Glass composition

Abstract

The invention relates to a glass composition, in particular a flat glass composition, comprising the following constituents in percentages by weight relative to the total weight of the composition: - from 60% to 70% SiO2; - from 2% to 18% Na2O; - from 8% to 28% CaO; - from 1% to 9% MgO; - from 0% to 2% Al2O3; - from 0% to 0.5% K2O, wherein the sum of the contents of CaO and MgO is from 14% to 32%, preferably with a sum of the contents by weight of Na2O, CaO and MgO ranging from 30 to 37%, and an MgO / (Na2O+CaO+MgO) ratio of greater than 0.16. The invention also relates to a method for preparing a flat glass comprising a composition according to the invention, and also to a glass sheet having a composition according to the invention.

Description

Description Title of the invention: Glass composition. TECHNICAL FIELD [1] The invention relates to a glass composition of the soda-lime-silica type, in particular flat glass composition, as well as to its manufacturing process and its use as a glazing. TECHNOLOGICAL BACKGROUND [2] Processes for manufacturing so-called float flat glass consist in melting a batch of raw materials in a glass furnace at temperatures that may reach 1550°C, and then forming a ribbon of molten glass on a bath of molten tin. The ribbon is then slowly cooled, annealed and cut. [3] As with any industrial process, there is a growing 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 energies or fuels, increasing the proportion of raw materials derived from recycling streams or industrial waste recovery in vitrifiable mixtures, or on energy recovery at the various stages of the process. [4] Flat glass generally has a chemical composition comprising (in percentages by weight relative to the weight of the composition):- from 69% to 74% silicon dioxide (SiO2);- from 10% to 16% sodium oxide (Na2O);- from 5% to 14% calcium oxide (CaO);- from 0% to 6% magnesium oxide (MgO);- from 0% to 3% aluminum oxide (Al2O3);- from 0% to 5% other oxides (Fe2O3, K2O, ZnO, SrO, B2O3, etc.). [5] These compositions are favored by manufacturers to ensure both the quality and durability of the finished product and the stability of the process. In particular, it is commonly accepted that the content of alkaline-earth oxides, particularly of CaO, must be kept relatively low in order to satisfy the floatability criterion (the working range ΔT=Tlog3.5-Tliq must be greater than 20°C, or even greater than 50°C) and to avoid the risk of devitrification. In particular, document US 5,071,796 describes flat glass compositions comprising a carefully controlled alkaline earth oxide (CaO + MgO) content (from 10.2 to 12% by weight), in view of the negative effect of calcium oxide on the working range. [6] Going against these preconceptions, the inventors have developed a soda-lime-silica glass composition that makes it possible to reduce the energy consumption required for its manufacture, while satisfying both the quality and durability requirements of the finished product, in particular in terms of hydrolytic resistance, and the industrial requirements, notably with regard to the risks of devitrification and with regard to fining capability. Thus, it is to the credit of the applicant to propose a glass composition, in particular a flat glass composition, which, surprisingly, solves all these problems. SUMMARY OF THE INVENTION [7] According to a first aspect, the present invention relates to a glass composition, in particular a flat glass composition, comprising the following constituents in percentages by weight relative to the weight of the composition:- from 60% to 70% SiO2;- from 2% to 18% Na2O;- from 8% to 28% CaO;- from 1% to 9% MgO;- from 0% to 2% Al2O3;- from 0% to 0.5% K2Owherein the sum of the contents of CaO and MgO is from 14 to 32%. In addition, the sum of the contents by weight of Na2O, CaO and MgO is preferably from 30 to 37%, and the MgO / (Na2O+CaO+MgO) ratio is preferably greater than 0.16. [8] It has in fact been demonstrated that these compositions have a viscosity range advantageous for floating over a lower temperature range than standard float glass, without the risk of devitrification and with a refining capability equivalent to that of standard soda-lime-silica glass, thus allowing significant energy savings. Indeed, surprisingly, the inventors found that the specific composition according to the invention did not exhibit a minimum quenching rate (or "critical quenching rate"). In other words, the composition according to the invention does not devitrify, regardless of the cooling rate of the glass. The Applicant thus proposes glass compositions which, despite relatively high liquidus temperatures and unfavorable working ranges, are compatible with industrial requirements in view of devitrification risks, contrary to preconceived ideas, and enable energy savings during manufacture. [9] According to a second aspect, the invention also relates to a method for manufacturing a glass, in particular 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. 

[10] According to other aspects, the invention also relates to a glass sheet having a composition according to the invention, a glazing comprising such a glass sheet according to the invention, and the use of such a glazing for building or motor vehicles. DETAILED DESCRIPTION 

[11] The general terms used in the present text are defined below. 

[12] The expression "comprising" encompasses the expression "consisting of". 

[13] The expression "from ... to ..." is to be understood as including the endpoints. 

[14] Unless expressly stated otherwise, "free of", within the meaning of the present invention, means a substance content by weight of less than or equal to 0.1%, preferably less than or equal to 0.05%, relative to the total weight of the composition. 

[15] The composition according to the invention is of the soda-lime-silica type and comprises the following constituents, in percentages by weight relative to the total weight of the composition:- from 60% to 70% SiO2;- from 2% to 18% Na2O;- from 8% to 28% CaO;- from 1% to 9% MgO;- from 0% to 2% Al2O3;- from 0% to 0.5% K2Owherein the sum of the contents of CaO and MgO is from 14 to 32%. 

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

[17] Advantageously, the composition according to the invention is a flat glass composition. In other words, it is compatible with industrial flat glass manufacturing methods, in particular float processes. 

[18] Advantageously, the MgO / (Na2O+CaO+MgO) ratio is greater than 0.16, for example from 0.16 to 0.21. Surprisingly, compositions having the particular combination of features according to the invention have a lower fining temperature (Tlog2) than standard compositions (the composition being globally more fluid over the working temperature range) without increasing the risk of devitrification (that is to say the formation of crystals, which disrupt the amorphous structure of the glass and can alter its optical and mechanical properties). Generally, a lower viscosity promotes nucleation and crystal growth. Thus, without wishing to be bound by any theory, the specific composition according to the invention appears to be unfavourable to crystal formation. 

[19] Advantageously, in the composition according to the invention, the sum of the contents by weight of SiO2, Na2O, CaO and MgO represents at least 95% by weight, preferably at least 98% by weight, relative to the total weight of the composition. 

[20] Preferably, in the composition according to the invention, the sum of the contents by weight of SiO2, Na2O, CaO, Al2O3, K2O and MgO represents at least 95%, preferably at least 98% by weight, more preferably at least 99% by weight, relative to the total weight of the composition. 

[21] Silicon dioxide (SiO2) is the main network-forming element in glass. Excessively low contents lead to a deterioration in the hydrolytic resistance of glass, particularly in basic medium. Conversely, high contents lead to excessive increase in glass viscosity, which is problematic for the melting of the mixture of raw material and for glass forming. The composition according to the invention comprises 60% to 70% by weight of SiO2. Preferably, the SiO2 content by weight is from 60% to 68%, more preferably from 62% to 67%, relative to the total weight of the composition. 

[22] Aluminium oxide (Al2O3) may be present in the composition according to the invention in a content of 0% to 2% by weight, relative to the weight of the composition. Preferably, the Al2O3 content is 0.5% to 2% by weight. 

[23] Alkaline oxides (e.g. Na2O and K2O) are network modifiers. They allow to reduce the high-temperature viscosity of the molten glass composition, but may also adversely affect the hydrolytic resistance of the glass. The composition according to the invention comprises 2% to 18% by weight of Na2O. Preferably, the Na2O content by weight is 2% to 16%, more preferably 3% to 14%, relative to the total weight of the composition. The composition according to the invention comprises 0% to 0.5% by weight of K2O. Preferably, the K2O content by weight is 0.05% to 0.5%, relative to the total weight of the composition. Preferably, the composition according to the invention is free of lithium oxide (Li2O). This oxide is generally used to facilitate glass melting and / or improve the mechanical properties of said glass. However, besides its scarcity and high cost, it tends to evaporate at high temperature, which may complicate precise control of the glass composition during melting. 

[24] Alkaline-earth oxides (e.g. CaO and MgO) are network modifiers. They allow to reduce the high-temperature viscosity of the molten glass composition, but may also increase the liquidus temperature and / or increase the risk of devitrification. The composition according to the invention comprises 8% to 28% by weight of CaO. Preferably, the CaO content by weight is at least 9%, more preferably at least 12%, even more preferably at least 14%, relative to the total weight of the composition. Preferably, the CaO content by weight is from 9% to 28%, more preferably from 12% to 25%, most preferably from 14% to 25%, relative to the total weight of the composition. The composition according to the invention comprises from 1% to 9% by weight of MgO, preferably from 1% to 8% by weight of MgO, relative to the total weight of the composition. More particularly, the MgO content by weight may be from 5% to 9%, preferably from 5% to 8%, relative to the total weight of the composition. 

[25] The sum of the CaO and MgO contents in the composition according to the invention is from 14 to 32%, relative to the total weight of the composition. Preferably, the sum of the contents by weight of CaO and MgO is from 16% to 32%, more preferably from 18% to 30%, relative to the total weight of the composition. This lowers the mixture viscosity at high temperatures, despite the increase in liquidus temperature, while maintaining good properties for glass fining and forming. 

[26] Preferably, the sum of the contents by weight of Na2O, CaO and MgO is from 25 to 40%, more preferably from 30% to 37%, relative to the total weight of the composition. 

[27] Advantageously, in the composition according to the invention, the ratio R of the sum of the SiO2 and Al2O3 contents by weight to the sum of the CaO, MgO and Na2O contents by weight is from 1.5 to 2.5, preferably from 1.7 to 2.1. This makes it possible to combine good glass properties. 

[28] Preferably, the composition according to the invention comprises a boron oxide (B2O3) content by weight of less than 1%, preferably less than 0.5%, relative to the total weight of the composition. More preferably, the composition according to the invention is free of boron oxide (B2O3). This oxide is generally added to reduce the melting temperature of the glass or to improve its thermal resistance. However, depending on the other oxides present and their content, it may modify the viscosity of the mixture or promote crystal formation (devitrification). Moreover, the use of boron may raise environmental concerns. 

[29] Preferably, the composition according to the invention is free of ZnO. This oxide is generally added for its beneficial effects on glass hardness, chemical resistance and optical properties. However, depending on the other oxides present and their content, it may modify the viscosity of the mixture or promote crystallization (devitrification), which may lead to a loss of transparency and / or affect the mechanical properties of the glass. 

[30] Preferably, the composition according to the invention is free of ZrO2. This oxide is often used to improve properties such as mechanical strength and glass hardness. However, this highly refractory oxide is dense and more difficult to melt, which increases the risk of unmelted material and / or the energy required for melting. Moreover, ZrO2-based glasses are more difficult to recycle. 

[31] Preferably, the composition according to the invention comprises a phosphorous oxide (P2O5) content by weight of less than 1%, preferably less than 0.5%, relative to the total weight of the composition. More preferably, the composition according to the invention is free of P2O5. This oxide generally reduces the glass working temperature at which the glass is processed, and saves energy. However, it may promote the formation of undesirable crystals, which may affect the transparency and mechanical properties of the glass. 

[32] Preferably, the composition according to the invention comprises a BaO content by weight of less than 1%, preferably less than 0.5%, relative to the total weight of the composition. Advantageously, the composition according to the invention is free of BaO. This oxide generally lowers viscosity, as do CaO and MgO, but has less impact on increasing liquidus temperature. However, barium may be toxic in certain forms, and requires precautions during glass manufacture and recycling. 

[33] Preferably, the composition according to the invention comprises a SrO content by weight of less than 1%, preferably less than 0.5%, relative to the total weight of the composition. Advantageously, the composition according to the invention is free of SrO. This oxide generally lowers viscosity, like CaO and MgO, but has less effect on increasing the liquidus temperature. However, besides its high cost and scarcity, it increases glass density and viscosity, and may also cause problems of homogeneity. 

[34] Preferably, the composition according to the invention comprises a titanium oxide (TiO2) content by weight of less than 0.5%, preferably less than 0.2%, relative to the total weight of the composition. More preferably, the composition according to the invention is free of TiO2. This oxide is generally used to reinforce the glass structure (scratch and wear resistance). However, it promotes nucleation and formation of undesirable crystals, which may affect the transparency and optical properties of the glass. 

[35] Preferably, the composition according to the invention comprises a SnO2 content by weight of less than 1%, preferably less than 0.5%, relative to the total weight of the composition. More preferably, the composition according to the invention is free of SnO2. This oxide may influence several glass properties, such as transparency, chemical resistance and electrical conductivity. However, in addition to its high cost, it increases glass viscosity and may tend to induce crystallization, defect formation or heterogeneity problems. 

[36] Preferably, in the composition according to the invention, the sum of the contents by weight of B2O3, SrO, BaO, ZrO2, TiO2, ZnO and SnO2 is less than 1%, preferably less than 0.5%, more preferably less than 0.2%, relative to the total weight of the composition. The particular composition according to the invention allows good properties to be obtained without requiring substantial contents of such oxides. 

[37] The composition according to the invention may comprise a Fe2O3 content by weight of up to 2%, relative to the total weight of the composition. This could be the case for automotive applications, for example. Preferably, the composition according to the invention comprises a Fe2O3 content by weight of at most 1%, preferably at most 0.5%, more preferably at most 0.2%, relative to the total weight of the composition. For the purposes of this invention, "Fe2O3 content" is taken to mean the total iron oxide content, in particular including iron(II) and iron(III) oxides. 

[38] The composition according to the invention may optionally contain other impurities, such as sulfur oxide (SO3), often in low contents (e.g. less than 0.1%). The SO3 element may notably originate from fining agents (such as sodium sulfate). 

[39] According to an advantageous embodiment, the composition according to the invention comprises the following constituents, in percentages by weight relative to the total weight of the composition:- from 60% to 68% SiO2;- from 2% to 18% Na2O;- from 9% to 28% CaO;- from 1% to 8% MgO;- from 0.5% to 2% Al2O3;- from 0% to 0.5% K2Owherein the sum of the contents of CaO and MgO is from 16 to 32%, preferably from 18% to 30%. This composition may further comprise one or more of the features previously described. 

[40] According to another advantageous embodiment, the composition according to the invention comprises the following constituents, in percentages by weight relative to the total weight of the composition:- from 62% to 67% SiO2;- from 2% to 16% Na2O;- from 12% to 25% CaO;- from 1% to 8% MgO;- from 0.5% to 2% Al2O3;- from 0% to 0.5% K2Owherein the sum of the contents of CaO and MgO is from 16 to 32%, preferably from 18% to 30%. This composition may further comprise one or more of the features previously described. 

[41] The composition according to the invention has the advantage of being able to be melted and transformed into a glass ribbon at temperatures lower than those usually used for the manufacture of standard flat glass, in particular by the float method. Additionally, it can also be refined at lower temperatures (below 1400°C), resulting in high optical-quality glass, without unmelted particles and / or bubbles and without risk of devitrification. The composition according to the invention may also have other advantageous properties in terms of processability. 

[42] The composition according to the invention may have a liquidus temperature (Tliq) greater than 1050°C, preferably greater than 1060°C, for example from 1060°C to 1350°C. The liquidus temperature (Tliq) represents the temperature above which the mixture is completely liquid (no more coexistence of liquid and solid forms). Thus, the compositions according to the invention have a higher liquidus temperature than standard compositions. 

[43] The composition according to the invention can have a fining temperature (Tlog2) below 1400°C, preferably below 1380°C. Preferably, the fining temperature (Tlog2) is from 1100°C to 1400°C, more preferably from 1100°C to 1380°C, and even more preferably from 1100°C to 1370°C. The fining temperature (Tlog2) represents the temperature at which the molten glass composition has a viscosity of 100 poise (to facilitate fining of the molten glass). 

[44] The composition according to the invention may have a working range (ΔT=Tlog3.5-Tliq) of less than 20°C, for example less than 10°C. The temperature Tlog3.5 is the temperature at which the molten glass composition reaches a viscosity v such that log v=3.5 (viscosity of 3160 poise) and represents the temperature beyond which the viscosity is too low to carry out glass forming. Thus, the working range ΔT corresponds to the temperature zone where the molten mixture can be "formed" and can spread sufficiently over the tin bath. From an industrial standpoint, compositions having a ΔT greater than 50°C are generally preferred in order to limit the risk of crystallization during forming on the float bath. Unexpectedly, et despite a low working range ΔT, the compositions according to the invention present no risk of crystallization and are compatible with the method for producing flat glass by a float process. They also reduce energy consumption by taking advantage of a lower fining temperature. 

[45] 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. 

[46] The mixture of raw materials is prepared according to known techniques in order to obtain a target composition according to the invention. The raw materials may be selected, for example, from synthetic mineral materials and / or natural mineral materials. Examples of raw materials that may be used include sand, feldspar, dolomite, talc, wollastonite, sodium carbonate, sodium hydroxide, limestone, sodium sulfate, etc. 

[47] The mixture of raw materials is then heated, generally in a furnace, until a molten glass is obtained. The temperature and duration of heating vary according to the quality of the glass required, and in particular according to the degree of tolerance in terms of unmelted particles (referred to as "unmelted inclusions") and bubbles. Advantageously, in the case of compositions according to the invention, the furnace operating temperature can be below 1400°C, for example between 1200°C and 1400°C. Well-known glass melting techniques may be used to transform the mixture of raw material into molten glass. 

[48] The molten glass mixture is then introduced onto a bath of molten tin to form a glass ribbon (the well-known "float" process). The glass ribbon may then be annealed in an annealing furnace. 

[49] Advantageously, in the method according to the invention, the melting step is carried out at a temperature below 1400°C, preferably at a temperature below 1380°C. 

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

[51] The invention also relates to a glazing comprising a glass sheet according to the invention. 

[52] The invention also relates to the use of a glazing according to the invention for building or automotive applications, preferably for building applications. Examples 

[53] The invention is illustrated by the non-limiting examples below. The compositions according to the invention I1 to I3 and the comparative composition C1 were prepared from the following industrial-grade raw materials: sand, sodium carbonate, limestone, dolomite and cullet. These compositions are detailed in Table 1 below. Unless otherwise indicated, the indicated percentages are percentages by weight. [Table 1] Composition(in %)C1I1I2I3SiO272.764.366.068.7Al2O30.720.240.941.0Na2O13.613.116.96CaO1015.29.519.0MgO2.47.46.65.0K2O-0.080.270.13Properties:    Tliq (in °C, ± 5°C)1021114010701353Tlog2 (in °C, ± 5°C)1451132113731378ΔT = Tlog3.5 - Tliq (°C)> 50°C-57°C+9°C-214°CDGG (mg)33.8933.733.5not determinedOptical qualitiesat 1450°C: no defect at 1370°C: presence of a large number of defects (bubbles and unmelted inclusions)at Tlog2: no defects (with and without fining agents)at Tlog2: no defects (with and without fining agents)at Tlog2: no defects (with and without fining agents) 

[54] The compositions are produced with and without the addition of fining agents, by melting the raw materials in an electric furnace at a glassmaking temperature close to the estimated fining temperature (Tlog2) of the compositions. Glass plates are produced by cutting after forming the mixture of materials on a tin bath using the well-known float process. 

[55] The properties of the resulting compositions and glasses are also measured and listed in Table 1 above (liquidus temperatures, fining temperature, etc.). The fining temperature is determined by viscometry in accordance with ISO 7884-2:1987. 

[56] It may be noted that the liquidus temperature of composition C1 is low and comfortably satisfies the floatability criterion (ΔT = Tlog3.5 - Tliq > 20°C). In contrast, the liquidus temperatures of compositions I1 to I3 according to the invention are higher and do not satisfy this criterion. However, the compositions according to the invention have a substantially lower fining temperature than composition C1. Despite a high liquidus temperature and an unfavourable working range, the glasses resulting from compositions according to the invention have no defects (bubbles or unmelted particles) and did not devritrify. However, these glasses are produced at a lower temperature than the usual temperatures for standard compositions. It should be noted that compositions C1 and I2 were also prepared at the same temperature of 1370°C in a flame furnace, using Pt-Rh crucibles. Here again, composition I2 did not devitrify and exhibits no defects. Composition C1, however, has very many defects (unmelted inclusions or bubbles). 

[57] Hydrolytic resistance is also evaluated using the DGG (Deutsche Glass Gesellschaft) method. In addition to the amount of alkalis solubilized in water, this method gives the mass of dry residue. The method involves immersing 10 grams of crushed glass grains (previously sieved to a particle size of 355 to 400 micrometers) in beakers of demineralized water. The whole is boiled for 5 hours using an oil bath. After rapid cooling, the solution is filtered and a determined volume of filtrate is evaporated to dryness. The weight of dry matter obtained is used to calculate the amount 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 is 2 g / L methyl red in 60% ethanol). The results show that the compositions according to the invention have hydrolytic resistance equivalent to that of standard glasses. 

[58] The energy saving of the compositions according to the invention, compared to the reference composition C1, is also evaluated by taking into account the calculated enthalpy change (in kWh per tonne of glass) and the idling losses (which represent the heat losses in the furnace, in kWh per tonne of glass, calculated by taking into account the specific pull). The specific pull represents the amount of glass that can be processed per unit of time and unit area (in tonnes of glass per m2.hour). The results are shown in Table 2 below. [Table 2] CompositionC1I1I2I3Enthalpy change ΔH of the raw material mixture (kWh / tonne)690682684702Specific pull (tonnes / m2 / h)0.0720.1350.1140.110Idling loss (kWh / tonne)16287103107ΔH + idling loss(kWh / tonne)852769787808Energy saving ΔE (relative to composition C1)--9.78%-7.62%-5.11% 

[59] While the enthalpy changes of the raw materials are of the same order of magnitude, once in the furnace the compositions according to the invention consume less energy than the reference composition C1 (energy saving ranging from about 5% to 10%). In fact, the significant reduction in production temperature (owing to a lower fining temperature of the compositions) makes it possible to significantly increase the specific pull. These advantages are achieved thanks to the compositions according to the invention having a higher alkaline-earth oxide content. 

[60] Thus, it has been demonstrated that, contrary to accepted preconceptions, it is possible to obtain float glass without defects or devitrification from compositions according to the invention, despite a higher liquidus temperature than standard compositions, and despite an unfavourable working range ΔT. 

Claims

 1. A glass composition, in particular a flat glass composition, comprising the following constituents in percentages by weight relative to the total weight of the composition:- from 60% to 70% SiO2;- from 2% to 18% Na2O;- from 8% to 28% CaO;- from 1% to 9% MgO;- from 0% to 2% Al2O3;- from 0% to 0.5% K2Owherein the sum of the contents of CaO and MgO is from 14 to 32%,the sum of the contents by weight of Na2O, CaO and MgO is from 30 to 37%, andthe MgO / (Na2O+CaO+MgO) ratio is greater than 0.16. 2. The composition according to claim 1, wherein the sum of the contents by weight of SiO2, Na2O, CaO and MgO represents at least 95% by weight, preferably at least 98% by weight, relative to the total weight of the composition. 3. The composition according to any one of the preceding claims, wherein the sum of the contents by weight of SiO2, Na2O, CaO, Al2O3, K2O and MgO represents at least 95%, preferably at least 98% by weight, more preferably at least 99% by weight, relative to the total weight of the composition. 4. The composition according to any one of the preceding claims, wherein the SiO2 content by weight is from 60% to 68%, preferably from 62% to 67%, relative to the total weight of the composition. 5. The composition according to any one of the preceding claims, wherein the CaO content by weight is at least 9%, preferably at least 12%, more preferably at least 14%, relative to the total weight of the composition. 6. The composition according to any one of the preceding claims, wherein the sum of the contents by weight of CaO and MgO is from 16% to 32%, preferably from 18% to 30%, relative to the total weight of the composition. 7. The composition according to any one of the preceding claims, the composition comprising a boron oxide (B2O3) content by weight of less than 1%, preferably the composition being free of boron oxide (B2O3). 8. The composition according to any one of the preceding claims, the composition being free of ZnO. 9. The composition according to any one of the preceding claims, the composition being free of ZrO2. 10. The composition according to any one of the preceding claims, the composition comprising a BaO content by weight of less than 1%, preferably less than 0.5%, relative to the total weight of the composition, more preferably the composition being free of BaO. 11. The composition according to any one of the preceding claims, the composition comprising a SrO content by weight of less than 1%, preferably less than 0.5%, relative to the total weight of the composition, more preferably the composition being free of SrO. 12. The composition according to any one of the preceding claims, the composition comprising a titanium oxide (TiO2) content by weight of less than 0.5%, preferably less than 0.2%, relative to the total weight of the composition, more preferably the composition being free of TiO2. 13. The composition according to any one of the preceding claims, the composition comprising a SnO2 content by weight of less than 1%, preferably less than 0.5%, relative to the total weight of the composition, more preferably the composition being free of SnO2. 14. The composition according to any one of the preceding claims, wherein the sum of the contents by weight of B2O3, SrO, BaO, ZrO2, TiO2, ZnO and SnO2 is less than 1%, preferably less than 0.5%, more preferably less than 0.2%, relative to the total weight of the composition. 15. The composition according to any one of the preceding claims, the composition comprising a Fe2O3 content by weight of at most 1%, preferably at most 0.5%, more preferably at most 0.2%, relative to the total weight of the composition. 16. The composition according to any one of the preceding claims, the composition having a fining temperature (Tlog2) below 1400°C, preferably below 1380°C. 17. 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 any one of claims 1 to 16;- a step of forming the molten mixture into a glass ribbon by floating. 18. The method according to claim 17, wherein the melting step is carried out at a temperature below 1400°C, preferably at a temperature below 1380°C. 19. A glass sheet having a composition according to any one of claims 1 to 16. 20. A glazing comprising a glass sheet according to claim 19.