Abstract:
A glass for chemical tempering is provided. The glass contains as represented by mole percentage based on the following oxides, from 61 to 77% of SiO 2 , from 11 to 18% of Al 2 O 3 , from 3 to 15% of MgO, from 0 to 0.5% of CaO, from 0 to 4% of ZrO 2 , from 8 to 18% of Na 2 O and from 0 to 1.9% of K 2 O; wherein an R value calculated by the following formula by using contents of the respective components, is at least 0.66: 
       R=0.029×SiO 2 +0.021×Al 2 O 3 +0.016×MgO−0.004×CaO+0.016×ZrO 2 +0.029×Na 2 O+0×K 2 O−2.002

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method for producing chemically tempered glass which is suitable for e.g. a cover glass for a display device, such as a mobile device such as a cell phone or a personal digital assistance (PDA), a large-sized flat screen television such as a large-sized liquid crystal television or a large-sized plasma television, or a touch panel. 
       BACKGROUND ART 
       [0002]    In recent years, for a display device such as a mobile device, a liquid crystal television or a touch panel, a cover glass (protective glass) has been used in many cases in order to protect the display and to improve the appearance. 
         [0003]    For such a display device, weight reduction and thickness reduction are required for differentiation by a flat design or for reduction of the load for transportation. Therefore, a cover glass to be used for protecting a display is also required to be thin. However, if the thickness of the cover glass is made to be thin, the strength is lowered, and there has been a problem such that the cover glass itself is broken by e.g. a shock due to falling or flying of an object in the case of a installed type or by dropping during the use in the case of a portable device, and the cover glass cannot accomplish the essential role to protect a display device. 
         [0004]    In order to solve the above problem, it is conceivable to improve the strength of the cover glass, and as such a method, a method to form a compressive stress layer on a glass surface is commonly known. 
         [0005]    The method to form a compressive stress layer on a glass surface, may typically be an air quenching tempering method (physical tempering method) wherein a surface of a glass plate heated to near the softening point is quenched by air cooling or the like, or a chemical tempering method wherein alkali metal ions having a small ion radius (typically Li ions or Na ions) at a glass plate surface are exchanged with alkali ions having a larger ion radius (typically K ions) by ion exchange at a temperature lower than the glass transition point. 
         [0006]    As mentioned above, the thickness of the cover glass is required to be thin. However, if the air quenching tempering method is applied to a thin glass plate having a thickness of less than 1 mm, as required for a cover glass, the temperature difference between the surface and the inside tends not to arise, and it is thereby difficult to form a compressive stress layer, and the desired property of high strength cannot be obtained. Therefore, a cover glass tempered by the latter chemical tempering method is usually used. 
         [0007]    As such a cover glass, one having soda lime glass chemically tempered is widely used (e.g. Patent Document 1). 
         [0008]    Soda lime glass is inexpensive and has a feature that the surface compressive stress S of a compressive stress layer formed at the surface of the glass by the chemical tempering can be made to be at least 200 MPa, but there has been a problem that it is difficult to make the thickness t of the compressive stress layer to be at least 30 μm. 
         [0009]    Therefore, one having SiO 2 —Al 2 O 3 —Na 2 O type glass different from soda lime glass, chemically tempered, has been proposed for such a cover glass (e.g. Patent Document 2). 
         [0010]    Such SiO 2 —Al 2 O 3 —Na 2 O type glass has a feature that it is possible not only to make the above S to be at least 200 MPa but also to make the above t to be at least 30 μm. 
       PRIOR ART DOCUMENTS 
     Patent Documents 
       [0011]    Patent Document 1: JP-A-2007-11210 
         [0012]    Patent Document 2: U.S. Patent Application Publication No. 2008/0286548 
       DISCLOSURE OF INVENTION 
     Technical Problem 
       [0013]    In the above-described application, etc., ion exchange treatment for chemical tempering is usually carried out by immersing glass containing sodium (Na) in a molten potassium salt, and as such a potassium salt, potassium nitrate or a mixed salt of potassium nitrate and sodium nitrate, is used. 
         [0014]    In such ion exchange treatment, ion exchange of Na in the glass with potassium (K) in the molten salt is carried out. Therefore, if the ion exchange treatment is repeated by using the same molten salt, the Na concentration in the molten salt increases. 
         [0015]    If the Na concentration in the molten salt increases, the surface compressive stress S of the chemically tempered glass decreases, and therefore, there has been a problem that it is necessary to strictly watch the Na concentration in the molten salt and to frequently carry out replacement of the molten salt, so that S of the chemically tempered glass will not become lower than the desired value. 
         [0016]    It is desired to reduce the frequency of such replacement of the molten salt, and it is an object of the present invention to provide a method for producing chemically tempered glass, whereby such a problem can be solved. 
       Solution to Problem 
       [0017]    The present invention provides a method for producing chemically tempered glass, which comprises repeating ion exchange treatment of immersing glass in a molten salt to obtain chemically tempered glass, wherein the glass comprises, as represented by mole percentage based on the following oxides, from 61 to 77% of SiO 2 , from 1 to 18% of Al 2 O 3 , from 3 to 15% of MgO, from 0 to 5% of CaO, from 0 to 4% of ZrO 2 , from 8 to 18% of Na 2 O and from 0 to 6% of K 2 O; the total content of SiO 2  and Al 2 O 3  is from 65 to 85%; the total content of MgO and CaO is from 3 to 15%; and R calculated by the following formula by using contents of the respective components, is at least 0.66 (hereinafter sometimes referred to as the first invention). Further, the glass to be used here may be referred to as the first glass of the present invention, and, for example, SiO 2  in the following formula is the content of SiO 2  as represented by mole percentage. 
         [0000]      R=0.029×SiO 2 +0.021×Al 2 O 3 +0.016×MgO−0.004×CaO+0.016×ZrO 2 +0.029×Na 2 O+0×K 2 O−2.002
 
         [0018]    The total content of SiO 2 , Al 2 O 3 , MgO, CaO, ZrO 2 , Na 2 O and K 2 O in the first glass of the present invention is typically at least 98.5%. 
         [0019]    Further, the present invention provides a method for producing chemically tempered glass, which comprises repeating ion exchange treatment of immersing glass in a molten salt to obtain chemically tempered glass, wherein the glass comprises, as represented by mole percentage based on the following oxides, from 61 to 77% of SiO 2 , from 1 to 18% of Al 2 O 3 , from 3 to 15% of MgO, from 0 to 5% of CaO, from 0 to 4% of ZrO 2 , from 8 to 18% of Na 2 O, from 0 to 6% of K 2 O and at least one component selected from B 2 O 3 , SrO and BaO; the total content of SiO 2  and Al 2 O 3  is from 65 to 85%; the total content of MgO and CaO is from 3 to 15%; and R′ calculated by the following formula by using contents of the respective components, is at least 0.66 (hereinafter sometimes referred to as the second invention). Further, the glass to be used here may be referred to as the second glass of the present invention. 
         [0000]      R′=0.029×SiO 2 +0.021×Al 2 O 3 +0.016×MgO−0.004×CaO+0.016×ZrO 2 +0.029×Na 2 O+0×K 2 O+0.028×B 2 O 3 +0.012×SrO+0.026×BaO−2.002
 
         [0020]    The total content of SiO 2 , Al 2 O 3 , MgO, CaO, ZrO 2 , Na 2 O, K 2 O, B 2 O 3 , SrO and BaO in the second glass of the present invention is typically at least 98.5%. 
         [0021]    Further, the present invention provides a method for producing chemically tempered glass, which comprises repeating ion exchange treatment of immersing glass in a molten salt to obtain chemically tempered glass, wherein the glass comprises, as represented by mole percentage based on the following oxides, from 61 to 77% of SiO 2 , from 1 to 18% of Al 2 O 3 , from 3 to 15% of MgO, from 0 to 5% of CaO, from 0 to 4% of ZrO 2 , from 8 to 18% of Na 2 O, from 0 to 6% of K 2 O and at least one component selected from B 2 O 3 , SrO, BaO, ZnO, Li 2 O and SnO 2 ; the total content of SiO 2  and Al 2 O 3  is from 65 to 85%; the total content of MgO and CaO is from 3 to 15%; and R″ calculated by the following formula by using contents of the respective components, is at least 0.66 (hereinafter sometimes referred to as the third invention). Further, the glass to be used here may be referred to as the third glass of the present invention. 
         [0000]      R″=0.029×SiO 2 +0.021×Al 2 O 3 +0.016×MgO−0.004×CaO+0.016×ZrO 2 +0.029×Na 2 O+0×K 2 O+0.028×B 2 O 3 +0.012×SrO+0.026×BaO+0.019×ZnO+0.033×Li 2 O+0.032×SnO 2 −2.002
 
         [0022]    Total content of SiO 2 , Al 2 O 3 , MgO, CaO, ZrO 2 , Na 2 O, K 2 O, B 2 O 3 , SrO, BaO, ZnO, Li 2 O and SnO 2  in the third glass of the present invention is typically at least 98.5%. 
         [0023]    Further, the present invention provides a method for producing chemically tempered glass, which comprises repeating ion exchange treatment of immersing glass in a molten salt to obtain chemically tempered glass, wherein the glass comprises, as represented by mole percentage based on the following oxides, from 62 to 77% of SiO 2 , from 1 to 18% of Al 2 O 3 , from 3 to 15% of MgO, from 0 to 5% of CaO, from 0 to 4% of ZrO 2  and from 8 to 18% of Na 2 O; the total content of SiO 2  and Al 2 O 3  is from 65 to 85%; the total content of MgO and CaO is from 3 to 15%; and the glass contains no K 2 O (hereinafter sometimes referred to as the fourth invention). The first, second, third and fourth glasses of the present invention will be generally referred to as the glass of the present invention. 
         [0024]    Further, the present invention provides the method for producing chemically tempered glass, wherein SiO 2  is at least 61%, Al 2 O 3  is from 3 to 12%, MgO is at most 12% and CaO is from 0 to 3%. 
         [0025]    Further, the present invention provides the method for producing chemically tempered glass, wherein ZrO 2  is at most 2.5% and Na 2 O is at least 10%. 
         [0026]    Further, the present invention provides the method for producing chemically tempered glass, wherein Al 2 O 3  is at least 9% and CaO is from 0 to 2%. 
         [0027]    Further, the present invention provides the method for producing chemically tempered glass, wherein the total content of SiO 2 , Al 2 O 3 , MgO, CaO, ZrO 2 , Na 2 O and K 2 O, is at least 98.5%. 
         [0028]    Further, the present invention provides the method for producing chemically tempered glass, wherein a compressive stress layer formed at the surface of the chemically tempered glass has a thickness of at least 10 μm and a surface compressive stress of at least 200 MPa. 
         [0029]    Further, the present invention provides the method for producing chemically tempered glass, wherein the chemically tempered glass is a glass plate having a thickness of at most 1.5 mm. 
         [0030]    Further, the present invention provides the method for producing chemically tempered glass, wherein the chemically tempered glass is a cover glass. 
         [0031]    The present inventors have considered that there may be a relation between the composition of Na-containing glass and such a phenomenon that by repeating ion exchange treatment of immersing the Na-containing glass in a molten potassium salt many times to obtain chemically tempered glass, the Na concentration in the molten potassium salt increases and at the same time, the surface compressive stress of the chemically tempered glass becomes small, and have conducted the following experiment. 
         [0032]    Firstly, 29 types of glass plates were prepared which had compositions as represented by mol % in Tables 1 to 3 and each of which had a thickness of 1.5 mm and a size of 20 mm×20 mm and had both sides mirror-polished with cerium oxide. 
         [0033]    The glass transition points Tg (unit: ° C.) and Young&#39;s modulus E (unit: GPa) of these glasses are also shown in the same Tables. 
         [0034]    Here, those provided with * are ones calculated from the compositions. 
         [0035]    Tg was measured as follows. That is, by means of a differential thermal dilatometer, the elongation percentage of glass was measured to a yield point when the temperature was raised from room temperature at a rate of 5° C./min using quartz glass as a reference sample, and the temperature corresponding to a folding point in the obtained thermal expansion curve was taken as the glass transition point. 
         [0036]    E was measured by an ultrasonic pulse method with respect to a glass plate having a thickness of from 5 to 10 mm and a size of 3 cm×3 cm. 
         [0037]    These 29 types of glass plates were subjected to ion exchange of immersing for 10 hours in a molten potassium salt having a KNO 3  content of 100% and having a temperature of 400° C. to obtain chemically tempered glass plates, whereupon their surface compressive stresses CS1 (unit: MPa) were measured. Here, glass A27 is glass used for a cover glass for a mobile device. 
         [0038]    Further, these 29 types of glass plates were subjected to ion exchange of immersing for 10 hours in a molten potassium salt having a KNO 3  content of 95% and a NaNO 3  content of 5% and having a temperature of 400° C. to obtain chemically tempered glass plates, and their surface compressive stresses CS2 (unit: MPa) were measured. Here, CS1 and CS2 were measured by means of a surface stress meter FSM-6000, manufactured by Orihara Manufacturing Co., Ltd. 
         [0039]    CS1 and CS2 are shown together with their ratio r=CS2/CS1 in the corresponding rows in Tables 1 to 3. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Glass 
                 1 
                 2 
                 A1 
                 A2 
                 A3 
                 A4 
                 A5 
                 A6 
                 A7 
                 A8 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SiO 2   
                 73.0 
                 72.0 
                 64.3 
                 64.3 
                 64.3 
                 64.3 
                 63.8 
                 63.8 
                 64.3 
                 64.3 
               
               
                 Al 2 O 3   
                 7.0 
                 6.0 
                 6.5 
                 7.0 
                 6.5 
                 7.0 
                 7.0 
                 7.5 
                 6.0 
                 6.0 
               
               
                 MgO 
                 6.0 
                 10.0 
                 11.0 
                 11.0 
                 11.0 
                 11.0 
                 11.0 
                 11.0 
                 11.5 
                 12.0 
               
               
                 CaO 
                 0 
                 0 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 SrO 
                 0 
                 0 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 BaO 
                 0 
                 0 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 ZrO 2   
                 0 
                 0 
                 2.0 
                 1.5 
                 1.5 
                 1.0 
                 1.5 
                 1.0 
                 2.0 
                 1.5 
               
               
                 Na 2 O 
                 14.0 
                 12.0 
                 12.0 
                 12.0 
                 12.5 
                 12.5 
                 12.5 
                 12.5 
                 12.0 
                 12.0 
               
               
                 K 2 O 
                 0 
                 0 
                 4.0 
                 4.0 
                 4.0 
                 4.0 
                 4.0 
                 4.0 
                 4.0 
                 4.0 
               
               
                 Tg 
                 617 
                 647 
                 615 
                 617 
                 608 
                 603 
                 614 
                 610 
                 615 
                 609 
               
               
                 E 
                 70.8 
                 73.1 
                 75.8 
                 75.3 
                 74.9 
                 74.4 
                 75.1 
                 74.8 
                 75.8 
                 75.3 
               
               
                 CS1 
                 888 
                 900 
                 1049 
                 1063 
                 1035 
                 1047 
                 1063 
                 1046 
                 1020 
                 1017 
               
               
                 CS2 
                 701 
                 671 
                 589 
                 593 
                 601 
                 590 
                 601 
                 599 
                 588 
                 579 
               
               
                 r 
                 0.79 
                 0.75 
                 0.56 
                 0.56 
                 0.58 
                 0.56 
                 0.57 
                 0.57 
                 0.58 
                 0.57 
               
               
                 R 
                 0.76 
                 0.72 
                 0.55 
                 0.56 
                 0.56 
                 0.56 
                 0.56 
                 0.56 
                 0.55 
                 0.55 
               
               
                 R′ 
                 0.76 
                 0.72 
                 0.56 
                 0.56 
                 0.57 
                 0.57 
                 0.56 
                 0.56 
                 0.56 
                 0.56 
               
               
                 R″ 
                 0.76 
                 0.72 
                 0.56 
                 0.56 
                 0.57 
                 0.57 
                 0.56 
                 0.56 
                 0.56 
                 0.56 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Glass 
                 A9 
                 A10 
                 A11 
                 A12 
                 A13 
                 A14 
                 A15 
                 A16 
                 A17 
                 A18 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SiO 2   
                 64.3 
                 64.3  
                 64.3  
                 64.3  
                 64.3 
                 65.3  
                 64.3  
                 60.3  
                 56.3  
                 64.3 
               
               
                 Al 2 O 3   
                 7.2 
                 7.0 
                 6.0 
                 6.0 
                 8.0 
                 7.0 
                 10.0  
                 11.5  
                 15.5  
                 8.0 
               
               
                 MgO 
                 11.0 
                 11.0  
                 12.5  
                 13.0  
                 11.0 
                 11.0  
                 8.5 
                 11.0  
                 11.0  
                 10.5 
               
               
                 CaO 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 SrO 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 BaO 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 ZrO 2   
                 0.5 
                 1.5 
                 1.0 
                 0.5 
                 0.5 
                 0.5 
                 0   
                 0   
                 0   
                 0.5 
               
               
                 Na 2 O 
                 12.7 
                 11.5  
                 12.0  
                 12.0  
                 12.0 
                 12.0  
                 13.0  
                 13.0  
                 13.0  
                 12.5 
               
               
                 K 2 O 
                 4.0 
                 4.5 
                 4.0 
                 4.0 
                 4.0 
                 4.0 
                 4.0 
                 4.0 
                 4.0 
                 4.0 
               
               
                 Tg 
                 597 
                 599*   
                 586*   
                 582*   
                 614 
                 591*   
                 602*   
                 608*   
                 633*   
                 608 
               
               
                 E 
                 73.6 
                 75.6  
                 75.2  
                 74.6  
                 74.8 
                 74.1  
                 72*   
                 74*   
                 75*   
                 74.4 
               
               
                 CS1 
                 1003 
                 1013    
                 984    
                 963    
                 954 
                 983    
                 1072    
                 1145    
                 1221    
                 1024 
               
               
                 CS2 
                 588 
                 564    
                 561    
                 546    
                 576 
                 574    
                 640    
                 641    
                 647    
                 582 
               
               
                 r 
                 0.59 
                  0.56 
                  0.57 
                  0.57 
                 0.60 
                  0.58 
                  0.60 
                  0.56 
                  0.53 
                 0.57 
               
               
                 R 
                 0.57 
                  0.54 
                  0.55 
                  0.55 
                 0.56 
                  0.57 
                  0.59 
                  0.54 
                  0.51 
                 0.57 
               
               
                 R′ 
                 0.57 
                  0.55 
                  0.56 
                  0.56 
                 0.57 
                  0.57 
                  0.59 
                  0.54 
                  0.51 
                 0.57 
               
               
                 R″ 
                 0.57 
                  0.55 
                  0.56 
                  0.56 
                 0.57 
                  0.57 
                  0.59 
                  0.54 
                  0.51 
                 0.57 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Glass 
                 A19 
                 A20 
                 A21 
                 A22 
                 A23 
                 A24 
                 A25 
                 A26 
                 A27 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SiO 2   
                 64.3  
                 63.5  
                 66.0  
                 64.5  
                 65.0  
                 63.5  
                 64.3 
                 71.3  
                 66.7  
               
               
                 Al 2 O 3   
                 8.5 
                 10.5  
                 9.0  
                 9.0  
                 5.0  
                 5.0 
                 6.0 
                 2.0 
                 10.8  
               
               
                 MgO 
                 10.5  
                 9.0  
                 8.0  
                 12.0  
                 12.0  
                 8.0 
                 11.0 
                 10.4  
                 6.2  
               
               
                 CaO 
                 0.1 
                 0   
                 0   
                 0   
                 0.5  
                 4.0 
                 0.1 
                 0.3 
                 0.6  
               
               
                 SrO 
                 0.1 
                 0   
                 0   
                 0   
                 0   
                 0   
                 0.1 
                  0.03 
                 0   
               
               
                 BaO 
                 0.1 
                 0   
                 0   
                 0   
                 0   
                 0   
                 0.1 
                  0.02 
                 0   
               
               
                 ZrO 2   
                 0   
                 0   
                 0   
                 0   
                 0   
                 1.3 
                 2.5 
                 0.5 
                 0   
               
               
                 Na 2 O 
                 12.5  
                 15.0  
                 15.0  
                 11.5  
                 11.0  
                 9.4 
                 12.0 
                 10.8  
                 13.2  
               
               
                 K 2 O 
                 4.0 
                 2.0  
                 2.0  
                 3.0  
                 6.5  
                 8.9 
                 4.0 
                 4.6 
                 2.4  
               
               
                 Tg 
                 594*   
                 598*    
                 599    
                 648    
                 568*    
                 580*   
                 620 
                 566*   
                 595    
               
               
                 E 
                 73*   
                 74*   
                 72*   
                 75*   
                 71*   
                 70*   
                 78 
                 71*   
                 72*   
               
               
                 CS1 
                 985    
                 1190     
                 1054     
                 919    
                 746    
                 668    
                 1019 
                 664    
                 1039     
               
               
                 CS2 
                 577    
                 752    
                 722    
                 516    
                 382    
                 240    
                 571 
                 407    
                 679    
               
               
                 r 
                  0.59 
                 0.63 
                 0.69 
                 0.56 
                 0.51 
                  0.36 
                 0.56 
                  0.61 
                 0.65 
               
               
                 R 
                  0.57 
                 0.64 
                 0.66 
                 0.58 
                 0.50 
                  0.35 
                 0.55 
                  0.59 
                 0.64 
               
               
                 R′ 
                  0.58 
                 0.64 
                 0.66 
                 0.58 
                 0.50 
                  0.35 
                 0.56 
                  0.59 
                 0.64 
               
               
                 R″ 
                  0.58 
                 0.64 
                 0.66 
                 0.58 
                 0.50 
                  0.35 
                 0.56 
                  0.59 
                 0.64 
               
               
                   
               
             
          
         
       
     
         [0040]    From these results, it has been found that there is a high correlation between R calculated by above formula (shown in Tables 1 to 3) and the above r.  FIG. 1  is a scatter graph to make this point clear wherein the abscissa represents R and the ordinate represents r, and the straight line in the Fig. represents r=1.033×R−0.0043, and the correlation coefficient is 0.97. 
         [0041]    Further, values of the above R′ and R″ are also shown below the row for R in Tables 1 to 3. 
         [0042]    From the above correlation found by the present inventors, the following is evident. That is, in order to reduce the frequency of replacement of the molten salt, glass having a less degree of decrease in the surface compressive stress S due to an increase of the Na concentration i.e. glass having the above r being large, may be used, and for such a purpose, the above R of the glass may be made to be large. 
         [0043]    Further, r of conventional glass A27 is 0.65, and when R is made to be at least 0.66, r becomes roughly at least 0.68 i.e. is distinctly larger than glass A27, whereby it becomes possible to remarkably reduce the frequency of replacement of the molten salt, or to substantially relax the watching of the molten salt. 
         [0044]    The strength of the chemically tempered glass depends largely on the surface compressive stress, and the smaller the surface compressive stress, the lower the strength of the chemically tempered glass. Therefore, the surface compressive stress obtainable by the chemical tempering treatment is required to be at least 68% as compared with the surface compressive stress when the Na concentration in the molten salt is 0%, i.e. r is required to be at least 0.68. From this viewpoint, when the Na concentration in the molten salt is represented by C, the useful range of C is the range which satisfies the following formula. 
         [0000]      0.68≦( r− 1)× C/ 5+1
 
         [0045]    Thus, C≦1.6/(1−r) must be satisfied. 
         [0046]    If r is less than 0.68, the decrease ratio of the surface compressive stress of the chemically tempered glass due to an increase of the Na concentration in the molten salt is large, whereby such a molten salt is useful only within a narrow range where the Na concentration is less than 5.0%, and the frequency of replacement increases. When r is 0.75, 0.79 and 0.81, the molten salt becomes useful within a wide range of the Na concentration where the Na concentration is at most 6.4%, at most 7.6% and at most 8.4%, respectively, and thus, when r is 0.75, 0.79 and 0.81, the frequency of replacement can be suppressed to be 78%, 66% and 59%, respectively, as compared with the case where r is 0.68. Accordingly, r is preferably at least 0.70, more preferably at least 0.75, further preferably at least 0.79, particularly preferably at least 0.81. 
         [0047]    On the other hand, if r is less than 0.68, the change in the surface compressive stress S of the chemically tempered glass due to a change of the Na concentration in the molten salt is large, whereby adjustment of the surface compressive stress tends to be difficult, and watching of the Na concentration in the molten salt is required to be strict. 
         [0048]    Further, when glasses 1 and 2 having r being largest among 29 types of glasses, are compared with other 27 types of glasses, they are common in that they contain no K 2 O. On the other hand, the coefficient relating to K 2 O in the above formula for calculation of R is 0 and is substantially small as compared with the coefficient of 0.029 relating to Na 2 O being the same alkali metal oxide, and this explains such a point. 
         [0049]    The present invention has been accomplished on the basis of the above finding. 
       Advantageous Effects of Invention 
       [0050]    According to the present invention, the decrease ratio of the surface compressive stress S of chemically tempered glass due to an increase of the Na concentration in the molten salt can be made small, whereby it is possible to relax the watching of the Na concentration in the molten salt and to reduce the frequency of replacement of the molten salt. 
         [0051]    Further, the decrease ratio of S of chemically tempered glass immediately before replacement of the molten salt to S of chemically tempered glass obtained by the first ion exchange treatment becomes small, whereby variation in S among lots can be made small. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0052]      FIG. 1  is a graph showing the relation between R obtained by calculation from the glass composition and the decrease ratio r of the surface compressive stress due to an increase of the Na concentration in the molten potassium salt. 
           [0053]      FIG. 2  is a graph showing the relation between R′ obtained by calculation from the glass composition and the decrease ratio r of the surface compressive stress due to an increase of the Na concentration in the molten potassium salt. The straight line in the Fig. represents r=1.048×R′−0.0135, and the correlation coefficient is 0.98. The glasses used for the preparation of this graph are 67 types of glasses in total i.e. 29 types of glasses in Tables 1 to 3, 20 types of glasses in Tables 4 and 5 given hereinafter, 7 types of glasses 23 to 29 in Table 6 given hereinafter, 5 types of glasses 36 to 40 in Table 7 given hereinafter, and 6 types of glasses 41 to 46 in Table 8 given hereinafter. 
           [0054]      FIG. 3  is a graph showing the relation between R″ obtained by calculation from the glass composition and the decrease ratio r of the surface compressive stress due to an increase of the Na concentration in the molten potassium salt. The straight line in the Fig. represents r=1.014×R″+0.0074, and the correlation coefficient is 0.95. The glasses used for the preparation of this graph are 94 types of glasses in total i.e. 29 types of glasses in Tables 1 to 3, 20 types of glasses in Tables 4 and 5 given hereinafter, 7 types of glasses 23 to 29 in Table 6 given hereinafter, 5 types of glasses 36 to 40 in Table 7 given hereinafter, 6 types of glasses 41 to 46 in Table 8 given hereinafter, 8 types of glasses 49, 51 to 55, 57 and 58 in Table 9 given hereinafter, 8 types of glasses 59 to 64, 66 and 68 in Table 10 given hereinafter, 5 types of glasses 69, 73, 74, 77 and 78 in Table 11 given hereinafter, and 6 types of glasses 79 to 82, 84 and 85 in Table 12 given hereinafter. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0055]    The surface compressive stress S of chemically tempered glass to be produced by the method of the present invention (hereinafter sometimes referred to as chemically tempered glass of the present invention) is typically at least 200 MPa, but in the case of a cover glass, etc., S is preferably at least 400 MPa, more preferably at least 550 MPa, particularly preferably more than 700 MPa. Further, S is typically at most 1,200 MPa. 
         [0056]    The thickness t of the compressive stress layer of chemically tempered glass of the present invention is typically at least 10 μm, preferably at least 30 μm, more preferably more than 40 μm. Further, t is typically at most 70 μm. 
         [0057]    In the present invention, the molten salt is not particularly limited so long as Na in the glass surface layer cab be ion exchanged with K in the molten salt, and it may, for example, be molten potassium nitrate (KNO 3 ). 
         [0058]    In order to make it possible to carry out the above ion exchange, the molten salt is required to be a molten salt containing K, but there is no other restriction so long as the object of the present invention is not impaired. As the molten salt, the above-mentioned molten KNO 3  is usually used, but one containing, in addition to KNO 3 , at most about 5% of NaNO 3 , is also commonly used. Further, in the molten salt containing K, the proportion of K ions in cations is typically at least 0.7 by molar ratio. 
         [0059]    Ion exchange treatment conditions to form a chemically tempered layer (compressive stress layer) having a desired surface compressive stress may vary depending upon e.g. the thickness in the case of a glass plate. However, it is typical to immerse a glass substrate in molten KNO 3  at from 350 to 550° C. for from 2 to 20 hours. From the economical viewpoint, the immersion is carried out under conditions of from 350 to 500° C. and from 2 to 16 hours, and more preferably, the immersion time is from 2 to 10 hours. 
         [0060]    In the method of the present invention, ion exchange treatment is repeated typically in such a manner that glass is immersed in the molten salt to carry out ion exchange treatment to form chemically tempered glass, then the chemically tempered glass is taken out from the molten salt and then, another glass is immersed in the molten salt to form chemically tempered glass, and then such chemically tempered glass is taken out from the molten salt. 
         [0061]    The thickness of glass is from 0.4 to 1.2 mm, and the thickness t of a compressive stress layer of one having a glass plate made of glass of the present invention chemically tempered, is at least 30 μm, and the surface compressive stress S is preferably at least 550 MPa. Typically, t is from 40 to 60 μm, and S is from 650 to 820 MPa. 
         [0062]    A glass plate for a display device of the present invention is usually obtained by chemically tempering a glass plate obtained by processing a glass plate made of glass of the present invention by e.g. cutting, hole making, polishing, etc. 
         [0063]    The thickness of the glass plate for a display device of the present invention is typically from 0.3 to 2 mm, usually from 0.4 to 1.2 mm. 
         [0064]    The glass plate for a display device of the present invention is typically a cover glass. 
         [0065]    A method for producing a glass plate made of glass of the present invention is not particularly limited, and for example, various raw materials are mixed in proper amounts, heated and melted at from about 1,400 to 1,700° C. and then homogenized by defoaming, stirring or the like and formed into a plate by a well known float process, downdraw method or press method, which is annealed and then cut into a desired size to obtain the glass plate. 
         [0066]    The glass transition point Tg of the glass of the present invention is preferably at least 400° C. If it is lower than 400° C., the surface compressive stress is likely to be relaxed during the ion exchange, and no adequate stress may be obtained. Tg is typically at least 570° C. 
         [0067]    The Young&#39;s modulus E of the glass of the present invention is preferably at least 66 MPa. If it is less than 66 MPa, the fracture toughness tends to be low, and the glass tends to be easily broken. In a case where it is used for the production of a glass plate for a display device of the present invention, E of the glass of the present invention is preferably at least 67 MPa, more preferably at least 68 MPa, further preferably at least 69 MPa, particularly preferably at least 70 MPa. 
         [0068]    Now, the composition of the glass of the present invention will be described by using contents represented by mole percentage unless otherwise specified. 
         [0069]    SiO 2  is a component to constitute a glass matrix and is essential. If it is less than 61%, the change in the surface compressive stress due to the NaNO 3  concentration in the KNO 3  molten salt tends to be large, and cracking is likely to be formed when the glass surface is damaged, the weather resistance tends to deteriorate, the specific gravity tends to increase, or the liquid phase temperature tends to increase whereby the glass tends to be instable. It is preferably at least 62%, typically at least 63%. Further, in the fourth glass of the present invention, SiO 2  is at least 62%. 
         [0070]    If SiO 2  exceeds 77%, the temperature T2 at which the viscosity becomes 10 2  dPa·s and the temperature T4 at which the viscosity becomes 10 4  dPa·s will increase, whereby melting or molding of glass tends to be difficult, or the weather resistance tends to deteriorate. It is preferably at most 76%, more preferably at most 75%, further preferably at most 74%, particularly preferably at most 73%. 
         [0071]    Al 2 O 3  is a component to improve the ion exchange performance and weather resistance, and is essential. If it is less than 1%, it tends to be difficult to obtain the desired surface compressive stress S or compressive stress layer thickness t by ion exchange, or the weather resistance tends to deteriorate. It is preferably at least 3%, more preferably at least 4%, further preferably at least 5%, particularly preferably at least 6%, typically at least 7%. If it exceeds 18%, the change in the surface compressive stress due to the NaNO 3  concentration in the KNO 3  molten salt tends to be large, T2 or T4 tends to increase, whereby melting or molding of glass tends to be difficult, or the liquid phase temperature tends to be high, whereby devitrification is likely to occur. It is preferably at most 12%, more preferably at most 11%, further preferably at most 10%, particularly preferably at most 9%, typically at most 8%. 
         [0072]    In a case where it is particularly desired to minimize the change in the surface compressive stress due to the NaNO 3  concentration in the KNO 3  molten salt, Al 2 O 3  is preferably less than 6%. 
         [0073]    The total content of SiO 2  and Al 2 O 3  is typically from 66 to 83%. 
         [0074]    MgO is a component to improve the melting property, and is essential. If it is less than 3%, the melting property or Young&#39;s modulus tends to deteriorate. It is preferably at least 4%, more preferably at least 5%, further preferably at least 6%. In a case where it is particularly desired to increase the melting property, MgO is preferably more than 7%. 
         [0075]    If MgO exceeds 15%, the change in the surface compressive stress due to the NaNO 3  concentration in the KNO 3  molten salt tends to be large, the liquid phase temperature tends to increase, whereby devitrification is likely to occur, or the ion exchange rate tends to deteriorate. It is preferably at most 12%, more preferably at most 11%, further preferably at most 10%, particularly preferably at most 8%, typically at most 7%. 
         [0076]    CaO may be contained up to 5% in order to improve the melting property at a high temperature or to prevent devitrification, but it is likely to increase the change in the surface compressive stress due to the NaNO 3  concentration in the KNO 3  molten salt, or to lower the ion exchange rate or the durability against cracking. In a case where CaO is contained, its content is preferably at most 3%, more preferably at most 2%, further preferably at most 1.5%, particularly preferably at most 1%, most preferably at most 0.5%, and typically, no CaO is contained. 
         [0077]    In a case where CaO is contained, the total content of MgO and CaO is preferably at most 15%. If it exceeds 15%, the change in the surface compressive stress due to the NaNO 3  concentration in the KNO 3  molten salt tends to be large, or the ion exchange rate or the durability against cracking is likely to deteriorate. It is preferably at most 14%, more preferably at most 13%, further preferably at most 12%, particularly preferably at most 11%. 
         [0078]    Na 2 O is a component to reduce the change in the surface compressive stress due to a NaNO 3  concentration in the KNO 3  molten salt, to form a surface compressive stress layer by ion exchange, or to improve the melting property of glass, and is essential. If it is less than 8%, it becomes difficult to form a desired surface compressive stress layer by ion exchange, or it becomes difficult to melt or mold the glass as T2 or T4 increases. It is preferably at least 9%, more preferably at least 10%, further preferably at least 11%, particularly preferably at least 12%. If Na 2 O exceeds 18%, the weather resistance tends to deteriorate, or cracking is likely to form from an indentation. It is preferably at most 17%, more preferably at most 16%, further preferably at most 15%, particularly preferably at most 14%. 
         [0079]    K 2 O is not essential but is a component to increase the ion exchange rate, and thus, it may be contained up to 6%. If it exceeds 6%, the change in the surface compressive stress due to a NaNO 3  concentration in the KNO 3  molten salt becomes large, cracking is likely to be formed from an indentation, or the weather resistance tends to deteriorate. It is preferably at most 4%, more preferably at most 3%, further preferably at most 1.9%, particularly preferably at most 1%, and typically no K 2 O is contained. Here, the fourth glass of the present invention contains no K 2 O. 
         [0080]    In a case where K 2 O is contained, the total content R 2 O of Na 2 O and K 2 O is preferably from 8.5 to 20%. If the total content exceeds 20%, the weather resistance tends to deteriorate, or cracking is likely to be formed from an indentation. The total content is preferably at most 19%, more preferably at most 18%, further preferably at most 17%, particularly preferably at most 16%. On the other hand, if R 2 O is less than 8.5%, the melting property of glass tends to deteriorate. It is preferably at least 9%, more preferably at least 10%, further preferably at least 11%, particularly preferably at least 12%. 
         [0081]    ZrO 2  is not an essential component, but may be contained up to 4%, for example, to increase the surface compressive stress or to improve the weather resistance. If it exceeds 4%, the change in the surface compressive stress due to a NaNO 3  concentration in the KNO 3  molten salt becomes large, or the resistance against cracking tends to deteriorate. It is preferably at most 2.5%, more preferably at most 2%, further preferably at most 1%, particularly preferably at most 0.5%, and typically no ZrO 2  is contained. 
         [0082]    The glass of the present invention essentially comprises the above-described components, but may contain other components within a range not to impair the object of the present invention. In a case where such other components are contained, the total content of such components is preferably at most 5%, more preferably at most 3%, particularly preferably at most 2%, typically less than 1.5%. Now, such other components will be exemplified. 
         [0083]    SrO may be contained in order to improve the melting property at a high temperature or to prevent devitrification, but it is likely to increase the change in the surface compressive stress due to a NaNO 3  concentration in the KNO 3  molten salt, or to decrease the ion exchange rate or the durability against cracking. The content of SrO is preferably at most 1%, more preferably at most 0.5%, and typically no SrO is contained. 
         [0084]    BaO may be contained in order to improve the melting property at a high temperature or to prevent devitrification, but it may increase the change in the surface compressive stress due to a NaNO 3  concentration in the KNO 3  molten salt, or to decrease the ion exchange rate or the durability against cracking. The content of BaO is preferably at most 1%, more preferably at most 0.5%, and typically no BaO is contained. 
         [0085]    The total content RO of MgO, CaO, SrO and BaO is preferably at most 15%. If the total content exceeds 15%, the change in the surface compressive stress due to a NaNO 3  concentration in the KNO 3  molten salt becomes large, or the ion exchange rate or the durability against cracking tends to deteriorate. The total content is preferably at most 14%, more preferably at most 13%, further preferably at most 12%, particularly preferably at most 11%. 
         [0086]    ZnO may be contained in order to improve the melting property of glass at a high temperature, but in such a case, the content is preferably at most 1%. In the production by a float process, it is preferably controlled to be at most 0.5%. If it exceeds 0.5%, it is likely to be reduced during the float forming to form a product defect. Typically no ZnO is contained. 
         [0087]    B 2 O 3  is preferably at most 5% in order to improve the melting property. If it exceeds 5%, homogeneous glass tends to be hardly obtainable, and molding of glass is likely to be difficult. It is preferably at most 4%, more preferably at most 3%, further preferably at most 1.7%, further preferably at most 1%, particularly preferably at most 0.5%, and typically no B 2 O 3  is contained. 
         [0088]    In a case where SrO, BaO or B 2 O 3  is contained, the above-mentioned R′ is preferably at least 0.66. 
         [0089]    Further, the second glass of the present invention contains at least one component selected from B 2 O 3 , SrO and BaO. 
         [0090]    TiO 2  is likely to deteriorate the visible light transmittance and to color glass to be brown when it is coexistent with Fe ions in the glass, and therefore, it is preferably at most 1%, if contained, and typically, it is not contained. 
         [0091]    Li 2 O is a component to lower the strain point and to bring about a stress relaxation thereby to make it difficult to stably obtain a surface compressive stress layer and therefore is preferably at most 4.3%, more preferably at most 3%, further preferably at most 2%, particularly preferably at most 1%, and typically, no Li 2 O is contained. 
         [0092]    SnO 2  may be contained, for example, in order to improve the weather resistance, but even in such a case, the content is preferably at most 3%, more preferably at most 2%, further preferably at most 1%, particularly preferably at most 0.5%, and typically no SnO 2  is contained. 
         [0093]    Further, the third glass of the present invention contains at least one component selected from B 2 O 3 , SrO, BaO, ZnO, Li 2 O and SnO 2 . 
         [0094]    As a clarifying agent at the time of melting glass, SO 3 , a chloride or a fluoride may suitably be contained. However, in order to increase the visibility of display devices such as touch panels, it is preferred to reduce contamination by impurities such as Fe 2 O 3 , NiO or Cr 2 O 3  having an absorption in a visible light range in raw materials as far as possible, and the content of each of them is preferably at most 0.15%, more preferably at most 0.1%, particularly preferably at most 0.05%, as represented by mass percentage. 
         [0095]    In the first glass of the present invention, the above-mentioned R is at least 0.66, but when at least one component selected from B 2 O 3 , SrO, BaO, ZnO, Li 2 O and SnO 2  is contained, the total content of such components is preferably at most 5 mol %, more preferably at most 4%, further preferably at most 3%, particularly preferably at most 2%, typically less than 1.5%. 
         [0096]    In the second glass of the present invention, the above-mentioned R′ is at least 0.66, but when at least one component selected from ZnO, Li 2 O and SnO 2  is contained, the total content of such components is preferably at most 5 mol %, more preferably at most 4%, further preferably at most 3%, particularly preferably at most 2%, typically less than 1.5%. 
         [0097]    In the third glass of the present invention, the above-mentioned R″ is at least 0.66, but the total content of SiO 2 , Al 2 O 3 , MgO, CaO, ZrO 2 , Na 2 O, K 2 O, B 2 O 3 , SrO, BaO, ZnO, Li 2 O and SnO 2  is preferably more than 95 mol %, more preferably more than 96%, further preferably more than 97%, particularly preferably more than 98%, typically more than 98.5%. 
         [0098]    In the present invention, the method of repeating ion exchange treatment of glass is not particularly limited and may, for example, be carried out as follows. That is, 100 glass plates containing Na and having a size of from 150 to 600 cm 2  are put in a basket provided with slits, so that each glass plate is placed between adjacent slits so that glass plates are not in contact with one another. In a tank having a capacity of 100,000 cm 3  filled with a molten potassium salt of 400° C., the basket is immersed for 8 hours to carry out ion exchange treatment, and then, the basket is taken out. Then, a basket having other glass plates put therein is immersed in the above tank, and ion exchange treatment is repeated. 
       EXAMPLES 
       [0099]    Glasses 1 and 2 in Table 1 and glass A21 in Table 3 are Examples of the glass of the present invention, and they were prepared as follows. That is, raw materials for the respective components were blended to have compositions as represented by mole percentage in columns for SiO 2  to K 2 O in the Tables and melted at a temperature of from 1,550 to 1,650° C. for from 3 to 5 hours by means of a platinum crucible. During the melting, a platinum stirrer was inserted in molten glass, and the glass was stirred for 2 hours and homogenized. Then, the molten glass was cast and formed into a plate and annealed to room temperature at a cooling rate of 1° C./min. 
         [0100]    Further, glasses in Examples 3 to 29 and 36 to 46 having compositions as represented by mole percentage in columns for SiO 2  to K 2 O in Tables 4 to 8, and glasses in Examples 49 to 82, 84 and 85 having compositions as represented by mole percentage in columns for SiO 2  to SnO 2  in Tables 9 to 12, were prepared in the same manner as the preparation of the above glasses 1, 2 and A21. 
         [0101]    With respect to these glasses, Tg (unit: ° C.), the Young&#39;s modulus E (unit: MPa), R, R′, R″, CS1 (unit: MPa), CS2 (unit: MPa) and r are shown in the Tables. Further, Tg in Examples 13 to 17, 36 to 38, 41 to 46, 61, 63, 75, 77 to 82 and 84, and E in Examples 13 to 18, 20, 23 to 25, 28, 36 to 40, 43 to 46 and 79 to 82, were obtained by calculation or assumption from the compositions, and with respect to Examples 50, 56, 65, 67, 70 to 72, 75 and 76, CS1, CS2 and r could not be accurately measured and thus were obtained by calculation or assumption from the compositions. The glasses in Examples 41 and 42 are not the glass of the present invention, and MgO is less than 3%, the Young&#39;s modulus is also low, and the fracture strength is likely to be small. 
         [0102]    With respect to the glasses in Examples 30 to 35 in Tables 6 and 7, in Examples 47 and 48 in Table 8 and in Example 83 in Table 12, melting as described above was not carried out, and Tg, E, CS1, CS2 and r shown in these Tables were obtained by calculation of assumption from the compositions. 
         [0103]    Examples 3 to 30, 32 to 35, 41, 42, 47, 49 to 80, 84 and 85 are Examples of the present invention. Further, Examples 41, 42 and 56 to 78 are Reference Examples of the first invention, and Examples 16, 35, 42, 79 and 80 are Reference Examples of the fourth invention. 
         [0104]    Examples 31, 37 to 40, 43 to 46, 48, 82 and 83 are Comparative Examples of the present invention, and Examples 36 and 81 are Reference Examples. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Ex. 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SiO 2   
                 75.5 
                 73.0 
                 73.0 
                 73.0 
                 73.0 
                 73.2 
                 72.0 
                 72.0 
                 72.0 
                 72.0 
               
               
                 Al 2 O 3   
                 4.9 
                 5.0 
                 5.0 
                 7.0 
                 7.0 
                 7.0 
                 7.0 
                 7.0 
                 6.0 
                 6.0 
               
               
                 MgO 
                 5.9 
                 8.0 
                 10.0 
                 5.5 
                 5.5 
                 5.5 
                 10.0 
                 9.0 
                 12.0 
                 14.0 
               
               
                 CaO 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 ZrO 2   
                 0 
                 0 
                 0 
                 0.5 
                 0.5 
                 0.3 
                 0 
                 0 
                 0 
                 0 
               
               
                 Na 2 O 
                 13.7 
                 14.0 
                 12.0 
                 14.0 
                 14.0 
                 14.0 
                 11.0 
                 12.0 
                 10.0 
                 8.0 
               
               
                 K 2 O 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 Tg 
                 586 
                 600 
                 632 
                 625 
                 617 
                 620 
                 674 
                 660 
                 678 
                 701 
               
               
                 E 
                 69.7 
                 70.6 
                 72.9 
                 73.0 
                 72.3 
                 74.6 
                 72.8 
                 72.3 
                 74.3 
                 73.3 
               
               
                 R 
                 0.78 
                 0.75 
                 0.73 
                 0.76 
                 0.76 
                 0.77 
                 0.71 
                 0.73 
                 0.69 
                 0.67 
               
               
                 R′ 
                 0.78 
                 0.75 
                 0.73 
                 0.76 
                 0.76 
                 0.77 
                 0.71 
                 0.73 
                 0.69 
                 0.67 
               
               
                 R″ 
                 0.78 
                 0.75 
                 0.73 
                 0.76 
                 0.76 
                 0.77 
                 0.71 
                 0.73 
                 0.69 
                 0.67 
               
               
                 CS1 
                 684 
                 810 
                 895 
                 915 
                 870 
                 889 
                 940 
                 963 
                 862 
                 681 
               
               
                 CS2 
                 575 
                 651 
                 637 
                 719 
                 696 
                 699 
                 667 
                 711 
                 595 
                 502 
               
               
                 r 
                 0.84 
                 0.80 
                 0.71 
                 0.79 
                 0.80 
                 0.79 
                 0.71 
                 0.74 
                 0.69 
                 0.74 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Ex. 
                 13 
                 14 
                 15 
                 16 
                 17 
                 18 
                 19 
                 20 
                 21 
                 22 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SiO 2   
                 71.7  
                 71.4  
                 70.0  
                 70.1  
                 71.1  
                 73.6  
                 72.4 
                 74.0  
                 72.0 
                 73.6 
               
               
                 Al 2 O 3   
                 7.1  
                 8.2  
                 9.0  
                 6.0  
                 9.3  
                 6.5  
                 7.5 
                 7.0  
                 7.0 
                 7.0 
               
               
                 MgO 
                 8.1  
                 6.1  
                 7.0  
                 10.3  
                 4.1  
                 6.0  
                 6.0 
                 5.0  
                 7.0 
                 6.0 
               
               
                 CaO 
                 0   
                 0   
                 0   
                 0   
                 0   
                 0   
                 0 
                 0   
                 0 
                 0 
               
               
                 ZrO 2   
                 0   
                 0   
                 0   
                 0.63 
                 0   
                 0   
                 0 
                 0   
                 0 
                 0 
               
               
                 Na 2 O 
                 13.1  
                 14.3  
                 14.0  
                 12.0  
                 15.5  
                 13.9  
                 14.1 
                 14.0  
                 14.0 
                 13.4 
               
               
                 K 2 O 
                 0   
                 0   
                 0   
                 1.0  
                 0   
                 0   
                 0 
                 0   
                 0 
                 0 
               
               
                 Tg 
                 603*    
                 603*    
                 609*    
                 596*    
                 603*    
                 613    
                 628 
                 613    
                 623 
                 626 
               
               
                 E 
                 74*   
                 72*   
                 73*   
                 75*   
                 71*   
                 72*   
                 69.3 
                 71*   
                 69.7 
                 69.3 
               
               
                 R 
                 0.74 
                 0.75 
                 0.74 
                 0.68 
                 0.77 
                 0.77 
                 0.76 
                 0.78 
                 0.75 
                 0.76 
               
               
                 R′ 
                 0.74 
                 0.75 
                 0.74 
                 0.68 
                 0.77 
                 0.77 
                 0.76 
                 0.78 
                 0.75 
                 0.76 
               
               
                 R″ 
                 0.74 
                 0.75 
                 0.74 
                 0.68 
                 0.77 
                 0.77 
                 0.76 
                 0.78 
                 0.75 
                 0.76 
               
               
                 CS1 
                 963    
                 972    
                 1065     
                 952    
                 936    
                 816    
                 926 
                 811    
                 917 
                 881 
               
               
                 CS2 
                 725    
                 753    
                 790    
                 667    
                 748    
                 667    
                 711 
                 662    
                 689 
                 718 
               
               
                 r 
                 0.75 
                 0.77 
                 0.74 
                 0.70 
                 0.80 
                 0.82 
                 0.77 
                 0.82 
                 0.75 
                 0.81 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                 Ex. 
                 23 
                 24 
                 25 
                 26 
                 27 
                 28 
                 29 
                 30 
                 31 
                 32 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SiO 2   
                 72.4  
                 73.7  
                 72.3  
                 73.0 
                 72.6 
                 73.4  
                 72.5  
                 77.0 
                 60.0 
                 77.0 
               
               
                 Al 2 O 3   
                 7.0  
                 8.1  
                 5.9  
                 8.0 
                 7.0 
                 7.0  
                 6.2  
                 3.0 
                 12.0 
                 3.0 
               
               
                 MgO 
                 6.0  
                 4.0  
                 7.9  
                 6.0 
                 7.0 
                 5.0  
                 8.5  
                 3.0 
                 10.0 
                 12.0 
               
               
                 CaO 
                 0   
                 0   
                 0   
                 0 
                 0 
                 0   
                 0   
                 0 
                 0 
                 0 
               
               
                 ZrO 2   
                 0   
                 0   
                 0   
                 0 
                 0 
                 0   
                 0   
                 0 
                 0 
                 0 
               
               
                 Na 2 O 
                 14.6  
                 14.1  
                 13.9  
                 13.0 
                 13.4 
                 14.6  
                 12.8  
                 17.0 
                 18.0 
                 8.0 
               
               
                 K 2 O 
                 0   
                 0   
                 0   
                 0 
                 0 
                 0   
                 0   
                 0 
                 0 
                 0 
               
               
                 Tg 
                 603    
                 625    
                 612    
                 654 
                 631 
                 604    
                 627    
                 552 
                 592 
                 613 
               
               
                 E 
                 72*   
                 70*   
                 73*   
                 70.0 
                 69.9 
                 71*   
                 70.2  
                 68 
                 76 
                 76 
               
               
                 R 
                 0.76 
                 0.78 
                 0.75 
                 0.76 
                 0.75 
                 0.78 
                 0.74 
                 0.84 
                 0.67 
                 0.72 
               
               
                 R′ 
                 0.76 
                 0.78 
                 0.75 
                 0.76 
                 0.75 
                 0.78 
                 0.74 
                 0.84 
                 0.67 
                 0.72 
               
               
                 R″ 
                 0.76 
                 0.78 
                 0.75 
                 0.76 
                 0.75 
                 0.78 
                 0.74 
                 0.84 
                 0.67 
                 0.72 
               
               
                 CS1 
                 835    
                 855    
                 883    
                 941 
                 925 
                 807    
                 915    
                 1100 
                 1400 
                 1000 
               
               
                 CS2 
                 681    
                 683    
                 678    
                 725 
                 696 
                 656    
                 688    
                 957 
                 896 
                 730 
               
               
                 r 
                 0.82 
                 0.80 
                 0.77 
                 0.77 
                 0.75 
                 0.81 
                 0.75 
                 0.87 
                 0.64 
                 0.73 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
               
                 Ex. 
                 33 
                 34 
                 35 
                 36 
                 37 
                 38 
                 39 
                 40 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SiO 2   
                 77.0 
                 77.0 
                 77.0 
                 68.3  
                 66.4  
                 66.0  
                 64.0  
                 65.5  
               
               
                 Al 2 O 3   
                 3.0 
                 3.0 
                 3.0 
                 6.0  
                 6.0  
                 7.0  
                 5.4 
                 5.0  
               
               
                 MgO 
                 3.0 
                 3.0 
                 3.0 
                 10.5  
                 10.8  
                 11.0  
                 5.4 
                 12.0  
               
               
                 CaO 
                 3.0 
                 0 
                 0 
                 0   
                 0   
                 0   
                 4.0 
                 0   
               
               
                 SrO 
                 0 
                 0 
                 0 
                 0   
                 0   
                 0   
                 0   
                 0   
               
               
                 BaO 
                 0 
                 0 
                 0 
                 0   
                 0   
                 0   
                 0   
                 0   
               
               
                 ZrO 2   
                 0 
                 4.0 
                 0 
                 1.3  
                 1.9  
                 0   
                 2.5 
                 2.5  
               
               
                 Na 2 O 
                 14.0 
                 13.0 
                 11.0 
                 12.0  
                 12.0  
                 12.0  
                 9.6 
                 10.0  
               
               
                 K 2 O 
                 0 
                 0 
                 6.0 
                 2.0  
                 3.0  
                 4.0  
                 9.1 
                 5.0  
               
               
                 Tg 
                 574 
                 610 
                 570 
                 601*    
                 599*    
                 587*    
                 575    
                 632    
               
               
                 E 
                 70 
                 73 
                 63 
                 75*   
                 75*   
                 73*   
                 69*   
                 76*   
               
               
                 R 
                 0.74 
                 0.78 
                 0.66 
                 0.64 
                 0.60 
                 0.58 
                  0.36 
                 0.52 
               
               
                 R′ 
                 0.74 
                 0.78 
                 0.66 
                 0.64 
                 0.60 
                 0.58 
                  0.36 
                 0.52 
               
               
                 R″ 
                 0.74 
                 0.78 
                 0.66 
                 0.64 
                 0.60 
                 0.58 
                  0.36 
                 0.52 
               
               
                 CS1 
                 1000 
                 1200 
                 800 
                 988    
                 1002     
                 876    
                 686    
                 847    
               
               
                 CS2 
                 740 
                 996 
                 600 
                 652    
                 616    
                 542    
                 262    
                 482    
               
               
                 r 
                 0.74 
                 0.83 
                 0.75 
                 0.66 
                 0.61 
                 0.62 
                  0.38 
                 0.57 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 8 
               
               
                   
               
               
                 Ex. 
                 41 
                 42 
                 43 
                 44 
                 45 
                 46 
                 47 
                 48 
               
               
                   
               
             
             
               
                 SiO 2   
                  64.2 
                 64.4  
                 64.3  
                 64.3  
                 64.3  
                 64.3  
                 64.3  
                 60.3  
               
               
                 Al 2 O 3   
                  12.6 
                 14.0  
                 8.0 
                 8.0 
                 8.0 
                 8.0 
                 11.5  
                 13.5  
               
               
                 B 2 O 3   
                   9.6 
                 6.9  
                 0   
                 0   
                 0   
                 0   
                 0   
                 0   
               
               
                 MgO 
                 0 
                 0   
                 6.5 
                 3.5 
                 5.5 
                 4.5 
                 9.0 
                 11.0  
               
               
                 CaO 
                 0 
                 0.1  
                 0.1 
                 3.1 
                 1.1 
                 2.1 
                 0.1 
                 0.1  
               
               
                 SrO 
                 0 
                 0   
                 4.1 
                 0.1 
                 2.6 
                 1.6 
                 0.1 
                 0.1  
               
               
                 BaO 
                 0 
                 0   
                 0.1 
                 4.1 
                 1.6 
                 2.6 
                 0.1 
                 0.1  
               
               
                 ZrO 2   
                 0 
                 0   
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0   
                 0   
               
               
                 Na 2 O 
                  13.6 
                 14.1  
                 12.5  
                 12.5  
                 12.5  
                 12.5  
                 14.9  
                 15.0  
               
               
                 K 2 O 
                 0 
                 0.5  
                 4.0 
                 4.0 
                 4.0 
                 4.0 
                 0   
                 0   
               
               
                 Tg 
                 602*  
                 615*    
                 598*   
                 608*   
                 596*   
                 601*   
                 615*   
                 625*    
               
               
                 E 
                 64  
                 65    
                 72*   
                 69*   
                 71*   
                 70*   
                 76*   
                 78*   
               
               
                 R 
                   0.52 
                 0.57 
                  0.50 
                  0.44 
                  0.48 
                  0.46 
                  0.68 
                 0.64 
               
               
                 R′ 
                   0.79 
                 0.76 
                  0.56 
                  0.55 
                  0.56 
                  0.55 
                  0.68 
                 0.64 
               
               
                 R″ 
                   0.79 
                 0.76 
                  0.56 
                  0.55 
                  0.56 
                  0.55 
                  0.68 
                 0.64 
               
               
                 CS1 
                 857  
                 1024     
                 938    
                 844    
                 903    
                 901    
                 1200    
                 1400     
               
               
                 CS2 
                 698  
                 793    
                 530    
                 474    
                 523    
                 511    
                 804    
                 854    
               
               
                 r 
                   0.81 
                 0.77 
                  0.56 
                  0.56 
                  0.58 
                  0.57 
                  0.67 
                 0.61 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 9 
               
               
                   
               
               
                 Ex. 
                 49 
                 50 
                 51 
                 52 
                 53 
                 54 
                 55 
                 56 
                 57 
                 58 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SiO 2   
                 66.6 
                 66.6 
                 66.6 
                 72.8 
                 72.8 
                 72.7 
                 63.6 
                 64.7 
                 61.7 
                 66.7 
               
               
                 Al 2 O 3   
                 5.6 
                 12.5 
                 12.5 
                 4.5 
                 10.2 
                 6.8 
                 6.8 
                 2.8 
                 2.8 
                 8.3 
               
               
                 B 2 O 3   
                 5.6 
                 4.2 
                 4.2 
                 4.5 
                 3.4 
                 2.3 
                 2.3 
                 8.3 
                 8.3 
                 8.3 
               
               
                 MgO 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 9.1 
                 0 
                 0 
                 0 
               
               
                 ZnO 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 2.0 
                 5.0 
                 0 
               
               
                 Li 2 O 
                 0 
                 0 
                 0.1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 Na 2 O 
                 22.2 
                 16.7 
                 16.6 
                 18.2 
                 13.6 
                 18.2 
                 18.2 
                 22.2 
                 22.2 
                 16.7 
               
               
                 Tg 
                 562 
                 591 
                 586 
                 569 
                 605 
                 561 
                 571 
                 556 
                 549 
                 572 
               
               
                 E 
                 74.4 
                 70.9 
                 70.0 
                 74.2 
                 69.6 
                 70.9 
                 72.2 
                 75.4 
                 69.0 
                 70.2 
               
               
                 R 
                 0.69 
                 0.68 
                 0.67 
                 0.73 
                 0.72 
                 0.78 
                 0.66 
                 0.58 
                 0.49 
                 0.59 
               
               
                 R′ 
                 0.85 
                 0.79 
                 0.79 
                 0.86 
                 0.81 
                 0.84 
                 0.72 
                 0.81 
                 0.72 
                 0.82 
               
               
                 R″ 
                 0.85 
                 0.79 
                 0.79 
                 0.86 
                 0.81 
                 0.84 
                 0.72 
                 0.85 
                 0.82 
                 0.82 
               
               
                 CS1 
                 685 
                 1250 
                 1138 
                 682 
                 985 
                 642 
                 1058 
                 950 
                 1030 
                 925 
               
               
                 CS2 
                 628 
                 1025 
                 931 
                 622 
                 820 
                 525 
                 760 
                 808 
                 782 
                 831 
               
               
                 r 
                 0.92 
                 0.82 
                 0.82 
                 0.91 
                 0.83 
                 0.82 
                 0.72 
                 0.85 
                 0.76 
                 0.90 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 10 
               
               
                   
               
               
                 Ex. 
                 59 
                 60 
                 61 
                 62 
                 63 
                 64 
                 65 
                 66 
                 67 
                 68 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SiO 2   
                 66.6 
                 66.6 
                 64.6 
                 66.7 
                 64.6  
                 64.6 
                 72.8 
                 63.7 
                 63.7 
                 63.6 
               
               
                 Al 2 O 3   
                 16.7 
                 16.7 
                 16.7 
                 12.5 
                 12.5  
                 12.5 
                 3.4 
                 4.5 
                 3.4 
                 2.3 
               
               
                 B 2 O 3   
                 5.6 
                 5.6 
                 5.6 
                 4.2 
                 4.2  
                 4.2 
                 10.2 
                 13.6 
                 10.2 
                 6.8 
               
               
                 MgO 
                 0 
                 0 
                 0 
                 0 
                 0   
                 0 
                 0 
                 9.1 
                 9.1 
                 9.1 
               
               
                 ZnO 
                 0 
                 0 
                 0 
                 0 
                 2.0  
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 Li 2 O 
                 0 
                 2.0 
                 0 
                 2.0 
                 0   
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 Na 2 O 
                 11.1 
                 9.1 
                 11.1 
                 14.6 
                 16.7  
                 16.7 
                 13.6 
                 9.1 
                 13.6 
                 18.2 
               
               
                 SnO 2   
                 0 
                 0 
                 2.0 
                 0 
                 0   
                 2.0 
                 0 
                 0 
                 0 
                 0 
               
               
                 Tg 
                 634 
                 618 
                 630 
                 553 
                 592*    
                 605 
                 571 
                 552 
                 563 
                 563 
               
               
                 E 
                 65.4 
                 65.6 
                 63.3 
                 72.6 
                 68.3  
                 68.5 
                 71.1 
                 65.8 
                 72.1 
                 73.5 
               
               
                 R 
                 0.60 
                 0.54 
                 0.54 
                 0.62 
                 0.62 
                 0.62 
                 0.58 
                 0.35 
                 0.46 
                 0.56 
               
               
                 R′ 
                 0.76 
                 0.70 
                 0.70 
                 0.74 
                 0.74 
                 0.74 
                 0.86 
                 0.73 
                 0.74 
                 0.75 
               
               
                 R″ 
                 0.76 
                 0.77 
                 0.76 
                 0.80 
                 0.77 
                 0.80 
                 0.86 
                 0.73 
                 0.74 
                 0.75 
               
               
                 CS1 
                 915 
                 932 
                 897 
                 1090 
                 1123     
                 1229 
                 700 
                 586 
                 750 
                 1016 
               
               
                 CS2 
                 688 
                 705 
                 744 
                 874 
                 917    
                 951 
                 630 
                 398 
                 540 
                 701 
               
               
                 r 
                 0.75 
                 0.76 
                 0.83 
                 0.80 
                 0.82 
                 0.77 
                 0.90 
                 0.68 
                 0.72 
                 0.69 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 11 
               
               
                   
               
               
                 Ex. 
                 69 
                 70 
                 71 
                 72 
                 73 
                 74 
                 75 
                 76 
                 77 
                 78 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SiO 2   
                 63.6 
                 63.7 
                 63.7 
                 63.7 
                 63.7 
                 66.7 
                 68.3  
                 68.3 
                 61.6  
                 61.6  
               
               
                 Al 2 O 3   
                 9.1 
                 6.8 
                 4.5 
                 13.6 
                 10.2 
                 2.8 
                 3.4  
                 6.8 
                 16.7  
                 12.5  
               
               
                 B 2 O 3   
                 9.1 
                 6.8 
                 4.5 
                 4.5 
                 3.4 
                 8.3 
                 10.2  
                 6.8 
                 5.6  
                 4.2  
               
               
                 MgO 
                 9.1 
                 9.1 
                 9.1 
                 9.1 
                 9.1 
                 0 
                 0   
                 0 
                 0   
                 0   
               
               
                 ZnO 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0   
                 0 
                 5.0  
                 5.0  
               
               
                 ZrO 2   
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 4.5  
                 4.5 
                 0   
                 0   
               
               
                 Na 2 O 
                 9.1 
                 13.6 
                 18.2 
                 9.1 
                 13.6 
                 22.2 
                 13.6  
                 13.6 
                 11.1  
                 16.7  
               
               
                 Tg 
                 576 
                 571 
                 562 
                 650 
                 598 
                 574 
                 571*    
                 589 
                 643*    
                 577*    
               
               
                 E 
                 64.8 
                 72.5 
                 73.9 
                 72.3 
                 71.1 
                 78.3 
                 68.5  
                 68 
                 76.7  
                 68.1  
               
               
                 R 
                 0.44 
                 0.53 
                 0.61 
                 0.54 
                 0.60 
                 0.63 
                 0.52 
                 0.59 
                 0.46 
                 0.53 
               
               
                 R′ 
                 0.70 
                 0.72 
                 0.74 
                 0.67 
                 0.69 
                 0.87 
                 0.80 
                 0.78 
                 0.61 
                 0.65 
               
               
                 R″ 
                 0.70 
                 0.72 
                 0.74 
                 0.67 
                 0.69 
                 0.87 
                 0.80 
                 0.78 
                 0.71 
                 0.74 
               
               
                 CS1 
                 709 
                 950 
                 930 
                 740 
                 1102 
                 837 
                 940    
                 1020 
                 963    
                 1246     
               
               
                 CS2 
                 502 
                 665 
                 660 
                 488 
                 786 
                 769 
                 780    
                 826 
                 698    
                 927    
               
               
                 r 
                 0.71 
                 0.70 
                 0.71 
                 0.66 
                 0.71 
                 0.92 
                 0.83 
                 0.81 
                 0.72 
                 0.74 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 12 
               
               
                   
               
               
                 Ex. 
                 79 
                 80 
                 81 
                 82 
                 83 
                 84 
                 85 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SiO 2   
                 64.0 
                 63.0 
                 61.0 
                 65.3 
                 66.7 
                 68.0 
                 68.0 
               
               
                 Al 2 O 3   
                 11.0 
                 12.0 
                 11.0 
                 7.0 
                 3.6 
                 9.0 
                 10.0 
               
               
                 MgO 
                 9.0 
                 7.0 
                 13.0 
                 11.2 
                 12.1 
                 8.0 
                 8.0 
               
               
                 CaO 
                 0 
                 0 
                 0 
                 0 
                 1.1 
                 0 
                 0 
               
               
                 SrO 
                 0 
                 0 
                 0 
                 0 
                 0.6 
                 0 
                 0 
               
               
                 ZrO 2   
                 0 
                 0 
                 0.8 
                 0.5 
                 0.7 
                 0 
                 0 
               
               
                 Na 2 O 
                 15.0 
                 17.0 
                 14.2 
                 9.0 
                 11.0 
                 15.0 
                 14.0 
               
               
                 K 2 O 
                 1.0 
                 1.0 
                 0 
                 7.0 
                 4.2 
                 0 
                 0 
               
               
                 Tg 
                 607 
                 600 
                 618 
                 600 
                 574 
                 632 
                 663 
               
               
                 E 
                 74.5 
                 73.0 
                 79.8 
                 71.3 
                 74.4 
                 71.1 
                 72.1 
               
               
                 R 
                 0.66 
                 0.68 
                 0.63 
                 0.49 
                 0.53 
                 0.72 
                 0.71 
               
               
                 R′ 
                 0.66 
                 0.68 
                 0.63 
                 0.49 
                 0.53 
                 0.72 
                 0.71 
               
               
                 R″ 
                 0.66 
                 0.68 
                 0.63 
                 0.49 
                 0.53 
                 0.72 
                 0.71 
               
               
                 CS1 
                 1178 
                 1223 
                 1231 
                 646 
                 500 
                 1141 
                 1189 
               
               
                 CS2 
                 817 
                 859 
                 810 
                 376 
                 260 
                 839 
                 855 
               
               
                 r 
                 0.69 
                 0.70 
                 0.66 
                 0.58 
                 0.52 
                 0.74 
                 0.72 
               
               
                   
               
             
          
         
       
     
       INDUSTRIAL APPLICABILITY 
       [0105]    The method of the present invention is useful for the production of e.g. a cover glass for display devices. Further, it is useful also for the production of e.g. a solar cell substrate or a window glass for aircrafts. 
         [0106]    The entire disclosures of Japanese Patent Application No. 2011-114783 filed on May 23, 2011 and Japanese Patent Application No. 2011-247766 filed on Nov. 11, 2011 including specifications, claims, drawings and summaries are incorporated herein by reference in their entireties.