a second layer (9b) is applied to the first layer (9a).