BACKGROUND OF THE INVENTION 
     The invention relates to novel glass compositions of the silica-soda-lime type which are intended for the production of flat glass, bottles or flasks. Although the invention is not limited to such an application, it will be described more particularly with reference to motor-vehicle applications. 
     Windows intended for the motor-vehicle industry are subject to various requirements, especially with regard to their optical properties; these requirements are governed by regulations, for example those regarding the light transmission of a windscreen or those concerned with the user&#39;s comfort, for example with regard to energy transmission, or else those concerned with the aesthetic appearance, especially with regard to colour. 
     Moreover, manufacturers have for some time been focusing on a new requirement--that regarding UV transmission. The purpose of this new tendency is in particular a wish for the best possible protection of the skin from sunlight, which avoids suntanning and sunstroke. Another purpose of this tendency is to reduce fading of fabrics with which the interior of motor vehicles are furnished. 
     It is known that iron, in its ferric ion form Fe 3+ , i.e. in the Fe 2  O 3  form, makes it possible to absorb in the ultraviolet. Moreover, it is known that iron, in its ferrous ion form Fe 2+ , i.e. in the FeO form, makes it possible to absorb in the infrared and therefore to absorb energy. 
     It is thus known that controlling the redox (oxidation-reduction coefficient) of a glass composition makes it possible to regulate the absorption in the ultraviolet compared with that in the infrared with regard to the action of iron. 
     Moreover, it has already been described, especially in Patent Application WO 94/14716, that cerium oxide CeO 2  has an effect on the UV absorption. However, introducing this oxide increases the cost of the composition very significantly because of the cost of the batch materials which allow this oxide to be introduced. 
     It has already also been described, especially in Patent U.S. Pat. No. 5,478,783, that titanium oxide TiO 2  also has an effect on the UV absorption. Although the cost of introducing TiO 2  into a glass matrix is less than that of CeO 2 , it nevertheless remains very high. 
     It is also known, especially from document WO 94/14716, to combine the two oxides CeO 2  and TiO 2 , still for the purpose of achieving better UV absorption. Of course, such a combination does not allow the additional cost of introducing these oxides into a glass matrix to be reduced. 
     SUMMARY OF THE INVENTION 
     The inventors were thus given the task of designing glass compositions of the silica-soda-lime type which possess enhanced UV absorption having an additional production cost of less than that involved in the abovementioned techniques. 
     This objective was achieved according to the invention by a glass composition of the silica-soda-lime type having a UV transmission such that T UV .ISO is less than 15%, and preferably less than 13%, for a glass thickness of between 2.85 mm and 4.85 mm, especially one equal to 3.85 mm, and containing the oxides below, within the following weight limits: ##STR2## where Fe 2  O 3  is the total iron. 
     The Fe 2  O 3  oxide content is advantageously such that: 
     
         0.8%≦Fe.sub.2 O.sub.3 ≦1.3% 
    
     Preferably, the glass composition has a T UV .ISO of less than 10% for a glass thickness of between 2.85 mm and 4.85 mm, and preferably of between 3 and 4 mm. 
     It seems in fact that the simultaneous presence of the two oxides, Fe 2  O 3  and WO 3 , leads to an improvement in the UV absorption of glass compositions of the silica-soda-lime type. The results obtained with regard to UV transmission are quite comparable with those obtained using the techniques involving the oxide TiO 2 . However, the cost of producing the composition according to the invention id markedly less than that of the prior techniques, especially because of the cost of the batch materials. 
     Batch materials containing WO 3  may be of various kinds; they may, for example, be pure WO 3  or yellow or blue wolfram which contains 98.5% WO 3 . In order to reduce the production cost even more, batch materials containing WO 3  day be ores, such as scheelite, consisting mainly of CaWO 4 , or wolframite, consisting mainly of Fe Mn(WO 3 ). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to a preferred embodiment of the invention, the overall energy transmission T E  is less than 50%, and preferably less than 45%, for a thickness of 3.85 mm. Such requirements correspond especially to those for motor-vehicle applications that are required to ensure the comfort of the individuals in the passenger compartment. More preferably still, the redox, i.e. the FeO/Fe 2  O 3  ratio, is between 0.12 and 0.29 and preferably between 0.15 and 0.26. 
     According to a first embodiment, and more particularly for motor-vehicle applications such as windscreens or rear windows, the overall light transmission factor under illuminant A (T LA ) is greater than or equal to 70%. 
     According to another embodiment of the invention, and more particularly for motor-vehicle applications such as side windows, the overall light transmission factor under illuminant A (T LA ) is less than or equal to 35%. 
     According to a third variant of the invention, and more particularly for applications such as motor-vehicle sunroofs, the overall, light transmission factor under illuminant A (T LA ) is less than or equal to 10%. According to an alternative embodiment, the Fe 2  O 3  content is advantageously greater than 1.5%. 
     According to a preferred embodiment of the invention, the glass composition comprises the constituents below, within the following weight limits: 
     
         ______________________________________  SiO.sub.2        68.5-74%  CaO    7-10%  MgO   0-5%  Al.sub.2 O.sub.3          0-1.5%  K.sub.2 O        0-1%  Na.sub.2 O         13-16%.______________________________________ 
    
     More preferably still, the glass composition comprises the constituents below within the following weight limits: 
     
         ______________________________________  SiO.sub.2        70-73%  CaO    8-10%  MgO   0-4%  Al.sub.2 O.sub.3        0-1%  K.sub.2 O        0-1%  Na.sub.2 O         13-16%.______________________________________ 
    
     The silica is maintained within the relatively narrow limits for the following reasons: above approximately 74%, the viscosity of the glass and its ability to devitrify greatly increase, making it much more difficult to melt and to cast onto a bath of tin; below 68.5%, the hydrolytic resistance of the glass decreases very rapidly and the transmission in the visible also decreases. 
     This reduction in the hydrolytic resistance of the glass may be at least partly compensated for by the introduction of Al 2  O 3 , but this oxide helps to increase its viscosity and to reduce the transmission in the visible; it can therefore be used only in a very small amount. 
     The alkali metal oxides Na 2  O and K 2  O make it possible for the glass to melt easily and for its high-temperature viscosity to be adjusted. K 2  O is advantageously used with contents of less than 1%; the K 2  O concentration can be increased, but mainly only to the detriment of Na 2  O, this possibly contributing to an increase in the viscosity. The sum of the Na 2  O and K 2  O weight contents is preferably greater than 15%. 
     The alkaline-earth metal oxides introduced into the glasses according to the invention have the overall effect of raising the high-temperature viscosity. The CaO content must not exceed 10% in order to keep the devitrification of the glasses within acceptable limits. 
     With regard to the oxide MgO, according to a first embodiment of the invention, its content is advantageously greater than 2%, especially from an economic standpoint. 
     According to another embodiment, its content is less than 2%; it has been shown that such MgO contents characterize the composition according to the invention by a shift in the maximum of the FeO absorption band to long wavelengths. Limiting the MgO concentration in the glasses of the invention to 2%, and preferably eliminating it therefrom, by intentional addition, make it possible actually to increase their ability to absorb in the infrared. Complete elimination of MgO, which has a major effect on the viscosity, may be at least partly compensated for by increasing the Na 2  O and/or SiO 2  content. 
     BaO, which makes it possible to increase the light transmission, may be added to the compositions according to the invention in contents of less than 4%. This is because BaO has a much smaller influence on the viscosity of the glass than MgO and CaO. Within the context of the invention, increasing the BaO content is mainly to the detriment of the alkali metal oxides, MgO and especially CaO. Any significant increase in BaO therefore helps to increase the viscosity of the glass, especially at low temperatures. In addition, introducing a high concentration of BaO substantially increases the cost of the composition and has a tendency to reduce the hydrolytic resistance of the glass. When the glasses of the invention contain barium oxide, the concentration of this oxide is preferably between 0.5 and 3.5% by weight. 
     Apart from meeting the limits defined above with regard to the variation in the content of each alkaline-earth metal oxide, it is preferable to limit the sum of the MgO, CaO and BaO concentrations to a value of less than or equal to 12% in order to obtain the desired transmission properties. 
     With regard to the oxide WO 3 , a relatively high content may be accompanied by a yellowish coloration of the glass. According to one embodiment of the invention, the glass composition furthermore comprises the oxide CeO 2  with a weight content of less than or equal to 2.2%, and preferably less than 1.6%. More preferably still, the glass composition comprises the oxide CeO 2 , within the following weight limits: 
     
         0.2≦CeO.sub.2 ≦1.5%. 
    
     This oxide actually makes it possible, in combination with WO 3 , to obtain the required properties and to avoid yellowish coloration when the WO 3  contents are relatively high. Advantageously, it is thus possible to obtain a T UV .ISO ≦10%. 
     According to another embodiment of the invention, the glass composition comprises the oxide TiO 2  with a weight content of less than or equal to 1% and preferably greater than 0.1%. The oxide TiO 2  may be present in combination with the oxide CeO 2 . 
     Likewise, in another embodiment, the glass composition comprises the oxide La 2  O 3  with a weight content of less than or equal to 2%, it being possible for this to be present in combination with the oxides CeO 2  and/or TiO 2 . Advantageously, when lanthanum oxide La 2  O 3  is used, it is provided by the batch materials containing the oxide CeO 2 . 
     When it is desired to produce coloured glasses, the glass compositions may furthermore comprise one or more colouring agents such as CoO, Se, Cr 2  O 3 , NiO, V 2  O 5 . 
     The glasses according to the invention may also contain up to 1% of other constituents which are provided by the impurities in the batch materials and/or because of the introduction of cullet into the batch mixture and/or which come from the use of a refining agent (SO 3 , Cl, Sb 2  O 3 , As 2  O 3 ). 
     To facilitate the melting operation, and especially to make it mechanically beneficial, the matrix advantageously has a temperature, corresponding to a viscosity η such that logη=2, of less than 1500° C. More preferably still, and especially for producing the substrate from a ribbon of glass obtained using the float technique, the matrix has a temperature T log η-3.5 corresponding to a viscosity η, expressed in poise, such that logη=3.5 and a liquidus temperature T liq  which satisfy the relationship: 
     
         T.sub.logη=3.5 -T.sub.liq &gt;20° C. 
    
     and preferably the relationship: 
     
         T.sub.logη=3.5 -T.sub.liq &gt;50° C. 
    
     Further details and advantageous characteristics will emerge below from the description of illustrative embodiments according to the invention. 
     Several series of glasses were produced from the compositions given in the following tables. All these glasses were produced under more or less the same oxidation-reduction conditions--their redox is between 0.12 and 0.29. 
     This table also gives the values of the following properties, measured for thicknesses of 4.85 mm or 3.85 mm, or 3.15 mm or 2.85 mm: 
     the overall light transmission factor under illuminant A (T LA ) between 380 and 780 nm, 
     the overall energy transmission factor T E  integrated between 295 and 2500 nm according to the Parry Moon (Mass 2) standard, 
     the ultraviolet transmission factor integrated between 295 and 380 nm, T UV .ISO, according to the ISO 9050 standard, and 
     the dominant wavelength under illuminant D 65  (λ d ). 
     The tables also show the temperatures T log η=2 and T log η=3.5 corresponding to the viscosities, expressed in poise, such that logη=2 and logη=3.5, as well as the liquidus temperature T liq . 
     The first glass, called R, is a reference glass whose composition is standard for windows intended for the motor-vehicle industry. 
     
                                           TABLE 1__________________________________________________________________________     R   1   2   3   4   5   6    7    8__________________________________________________________________________SiO.sub.2 71.6         72.62             72.63                 72.63                     72.86                         72.75                             72.57                                  72.5 71.70CaO       8.6 8.63             8.63                 8.63                     8.68                         8.66                             8.64 8.64 8.63MgO       3.7 0.30             0.30                 0.30                     0.30                         0.30                             0.30 0.30 0.30Al.sub.2 O.sub.3     0.6 0.90             0.90                 0.90                     0.90                         0.90                             0.90 0.90 0.90K.sub.2 O 0.2 0.30             0.34                 0.34                     0.30                         0.30                             0.30 0.30 0.30Na.sub.2 O     14.2         15.29             15.29                 15.29                     15.37                         15.35                             15.31                                  15.30                                       15.30SO.sub.3  0.2 0.30             0.30                 0.30                     0.30                         0.30                             0.30 0.30 0.30Fe.sub.2 O.sub.3     0.86         1.21             1.21                 1.23                     1.12                         1.14                             1.13 1.2  1.12WO.sub.3  0   0.45             0.40                 0.45                     0.10                         0.25                             0.5  1.2  0.45CeO.sub.2 0   0   0   0   0   0   0    0    1.00Redox     0.28         0.15             0.16                 0.16                     0.14                         0.17                             0.175                                  0.184                                       0.19T.sub.LA (%), 3.85 mm     71  70.5             71.5                 70.3                     72.3                         70.8                             69.7 66.6 70T.sub.E (%), 3.85     43.5         43.9             43.5                 44.9                     45.5                         43.7                             42.4 38.2 42.3T.sub.UV.ISO (%), 3.85 mm     18.5         11.6             12  11.3                     14.7                         13.3                             12.6 11.4 8.0.sub.d (D.sub.65) nm     502 553 555 557 541 537 545  537  550T.sub.logη=2 (° C.)     1436         1457             1457                 1457                     1457                         1457                             1457 1457 1457T.sub.logη=3.5 (° C.)     1101         1107             1107                 1107                     1107                         1107                             1107 1107 1107T.sub.liq (° C.)     1040         1002             1002                 1002                     1002                         1002                             1002 1002 1002__________________________________________________________________________ 
    
     
                       TABLE 2______________________________________9           10      11       12    13    14______________________________________SiO.sub.2   71.6    71.2    71.2   72.2  68.7  68.9CaO     8.6     8.7     8.7    8.65  8.2   8.2MgO     0.3     0.3     0.3    0.3   3.8   3.8Al.sub.2 O.sub.3   0.9     0.9     0.9    0.9   0.6   0.6Na.sub.2 O + K.sub.2 O   15.5    15.7    15.7   15.6  15.13 14.9Fe.sub.2 O.sub.3   0.77    0.82    1.3    1.3   0.75  0.75WO.sub.3   0.95    1       0.35   0.3   0.5   0.6CeO.sub.2   1       1       0.42   0.4   1.9   1.9Redox   0.22    0.24    0.14   0.13  0.22  0.22T.sub.LA (%),   74.5    71.7    71.9   71.2  72.8  72.33.85 mmT.sub.E (%),   47.8    43.5    46.5   45.5  47.8  46.93.85 mmT.sub.UV.ISO (%),   10      9.5     9.1    8.6   7.4   6.93.85 mm.sub.d (D.sub.65) nm   542     529     554    556   537   522______________________________________ 
    
     
                       TABLE 3______________________________________    15    16         17      18______________________________________SiO.sub.2  71      71         71    71CaO + MgO  8.7     8.7        8.7   8.7Al.sub.2 O.sub.3      0.9     0.9        0.9   0.9Na.sub.2 O + K.sub.2 O      15.6    15.6       15.6  15.6Fe.sub.2 O.sub.3      1.2     1.2        1.32  1.3WO.sub.3   0.45    0.45       1     1CeO.sub.2  0       0          0.4   0.4La.sub.2 O.sub.3      1       3          0     1Redox      0.18    0.18       0.18  0.18Thickness  3.35    3.47       3.48  3.58in mmT.sub.LA (%),      71      71         71    713.85 mmT.sub.E (%),      44      44.3       45.4  45.63.85 mmT.sub.UV.ISO (%),      13.4    12.4       8.2   7.43.85 mm.sub.d (D.sub.65) nm      537     542        558   560______________________________________ 
    
     
                       TABLE 4______________________________________19           20       21       22     23______________________________________SiO.sub.2   68.7     68.7     68.7   68.9   68.7CaO     8.2      8.2      8.2    8.2    8.2MgO     3.8      3.8      3.8    3.8    3.8Al.sub.2 O.sub.3   0.6      0.6      0.6    0.6    0.6Na.sub.2 O + K.sub.2 O   15.13    15.13    15.13  14.9   15.13Fe.sub.2 O.sub.3   0.63     0.73     0.72   0.73   0.73WO.sub.3   0.5      0.5      0.5    0.6    0.6CeO.sub.2   1.9      1.9      1.9    1.9    1.9Redox   0.18     0.22     0.24   0.22   0.27T.sub.LA (%),   75.9     72.8     70.6   72.3   703.85 mmT.sub.E (%),   53.5     47.8     44.2   46.9   42.63.85 mmT.sub.UV.ISO (%),   8        7.4      7      6.9    73.85 mm.sub.d (D.sub.65) nm   547      537      522    536    517______________________________________ 
    
     
                       TABLE 5______________________________________24           25       26       27     28______________________________________SiO.sub.2   72.9     68.9     68.9   72     72CaO     8.6      8.2      8.2    8.6    8.6MgO     0        3.8      3.8    0      0Al.sub.2 O.sub.3   0.75     0.6      0.6    0.75   0.75Na.sub.2 O + K.sub.2 O   15.36    14.9     14.9   15.5   15.5Fe.sub.2 O.sub.3   1.3      0.73     0.73   0.83   0.83WO.sub.3   0.4      0.6      0.6    0.4    0.4CeO.sub.2   0.42     1.9      1.9    1.55   1.55Redox   0.14     0.22     0.165  0.17   0.135T.sub.LA (%),   66.8     *        71.2   68.4   73.44.85 mmT.sub.E (%),   39.0     *        46.4   38.5   47.14.85 mmT.sub.UV.ISO (%),   5.9      *        4.8    6.5    6.34.85 mmT.sub.LA (%),   71.8     72.3     75     72.5   76.83.85 mmT.sub.E (%),   45.1     46.9     52.5   44.3   52.93.85 mmT.sub.UV.ISO (%),   8.6      6.9      7      9.1    8.83.85 mm.sub.d (D.sub.65) nm   554      522      553    518    553T.sub.LA (%),   74.6     75.4     77.8   75.7   79.33.15 mmT.sub.E (%),   50.4     52.1     57.6   49.2   57.73.15 mmT.sub.UV.ISO (%),   11.5     9.3      9.4    11.7   11.33.15 mmT.sub.LA (%),   76.1     76.8     79.1   77.1   80.42.85 mmT.sub.E (%),   52.9     54.6     59.9   51.7   60.02.85 mmT.sub.UV.ISO (%),   13.1     10.7     10.8   13.2   12.72.85 mm______________________________________ 
    
     In Table 6, the transmission values are calculated based on a model. 
     
                       TABLE 6______________________________________          29    30______________________________________SiO.sub.2        73.30   73.75CaO              7.70    7.70MgO              0       0Al.sub.2 O.sub.3 1.00    1.00Na.sub.2 O + K.sub.2 O            15.10   15.10Fe.sub.2 O.sub.3 0.70    1.05WO.sub.3         0.40    0.40CeO.sub.2        1.80    1.00Redox            0.2     0.15T.sub.LA (%),    75.7    73.33.85 mmT.sub.E (%),     49.8    47.83.85 mmT.sub.UV.ISO (%),            8.5     8.53.85 mm______________________________________ 
    
     
                       TABLE 7______________________________________      31   32         33     34______________________________________SiO.sub.2    72     72         72.2 72.2CaO          8.6    8.6        8.6  8.6MgO          0      0          0    0Al.sub.2 O.sub.3        0.75   0.75       0.9  0.9Na.sub.2 O + K.sub.2 O        15.5   15.5       15.55                               15.55Fe.sub.2 O.sub.3        0.83   0.83       1.1  1.1WO.sub.3     0.40   0.20       0.25 0.2TiO.sub.2    0      0.2        0.25 0.3CeO.sub.2    1.55   1.55       0.8  0.8Redox        0.135  *          *    *T.sub.LA  (%),        73.4   73.4       70.4 71.23.85 mmT.sub.E  (%),        47.1   47.1       43.8 44.63.85 mmT.sub.UV.I8O 6.3),  6.6        6.2  6.03.85 mm______________________________________ *: Values not measured 
    
     First of all, these results demonstrate that these glass compositions may be melted using conventional techniques and are compatible with the usual techniques for manufacturing flat glass. 
     Moreover, the optical properties are satisfactory and, more particularly, the UV transmission T UV .ISO is less than 15% or even less than 13%, which means that the fabrics with which the passenger compartment are finished and the skin of passengers are protected.
The subject of the invention is a glass composition of the silica-soda-lime type, having a UV transmission such that T UV.ISO <15%, and preferably T UV.ISO <=13%, for a glass thickness of between 2.85 mm and 4.85 mm, and containing the oxides below, within the following weight limits: ##STR1##