Flat glass composition.

Abstract

The invention relates to a flat glass composition comprising the following constituents, their sum representing at least 95% of the composition, the percentages being expressed by mass relative to the total mass of the composition: - from 60% to 75% of silicon dioxide (SiO2); - from 8% to 12% of sodium oxide (Na2O); - from 6% to 12% of calcium oxide (CaO); - from 6% to 10% of aluminum oxide (Al2O3); - from 0% to 5% of potassium oxide (K2O); - from 0% to 3.5% of magnesium oxide (MgO); in which the sum of the mass contents of CaO and MgO is from 6% to 12%. The invention also relates to a process for preparing a flat glass comprising a composition according to the invention, as well as a glass sheet comprising a composition according to the invention.

Description

Title of the invention: Flat glass composition. Technical field

[0001] The invention relates to a flat glass composition (or float glass), of the soda-lime-silica type, in particular for applications in the building or automotive fields. The present invention also relates to a method for manufacturing such flat glass, and its use as glazing. TECHNOLOGICAL BACKGROUND

[0002] The manufacturing processes for so-called float glass consist of melting a mixture of raw materials in a glass furnace at temperatures ranging from 1300 to 1700°C, then forming a ribbon of molten glass on a bath of molten tin. The ribbon is then slowly cooled, annealed and then cut.

[0003] As with any industrial process, there is an increased need to decarbonize flat glass production processes in order to limit their impact on the environment. Development efforts are therefore generally focused on the use of renewable energy or fuels, increasing the proportion of raw materials from recycling or industrial waste recovery channels in vitrifiable mixtures, or even energy recovery at the various stages of the process.

[0004] At the same time, float glass compositions are subject to standards in order to obtain a set of desired physical or mechanical properties and to define quality criteria. Thus, conventional glass products are of the soda-lime-silica type and have the following chemical composition, expressed by convention in oxides of the constituent elements (in mass percentages): - from 69% to 74% silicon dioxide (SiO2); - from 5% to 14% calcium oxide (CaO); - from 10% to 16% sodium oxide (Na2O); - from 0% to 6% magnesium oxide (MgO); - from 0% to 3% aluminum oxide (A12O3); - from 0% to 5% of other oxides (Fe2O3, K2O, ZnO, SrO, B2O3, etc.).

[0005] These standard compositions allow the production of flat glasses of satisfactory quality and durability (optical qualities, vitrification, refining, resistance to surface abrasion, surface durability to weathering, etc.) with controlled process constraints and production costs (including various aspects: minimized melting temperature, less corrosion of refractories, hardenability, etc.).

[0006] To reduce environmental impacts, it is proposed to reduce the energies required for melting raw materials. For example, document US 5,071,796 proposes flat glass compositions having lower melting temperatures. These compositions are notably obtained by slightly reducing the silica content and compensating for it with a higher alkali content (Na2O). However, the glasses obtained have a substantially higher thermal expansion coefficient (TEC) than standard float glasses (10.44 x 106 °C 1 compared to 8.62 x 106 T1), which may alter their thermal shock resistance properties.

[0007] Document WO 2014 / 128714 describes flat glass compositions of the soda-lime-silica type which make it possible to save energy during production and also to reduce carbon dioxide (CO2) emissions linked to the reaction of the raw materials. These compositions are obtained by substituting, in the raw materials used, sodium carbonate and limestone (sources of Na2O and CaO) with a pentahydrated borax (source of B2O3). They also make it possible to reduce the coefficient of thermal expansion and improve surface durability to weathering. However, the boron compounds used are harmful to health (classified as "CMR").

[0008] More generally, a change in composition by varying one or more constituents often has adverse effects on one or more other properties (such as hydrolytic resistance, high temperature viscosity, reduction of the temperature range in which the glass can be formed without devitrification, etc.). It is therefore complex to modify and formulate float glass compositions, allowing both to reduce environmental impacts and to satisfy the various production and performance criteria (including the conditions for good "floatability" and refining, durability of the glass, etc.).

[0009] Thus, alternative compositions are still being researched and there is still a need for compositions that can reduce environmental impacts without increasing production costs and without deteriorating the thermal, mechanical and durability properties of the glasses.

[0010] It is to the applicant's credit to propose a flat glass composition which, surprisingly, makes it possible to reduce the carbon footprint of the product, while satisfying both the process constraints and the quality and durability requirements. Summary of the invention

[0011] According to a first aspect, the invention relates to a flat glass composition comprising the following constituents, their sum representing at least 95% of the composition, the percentages being expressed in mass relative to the total mass of the composition: - 60% to 75% silicon dioxide (SiO2); - 8% to 12% sodium oxide (Na2O); - from 6% to 12% calcium oxide (CaO); - from 6% to 10% aluminum oxide (A12O3); - from 0% to 5% potassium oxide (K2O); - from 0% to 3.5% magnesium oxide (MgO); in which the sum of the mass contents of CaO and MgO is 6% to 12%.

[0012] The composition according to the invention has the advantage of allowing the use of a greater variety of alternative raw materials. Surprisingly, the inventors have demonstrated that, although the composition according to the invention has a higher refining temperature (Tiog2) than standard compositions, it allows a significant reduction in overall CO2 emissions, taking into account both emissions linked to the melting of raw materials (reactions of raw materials), emissions linked to the combustion of natural gas (energy required for the temperature increase) and other emissions linked to the use and production of synthetic raw materials.

[0013] The invention also relates, according to a second aspect, to a method for manufacturing a flat glass, comprising: - a step of melting a mixture of selected raw materials so as to obtain a target composition according to the invention; - a step of forming the molten mixture into a glass ribbon by floating.

[0014] According to another aspect, the invention also relates to a window comprising a composition according to the invention.

[0015] The invention relates to the use of a composition according to the invention as glazing, preferably for buildings. DETAILED DESCRIPTION

[0016] The general terms used in this text are defined below.

[0017] The expression “comprising” encompasses the expression “consisting of”.

[0018] The expression “from ... to ...” must be understood inclusively.

[0019] Unless explicitly stated, the term "free", within the meaning of the present invention, means a mass content of substance less than or equal to 0.1%, preferably less than or equal to 0.05% relative to the total mass of the composition.

[0020] The composition according to the invention comprises the following constituents, their sum representing at least 95% of the composition, the percentages being expressed by mass relative to the total mass of the composition: - 60% to 75% silicon dioxide (SiO2); - 8% to 12% sodium oxide (Na2O); - from 6% to 12% calcium oxide (CaO); - from 6% to 10% aluminum oxide (A12O3); - from 0% to 5% potassium oxide (K2O); - from 0% to 3.5% magnesium oxide (MgO); in which the sum of the mass contents of CaO and MgO is 6% to 12%.

[0021] It is understood that this is the composition expressed, by convention, in oxides of the constituent elements (SiO2, Na2O, CaO, K2O, Al2O3, Fe2O3, etc.). Indeed, glass is a substance of variable composition, resulting from complex reactions forming a random network. Although conventionally the compositions of glass are expressed in oxides of different elements, glass is not a mixture of these different oxides and does not contain these oxides as such.

[0022] In the composition according to the invention, the sum of the mass contents of SiO2, Na2 O, CaO, Al2O3, K2O and MgO represents at least 95%, relative to the total mass of the composition. More preferably, the sum of the mass contents of SiO2, Na2 O, CaO, Al2O3, K2O and MgO represents at least 98%, relative to the total mass of the composition.

[0023] The composition according to the invention comprises 60% to 75% of SiO2. Silicon dioxide (SiO2), also called silica herein, is the main network-forming element in glass. Too low contents lead to a deterioration in the hydrolytic resistance of the glass, particularly in a basic medium. On the other hand, contents above 75% lead to too great an increase in the viscosity of the glass, which is problematic for melting the mixture of raw materials and for forming the glass. The silica can be provided by raw materials such as sand, cullet, feldspar and / or wollastonite. Preferably, in the composition according to the invention, the mass content of SiO2 is at most 73%, more preferably at most 71%, relative to the total mass of the composition. Preferably, in the composition according to the invention, the mass content of SiO2 is from 65% to 73%, more preferably from 65% to 71%, relative to the total mass of the composition..

[0024] The composition according to the invention comprises 6% to 10% of A12O3. Aluminum oxide (A12O3) is also a network-forming element in glass. According to ISO 16293, applicable to building glazing, the standard content of A12O3 is 0% to 3%. Going against this prejudice, the inventors have developed a glass composition compatible with the float process which, surprisingly, makes it possible to significantly reduce the carbon footprint of the glass produced. while maintaining satisfactory properties in terms of quality, durability and processability. Aluminum oxide can in particular be provided by raw materials such as aluminosilicates, in particular feldspars. Preferably, in the composition according to the invention, the mass content of Al2O3 is 7% to 10%, more preferably 8% to 10%, even more preferably 8% to 9%, relative to the total mass of the composition.

[0025] Alkali oxides (Na2O and K2O) are network modifiers. They make it possible to reduce the high-temperature viscosity of the molten glass composition but can also have a negative impact on the hydrolytic resistance of the glass. These elements can, for example, be provided by raw materials such as sodium carbonate, cullet, aluminosilicates, in particular feldspars. The composition according to the invention comprises 8% to 12% of Na2O. Preferably, the mass content of Na2O is 8% to 11%, more preferably 9% to 11%, relative to the total mass of the composition. The composition according to the invention comprises 0% to 5% of K2O. Preferably, in the composition according to the invention, the mass content of K2O is from 0.1% to 5%, more preferably from 0.5% to 5%, even more preferably from 1% to 4%, relative to the total mass of the composition.

[0026] Alkaline earth oxides (CaO and MgO) are network modifiers. They make it possible to reduce the high temperature viscosity of the molten glass composition but can also increase the liquidus temperature and / or increase the risks of devitrification. These elements can for example be provided in the mixture of raw materials by limestone, wollastonite, talc, lime, cullet and / or dolomite. The composition according to the invention comprises 6% to 12% of CaO. Preferably, the mass content of CaO is at most 11%, preferably at most 10%, even more preferably at most 9%, relative to the total mass of the composition. Preferably, the mass content of CaO is from 6% to 11%, more preferably from 7% to 10%, even more preferably from 7% to 9%, relative to the total mass of the composition. The composition according to the invention comprises 0% to 3.5% of MgO.Preferably, the mass content of MgO is at most 3%, preferably at most 2.5%, even more preferably at most 2%, relative to the total mass of the composition. Preferably, the mass content of MgO is from 0.1% to 3.5%, more preferably from 0.1% to 3%, more preferably from 0.5% to 2.5%, even more preferably from 1% to 2%, relative to the total mass of the composition.

[0027] Preferably, the sum of the mass contents of CaO and MgO is at most 11%, preferably at most 10%, more preferably at most 9%, relative to the total mass of the composition. Preferably, the sum of the mass contents of CaO and MgO is 6% to 11%, more preferably 7% to 10%, better still 7% to 9% relative to the total mass of the composition. This makes it possible to optimize the carbon footprint while maintaining or even improving the forming properties (in particular the buoyancy criterion) and the durability of the glass.

[0028] Preferably, the sum of the mass contents of Na2O and K2O is less than 15%, relative to the total mass of the composition.

[0029] Advantageously, in the composition according to the invention, the sum of the mass contents of Na2O and CaO is from 14% to 20%, preferably from 14% to 19%, relative to the total mass of the composition. This makes it possible to obtain good properties, particularly in terms of durability of the glass.

[0030] Advantageously, in the composition according to the invention, the ratio R of the sum of the mass contents of SiO2 and Al2O3 to the sum of the mass contents of CaO, MgO, Na2O and K2O, is 3 to 4. This makes it possible to maintain good properties for the forming and durability of the glass.

[0031] Preferably, the composition according to the invention is free of boron oxide (B2O3).

[0032] Preferably, the composition according to the invention is free of lithium oxide (Li2O), particularly for cost reasons.

[0033] Preferably, the composition according to the invention is free of ZnO and / or ZrO2.

[0034] Preferably, the composition according to the invention comprises a mass content of BaO and / or SrO of at most 1%, preferably at most 0.5%, more preferably at most 0.2%, relative to the total mass of the composition. In other words, the sum of the BaO and SrO contents is less than or equal to these percentages. More preferably, the composition according to the invention is free of BaO and / or SrO, in particular for cost reasons.

[0035] Preferably, the composition according to the invention comprises a mass content of Fe2O3 of at most 2%, preferably at most 1%, more preferably at most 0.6%, relative to the total mass of the composition.

[0036] The composition according to the invention may optionally contain other impurities, such as for example sulfur oxide (SO3), often in low levels (for example less than 1%). The SO3 element may in particular come from refining agents (such as sodium sulfate).

[0037] According to a preferred embodiment, the composition according to the invention comprises the following constituents, the percentages being expressed by mass relative to the total mass of the composition: - from 60% to 71% SiO2; - from 8% to 11% of Na2O; - from 6% to 11% CaO, preferably from 6% to 10% CaO; - from 6% to 10% of A12O3; - from 0% to 5% K2O; - from 0% to 3% of MgO; and the sum of the mass contents of CaO and MgO being 6 to 11%. This composition may further comprise one or more of the characteristics previously described.

[0038] According to another preferred embodiment, the composition according to the invention comprises the following constituents, the percentages being expressed by mass relative to the total mass of the composition: - from 60% to 71% of SiO2, preferably from 65% to 71% of SiO2; - from 8% to 11% of Na2O; - from 6% to 10% CaO; - from 6% to 10% of A12O3; - from 0% to 5% K2O; - from 0% to 3% of MgO; and the sum of the mass contents of CaO and MgO being 6% to 10%. This composition may further comprise one or more of the characteristics described above.

[0039] The composition according to the invention has the advantage of being able to be melted and transformed into a glass ribbon at temperatures which can be used for the manufacture of flat glass, in particular according to the float process. In addition, it has very good properties, in particular an improved service life. The composition according to the invention may also have other advantageous properties in terms of process stability.

[0040] The composition according to the invention may have a refining temperature (Tiog2 ) greater than 1450°C, or even greater than 1500°C. Preferably, the refining temperature (Tiog2) is less than 1700°C, more preferably less than 1600°C. The refining temperature (Tiog2) represents the temperature at which the molten glass composition has a viscosity of 100 poises (making it easier to refining the molten glass). Thus, the compositions according to the invention have a higher refining temperature than standard compositions.

[0041] The composition according to the invention may have a liquidus temperature (Tiiq) of at most 1200°C, for example from 1000°C to 1150°C. The liquidus temperature (Tiiq) represents the temperature from which the mixture is entirely liquid (no more coexistence of liquid and solid forms).

[0042] Advantageously, the composition according to the invention has a forming margin (AT=Tiog3,5-Tiiq), greater than 50°C, preferably greater than 80°C, more preferably greater than 100°C. The temperature Tiog3.5, is the temperature at which the molten glass composition reaches a viscosity v such that log v=3.5 (viscosity of 3160 Poises) and represents the temperature beyond which the viscosity is too low to operate the forming of the glass. Thus the forming margin AT corresponds to the temperature zone where the molten mixture can be "formed" and can spread sufficiently on the tin bath. On an industrial level, it is generally preferred to use compositions which have an AT greater than 20°C, or even greater than 50°C (floatability criterion) in order to limit the risks of crystallization or devitrification during forming on the float. Against all expectations, the compositions according to the invention have a particularly advantageous forming margin AT.

[0043] The composition according to the invention may have a lower annealing temperature (Tiogi4.5) of at least 500°C, for example from 500°C to 600°C. The lower annealing temperature (Tiogi45) represents the temperature below which the glass no longer has viscous behavior (viscosity of the order of 1014.5 Poises) and makes it possible to evaluate the temperature resistance of a glass.

[0044] The compositions according to the invention allow the use of a greater diversity of raw materials, including those which contain, among others, the element aluminum. The compositions according to the invention have higher liquidus and refining temperatures than standard compositions. Despite this, and unexpectedly, the compositions according to the invention are not only compatible with the process for producing flat glass by float but also allow a significant reduction in overall CO2 emissions by allowing more diversified raw materials. Indeed, without wishing to be bound by any theory, it is observed that even if the compositions require a higher furnace temperature, the mixture of raw materials "melts" better while requiring less energy.

[0045] The present invention also relates to a method for manufacturing a flat glass, comprising: - a step of melting a mixture of selected raw materials so as to obtain a target composition according to the invention; - a step of forming the molten mixture into a glass ribbon by floating.

[0046] The mixture of raw materials is prepared according to known techniques so as to result in a target composition according to the invention. The raw materials may be selected, for example, from synthetic mineral materials and / or natural mineral materials. In the case in particular of natural mineral materials, the constituent elements may be assayed according to appropriate techniques (for example chemical analysis, X-ray diffraction, etc.) and, on the basis of this initial determination of the composition of the mineral matter, the necessary proportions of components (silica, carbonate, limestone, etc.) are calculated and adjusted to arrive at the target composition. Examples of raw materials that can be used include sand, wollastonite, aluminosilicates, sodium carbonate, limestone, dolomite, lime, talc, cullet, etc. According to an advantageous embodiment of the process, the mixture of selected raw materials comprises aluminosilicates, for example sodium, potassium and / or calcium.

[0047] The mixture of raw materials is then heated, generally in a furnace, until a molten glass is obtained. The heating is carried out at a higher or lower temperature and for a longer or shorter time depending on the quality of the glass required, in particular depending on the degree of tolerance for unmelted particles (called "unmelted") and bubbles. Generally, the maximum heating temperature of the molten glass is between 1200 and 1700°C. For the transformation of the mixture of raw materials into molten glass, glass melting techniques well known to those skilled in the art can be used. This transformation can be carried out in any type of furnace such as an electric electrode furnace, an overhead burner furnace such as a transverse burner furnace or a loop furnace.

[0048] The molten glass is then introduced onto a bath of molten tin to form a glass ribbon (the well-known "foat" process). The glass ribbon can then be annealed using an annealing furnace.

[0049] The present invention also relates to a mixture of raw materials selected so as to obtain a target composition according to the invention. Preferably, the mixture of raw materials comprises aluminosilicates, in particular sodium, potassium and / or calcium.

[0050] According to another aspect, the invention relates to a glass sheet having a composition according to the invention.

[0051] The present invention also relates to a glazing comprising a glass sheet according to the invention. The present invention also relates to the use of a glazing according to the invention for buildings or automobiles, preferably for buildings. Examples

[0052] The invention is illustrated using the non-limiting examples below where two compositions are compared, in particular in terms of CO2 emissions: - a known comparative composition “A”, representative of a classic “float” glass composition - a composition “B” according to the invention. The element contents of these compositions are detailed in Table 1.

[0053] The compositions are made by melting the raw materials in a furnace and glass plates are obtained by cutting after forming the mixture of materials on a tin bath according to the well-known float process.

[0054] The properties of the compositions and glasses obtained are also evaluated and listed in Table 1 below (liquidus temperatures, forming margin, etc.). [Tables 1] Composition: A (comparative) B (invention) SiO2 72.7 69.2 Na2O 14.0 10.7 CaO 10.4 7.2 A12O2 0.6 8.3 k2o - 3.0 MgO 1.9 1.4 Properties: Tlog2 (in °C, 1522 °C) (en °C, + 5°C) 554 582 Thiogl4.5 (°C) 507 532 Tliq (en °C, + 5°C) 1000 1070 AT = Tlog3.5 - Tliq (°C) 90 145 TEC (107 K1) 90.07+8.5, DG +15 (mg) Na2O : 9.1+0.3 Residue : 27 + 1.4 Na2O : 2.3 + 0.3 Residue : 10.1 + 1.4

[0055] Composition B according to the invention has a refining temperature (Tiog2) substantially higher than standard composition A (1562°C versus 1422°C). But this remains acceptable on an industrial level. As regards the glass transition temperature (Tg) and the lower annealing temperature (Tiog4.5), they remain comparable to those of standard composition A. Advantageously, and surprisingly, composition B according to the invention has a particularly wide AT forming margin (of 145°C). In addition, composition B according to the invention shows a good coefficient of thermal expansion (TEC).

[0056] Hydrolytic resistance is also evaluated using the DGG (Deutsche Glass Gesellschaft) method. This method applies to all types of glass and allows to give, in addition to the quantity of alkalis dissolved in water, the mass of dry residue. This method consists of immersing 10 grams of crushed glass grains (previously sieved to obtain a particle size of 355 to 400 micrometers) in beakers of demineralized water. The whole is brought to a boil for 5 hours using an oil bath. After rapid cooling, the solution is filtered and a determined volume of the filtrate is evaporated to dryness. The weight of the dry matter obtained allows to calculate the quantity of glass dissolved in water (in milligrams, per gram of glass tested). The amount of alkali is also measured by titration using a 0.01 mol / L hydrochloric acid solution (the color indicator being methyl red at 2g / L in 60% ethanol).The results show that the composition according to the invention has very good hydrolytic resistance.

[0057] To obtain each of the two compositions A and B detailed above, two mixtures of raw materials were used, in order to evaluate the impact of the compositions according to the invention on CO2 emissions (whatever the raw materials used): - two mixtures Al and B1 are made with the same types of raw materials called “carbonated”, the proportions of which are adapted so as to obtain the compositions according to A and B respectively; - two other mixtures A2 and B2 are made with the same types of raw materials called “decarbonated”, with the proportions allowing the target compositions A and B to also be obtained. These raw material mixes are detailed in Table 2 below. [Tables 2] Raw materials (in kg / ton of glass) Al B1 A2 B2 Sand 703 370 557 270 Sodium feldspar 34 - 28 - Mixed feldspar - 469 - 465 Sodium carbonate 227 131 228 131 Dolomite 89 68 - - Talc - - 56 43 Limestone 136 85 - - Wollastonite - - 226 149 Sodium sulfate 11 11 11 11 CO2 emissions: (in kg / tonne of glass) - linked to the melting of raw materials 199 126 100 60 - linked to the energy of the furnace (combustion of natural gas: 202 kgC02 / MWh) 283 286 283 291 - linked to the raw material Na2CO3 (11 00 kgco2 / tonne of Na2CO3) 251 144 252 145 Overall balance 733 556 635 495

[0058] The CO2 emissions are calculated for each composition and each of the raw material mixtures. The results are listed in Table 2. They show that the composition according to the invention allows a reduction in overall CO2 emissions of 24% in the first case of mixtures of "carbonated" raw materials. A significant reduction in emissions is also observed in the second case of mixtures of "decarbonated" raw materials (reduction of 22%). Thus, regardless of the raw materials used ("decarbonated" or not), the composition according to the invention allows a significant reduction in overall carbon dioxide emissions, which was not obvious in view of its higher Tiog2 refining temperature. Advantageously, by combining the composition according to the invention produced with the "decarbonated" raw materials, a reduction in overall emissions of 32% is even achieved compared to the reference composition.

[0059] In conclusion, the particular composition according to the invention not only allows a significant reduction in the carbon footprint of the product, but also to satisfy the quality and durability requirements of flat glass by combining all the desired properties (such as a melting temperature usable for the float process, a wide forming margin, a good coefficient of thermal expansion, very good surface durability, etc.).

Claims

Claims

1. A flat glass composition comprising the following constituents, their sum representing at least 95% of the composition, the percentages being expressed by mass relative to the total mass of the composition: - from 60% to 75% of silicon dioxide (SiO2); - from 8% to 12% of sodium oxide (Na2O); - from 6% to 12% of calcium oxide (CaO); - from 6% to 10% of aluminum oxide (A12O3); - from 0% to 5% of potassium oxide (K2O); - from 0% to 3.5% of magnesium oxide (MgO); in which the sum of the mass contents of CaO and MgO is from 6 to 12%.

2. Composition according to claim 1, in which the sum of the mass contents of SiO2, Na2O, CaO, Al2O3, K2O and MgO represents at least 98%, relative to the total mass of the composition.

3. Composition according to any one of the preceding claims, in which the sum of the mass contents of CaO and MgO is at most 11%, preferably at most 10%, more preferably at most 9%, relative to the total mass of the composition.

4. Composition according to any one of the preceding claims, in which the sum of the mass contents of Na2O and K2O is less than 15%, relative to the total mass of the composition.

5. Composition according to any one of the preceding claims, in which the sum of the mass contents of Na2O and CaO is from 14% to 20%, preferably from 14% to 19%, relative to the total mass of the composition.

6. Composition according to any one of the preceding claims, in which the ratio R of the sum of the mass contents of SiO2 and Al2O3 to the sum of the mass contents of CaO, MgO, Na2 O and K2O, is 3 to 4.

7. A composition according to any preceding claim, the composition being free of boron oxide (B2O3) and zinc oxide (ZnO).

8. A composition according to any preceding claim, the composition being free of lithium oxide (Li2O).

9. Composition according to any one of the preceding claims, the composition comprising a mass content of BaO and / or SrO of at most 1%, preferably at most 0.5%, more preferably at most 0.2%, relative to the total mass of the composition.

10. Composition according to any one of the preceding claims, the composition comprising a mass content of Fe2O3 of at most 2%, preferably at most 1%, more preferably at most 0.6%, relative to the total mass of the composition.

11. Composition according to any one of the preceding claims, the composition having a forming margin AT=Tiog3j5-TUq greater than 50°C, preferably greater than 80°C, more preferably greater than 100°C.

12. A method of manufacturing a flat glass, comprising: - a step of melting a mixture of raw materials selected so as to obtain a target composition according to any one of claims 1 to 11; and - a step of forming the molten mixture into a glass ribbon by float.

13. Mixture of raw materials selected so as to obtain a target composition according to any one of claims 1 to 11, comprising aluminosilicates, preferably sodium, potassium and / or calcium.

14. A glass sheet having a composition according to any one of claims 1 to 11.

15. Glazing comprising a glass sheet according to claim 14.